LARGE-SCALE TESTS OF SEISMICALLY ENHANCED PLANAR WALLS FOR
RESIDENTIAL CONSTRUCTION
A Thesis
Presented to the faculty of the Department of Civil Engineering
California State University, Sacramento
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
Civil Engineering
by
Amy Kathleen Hopkins
SPRING
2013
LARGE-SCALE TESTS OF SEISMICALLY ENHANCED PLANAR WALLS FOR
RESIDENTIAL CONSTRUCTION
A Thesis
by
Amy Kathleen Hopkins
Approved by:
__________________________________, Committee Chair
Dr. Benjamin Fell, P.E.
__________________________________, Second Reader
Dr. Matthew Salveson, P.E.
____________________________
Date
ii
Student: Amy Kathleen Hopkins
I certify that this student has met the requirements for format contained in the University format
manual, and that this thesis is suitable for shelving in the Library and credit is to be awarded for
the thesis.
__________________________, Graduate Coordinator ___________________
Dr. Matthew Salveson, P.E.
Date
Department of Civil Engineering
iii
Abstract
of
LARGE-SCALE TESTS OF SEISMICALLY ENHANCED PLANAR WALLS FOR
RESIDENTIAL CONSTRUCTION
by
Amy Kathleen Hopkins
As part of an investigation to reduce seismic damage to partition walls and other finishes in lightframe residential buildings, twenty full-scale gypsum-sheathed walls, built with wood and coldformed steel framing members, were tested.
The experiments investigated the effects of
enhanced, inexpensive construction procedures with the objective to increase the racking strength
and stiffness of partition-type shear walls, lessening seismic deformations.
The stiffness,
strength, and damage progression of the specimens with varying wall length, openings,
orthogonal wall returns, tie-down and anchoring configurations, and interior and exterior
sheathings are reported in this thesis.
Iterative tests of specific interior wall geometries
determined the optimal construction techniques required to reduce deformations and improve lifecycle performance. The main improvement to these specimens over typical construction was the
use of construction adhesive and mechanical fasteners to attach the sheathing to the framing.
Additional enhancements included mid-height blocking, improved corner stud assemblies,
properly sized tie downs at the ends of wall segments, and bent straps on the exterior of planar
wood-framed walls. The stiffness, strength, and residual capacity of specimens with orthogonal
walls increased as compared to specimens with in-plane-only shear walls.
_______________________, Committee Chair
Dr. Benjamin Fell, P.E.
_______________________
Date
iv
ACKNOWLEDGEMENTS
The support of the National Science Foundation (NSF) is appreciated for the funding of
this research through a Network for Earthquake Engineering Simulation (NEESR) award CMMI1135029.
The collaboration with the following industry panel in developing the testing plans is
greatly appreciated: Ben Schmidt (consultant), Rene Vignos, Geoff Bomba and Ali
Roufegarinejad (Forell/Elsesser Engineers), Greg Luth (GPLA, Inc.), David Mar (Tipping-Mar),
Kelly Cobeen (WJE, inc.), John Osteraas (Exponent), and Reynaud Serrett (Santa Clara
University). The collaboration with, and material donations from, Simpson Strong-Tie is greatly
appreciated.
The specimens were built and tested in the Structures Laboratory at California State
University, Sacramento (CSUS). The staff of the CSUS Tech Shop, specifically James Ster and
Mike Newton, were indispensable in the development and manufacturing of the test frame and
providing their support on test days.
Members of the Carpenter Union Local 46 and
undergraduate students of the CSUS civil engineering department are appreciated for their
various assistance in the creation and installation of specimens during the testing process.
Most importantly, without the assistance of a team, including my advisors Benjamin Fell,
Gregory Deierlein, and Eduardo Miranda, and fellow students at CSUS, including Maxwell
Hardy, Vandalist Kith, Nelson Tejada, and at Stanford University, Scott Swensen, Cristian
Acevedo, and Ezra Jampole, this project would not have been completed. Lastly, the lectures
provided by the instructors of the CSUS Civil Engineering department are greatly appreciated for
improving my understanding of the force path within a structure and ensuring the test frame and
specimens were properly designed to represent walls in residential buildings.
v
TABLE OF CONTENTS
Page
Acknowledgements .......................................................................................................................... v
List of Tables ................................................................................................................................. xii
List of Figures ............................................................................................................................... xiv
Chapter
1. INTRODUCTION ...................................................................................................................... 1
1.1 Motivation ........................................................................................................................... 1
1.2 Objectives and Scope .......................................................................................................... 4
1.3 Organization and Outline .................................................................................................... 5
2. SUMMARY OF PREVIOUS STUDIES ON LIGHT-FRAME CONSTRUCTION ............... 10
2.1 Introduction ....................................................................................................................... 10
2.2 Current Seismic Design Approaches for Light-Frame Buildings...................................... 11
2.3 Past Research on Light-Frame Construction (Simulation and Testing) ............................ 12
2.4 Gypsum Sheathed Partition Walls by McMullin and Merrick (2001) .............................. 15
2.5 Exterior Walls by Arnold et al. (2003) .............................................................................. 16
2.6 Components of Partition Walls by Swensen et al. (2012) ................................................. 17
2.7 Typical Construction Review ............................................................................................ 19
2.7.1 Typical Wood-frame Construction .......................................................................... 19
2.7.2 Typical Steel-frame Construction ............................................................................ 20
3. TESTING PROGRAM ............................................................................................................. 33
3.1 Introduction ....................................................................................................................... 33
3.2 Test Setup .......................................................................................................................... 33
3.3 Wall Construction Details and Material Properties ........................................................... 35
vi
3.3.1 Wood Framing ......................................................................................................... 35
3.3.2 Steel Framing ........................................................................................................... 36
3.3.3 Sheathing ................................................................................................................. 36
3.4 Test Matrix ........................................................................................................................ 37
3.5 Loading History................................................................................................................. 38
3.6 Instrumentation.................................................................................................................. 39
4. EXPERIMENTAL RESULTS OF CYCLIC TESTED PLANAR WOOD-FRAMED
WALLS ..................................................................................................................................... 68
4.1 Introduction ....................................................................................................................... 68
4.2 Planar Control Test: W1 .................................................................................................... 69
4.2.1 Summary and Overall Behavior............................................................................... 70
4.2.2 Observed Behavior 0-0.5% Interstory Drift ............................................................. 70
4.2.3 Observed Behavior Post 0.5% Interstory Drift ........................................................ 72
4.3 Planar Wall Tests With Unibody Enhancements: W2 Through W6 ................................. 73
4.3.1 W2: Characteristics .................................................................................................. 74
4.3.1.1 W2: Summary and overall behavior .............................................................. 74
4.3.1.2 W2: Observed behavior 0-0.5% interstory drift ............................................ 75
4.3.1.3 W2: Observed behavior post 0.5% interstory drift ........................................ 76
4.3.2 W3: Characteristics .................................................................................................. 77
4.3.2.1 W3: Summary and overall Behavior ............................................................. 77
4.3.2.2 W3: Observed behavior 0-0.5% interstory drift ............................................ 78
4.3.2.3 W3: Observed behavior post 0.5% interstory drift ........................................ 79
4.3.3 W4: Characteristics .................................................................................................. 80
4.3.3.1 W4: Summary and overall behavior .............................................................. 81
vii
4.3.3.2 W4: Observed behavior 0-0.5% interstory drift ............................................ 81
4.3.3.3 W4: Observed behavior post 0.5% interstory drift ........................................ 82
4.3.4 W5: Characteristics .................................................................................................. 83
4.3.4.1 W5: Summary and overall behavior .............................................................. 84
4.3.4.2 W5: Observed behavior 0-0.5% interstory drift ............................................ 85
4.3.4.3 W5: Observed behavior post 0.5% interstory drift ........................................ 86
4.3.5 W6: Characteristics .................................................................................................. 87
4.3.5.1 W6: Summary and overall behavior ............................................................... 88
4.3.5.2 W6: Observed behavior 0-0.5% interstory drift ............................................ 88
4.3.5.3 W6: Observed behavior post 0.5% interstory drift ........................................ 89
4.4 Planar Wall Tests With End Returns and Unibody Enhancements: W7 and W8 ............. 90
4.4.1 W7: Characteristics .................................................................................................. 91
4.4.1.1 W7: Summary and overall behavior .............................................................. 91
4.4.1.2 W7: Observed behavior 0-0.5% interstory drift ............................................ 92
4.4.1.3 W7: Observed behavior post 0.5% interstory drift ........................................ 93
4.4.2 W8: Characteristics .................................................................................................. 94
4.4.2.1 W8: Summary and overall Behavior ............................................................. 95
4.4.2.2 W8: Observed behavior 0-0.5% interstory drift ............................................ 95
4.4.2.3 W8: Observed behavior post 0.5% interstory drift ........................................ 96
4.5 Planar Wall Tests With Openings, Varying Aspect Ratios and Unibody
Enhancements: W9 Through W11 .................................................................................... 97
4.5.1 W9: Characteristics .................................................................................................. 98
4.5.1.1 W9: Summary and overall behavior .............................................................. 99
4.5.1.2 W9: Observed behavior 0-0.5% interstory drift ............................................ 99
viii
4.5.1.3 W9: Observed behavior post 0.5% interstory d ........................................... 100
4.5.2 W10: Characteristics .............................................................................................. 101
4.5.2.1 W10: Summary and overall behavior .......................................................... 101
4.5.2.2 W10: Observed behavior 0-0.5% interstory drift ........................................ 102
4.5.2.3 W10: Observed behavior post 0.5% interstory drift .................................... 103
4.5.3 W11: Characteristics .............................................................................................. 103
4.5.3.1 W11: Summary and overall behavior .......................................................... 104
4.5.3.2 W11: Observed behavior 0-0.5% interstory drift ........................................ 104
4.5.3.3 W11: Observed behavior post 0.5% interstory drift .................................... 105
5. EXPERIMENTAL RESULTS OF CYCLIC TESTED PLANAR STEEL-FRAMED
WALLS ................................................................................................................................... 181
5.1 Introduction ..................................................................................................................... 181
5.2 Planar Control Test: S1 ................................................................................................... 182
5.2.1 Summary and Overall Behavior............................................................................. 182
5.2.2 Observed Behavior 0-0.5% Interstory Drift ........................................................... 182
5.2.3 Observed Behavior Post 0.5% Interstory Drift ...................................................... 183
5.3 Planar Wall Tests With Unibody Enhancements: S2 Through S4 .................................. 184
5.3.1 S2: Characteristics ................................................................................................. 186
5.3.1.1 S2: Summary and overall behavior ............................................................. 186
5.3.1.2 S2: Observed behavior 0-0.5% Interstory Drift ........................................... 187
5.3.1.3 S2: Observed behavior post 0.5% Interstory Drift ...................................... 188
5.3.2 S3: Characteristics ................................................................................................. 189
5.3.2.1 S3: Summary and overall behavior ............................................................. 189
5.3.2.2 S3: Observed behavior 0-0.5% interstory drift ............................................ 190
ix
5.3.2.3 S3: Observed behavior post 0.5% interstory drift ....................................... 191
5.3.3 S4: Characteristics ................................................................................................. 191
5.3.3.1 S4: Summary and overall behavior ............................................................. 192
5.3.3.2 S4: Observed behavior 0-0.5% interstory drift ............................................ 192
5.3.3.3 S4: Observed behavior Post 0.5% interstory drift ....................................... 193
6. EXPERIMENTAL RESULTS OF WOOD-FRAMED WALLS WITH EXTERIOR
SHEATHING CONDITIONS ................................................................................................ 218
6.1 Introduction ..................................................................................................................... 218
6.2 Planar Wall Tests With Unibody Enhancements and Exterior Sheathing ....................... 218
6.2.1 W-DG: Characteristics ........................................................................................... 219
6.2.1.1 W-DG: Summary and overall behavior ....................................................... 220
6.2.1.2 W-DG: Observed behavior 0-0.5% interstory drift ..................................... 220
6.2.1.3 W-DG: Observed behavior post 0.5% interstory drift ................................. 221
6.2.2 W-PLY: Characteristics ......................................................................................... 222
6.2.2.1 W-PLY: Summary and overall behavior ..................................................... 222
6.2.2.2 W-PLY: Observed behavior 0-0.5% interstory drift ................................... 223
6.2.2.3 W-PLY: Observed behavior post 0.5% interstory drift ............................... 224
6.2.3 W-STU: Characteristics ......................................................................................... 225
6.2.3.1 W-STU: Summary and overall behavior ..................................................... 226
6.2.3.2 W-STU: Observed behavior 0-0.5% interstory drift ................................... 226
6.2.3.3 W-STU: Observed behavior post 0.5% interstory drift ............................... 227
7. CONCLUSIONS AND FUTURE WORK ............................................................................. 250
7.1 Summary ......................................................................................................................... 250
7.1.1 Interior Wood-framed Specimens .......................................................................... 251
x
7.1.2 Interior Steel-framed Specimens............................................................................ 252
7.1.3 Exterior Wood-framed Specimens ......................................................................... 253
7.2 Conclusions for Unibody Construction Techniques ........................................................ 254
7.3 Future Work and Recommendations ............................................................................... 255
References .................................................................................................................................... 260
xi
LIST OF TABLES
Tables
Page
Table 2.1 Abbreviated test matrix and results for McMullin and Merrick ................................... 22
Table 3.1 Test matrix .................................................................................................................... 41
Table 3.2 Test matrix uplift constraints and locations ................................................................... 42
Table 3.3 Loading protocol ............................................................................................................ 42
Table 3.4 Nail and screw information............................................................................................ 43
Table 3.5 Time intervals for lumber and adhesive......................................................................... 43
Table 3.6 Reported requirements and capacities for Simpson Strong-tie wood tie-downs ........... 44
Table 3.7 Liquid Nails construction adhesive reported shear strength .......................................... 44
Table 3.8 Reported requirements and capacities for Simpson Strong-tie light-gage steel
tie-downs ....................................................................................................................... 44
Table 3.9 Loctite construction adhesive reported shear strength ................................................... 44
Table 3.10 Instruments for Specimens W1 through W6 ................................................................ 45
Table 3.11 Instruments for specimens W7, W8, and W10 ............................................................ 46
Table 3.12 Instruments for wood-framed specimens with returns and door openings ................. 47
Table 3.13 Instruments for exterior specimens (W-DG, W-PLY, and W-STU) ........................... 48
Table 3.14 Camera locations for time-lapse cameras .................................................................... 49
Table 4.1 One-sided stiffness and strength of interior wood-framed wall specimens ................ 107
Table 4.2 Anchor bolt pretension forces at beginning of test (lbs) .............................................. 108
Table 4.3 Tie down pretension forces at beginning of test (lbs) .................................................. 108
Table 5.1 One-sided stiffness and strength of steel walls ........................................................... 194
Table 5.2 Anchor bolt pretension forces at beginning of test (lbs) .............................................. 194
Table 5.3 Tie down pretension forces at beginning of test (lbs) .................................................. 194
xii
Table 6.1 Stiffness and strength of exterior wood-framed walls ................................................ 229
Table 6.2 Anchor bolt pretension forces at beginning of test (lbs) .............................................. 229
Table 6.3 Tie down pretension forces at beginning of test (lbs) .................................................. 229
Table 7.1 Summary of interior specimen behaviors .................................................................... 257
Table 7.2 Summary of exterior specimen behaviors.................................................................... 257
xiii
LIST OF FIGURES
Figures
Page
Figure 1.1 Inelastic structural behavior with large R-factor design contrasted against a limited
ductility system through elastic design .......................................................................... 8
Figure 1.2 Typical floor plan demonstrating additional wall length and strength capacity
available through (a) only including exterior walls in the lateral force resisting
system and (b) using all interior and exterior walls to resist seismic forces. ................. 9
Figure 2.1 Wood panel wall force-deformation response and associated damage states (van de
Lindt, 2004).................................................................................................................. 23
Figure 2.2 Construction framing elevation for specimens (a) type 1 and (b) type 2 of McMullin
and Merrick research ................................................................................................... 24
Figure 2.3 Construction framing elevation for (a) walls 1 and 3 and (b) walls 2 and 4 of Arnold
et al. research ............................................................................................................... 25
Figure 2.4 Typical specimen cross-section installed in frame for Arnold et al. research. ............. 26
Figure 2.5 Construction framing for (a) fastener and (b) panel tests of Swensen et al. research.. 27
Figure 2.6 Monotonic backbone curves for (a) wood-framed specimens and (b) steel-framed
specimens of the fastener tests of Swensen et al. research ......................................... 28
Figure 2.7 Cyclic backbone cuves for (a) wood-framed specimens and (b) steel-framed
specimens of the panel tests of Swensen et al. research .............................................. 29
Figure 2.8 Suggested corner stud assemblies for (a) corners of exterior walls and (b) at the
intersections of partition walls of wood-framed structures (ANSI, 2005) ................... 30
Figure 2.9 Suggested corner stud assemblies for (a) corners of exterior walls and (b) at the
intersections of partition walls of light-gage steel-framed structures
(NAFSA, 2000) ............................................................................................................ 31
xiv
Figure 2.10 Anchorage requirements for steel-framed specimens (NAFSA, 2000). ..................... 32
Figure 3.1 Loading protocol .......................................................................................................... 50
Figure 3.2 Test frame component indentification and dimensions ................................................ 51
Figure 3.3 Test frame with (a) 8 ft. specimen and (b) 16 ft. specimen installed............................ 52
Figure 3.4 Stiffener assemblies on the W8x48 base beam............................................................. 53
Figure 3.5 Bell crank used to rotate the forces applied to the specimen ....................................... 54
Figure 3.6 Loading link (a) as designed in the manufacturing plans and (b) as installed in the
test frame...................................................................................................................... 55
Figure 3.7 Out-of-plane restrictions for top loading beam; (a) Plan and (b) section view of the
beam and (c) Turnbuckles used to pretension rods ...................................................... 56
Figure 3.8 Section view of wall in test rig; (a) Wood-framed specimen and (b) steel-framed
specimen ...................................................................................................................... 57
Figure 3.9 Corner and end details; End of planar wall: (a) Wood-framed, (d) Steel-framed;
T-shape corner assembly: (b) Wood-framed, (e) Steel-framed; (c) Wood-framed
L-shape corner assembly.............................................................................................. 58
Figure 3.10 Wallboard layout for (a) 8 ft. walls, (b) 16 ft. walls, and (c) Exterior face of external
walls ........................................................................................................................... 59
Figure 3.11 Approximate lumber moisture level for wood specimens .......................................... 60
Figure 3.12 Samples of instrumentation used to measure frame and wallboard behaviors;
(a) Strain gage on loading link, (b) LVDT measuring stud uplift, and (c) string
potentiometer measureing specimen displacement .................................................... 61
Figure 3.13 Applied force versus average recorded strain used for link calibration .................... 62
Figure 3.14 Planar wall instrumentation (W1-W6 and S1-S2) ...................................................... 63
xv
Figure 3.15 Instrumentation for specimens with returns and no openings
(W7, W8, W10, S3 and S4) ........................................................................................ 64
Figure 3.16 Instrumentation for specimens with door openings (W9 and W11) ........................... 65
Figure 3.17 Exterior wall instrumentation (W-DG, W-PLY, and W-STU); (a) On interior face
and (b) on exterior face .............................................................................................. 66
Figure 3.18 Example of how damage propagation for each set of cycles is highlighted using
different colors ........................................................................................................... 67
Figure 4.1 South elevation construction framing and details for specimens W1-W6.................. 109
Figure 4.2 Out-of-plane measurements for specimen W1; (a) Locations of the measurements
and (b) Measured and calculated displacements ........................................................ 110
Figure 4.3 Force-deformation response for 0-0.5% interstory drift cycles for specimens
W1-W6....................................................................................................................... 111
Figure 4.4 Fastener damage states; (a) Paint and mud cracking over screw at 0.1% and
(b) Popped screw heat at 0.3% ................................................................................... 112
Figure 4.5 Specimen W1 damage illustration at (a) 0.5% interstory drift and (b) End of test..... 113
Figure 4.6 Axial forces in anchor bolts and tie downs for specimen W1, 0-0.5% interstory
drift cycles.................................................................................................................. 114
Figure 4.7 Force time-history for Specimen W1 in anchor bolts (a) #1 and (b) #2 over the
0-0.5% interstory drift cycles ..................................................................................... 115
Figure 4.8 Force-deformation response for 0-2.5% interstory drift cycles for specimens
W1-W6....................................................................................................................... 116
Figure 4.9 Wallboard separation and sliding in W1 at (a) negative and (b) positive specimen
deformations .............................................................................................................. 117
xvi
Figure 4.10 Axial forces in anchor bolts and tie downs for specimen W1, post 0.5% interstory
drift ........................................................................................................................... 118
Figure 4.11 Cyclic backbone curve comparisons for specimens W1-W6 ................................... 119
Figure 4.12 Images demonstrating gypsum wallboard failures as evidenced by residual paper
backing on (a) double end stud and (b) interior stud ................................................ 120
Figure 4.13 Images showing damages at bottom of sill plate of W2; (a) Crack and screws
popping on South face at 0.3% interstory drift, (b) Crack location compared to
top of sill plate (0.3%), and (c) Wallboard disengaged from studs at 2.0% ............. 121
Figure 4.14 Specimen W2 damage illustration at (a) 0.5% interstory drift and (b) End of test... 122
Figure 4.15 Displacement time history of wallboard sheathing and frame members for W2
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 123
Figure 4.16 Axial forces in anchor bolts and tie downs for specimen W2, 0-0.5% interstory
drift cycles ................................................................................................................ 124
Figure 4.17 Axial forces in anchor bolts and tie downs for specimen W2, post 0.5%
interstory drift ........................................................................................................... 125
Figure 4.18 Neoprene pads at anchor bolts for specimen W3 ..................................................... 126
Figure 4.19 Specimen W3 damage illustration at (a) 0.5% interstory drift and (b) End of test... 127
Figure 4.20 Displacement time history of wallboard sheathing and frame members for W3
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 128
Figure 4.21 Axial forces in anchor bolts and tie downs for specimen W3, 0-0.5% interstory
drift cycles ................................................................................................................ 129
xvii
Figure 4.22 Axial forces in anchor bolts and tie downs for Specimen W3, post 0.5%
interstory drift .......................................................................................................... 130
Figure 4.23 Stiffness enhanced tie down used for specimen W4 prior to installation ................. 131
Figure 4.24 Specimen W4 damage illustration at (a) 0.5% interstory drift and (b) End of test .. 132
Figure 4.25 Displacement time history of wallboard sheathing and frame members for W4
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 133
Figure 4.26 Axial forces in anchor bolts and tie downs for Specimen W4, 0-0.5% interstory
drift cycles ............................................................................................................... 134
Figure 4.27 End stud and tie down behavior in W4 at (a) positive and (b) negative specimen
deformations ............................................................................................................ 135
Figure 4.28 Axial forces in anchor bolts and tie downs for Specimen W4, post 0.5%
interstory drift .......................................................................................................... 136
Figure 4.29 Uplift constraint assembly consisting of tie down and bent strap for W5 ................ 137
Figure 4.30 Specimen W5 damage illustration at (a) 0.5% interstory drift and (b) End of test... 138
Figure 4.31 Displacement time history of wallboard sheathing and frame members for W5
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud. .......................................... 139
Figure 4.32 Axial forces in anchor bolts and tie downs for Specimen W5, 0-0.5% interstory
drift cycles ............................................................................................................... 140
Figure 4.33 Axial forces in anchor bolts and tie downs for Specimen W5, post 0.5%
interstory drift .......................................................................................................... 141
Figure 4.34 Specimen W6 damage illustration at (a) 0.5% interstory drift and (b) End of test .. 142
xviii
Figure 4.35 Displacement time history of wallboard sheathing and frame members for W6
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 143
Figure 4.36 Axial forces in anchor bolts and tie downs for Specimen W6, 0-0.5% interstory
drift cycles ............................................................................................................... 144
Figure 4.37 Axial forces in anchor bolts and tie downs for Specimen W6, post 0.5% interstory
drift .......................................................................................................................... 145
Figure 4.38 Construction framing for Specimens W7 and W8; (a) East elevation, (b) South
elevation, and (c) Plan view ..................................................................................... 146
Figure 4.39 Cyclic backbone curve comparisons for specimens W1 and W6 - W8 .................... 147
Figure 4.40 Observed damages of specimen W7; (a) Hairline crack formed at corner between
main wall and return wall, (b) Buckling of wallboard at corner, (c) Crack on
return wall caused by failure of corner stud assembly, and (d) Failure of stud
assembly ................................................................................................................... 148
Figure 4.41 Force-deformation response for 0-0.5% interstory drift cycles for specimens
W7 – W8 .................................................................................................................. 149
Figure 4.42 Specimen W7 damage illustration at (a) 0.5% interstory drift and (b) End of test . 150
Figure 4.43 Displacement time history of wallboard sheathing and frame members for W7
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 151
Figure 4.44 Axial forces in anchor bolts and tie downs for Specimen W7, 0-0.5% interstory
drift cycles .............................................................................................................. 152
Figure 4.45 Force-deformation response for 0-2.5% interstory drift cycles for specimens
W7 – W8 ................................................................................................................. 153
xix
Figure 4.46 Axial forces in anchor bolts and tie downs for Specimen W7, post 0.5%
interstory drift ........................................................................................................ 154
Figure 4.47 Specimen W8 damage illustration at (a) 0.5% interstory drift and (b) End of test .. 155
Figure 4.48 Displacement time history of wallboard sheathing and frame members for W8
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 156
Figure 4.49 Axial forces in anchor bolts and tie downs for Specimen W8, 0-0.5% interstory
drift cycles ............................................................................................................... 157
Figure 4.50 Improved corner stud assembly failure on specimen W8 ......................................... 158
Figure 4.51 Axial forces in anchor bolts and tie downs for Specimen W8, post 0.5%
interstory drift .......................................................................................................... 159
Figure 4.52 Cyclic backbone curve comparisons for specimens W6 and W8 – W11 ................. 160
Figure 4.53 Curve fits capturing the behavior for varying aspect ratios; (a) Strength capacity
and (b) stiffness ........................................................................................................ 161
Figure 4.54 Construction framing for Specimen W9; (a) East elevation, (b) South elevation,
and (c) Plan view ...................................................................................................... 162
Figure 4.55 Force-deformation response for 0-0.5% interstory drift cycles for specimens
W9 – W11 ................................................................................................................ 163
Figure 4.56 Specimen W9 damage illustration at (a) 0.5% interstory drift and (b) End of test . 164
Figure 4.57 Displacement time history of wallboard sheathing and frame members for W9
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 165
Figure 4.58 Axial forces in anchor bolts and tie downs for Specimen W9, 0-0.5% interstory
drift cycles ............................................................................................................... 166
xx
Figure 4.59 Force-deformation response for 0-2.5% interstory drift cycles for specimens
W9 – W11 ................................................................................................................ 167
Figure 4.60 Axial forces in anchor bolts and tie downs for Specimen W9, post 0.5%
interstory drift .......................................................................................................... 168
Figure 4.61 South elevation construction framing for Specimen W10 ........................................ 169
Figure 4.62 Specimen W10 damage illustration at 0.5% interstory drift .................................... 170
Figure 4.63 Displacement time history of wallboard sheathing and frame members for W10
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 171
Figure 4.64 Axial forces in anchor bolts and tie downs for Specimen W10, 0-0.5% interstory
drift cycles ............................................................................................................... 172
Figure 4.65 Specimen W10 damage illustration at end of test .................................................... 173
Figure 4.66 Axial forces in anchor bolts and tie downs for Specimen W10, post 0.5%
interstory drift .......................................................................................................... 174
Figure 4.67 South elevation construction framing for Specimen W11 ........................................ 175
Figure 4.68 Specimen W11 damage illustration at 0.5% interstory drift .................................... 176
Figure 4.69 Displacement time history of wallboard sheathing and frame members for W11
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud .......................................... 177
Figure 4.70 Axial forces in anchor bolts and tie downs for Specimen W11, 0-0.5%
interstory drift cycles .............................................................................................. 178
Figure 4.71 Specimen W10 damage illustration at end of test. ................................................... 179
Figure 4.72 Axial forces in anchor bolts and tie downs for Specimen W11, post 0.5%
interstory drift .......................................................................................................... 180
xxi
Figure 5.1 South elevation construction framing and details for specimens S1 and S2 .............. 195
Figure 5.2 Force-deformation response for 0-0.5% interstory drift cycles for S1 – S4 ............... 196
Figure 5.3 Specimen S1 damage illustration at (a) 0.5% interstory drift and (b) End of test ..... 197
Figure 5.4 Fastener damage states of screws popping showing (a) only a visible divot and
(b) cracked mud with screw fully disengaged from wallboard ................................. 198
Figure 5.5 Axial forces in anchor bolts and tie downs for specimen S1, 0-0.5% interstory
drift cycles ................................................................................................................. 199
Figure 5.6 Force-deformation response for 0-2.5% interstory drift cycles for S1 – S4 ............... 200
Figure 5.7 Axial forces in anchor bolts and tie downs for specimen S1, post 0-0.5%
Interstory drift ........................................................................................................... 201
Figure 5.8 Steel framing chosen to improve unibody enhancements features (a) grooved
flanges of studs improve bonding of construction adhesive as shown when
(b) wallboard was removed after test ........................................................................ 202
Figure 5.9 Cyclic backbone curve comparisons for specimens S1 – S4 ...................................... 203
Figure 5.10 Specimen S2 damage illustration at (a) 0.5% interstory drift and (b) End of test ... 204
Figure 5.11 Displacement time history of wallboard sheathing and frame members for S2
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 205
Figure 5.12 Axial forces in anchor bolts and tie downs for Specimen S2, 0-0.5% interstory
drift cycles ............................................................................................................... 206
Figure 5.13 Axial forces in anchor bolts and tie downs for Specimen S2, post 0.5%
interstory drift .......................................................................................................... 207
Figure 5.14 Construction framing for Specimens S3 and S4 (a) East elevation, (b) South
elevation, and (c) Plan view ..................................................................................... 208
xxii
Figure 5.15 Specimen S3 damage illustration at (a) 0.5% interstory drift and (b) End of test ... 209
Figure 5.16 Displacement time history of wallboard sheathing and frame members for S3
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 210
Figure 5.17 Axial forces in anchor bolts and tie downs for Specimen S3, 0-0.5% interstory
drift cycles ............................................................................................................... 211
Figure 5.18 Axial forces in anchor bolts and tie downs for Specimen S3, post 0.5%
interstory drift .......................................................................................................... 212
Figure 5.19 Spacers added to blocking of specimen W4; (a) Image of blocking spacer material
and (b) Locations of blocking in plan view of specimen ........................................ 213
Figure 5.20 Specimen S4 damage illustration at (a) 0.5% interstory drift and (b) End of test ... 214
Figure 5.21 Displacement time history of wallboard sheathing and frame members for S4
measuring (a) the horizontal displacement at bottom of wallboard and top of
frame and (b) vertical uplift of wallboard and end stud ........................................... 215
Figure 5.22 Axial forces in anchor bolts and tie downs for Specimen S4, 0-0.5% interstory
drift cycles ............................................................................................................... 216
Figure 5.23 Axial forces in anchor bolts and tie downs for Specimen S4, post 0.5% interstory
drift .......................................................................................................................... 217
Figure 6.1 Construction plans for exterior wall specimens featuring (a) the East elevation,
(b) the South elevation, and (c) plan view ................................................................ 230
Figure 6.2 Cyclic backbone curve comparisons for exterior specimens ..................................... 231
Figure 6.3 Force-deformation response for 0-0.5% interstory drift cycles for exterior walls .... 232
Figure 6.4 Force-deformation response for 0-2.5% interstory drift cycles for exterior walls ..... 233
xxiii
Figure 6.5 Specimen W-DG damage illustration at 0.5% interstory drift on (a) North
(Gypsum Wallboard) face and (b) South (DensGlass® Wallboard) face ................... 234
Figure 6.6 Displacement time history of wallboard as compared to the frame for W-DG
(a) interior and (b) exterior wallboard horizontal displacements; (c) interior and
(d) exterior wallboard vertical displacements ........................................................... 235
Figure 6.7 Axial forces in anchor bolts and tie downs for specimen W-DG for 0-0.5%
interstory drift cycles ................................................................................................ 236
Figure 6.8 Specimen W-DG damage illustration at end of test on (a) North (Gypsum
Wallboard) face and (b) South (DensGlass® Wallboard) face ................................... 237
Figure 6.9 Axial forces in anchor bolts and tie downs for specimen W-DG for post 0.5%
interstory drift cycles ................................................................................................ 238
Figure 6.10 Specimen W-PLY damage illustration at 0.5% interstory drift on (a) North
(Gypsum Wallboard) face and (b) South (Plywood) face ........................................ 239
Figure 6.11 Displacement time history of wallboard as compared to the frame for W-PLY
(a) interior and (b) exterior wallboard horizontal displacements; (c) interior and
(d) exterior wallboard vertical displacements .......................................................... 240
Figure 6.12 Axial forces in anchor bolts and tie downs for specimen W-PLY for 0-0.5%
interstory drift .......................................................................................................... 241
Figure 6.13 Specimen W-PLY damage illustration at end of test on (a) North (Gypsum
Wallboard) face and (b) South (Plywood) face ........................................................ 242
Figure 6.14 Axial forces in anchor bolts and tie downs for specimen W-PLY for post 0.5%
interstory drift cycles ............................................................................................... 243
xxiv
Figure 6.15 Construction of W-STU (a) after Densglass® sheathing is installed, (b) after
building paper and wire lath are installed, (c) after scratch coat, (d) after brown
coat, (e) after finish coat .......................................................................................... 244
Figure 6.16 Specimen W-STU damage illustration at 0.5% interstory drift on (a) North
(Gypsum Wallboard) face and (b) South (Stucco) face .......................................... 245
Figure 6.17 Displacement time history of wallboard as compared to the frame for W-STU
(a) interior and (b) exterior wallboard horizontal displacements; (c) interior and
(d) exterior wallboard vertical displacements .......................................................... 246
Figure 6.18 Axial forces in anchor bolts and tie downs for specimen W-STU for 0-0.5%
interstory drift cycles ............................................................................................... 247
Figure 6.19 Specimen W-STU damage illustration at end of test on (a) North (Gypsum
Wallboard) face and (b) South (Stucco) face ........................................................... 248
Figure 6.20 Axial forces in anchor bolts and tie downs for specimen W-STU for post 0.5%
interstory drift .......................................................................................................... 249
Figure 7.1 Cyclic backbone curves for wood and steel specimens; (a) Interior wood
specimens, (b) interior steel specimens, and (c) exterior wood specimens ............... 258
Figure 7.2 Cyclic backbone curves for interior and steel specimens; (a) Typical construction
specimens, (b) planar unibody specimens, and (c) unibody specimens with returns . 259
xxv
1
CHAPTER 1
INTRODUCTION
1.1
Motivation
Current seismic building code provisions use response modification coefficients, or R-
factors (R > 1), to reduce the required strength of the lateral force resisting system below the
elastic force demand in a design level event, thereby allowing for inelastic structural response and
an increase in the ductility capacity of the components (ASCE, 2010). Common values of the
response modification coefficient range from 3 to 8 for low and high ductility systems,
respectively, with 6.5 as a common R-value for light-frame residential structures. Figure 1.1
demonstrates the force-deformation relationship of a low R-value and high R-value response. As
the lateral system deforms inelastically during earthquake loading, both the load carrying
components and non-structural components may undergo significant damaged. In fact, because
of the brittle behavior of common finish materials such as stucco, drywall and plaster, the damage
to non-structural components typically accounts for the majority of post-event renovations costs
(Comerio, 1998). While the elastic design and damage-free response may be economically
infeasible for multi-story steel or reinforced concrete lateral systems, residential structures have
considerable differences in terms of common room dimensions, floor mass, and function. As part
of a multi-phase Network for Earthquake Engineering (NEES) project, this thesis investigates the
response of planar wall components during earthquake-type loading towards a limited-ductility
(elastic) design methodology for residential structures.
Light-frame residential structures include nonstructural and structural components framed
with common lumber cross-sections such as 2x4s and 2x6s or light gage rolled steel crosssections with exterior Oriented Strand Board (OSB) or Plywood sheathing and composite gypsum
board for interior finishes. Lateral loading from wind or earthquake is transferred from the floor
2
or roof mass to the foundation through exterior walls sheathed with OSB and nail fasteners at
minimum spacing distances to develop the strength of the panel board. However, unlike high-rise
steel and concrete structures, the strength of interior partitions and finishes in residential
structures are a larger percentage of the stiffness and strength of the main lateral load resisting
system. For example, McMullin and Merrick (2001) demonstrated the one-sided strength of
gypsum-sheathed shear walls to be as large as 200 lb/ft, while OSB-sheathed and plywoodsheathed walls have strengths up to 520 lb/ft and 870 lb/ft, (Breyer et al., 2007). In addition, the
mass carried by lateral components in residential structures are significantly smaller as compared
to the mass from concrete slabs in office buildings with steel and reinforced concrete lateral
systems. Through feasible load transfer mechanisms from the floor mass, to the non-structural
shear wall and, finally, to the foundation, the strength of the wall can be integrated into the lateral
system thereby allowing the entire structure to resist the seismic forces with improved strength
and performance.
Figure 1.2 demonstrates the additional wall length available through the
inclusion of the non-structural shear walls on a typical floor plan.
The experimental results presented in this thesis demonstrate the strength and stiffness
characteristics of individual planar shear walls with seismic enhancements to increase
performance in the context of a limited ductility system. A common attribute of light-framed
shear walls, regardless of the sheathing material, is the large amount of inelasticity needed to
reach the peak force capacity of the wall. Thus, mobilizing the wall to relatively high drift levels,
on the order of 1.0%-1.5% interstory drift, is needed to reach their full strength. However, the
same testing series demonstrated fastener damage begins as soon as 0.2% interstory drift, with
significant damage necessitating replacement at 1.0% and total loss of economic value at
approximately 2.0% (McMullin and Merrick, 2001). While this performance meets the life-safety
and collapse prevention limit state, the large interstory drifts are certainly beyond the immediately
3
occupancy and operational limit states (FEMA, 2004a, 2004b). To decrease the deformation
needed to achieve peak strength, the majority of the tests reported in this thesis utilize
construction adhesive between the wood or metal framing members and sheathing materials is
used to create a stiffness-enhanced, limited ductility response, such that the strength is governed
by the adhesive strength, rather than fastener spacing. Additional modifications include fastener
enhancements (Swensen, 2012), uplift constraints, and fiber-composite materials for interior and
exterior facades.
Within the proposed “unibody” construction methodology, the building components are
designed such that architectural and exterior structural walls act together in shear to resist
earthquake loads and deformations. By engineering the architectural building components of the
structure to contribute to the lateral force resisting system, it may be possible for the residence to
be damage free after a seismic event.
The planar walls tested within this investigation
demonstrate how this design procedure significantly increases the strength and stiffness of the
walls, thereby decreasing deformation demands, displacement-sensitive damage, and repair
cost/time.
Reducing damage and repairs is especially important for residential structures
considering a) prohibitively expensive earthquake-insurance premiums in regions of high seismic
risk, and b) safety risks associated with displacing a large percentage of the population after a
large earthquake. Referring to the latter point, the Katrina hurricane demonstrated the large toll
to society when residences are uninhabitable for an extended period. While the current structural
design approach may satisfy the life-safety limit state during the earthquake event itself, it will
arguably create a more desperate challenge during the post-event response. In fact, Kircher
estimates that 160,000 to 250,000 damaged homes and millions of displaced people in the case of
a large event in the California Bay-Area (2006).
4
1.2
Objectives and Scope
As a part of a research project funded by National Science Foundation (NSF) through
NEESR, the testing program described herein develops and validates a new seismic design
concept for components of light-frame residential construction with an overall aim to improve
life-cycle seismic performance.
Overall, the objectives of the large NEES project are to
investigate (a) small component tests and planar wall panel tests to develop and characterize
construction techniques of interior and exterior walls, (b) an economical low-force highdisplacement isolation system, (c) quasi-static tests of three dimensional room assemblies, and (d)
shake-table tests of a two-story residential building with seismic isolators and enhanced unibody
construction.
The wall specimens in this report follow from work at Stanford University, which
included component, fastener and material tests, along with 4 ft. x 4 ft. planar wall tests utilizing
construction adhesive between the framing members and wallboards. The results of this phase are
reported in Swensen et al. (2012) and summarized in Chapter 2 of this report. The full-scale
specimens tested at California State University, Sacramento investigate similar performance
while also introducing additional variables and details representative of current construction.
The experiments of this phase of the project investigated the effects of enhanced,
inexpensive construction procedures on full-scale planar specimens built with wood and coldformed steel framing members. The differing characteristic of these specimens, as compared to
typical construction techniques, was the use of construction adhesive and mechanical fasteners to
attach the sheathing to the framing members. To further investigate and reduce the effects of
deformations from earthquake-type loading, the additional objectives of the study are to
determine and document –
5
(1) The strength, stiffness, and damage progression of unibody planar wall specimens
with varying aspect ratios, openings, and orthogonal walls
(2) The optimal construction techniques required when construction adhesive is used to
install wallboard panels
The next two phases of the aforementioned NEES project, which involve the
development of an economical seismic base isolation system and quasi-static tests of room
assemblies, will be completed during the summer of 2013. The work described in this thesis is
used in the design and construction of the room assemblies, albeit these tests focus on the forcetransfer mechanism between the floor diaphragm and the planar shear walls. The final phase of
the project is a culmination of all phases of the project and features a shake table test of a twostory residential building with enhanced unibody construction and seismic isolator, and is
scheduled to be completed during the summer of 2014.
1.3
Organization and Outline
Chapter 2 summarizes pertinent literature and design codes of previous work to contrast
performance, while also aiding in the construction techniques for the specimens conducted as part
of this research. This includes a discussion of typical construction techniques for light frame
construction based on various standards and specifications. Summarized next are the results of an
experimental project by McMullin and Merrick (2001), which investigated the seismic behavior
of gypsum-sheathed walls of various configurations in conjunction with the CUREE-Caltech
Woodframe Project. Then, the results of Arnold et al (2003) are discussed, which investigated
behavior of typical exterior wood-framed walls as part of the Earthquake Damage Assessment
and Repair Project. The chapter concludes by discussing the experiments conducted by Swensen
et al.(2012), representing the initial phase of this project which performed component and small
6
scale panel tests to determine efficient fastening and joining techniques for gypsum sheathed
walls.
Chapter 3 details the experimental setup and test rig used for the large-scale tests
described in Chapters 4-6.
The chapter reviews the design of the specimens, test setup,
experimental specimen matrix, loading history, instrumentation, camera locations and testing
observations.
Chapters 4 through 6 include the test results for each of the twenty specimens tested
within this research. For each specimen, an explanation is provided for the construction details
that uniquely characterize the specimen within the test series. Then, the test results are presented
in categories of overall behavior, which includes the strength, stiffness, and primary mode of
failure, and the observed behaviors during the 0 through 0.5% interstory drift cycles and the post
0.5% interstory drift cycles, which include all observed and recorded responses of the specimen
to the applied displacements.
Chapter 4 reports the test results of the eleven interior walls of the wood-framed
experimental suite. First presented within the chapter are the results of a free-standing planar
specimen built using typical construction techniques for nonstructural partition walls and acted as
the control specimen for the remainder of the wood-framed tests. Next, the chapter provides the
results of five specimens, which featured unibody construction techniques for specimens with the
same geometry as the control test. Each of these specimens featured iterative improvements to
the construction details to increase the strength, stiffness, and damageability performance of the
specimen. Then, the chapter presents the results of two planar wall specimens with attached
orthogonal walls, which were constructed using the construction techniques of the best
performing planar specimen. These two specimens also featured iterative improvements to the
7
construction details to improve the performance of the specimen. Lastly, the chapter presents the
results of planar walls with varying aspect ratios, door openings, and orthogonal end returns.
Chapter 5 presents the test results of four interior walls with light-gage steel framing
members. Similar to the previous chapter, presented first are the results of a free-standing planar
specimen using typical construction to serve as the control specimen for the other three tests.
Presented next are the results of a planar specimen built with unibody construction techniques.
Finally, results are presented for two specimens featuring orthogonal end returns and similar
iterations to the construction techniques as the wood-framed specimens.
Chapter 6 presents test results of three wood-framed specimens representative of exterior
walls in residential structures. To simulate an external end-return condition, these specimens
featured shorter orthogonal walls attached to the planar wall to create a “C”-shaped specimen.
The interior faces of the specimens were sheathed using gypsum board and the techniques
described in Chapter 4.
The exterior faces of the specimens featured different external
sheathings, including fiberglass mat sheathing, plywood, and three-coat stucco applied over
fiberglass mat sheathing.
Chapter 7 summarizes the key observations and conclusions drawn from the experiments
that may influence the geometries and construction details used for the room assembly and fullscale shake-table tests. The chapter will also provide recommendations for future studies not
addressed as part of this investigation.
8
Force
R≅1 Response
Inelastic
Damage
R>1 Response
Inelastic Damage
Interstory Drift
Figure 1.1 Inelastic structural behavior with large R-factor design contrasted against
a limited ductility system through elastic design.
9
MASTER
BATH
MASTER
BEDROOM
GREAT ROOM
DINING
WIC
KITCHEN
BATH
BEDROOM
BEDROOM
(a)
MASTER
BATH
MASTER
BEDROOM
GREAT ROOM
DINING
WIC
KITCHEN
BATH
BEDROOM
BEDROOM
(b)
Figure 1.2 Typical floor plan demonstrating additional wall length and strength capacity
available through (a) only including exterior walls in the lateral force resisting system
and (b) using all interior and exterior walls to resist seismic forces.
10
CHAPTER 2
SUMMARY OF PREVIOUS STUDIES ON LIGHT-FRAME CONSTRUCTION
2.1
Introduction
Extensive work has been conducted to investigate the behavior of light-frame
components and systems subjected to earthquake-type loading, including experimental and
analytical studies which have led to the current understanding of the response of wood and lightgage steel-framed structures.
In general, a central goal of previous studies has been on
developing a high-ductility response, allowing for large inelastic force reduction factors in the
design of typical structures. However, as demonstrated in Figure 2.1, large ductility demands
necessitate an inelastic response and associated damage states that require extensive repairs to
structural and nonstructural components alike.
In this light, this chapter summarizes several testing programs that are most pertinent to
the experiments conducted in this study. While the components described herein are meant to
create a “limited ductility” system with little, to no, inelastic deformations, the design and
construction are meant to mimic typical details of current practice as closely as possible. In
addition to the experimental studies, typical construction details for wood and light-gage steel
framing are provided, informed from current design provisions. Where appropriate, and to lessen
the magnitude of change to current practice, these details are repeated in the test specimens
described in Chapters 3-6.
The chapter begins by giving a review of a large majority of all experimental and
analytical tests conducted on residential components and systems. Next, several studies are
identified as being the most applicable and comparable to the current work including the
experimental studies by McMullin and Merrick (2001), Arnold et al. (2003) and Swensen et al.
(2012). These studies are discussed in more detail within this chapter. Finally, a review of
11
common light-frame details is provided which are informed from current design provisions and
past work. These latter sections, compared with the specimen geometry and details in Chapters 4,
5 and 6, are meant to highlight any differences in the unibody specimen design.
2.2
Current Seismic Design Approaches for Light-Frame Buildings
Seismic design requirements for light-framed systems distinguish between those
constructed with (a) wood or steel shear panels and (b) other panel materials. For systems with
wood and steel panels, the design requirements of ASCE 7 Minimum Design Loads for Buildings
and other Structures (ASCE, 2010) specify large inelastic force reduction factors of R = 6.5 to 7.0
for bearing wall and building frame systems, respectively (Cobeen et al. 2004). For light-framed
systems of other materials, such as gypsum wallboard and stucco, ASCE 7 specifies much lower
factors of R = 2 to 2.5, implying lower ductility and higher design strengths, leading to stiffer
systems with much lower drift demands. The relative strengths and stiffnesses of these systems
are illustrated in the seismic force versus drift plot of Figure 1.1
Studies conducted as part of the development of FEMA P695 Quantification of Building
Seismic Performance Factors (2009) have shown that wood panel walls designed according to
current standards just barely meet the minimum collapse safety specified in FEMA P695. For
example, in Performance Group No. PG-9 (Short Period, High Aspect Ratio) describing multi- or
1&2 family dwellings with high aspect ratio shear walls, the average adjusted collapse margin
ratio (ACMR) measuring the ratio of spectral acceleration to cause collapse in 50% of scaled
ground motions to the Maximum Considered Earthquake (MCE) acceleration is 1.89. The
established acceptance ACMR is only 1.90, thus passing by a narrow margin. Moreover, the
FEMA P695 procedures, and seismic code provisions in general, are only concerned with seismic
12
collapse safety and do not explicitly address damage control necessary for continued occupancy
or to limit economic losses.
Unlike multi-story steel and concrete framed buildings, where the façade and partitions
are generally designed to accommodate large drifts, in low-rise residential construction, the
partitions and façade are integral with the structure and will undergo the same deformations as the
primary seismic force resisting system. Consequently, where the lateral systems are designed
with high R-values, the resulting large deformations will cause significant damage to the
partitions and facades.
On the other hand, these same partitions and facades can provide
additional strength and stiffness that tend to improve the seismic performance of wood and steel
shear panel systems.
In contrast to the high-ductility systems, the proposed “limited-ductility” unibody
systems with lower R-values and larger strength and stiffness have the potential to provide better
damage resistance.
However, as it is commonly perceived that higher R-factors provide
improved performance, these limited-ductility systems are not widely used in high-seismic
regions. Such perceptions are compounded by the fact that there has been very little, if any,
research to establish the collapse safety performance of high-strength limited ductility systems for
high seismic regions.
2.3
Past Research on Light-Frame Construction (Simulation and Testing)
The lateral strength of wood panel walls is developed through a composite action of the
sheathing material, fasteners, and framing elements, where a majority of the strength is generated
by the sheathing material in a shear deformation mode. Inelastic action at small levels of
deformation is developed primarily at the interface between the fasteners, sheathing and framing
through deformation of the fasteners (Stewart et al., 1988) and crushing of the wood under the
13
bearing load of the fastener. While the strength capacity changes, these mechanisms are also
similar for light-gage steel-framed walls with wood paneling (Serrette, 1997).
At larger
deformations, global inelastic effects are activated such as sheathing buckling and separation as
well as uplifting of the panel wall (Schmid et al., 1994). Due to this complex behavior, numerous
physical and analytical investigations have used fastener, component or full-scale tests and
analyses to explore the lateral shear strength and ductility of wood panel walls.
Research on wood panel walls extends back to the 1940s where the earlier work is
summarized in Carney (1975) and Peterson (1983).
discussed by van de Lindt (2004).
Later investigations (after 1982) are
The most prevalent of testing programs are full-scale
rectangular panel walls whose dimensions are typically 8-20 ft long, 6-8 ft high and 4-6 in. thick.
The panel strength capacity has been shown to increase with length (Patton-Mallory and Wolfe,
1985) and a decreased fastener spacing (Atherton, 1983), where the effective length is calculated
by subtracting the cumulative opening distances for doors and windows (Falk and Itani, 1987).
Referring to Figure 2.1 (van de Lindt, 2004), however, the lateral strength capacity is fully
developed only after significant inelastic behavior and damage to the fasteners, frame and
sheathing. While the maximum shear strength is maintained across a large range of lateral
deformation prior to strength degradation and eventual failure, the elastic region is indiscernible
in the figure and Vmax is developed only after significant damage. In fact, Ficcadenti et al.
(1998) found that the strength capacity decreases with an increased number of elastic loading
cycles, signifying that even at small displacements the components are damaged and display an
inelastic response.
Several studies (Dinehart and Shenton 1998, Dinehart et al. 1999, and Higgins 2001)
have investigated the performance of wood frame panel walls with a variety of active and passive
damping devices and concluded that damping significantly increases the energy dissipation
14
capacity of the walls. However, since these devices are activated at relatively large displacements
and velocities, the walls still develop the damage mechanisms listed in Figure 2.1 and would
require repair similar to an undamped component after a design level earthquake (van de Lindt,
2004).
Modeling efforts of wood panel shear walls range from predictive equations for strength
and stiffness (Easley et al., 1982) to nonlinear finite element modeling (Cheung and Itani, 1983).
Predictive equations are developed using physics based models to capture the inelastic
deformation modes described previously and are quite accurate in predicting the elastic stiffness
and ultimate strength capacity (McCutcheon 1985, Patton-Mallory and McCutcheon, 1987).
Studies have also shown good agreement with experimental results by representing panel walls
with uniaxial spring, beam and shell elements to model the behavior of fasteners, frame members
and sheathing, respectively (Itani and Cheung 1984, Itani and Robledo 1984, White and Dolan
1995). Other modeling techniques include a pair of diagonal springs to represent wood sheathing
(Itani et al., 1982), single degree of freedom systems (Dolan and Filiatrault, 1990), and multiple
degree of freedom systems (Foliente 1995, Dinehart and Shenton 2000, Folz and Filiatrault 2001)
calibrated from experimental programs to capture strength degradation and pinching behavior.
The Folz and Filiatrault (2001) model was developed into a program titled CASHEW (Cyclic
Analysis of SHEar Walls) and became a common tool in the engineering community to predict
panel wall behavior.
Numerical models to perform nonlinear dynamic models have been
developed by Tarabia and Itani (1997), Folz and Filiatrault (2004a, 2004b) and Collins et al.
(2005). A relatively new nonlinear dynamic software package named SAPWood (van de Lindt et
al, 2010) built on the previous models and incorporates shear deformations of the walls and outof-plane rotations of the floor and ceiling diaphragms.
15
Similar experimental and analytical work has also been performed on nonstructural
partition walls with gypsum sheathing. While partition walls have a smaller strength capacity as
compared to wood sheathing shear walls, the force-deformation response and damage
mechanisms illustrated in Figure 2.1 are quite similar (Rihal 1984, Oliva et al. 1990, Karacabeyli
1996, McMullin and Merrick, 2001) and also develop at small levels of interstory drift.
Moreover, several investigations (Patton-Mallory and Wolfe 1985, Filiatrault et al. 2002 and
2010) have demonstrated that gypsum wallboard and stucco finishes on wood shear walls can
significantly contribute to the strength and stiffness of the wall and decrease interstory drift ratios
by up to 40% (Vance and Smith, 1996). Kanvinde and Deierlein (2006) proposed physics based
predictive models for the strength and stiffness, demonstrating strength capacities of gypsum
walls on the order of 5000-8000 lbs.
2.4
Gypsum Sheathed Partition Walls by McMullin and Merrick (2001)
As part of the CUREE-Caltech Woodframe, a project funded by the Consortium of
Universities for Research in Earthquake Engineering (CUREE), Kurt McMullin and Dan Merrick
tested seventeen specimens designed to represent standard construction of gypsum wallboard
partition walls. The main purpose of this portion of the project was to gain understanding of the
behavior of gypsum partition walls built using the standard construction practices.
All specimens, which were 8-foot high and 16-foot long, were framed with 2 in. by 4 in.
nominal dimension framing lumber and sheathed on both sides with Standard grade, ½ in.
gypsum sheathing wallboard. The varying parameters of the test matrix included fastener type
and spacing, loading protocol, fastener type and spacing, loading protocol, wallboard and opening
layout, construction methods, influence of door and floor trim and repair strategies.
construction plans used for the specimens in this research are shown in Figure 2.2.
The
16
McMullin and Merrick determined that the overall behavior and levels of damage of the
specimen appeared to be related to the rigidity and geometry of the boundary elements of the
wall. Damage patterns of the specimens usually initiated during the 0.25% drift levels as cracks
at the wall penetrations and over fastener heads.
Additional damages included the global
buckling of large portions of panels, and the loss of portions of panel sections and crushing of
wallboard at corners during the larger displacements.
Table 2.1 shows the two-sided maximum strength achieved by the specimens - excluding
openings - in which the gypsum wallboards were installed with screws. The maximum loads
were sustained between 1% and 1.5% drift and usually initiated one of two failure modes. The
first failure mode consisted of wallboard separation from the frame caused by fasteners being
pulled through the back of the wallboard. The second failure consisted of failure of the taped
wall joints which allowed racking movement of the individual wallboard panels. Lastly, the team
determined that the rigid restraint from intersecting walls appeared to significantly increase lateral
strength and stiffness of the walls.
2.5
Exterior Walls by Arnold et al. (2003)
As part of the CUREE Earthquake Damage Assessment and Repair Project (EDA),
funded by the California Earthquake Authority (CEA), Arnold et al. researched the response of
typically constructed wood-frame walls on the first floor of two story buildings. The objective of
the research was to determine and document the behaviors of walls based on the visible condition
of the finishes, document the typical patterns to walls with openings, and determine the visual and
structural performance of repair methods.
The research consisted of two 8-foot tall by 16-foot long wall configurations with
openings made from 2 in. by 4 in. nominal dimension framing lumber. The construction framing
17
of the two geometries of specimens are shown in Figure 2.3. The interior face each specimen was
sheathed with 1/2 in. gypsum wallboard. The exterior face of each specimen was sheathed with a
typical 7/8 in. three-coat Portland cement plaster system (stucco) applied over an open frame.
Figure 2.4 shows the typical cross section for a specimen installed in the test frame.
The results of the first two specimens of the project were used to determine important
performance regimes and damage states for purposes of repair and assessment of the later
specimens. Five regimes of behavior were established consisting of 0-0.2% drift, 0.2-0.4%, 0.40.7%, 0.7% to ultimate strength, and ultimate strength to failure. The next two specimens were
tested to the established drift levels, repaired aesthetically and structurally, and retested to
determine the effects of the repair methods. The results of these tests were used to develop
qualitative and quantitative assessments of a stucco finished wall after an earthquake to assess the
condition of the wall.
The initial wall behavior was characterized by a very stiff, nearly linear elastic response
with minor cracking of finishes. Within the regimes through the ultimate strength, the wall
stiffness began to soften and propagation of the cracks occurred. Following the ultimate strength,
the wall behavior was characterized by significant deterioration of behavior during all cycles,
extensive damages to the finishes, and detachment of the stucco at the sill plate.
2.6
Components of Partition Walls by Swensen et al (2012)
Within the first phase of this research project, Swensen et al. investigated components of
gypsum partition walls to improve the damage resistance of low-rise residential structures during
earthquakes by unifying structural and architectural systems.
To determine the effect of fastening methods upon the behavior of walls, variations of
mechanical fasteners and construction adhesive connecting the gypsum and framing were tested
18
on specimens built using the construction plans in Figure 2.5a.
The results of the initial
component tests, illustrated in Figure 2.6, showed that enhanced drywall screws, which featured a
thicker and longer unthreaded shank, increased the strength of wood-framed specimens by about
20% over the conventional coarse threaded drywall screw, but did not increase the initial
stiffness. The tests featuring construction adhesive to install the wallboards showed increases in
the stiffness and strength on both wood- and steel-framed specimens. Lastly, the tests showed
that an installation using a combination of screws and adhesive increased the strength of
specimens even further.
Five tests, featuring 4 ft by 4 ft wood- and steel-framed wall specimens, with the
construction plans shown in Figure 2.5b, confirmed these results.
Within these tests, the
enhanced drywall screws showed increases in stiffness and strength by 60% and 20%,
respectively, over the conventional fasteners for the wood-framed wall, as illustrated in Figure
2.7. The specimens featuring adhesive and mechanical fasteners showed the walls to be twice the
strength and two to four times as stiff as the specimens with only conventional fasteners.
Additional tests were performed to determine the stiffness and strength of two different
methods of joining wallboard panels on specimens subjected to shear. The first joining technique
featured a paper joint tape with premixed joint compound. The second technique featured an
enhanced compound, formed by mixing a quick-drying powder compound with water and a liquid
concrete bonding adhesive, with fiberglass tape. The results of these tests showed that using a
fiberglass joint tape and enhanced joint compound provided a joint that was more than 70%
stronger than the joint with a pre-mixed compound and paper tape.
19
2.7
Typical Construction Review
The definition of “typical” light-frame construction varies based on geographical location
of the structure and the engineer/contractor’s preferences. As a result, many texts of standards
and specification provide the minimum requirements and construction guidelines to achieve the
desired performance of the structure. The minimum construction details for this project were
determined through review of the standards and specifications that included the International
Residential Building Code (2009), National Design Specification for Wood Construction
(ANSI/AF&PA NDS-2005, 2005), the sixth edition of the Design of Wood Structures (Breyer et
al. 2007), and the American Iron and Steel Institute Standards including AISI S200, S211, S212,
S213, & S230 (2007). Within this base of knowledge, there are many variations of type and
spacing of the fasteners and framing members that may be used to achieve the desired structure
response. The following sections present the chosen minimum construction details of “typical”
specimens to be used within this project.
2.7.1
Typical wood-frame construction.
The chosen typical framing members for wood-framed construction are nominally 2 in.
by 4 in. members of grade No. 3 or better lumber with the 4 in. dimension oriented to form the
thickness of the wall. The frame consists of a sill plate which connects the wall to the foundation,
studs which are placed vertically at 16 in. on center, and a top plate which connects the wall to
the floor or roof above. Multiple studs are arranged at the ends of walls and intersections to
provide for rigid attachment of interior and exterior finish materials. The typical configurations
suggested by the American Forest & Paper Association (AF&PA) are shown in Figure 2.8
(AF&PA, 2001). To provide overlapping at corners and intersections, the double top plates are
installed. Connections at the ends of the studs to the top or sole plates are made using 2-16d nails
20
at each end. The built up studs and double top plate are connected using 10d nails installed on the
face of the member at 12 in. on center. All nails used for framing are smooth-common nails.
The sill plates are anchored to the foundation with at least ½ in. diameter bolts spaced at
a maximum of 6 ft. on center with a nut and washer on each bolt. For structures with the seismic
design category of D, plate washers, with a minimum size of 0.229 in. by 3 in. by 3 in. in size, are
located between the sill plate and nut.
The most common sheathing for interior walls is gypsum drywall. Therefore, the typical
sheathing chosen for this research is 5/8 in. thick gypsum wallboard installed with 1-5/8 in. long
Type W or S screws with spacing of 7 in. on the edges and 12 in. on intermediate supports.
These wallboard panels typically come in 4 ft by 8 ft, 4 ft by 10 ft, and 4 ft by 12 ft sizes and may
be installed vertically or horizontally.
2.7.2
Typical steel-frame construction.
Cold-formed steel framing members are of similar sizes to those of wood framing and
must be at least 11/4 in. wide in the least dimension and from 0.033 inch (20 gauge) to 0.112 in.
(10 gauge) thick. Interior studs, with dimensions 1-5/8 in. by 3-5/8 in, and track, with dimension
1-1/2 in. by 3-5/8 in., are used for the walls of this research. The frame consists of c-shaped studs
and top and bottom tracks which are connected using connected using two No. 8 screws on each
end of the stud, one per flange. The corner details suggested by the North American Steel
Framing Alliance (NASFA) are shown in Figure 2.9 (NAFSA, 2000).
The bottom tracks of the walls may be anchored to the foundation under the same bolt
size and spacing requirements as the wood-framed specimens. However, to achieve this, NAFSA
suggests that a stud blocking with a minimum length of 6 in. is installed between the washer and
bottom track as shown in Figure 2.10.
21
The gypsum wallboard panels, used for the internal sheathing, are installed using with
No. 6 or larger screws drywall screws at the same spacing as the wood-framed specimens.
22
Table 2.1 Abbreviated test matrix and results for McMullin and Merrick
Test
no.
2
Loading
protocol
3
Monotonic
5
6
7
9
13
Top
Constraint
Free
Max load per unit
length (lb/ft)
399
Fixed
Floating edge
634
611
Cyclic
Free
378
Top sill
fastened
Monotonic
Cyclic
Construction
Method
664
Door
10
11
12
Wall
openings
Door and
window
520
560
506
Fixed
Floating edge
612
758
23
Moderate
damage (e.g.,
fastener yielding,
bearing,
cracking)
Force
Vmax  1500 lb
Substantial Major Failure (e.g.,
damage damage uplift failure,
(e.g., (e.g., wall- complete
board separation)
fastener
pull-out) separation
Deformation
Figure 2.1 Wood panel wall force-deformation response and associated damage states
(van de Lindt, 2004).
24
(a)
(b)
Figure 2.2 Construction framing elevation for specimens (a) type 1 and (b) type 2 of
McMullin and Merrick research.
25
(a)
(b)
Figure 2.3 Construction framing elevation for (a) walls 1 and 3 and (b) walls 2 and 4 of
Arnold et al. research.
26
Figure 2.4 Typical specimen cross-section installed in frame for Arnold et al. research.
27
(a)
(b)
Figure 2.5 Construction framing for (a) fastener and (b) panel tests
of Swensen et al. research.
28
Equiv. Load Per Fastener (lbs)
900
Adhesive + Screws
800
700
Adhesive
600
Coarse Threaded
500
400
300
200
100
0
0
0.02
0.04
0.06
0.08
0.1
Displacement (in)
(a)
(b)
Figure 2.6 Monotonic backbone curves for (a) wood-framed specimens and
(b) steel-framed specimens of the fastener tests of Swensen et al. research.
29
Racking Load (lbs/ft)
750
500
250
0
Adhesive + Screws
-250
Maxiscrew
-500
Coarse Threaded
-750
-2
-1
0
1
Drift Ratio (%)
(a)
(b)
Figure 2.7 Cyclic backbone curves for (a) wood-framed specimens and
(b) steel-framed specimens of the panel tests of Swensen et al. research.
2
30
(a)
(b)
Figure 2.8 Suggested corner stud assemblies for (a) corners of exterior walls and (b) at the
intersections of partition walls of wood-framed structures (ANSI, 2005).
31
(a)
(b)
Figure 2.9 Suggested corner stud assemblies for (a) corners of exterior walls and (b) at the
intersections of partition walls of light-gage steel-framed structures (NAFSA, 2000).
32
Figure 2.10 Anchorage requirements for steel-framed specimens (NAFSA, 2000).
33
CHAPTER 3
TESTING PROGRAM
3.1
Introduction
To investigate the seismic performance of enhanced nonstructural partition walls, twenty
test specimens, consisting of sixteen wood-framed walls and four light gage steel-framed walls,
were constructed and tested as a phase of a larger Network for Earthquake Engineering Research
(NEES) project. Later experiments in the project include full-scale, quasi-static room assembly
tests and full-scale shake-table tests.
The component experiments described herein were
conducted at California State University, Sacramento, expanding upon preliminary connector and
small-scale studies performed at Stanford University (Swensen et al., 2012).
The twenty test specimens described in this report are representative of partition and
exterior walls in residential construction, constructed to increase the stiffness, strength and
damage resistance of the structure during earthquake-type cyclic loading. Tables 3.1 and 3.2 list
the test variables, and construction details investigated in the study, including construction
adhesive between the frame and sheathing, uplift tie-down details, orthogonal end returns,
framing material, sheathing type, openings, and wall length. The walls were loaded using a
modified CUREE test protocol as illustrated in Table 3.3 and Figure 3.1.
3.2
Test Setup
To simulate the resisting forces that a planar wall would experience in the first floor of a
two story residential building, specimens were installed within a test frame as shown in Figures
3.2 and 3.3. The frame was also designed to prevent out of plane motions at the top and bottom
of each specimen. The bottom of the specimens were attached to an assembly composed of a
W8x48 base beam and HSS 4x7x1/2 members which were anchored to the strong floor of the
34
laboratory. Stiffener plate assemblies, shown in Figure 3.4, were added to the base beam at each
of the tie down locations to simulate the resistance of a concrete foundation to prevent uplift of
the tie down rod. Referring to Figure 3.2, a W8x28 beam was installed on top of the specimen to
transfer the shear forces into the wall and simulate the stiffness of a second-story wall above.
A 220-kip, ±10 in. stroke hydraulic actuator was used to load the specimens. The
actuator was placed vertically and attached to a bell crank composed of two 1.5 in. thick plates.
The bell crank, illustrated in Figure 3.5, allowed the specimen’s force to be applied horizontally
at the top of the wall. A 2 in. thick steel link was used to attach the bell crank to the W8x28
loading beam as shown in Figure 3.6.
Out-of-plane movement was restricted at the top of the specimen with 4 in. x 4 in. square
plates, covered with Teflon and grease, placed on either side of the loading beam web and
secured onto 1 in. diameter pre-tensioned steel rods fixed to the laboratory strong wall as shown
in Figure 3.7. After the specimens were installed in the test rig, approximately 5,000 lbs of
tension was applied to the rods with turnbuckles (Figure 3.7c) and the walls were secured into
place using the square plates to sandwich the loading beam into the correct position.
For all walls, a 2 in. by 12 in. piece of lumber was added to the top plate of the wall to
simulate a ceiling return. As illustrated in Figure 3.8, the top plates and ceiling members of the
specimens were attached to the loading beam with 1/2 in. diameter through bolts in a staggered
pattern at approximately 6 in. o.c. The sill plates of the specimens were attached to the base
beam with 3/4 in. anchor bolts at 16 in. o.c. and uplift constraints that varied in location and size
according to the test parameters as noted in Table 3.2.
35
3.3
Wall Construction Details and Material Properties
3.3.1
Wood Framing
All lumber used for the wood-framed specimens was Douglas Fir No 2 or better. To allow the
walls to handle the expected shear forces, the wood-framed walls were built with double 2x4 top
plates, a 3x4 sill plate, and 2x4 studs spaced at 16 in. on center. For the specimens with door
openings, the headers over the opening were 4x6 members. The framing was constructed using
8d, 16d, and 20d common nails and wood screws as listed in Table 3.4. The details in Figure
3.9a-c show how the ends of the planar walls and corner assemblies were constructed.
The uplift constraints used at the ends of walls and pier segments included Simpson
Strong-tie HDU tie downs, bent Simpson Strong-tie MST straps, and stiffness enhanced tie
downs as listed in Table 3.2. The manufacturer’s reported allowable tension loads for the HDU
tie downs are shown in Table 3.6 along with the required fasteners and wood member
thicknesses.
Lumber was purchased with a specified moisture level of 18%.
To determine the
approximate moisture levels of the specimens during the test for the testing environment, the
moisture level of the lumber for Specimen W-STU was tested during the interval between
framing and testing using an Extech Moisture Meter. Figure 3.10 illustrated the decrease in
moisture content during the three weeks after purchase, stabilizing at approximately 10%
moisture after the 23rd day. Using Figure 3.10 in conjunction with the time intervals listed in
Table 3.5 will provide the approximate moisture levels in the lumber at the time of the test.
Liquid Nails Heavy Duty Construction Adhesive was the adhesive used for the woodframed specimens. The adhesive of all specimens were given at least 48 hours of cure time to
obtain the manufacturer’s recommended shear strength of listed in Table 3.7.
36
3.3.2
Steel Framing
Using similar construction plans and procedures, the light-gage steel specimens were
constructed with top and bottom plates of 1-1/2 x 3-5/8 x 18 ga. track and 1-5/8 x 3-5/8 x 20 ga.
partition studs at 16 in. o.c. The frames were built using #10x1 in. self-tapping screws as listed in
Table 3.3. The details in Figure 3.9d-e illustrated the construction of the ends of the planar walls
and corner assemblies. Similar to the wood-framed specimens, the uplift constraints at the ends
were the steel-specific Simpson Strong-tie S/HDU tie downs. The manufacturer’s reported
allowable tension loads for the S/HDU tie downs are shown in Table 3.8 along with the required
fasteners and steel member thicknesses.
Loctite PL375 Heavy-Duty Construction Adhesive was used for the steel-framed
specimens. Similar to the wood-framed specimens, the adhesive was given at least 48 hours of
cure time to obtain the manufacturer’s recommended shear strengths listed in Table 3.9.
3.3.3
Sheathing
All specimens were sheathed with 5/8 in. gypsum type X wallboard fastened to the
framing with 1-5/8 in. drywall screws spaced at 7 in. on center on the edges and 12 in. in the
field. The sheathing, 4 ft. by 8 ft. panels, was installed horizontally as shown in Figure 3.10a.
For specimens that were longer than 8 ft, the joints were staggered as shown in Figure 3.10b.
Three of the wood-framed specimens featured an alternate sheathing material representative of an
exterior sheathing. The exterior wallboards of these specimens were installed horizontally with
staggered joints as shown in Figure 3.10c. The first test featured an exterior sheathing of 5/8 in.
DensGlass® fiberglass mat gypsum sheathing installed with the same techniques as the gypsum
wallboards. The second test featured 15/32 in. Structural I Sheathing installed with 10d nails
spaced at 6 in. on center on the edges and 12 in. in the field. The third and final specimen
featured an exterior finish of three-coat-7/8 in. Portland cement plaster over 5/8 in. DensGlass®
37
sheathing. The DensGlass® sheathing of this specimen was installed using the same techniques
as the first exterior specimen. Two layers of Grade D building paper, 17 gage wire lath, #14 x 3½ in. screws with ¼ in. rubber washers spaced at 4 in. on center on the edges and 7 in. on center
in the field, were used to install the stucco. The three coats of stucco were applied with a 3/8 in.
thick scratch coat, a 3/8 in. thick brown coat, and a 1/8 in. thick finish coat.
3.4
Test Matrix
Tables 3.1 and 3.2 show the parameters that were varied for each specimen of this
research. Specimens W1 through W11, discussed in Chapter 4, form the interior walls in the
wood-frame suite. The first specimen, W1, was an 8 ft. x 8 ft. planar wall designed using current
construction techniques. The behavior of this specimen was used as the control for the woodframed tests. Specimens W2 through W6 represented an iterative test series to determine the
construction details required to improve the performance of the 8 ft. x 8 ft. planar walls. Within
this series, the behavior of each specimen was analyzed and influenced the uplift constraints,
blocking, and other details of the next specimen. The details of specimen W6 were then applied
to specimens W7 and W8, which represented a shorter iterative test series to determine the details
required for 8 ft. x 8 ft. walls with orthogonal returns. The final three interior walls, specimens
W9 through W11, characterized a test series with varying aspect ratios and door openings
constructed with the successful details of specimens W6 and W9.
Specimens S1 through S4, discussed in Chapter 5, represented the steel-framed suite and
determined how the seismically enhanced details determine through the wood-framed tests could
be applied to steel-framed walls. The first specimen, S1, was an 8 ft. x 8 ft. planar wall built
using current construction techniques and acts as the control for this suite of tests. Specimen S2
was built by applying the successful details of specimen W6 to the steel-frame construction
38
techniques. Specimens S3 and S4 were 8 ft. x 8 ft. walls with orthogonal returns built using the
details from specimen W8, with a similar iterative approach to determine the necessary details for
these walls.
Specimens W-DG, W-PLY, and W-STU represented exterior walls in the wood-framed
suite. These specimens feature differing external sheathing materials applied to walls that were
constructed using the details of specimen W8.
The DensGlass® wallboard was chosen for
specimens W-DG and W-STU due to the material similarities to the interior gypsum wallboards
and moisture resistant and fire rated properties. On specimen W-STU, three-coat-7/8 in. stucco,
which is a common finish for residential buildings, was installed over the DensGlass ® wallboard
to determine which behaviors are caused solely by the finish when installed with the seismically
enhanced details. W-PLY, which featured plywood sheathing, explores the behavior of an
exterior wall built with this commonly used sheathing and the enhanced details of unibody
construction.
3.5
Loading History
An adapted version of the CUREE Simplified Loading History was used for this study.
This type of loading history, which has no trailing cycles, allows for accurate analytical model
calibration following from force-deformation behavior that better illustrates the stiffness and
strength degradation of the specimen. These models will utilize the results from this phase of
testing for the analysis and design of the two-story building of the final phase of this project. The
protocol, shown in Table 3.3 and Figure 3.1, follows from the protocol used to test the 4 ft. x 4 ft.
enhanced panel tests by Swensen et. al., 2012. However, twelve cycles of smaller displacements
were added to the protocol to capture the behavior between 0.05% and 0.1% interstory drift of the
stiffer system.
39
3.6
Instrumentation
Many instruments were used to record the frame and sheathing behaviors of the
specimens during the tests. These instruments included strain gages, linear variable differential
transformers (LVDTs) and string potentiometers, examples of which are shown in Figure 3.12.
The recorded frame behavior included the lateral displacement at the top and bottom of
the wall, uplift of the end studs, out of plane displacements at the top of the wall and axial forces
in the anchor bolts and tie downs. To record the deformation of the sheathing panels relative to
the frame, LVDTs were used to measure the vertical and horizontal displacements of the internal
and external sheathings at the bottom of the wall.
Six strain gages were placed on the faces of the loading link, shown in figure 3.6a, to
measure the lateral force that was applied to the specimen. A calibration test was performed to
determine the exact elastic modulus of the link by recording the strains in each of the six gages as
compression and tension forces up to approximately 20,000 lbs were applied. Using the linear
data between 20,000 lbs tension and 13,000 lbs compression to relate the applied force and
average recorded strain, as shown in Figure 3.13, the equations below were used to determine the
elastic modulus.
𝑃 = 𝜎𝑎𝑣𝑔 𝐴 = 𝜀𝑎𝑣𝑔 𝐸𝐴 = 121.81 (𝜀𝑎𝑣𝑔 )
(3.6.1)
𝐸 = 30452.5 𝑘𝑠𝑖
(3.6.2)
The locations for the instruments that measured the behaviors of the planar wood- and
steel-framed walls (W1-W6 and S1-S2) are illustrated in Figure 3.14 and described in Table 3.10.
Figure 3.15 and Table 3.11 show the locations of the instruments that measured the behavior of
the wood- and steel-framed walls with returns (W7, W8, W10, S3, and S4). The instrument
locations for the wood-framed specimens with returns and door openings (W9 and W11) are
shown in Figure 3.16 and discussed in Table 3.12.
Lastly, the instruments measuring the
40
behaviors of the exterior wood-framed walls (W-DG, W-PLY, and W-STU) are detailed in Figure
3.17 and Table 3.13.
The damage progression recorded for each specimen included observations of physical
damage and/or noises which occurred during each experiment. Along with notes, handheld and
mounted cameras were used to record the visual progression of the damage. The still cameras
took pictures at the peak and zero displacements of each cycle during the test and were used to
create time lapse videos of the test at the locations shown in Table 3.14. Different colors of
marker were used to highlight the observed damage of each set of cycles and show how it
propagated throughout the test. An example of how the different colors show the damage
propagation through the different sets of cycles is shown in Figure 3.18.
41
Table 3.1 Test matrix
Test
No.
1, 3,
4
2
Specimen
Frame
Material
W1
Adhesive
W3
6
7
8
9
10
11
12
13
W4
W5
W6
W7
W8
W9
W10
W11
14
W-DG
15
16
17
18
S1
S2
S3
S4
19
W-STU
None
Additional
Parameter
Enhanced
Joint
Compound
PCP pads at
anchor bolts
Wood
Yes
Blocking
T-Shape
Wood
L-Shape
None
Steel
T-Shape
Wood
W-PLY
Joint
Detail
None
W2
5
20
Return
L-Shape
Yes
Blocking
None
None
Yes
Blocking
Yes
Blocking
DensGlass® on
exterior
Stucco over
DensGlass® on
exterior
Plywood on
exterior
42
Table 3.2 Test matrix uplift constraints and locations
Specimen
Uplift Constraint
W1, W2
HDU-5
W3
HDU-8
W4
Enhanced
W5, W6
HDU-8 + Strap
W7
HDU-8
HDU-5
HDU-8
W8, W10
HDU-8
W9, W11
W-DG, W-PLY,
W-STU
S1, S2
S3, S4
HDU-5
Strap
HDU-8
S\HDU-6
S\HDU-6
S\HDU-9
Location
Ends of Wall
Ends of Returns
Ends of Returns
Ends of Main Wall
East End of Wall
East Side of Door
Ends of Returns
Both Sides of Door
Ends of Returns
Ends of Main Wall
Ends of Wall
Ends of Returns
Ends of Main Wall
Table 3.3 Loading protocol
Drift Amplitude
(%)
0.05
0.075
0.1
0.2
0.3
0.4
0.5
0.75
1.0
1.25
1.5
1.75
2.0
2.5
No.
Cycles
6
6
6
6
6
6
6
4
4
4
3
3
3
2
Displacement
Rate (in/min)
0.10
0.10
0.10
0.20
0.25
0.50
0.50
0.50
0.75
0.75
1.00
1.00
1.25
1.25
43
Table 3.4 Nail and screw information
8d common
10d common
Diameter
(in.)
0.131
0.148
Length
(in.)
2-1/2
3
16d common
0.162
3-1/2
20d common
0.192
4
#6 1-5/8 in. coarse
drywall screw
0.145
1-5/8
Wood-to-wood screw
0.150
3
Strap-to-wood screw
#14 x 3 in. Hexwasher-head selfdrilling screw
#10 1in self tapping
screw
#8 1-5/8 in. selfdrilling drywall screw
0.210
1-5/8
Built up studs
(corner details)
Strap to studs
0.210
3-1/2
Lath to framing
0.158
1
Name
Framing
Material
Location Used
Built up studs
Plywood to framing
End nail
End nail
(bottom sill to stud)
Wood
Drywall to framing
Framing connections
Steel
0.123
1-5/8
Drywall to framing
Table 3.5 Time intervals for lumber and adhesive
Lumber
Framed to
Adhesive Cure
Specimen
Purchased to
Sheathed (days)
Time (days)
Framed (days)
W1
3
16
n/a
1 (North Side)
39 (North Side)
W2
1
37 (South Side
3 (South Side)
W3
3
10
4
W4
4
9
4
W5
12
106
2
W6
12
112
2
W7
12
121
4
W8
12
133
3
W9
12
142
4
W10
12
153
3
W11
12
163
8
W-DG
5
8
5
7 (Gypsum)
9 (Gypsum)
W-PLY
5
8 (Plywood)
8 (Plywood)
92 (Gypsum)
5 (Gypsum)
W-STU
5
57 (Densglass®)
40 (Densglass®)
44
Table 3.6 Reported requirements and capacities for Simpson Strong-tie wood tie-downs
Min.
Wood
Member
Thickness
(in.)
Fasteners
Model No.
HDU5-SDS2.5
HDU8-SDS2.5
Anchor
Bolt Dia.
(in.)
SDS Screws
5/8
7/8
14-SDS ¼”x2½”
20-SDS ¼”x2½”
Allowable Tension Loads
(lbs)
Deflection at
Douglas Fir
Allowable
Load (in.)
3
3
5645
5980
0.115
0.084
Table 3.7 Liquid Nails construction adhesive reported shear strength
Cure Time (days)
1
2
7
Shear Strength
(psi)
225
300
425
Table 3.8 Reported requirements and capacities for Simpson Strong-tie light-gage steel tie-downs
Model
S/HDU6
S/HDU9
Fasteners
Anchor
Stud
Bolt Dia.
Fasteners
(in.)
5/8
12- #14
7/8
18- #14
Stud Member
Thickness
Tension
Load
Deflection
at Load
2-33 (2-20ga)
2-33 (2-20ga)
8495
11125
0.250
0.189
Table 3.9 Loctite construction adhesive reported shear strength
Cure Time (days)
1
14
Shear Strength
(psi)
26.3
42.3
45
Table 3.10 Instruments for Specimens W1 through W6
Instrument
Name
Type
Wall displacement (location 1)
D
String pot.
Actuator force
Load
Load cell
Wall displacement (location 2)
OOPE
String pot.
Wall displacement (location 3)
OOPW
String pot.
Wallboard displacement
Wallboard uplift (location 1)
GH
GUE
LVDT
LVDT
Wallboard uplift (location 2)
GUW
LVDT
Stud Uplift (location 1)
Stud Uplift (location 2)
Anchorage force (location 1)
Anchorage force (location 2)
Anchorage force (location 3)
Anchorage force (location 4)
Anchorage force (location 5)
Loading strut gage (location 1)
Loading strut gage (location 2)
Loading strut gage (location 3)
Loading strut gage (location 4)
Loading strut gage (location 5)
Loading strut gage (location 6)
Tie down gage (location 1)
Tie down gage (location 2)
Tie down gage (location 3)
Tie down gage (location 4)
Tie down gage (location 5)
Tie down gage (location 6)
Tie down gage (location 7)
Tie down gage (location 8)
SUE
SUW
E1
E2
E3
E4
E5
LS1
LS2
LS3
LS4
LS5
LS6
TDW1
TDW2
TDW3
TDW4
TDE1
TDE2
TDE3
TDE4
LVDT
LVDT
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Function
Global wall displacement at top
of wall (control disp)
Force of actuator
Out of plane displacement of
North-East of ceiling
Out of plane displacement of
South-West of ceiling
Horizontal disp. of wallboard
Uplift of East side of wallboard
Uplift of West side of
wallboard
Uplift of East king stud
Uplift of West king stud
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain on face of west tie down
Strain on face of west tie down
Strain on face of west tie down
Strain on face of west tie down
Strain on face of east tie down
Strain on face of east tie down
Strain on face of east tie down
Strain on face of east tie down
46
Table 3.11 Instruments for specimens W7, W8, and W10
Instrument
Name
Type
Wall displacement (location 1)
D
String pot.
Actuator force
Load cell
Wall displacement (location 2)
OOPE
String pot.
Wall displacement (location 3)
OOPW
String pot.
Wallboard displacement
Stud Uplift (location 1)
Stud Uplift (location 2)
GH
SUE
SUW
LVDT
LVDT
LVDT
Stud Uplift (location 3)
SUNE
LVDT
Wallboard uplift (location 1)
GUE
LVDT
Wallboard uplift (location 2)
GUW
LVDT
Stud Uplift (location 4)
Anchorage force (location 1)
Anchorage force (location 2)
Anchorage force (location 3)
Anchorage force (location 4)
Anchorage force (location 5)
Loading strut gage (location 1)
Loading strut gage (location 2)
Loading strut gage (location 3)
Loading strut gage (location 4)
Loading strut gage (location 5)
Loading strut gage (location 6)
Tie down gage (location 1)
Tie down gage (location 2)
Tie down gage (location 3)
Tie down gage (location 4)
Tie down gage (location 5)
Tie down gage (location 6)
Tie down gage (location 7)
Tie down gage (location 8)
Tie down gage (location 9)
Tie down gage (location 10)
Tie down gage (location 11)
Tie down gage (location 12)
SUSE
E1
E2
E3
E4
E5
LS1
LS2
LS3
LS4
LS5
LS6
TDNE1
TDNE2
TDNE3
TDNE4
TDE1
TDE2
TDE3
TDE4
TDSE1
TDSE2
TDSE3
TDSE4
LVDT
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Function
Global wall displacement at
top of wall (control disp)
Force of actuator
Out of plane displacement of
North-East of ceiling
Out of plane displacement of
South-West of ceiling
Horizontal disp. of wallboard
Uplift of East king stud
Uplift of West king stud
Uplift of king stud at NE
return
Uplift of East side of
wallboard
Uplift of West side of
Wallboard
Uplift of king stud at SE return
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain on face of NE tie down
Strain on face of NE tie down
Strain on face of NE tie down
Strain on face of NE tie down
Strain on face of E tie down
Strain on face of E tie down
Strain on face of E tie down
Strain on face of E tie down
Strain on face of SE tie down
Strain on face of SE tie down
Strain on face of SE tie down
Strain on face of SE tie down
47
Table 3.12 Instruments for wood-framed specimens with returns
and door openings (W9 and W11)
Instrument
Name
Type
Wall displacement (location 1)
D
String pot.
Actuator force
Load cell
Wall displacement (location 2)
OOPE
String pot.
Wall displacement (location 3)
OOPW
String pot.
Wallboard displacement
Stud Uplift (location 1)
Stud Uplift (location 2)
Stud Uplift (location 3)
Wallboard uplift (location 1)
GH
SUE
SUW
SUNE
GUE
LVDT
LVDT
LVDT
LVDT
LVDT
Wallboard uplift (location 2)
GUDE
LVDT
Stud Uplift (location 4)
SUDE
LVDT
Anchorage force (location 1)
Anchorage force (location 2)
Anchorage force (location 3)
Anchorage force (location 4)
Anchorage force (location 5)
Loading strut gage (location 1)
Loading strut gage (location 2)
Loading strut gage (location 3)
Loading strut gage (location 4)
Loading strut gage (location 5)
Loading strut gage (location 6)
Tie down gage (location 1)
Tie down gage (location 2)
Tie down gage (location 3)
Tie down gage (location 4)
Tie down gage (location 5)
Tie down gage (location 6)
Tie down gage (location 7)
Tie down gage (location 8)
Tie down gage (location 9)
Tie down gage (location 10)
Tie down gage (location 11)
Tie down gage (location 12)
E1
E2
E3
E4
E5
LS1
LS2
LS3
LS4
LS5
LS6
TDNE1
TDNE2
TDNE3
TDNE4
TDE1
TDE2
TDE3
TDE4
TDD1
TDD2
TDD3
TDD4
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Function
Global wall displacement at top of
wall (control disp)
Force of actuator
Out of plane displacement of
North-East of ceiling
Out of plane displacement of
South-West of ceiling
Horizontal disp. of wallboard
Uplift of East king stud
Uplift of West king stud
Uplift of king stud at NE return
Uplift of East side of wallboard
Uplift of Wallboard of East side of
doorway
Uplift of king stud at East side of
doorway
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain on face of NE tie down
Strain on face of NE tie down
Strain on face of NE tie down
Strain on face of NE tie down
Strain on face of E tie down
Strain on face of E tie down
Strain on face of E tie down
Strain on face of E tie down
Strain on face of doorway tie down
Strain on face of doorway tie down
Strain on face of doorway tie down
Strain on face of doorway tie down
48
Table 3.13 Instruments for exterior specimens (W-DG, W-PLY, and W-STU)
Instrument
Name
Type
Wall displacement (location 1)
D
String pot.
Exterior wall displacement
EH
LVDT
Wall displacement (location 2)
OOPE
String pot.
Wall displacement (location 3)
OOPW
String pot.
Wallboard displacement
Stud Uplift (location 1)
Stud Uplift (location 2)
Stud Uplift (location 3)
Wallboard uplift (location 1)
GH
SUE
SUW
SUNE
GUE
LVDT
LVDT
LVDT
LVDT
LVDT
Wallboard uplift (location 2)
GUW
LVDT
Exterior wall uplift
EUE
LVDT
Anchorage force (location 1)
Anchorage force (location 2)
Anchorage force (location 3)
Anchorage force (location 4)
Anchorage force (location 5)
Loading strut gage (location 1)
Loading strut gage (location 2)
Loading strut gage (location 3)
Loading strut gage (location 4)
Loading strut gage (location 5)
Loading strut gage (location 6)
Tie down gage (location 1)
Tie down gage (location 2)
Tie down gage (location 3)
Tie down gage (location 4)
Tie down gage (location 5)
Tie down gage (location 6)
Tie down gage (location 7)
Tie down gage (location 8)
Tie down gage (location 9)
Tie down gage (location 10)
Tie down gage (location 11)
Tie down gage (location 12)
E1
E2
E3
E4
E5
LS1
LS2
LS3
LS4
LS5
LS6
TDNE1
TDNE2
TDNE3
TDNE4
TDW1
TDW2
TDW3
TDW4
TDE1
TDE2
TDE3
TDE4
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Strain gage
Function
Global wall displacement at top
of wall (control disp)
Horizontal disp. of exterior wall
Out of plane displacement of
North-East of ceiling
Out of plane displacement of
South-West of ceiling
Horizontal disp. of wallboard
Uplift of East king stud
Uplift of West king stud
Uplift of king stud at NE return
Uplift of East side of wallboard
Uplift of West side of
wallboard
Uplift of East side of exterior
wall
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Force at shear anchor
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain at face of loading strut
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
Strain on face of tie down
49
Table 3.14 Camera Locations for Time Lapse Videos
Specimen
Camera #1
North Face
Whole Wall
North Face
Whole Wall
North Face
Whole Wall
North Face
Whole Wall
North Face
Whole Wall
North Face
Whole Wall
Camera #2
South Face
West Corner
South Face
East Corner
South Face
West Corner
South Face
West Corner
South Face
West Corner
South Face
West Corner
South Face
East Corner
South Face
East Corner
South Face
East Corner
South Face
East Corner
W7
North Face
Whole Wall
South Face
West Corner
South Face
East Corner
W8
North Face
Whole Wall
South Face
West Corner
South Face
East Corner
W9
North Face
Whole Wall
South Face
West Side Door
South Face
East Side Door
W10
South Face
Whole Wall
South Face
West Corner
South Face
East Corner
South Face
Whole Wall
North Face
Whole Wall
North Face
Whole Wall
North Face
Whole Wall
South Face
West Side Door
North Face
West Corner
North Face
West Corner
North Face
West Corner
South Face
East Side Door
South Face
Whole Wall
South Face
Whole Wall
South Face
Whole Wall
W1
W2
W3
W4
W5
W6
W11
W-DG
W-PLY
W-STU
Camera #3
Camera #4
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
East Return
East Face
South Corner
East Return
East Face
South Corner
South Face
Over Doorway
East Return
East Face
South Corner
South Face
Over Doorway
South Face
East Corner
South Face
East Corner
South Face
East Corner
50
Figure 3.1 Loading protocol.
Figure 3.2 Test frame component identification and dimensions.
51
52
(a)
(b)
Figure 3.3 Test frame with (a) 8 ft. specimen and (b) 16 ft. specimen installed.
53
Figure 3.4 Stiffener assemblies on the W8x48 base beam.
54
Figure 3.5 Bell crank used to rotate the forces applied to the specimen.
55
(a)
(b)
Figure 3.6 Loading link (a) as designed in the manufacturing plans and
(b) as installed in the test frame.
56
(a)
(b)
(c)
Figure 3.7 Out-of-plane restrictions for top loading beam;
(a) Plan and (b) section view of the beam and
(c) Turnbuckles used to pretension rods.
57
/2"x3-5/8 18ga. Track
(a)
(b)
Figure 3.8 Section view of wall in test rig;
(a) Wood-framed specimen and (b) steel-framed specimen.
58
(a)
(d)
(b)
(e)
(c)
Figure 3.9 Corner and end details;
End of planar wall: (a) Wood-framed, (d) Steel-framed
T-shape corner assembly: (b) Wood-framed, (e) Steel-framed
(c) Wood-framed L-shape corner assembly.
59
(a)
(b)
(c)
Figure 3.10: Wallboard layout for
(a) 8 ft. walls, (b) 16 ft. walls, and
(c) Exterior face of 8 ft. external walls.
60
Figure 3.11 Approximate lumber moisture level for wood specimens.
61
(a)
(b)
(c)
Figure 3.12 Samples of instrumentation used to measure frame and wallboard behaviors;
(a) Strain gage on loading link, (b) LVDT measuring stud uplift, and
(c) string potentiometer measuring specimen displacement.
62
Figure 3.13 Applied force versus average recorded strain used for link calibration.
63
Figure 3.14 Planar wall instrumentation (W1-W6 and S1-S2).
64
Figure 3.15 Instrumentation for specimens with returns and no openings
(W7, W8, W10, S3 and S4).
65
Figure 3.16 Instrumentation for specimens with door openings (W9 and W11).
66
(a)
(b)
Figure 3.17 Exterior wall instrumentation (W-DG, W-PLY, and W-STU);
(a) On interior face and (b) on exterior face.
67
Figure 3.18 Example of how damage propagation for each set of cycles is
highlighted using different colors.
68
CHAPTER 4
EXPERIMENTAL RESULTS OF CYCLIC TESTED PLANAR WOOD-FRAMED WALLS
4.1
Introduction
This chapter discusses the results of the eleven interior wood-framed planar walls, listed
as W1-W11 in Table 3.1, which will contribute to the lateral force resisting system of residential
buildings built with unibody construction techniques.
Within this chapter, the specimen
geometry, construction details, and behavior of each specimen during the 0-0.5% and post 0.5%
interstory drifts will be presented for each wall. The behavior discussion is segmented in this
way due to the context of the overall project methodology to design a limited ductility structure,
with minimal damage during design level earthquake events. Thus, it is valuable to provide a
detailed and separated presentation of the results up to, and including, the point where the
specimens show significant damage and inelastic behavior.
For most of the specimens,
significant damage occurred at deformation cycles corresponding to 0.2% and 0.3%interstory
drift with strength loss occurring soon thereafter.
However, several specimens exhibited a
somewhat more ductile behavior with a strength capacity near 0.5% drift.
To summarize and compare the effects of the construction details investigated in the test
matrix for the wood-framed walls discussed in Chapter 3, the strength, stiffness, and damage
progression of each specimen are reported. Owing to the inherent nonlinear response of the lightframed wall specimens, the reported stiffness in Table 4.1 is the secant stiffness calculated with
the maximum forces sustained during the first cycle to +/-0.1% interstory drift. In addition, it
allows for comparison between the specimens discussed in this thesis with the previous phase of
4 ft. x 4 ft. wall panels tested at Stanford University (Swensen et al, 2012). However, it should be
noted that the specimen stiffness at earlier cycles (0.05 and 0.075%) are larger. Also listed in
69
Table 4.1 is the one-sided maximum strength capacity in pounds per linear foot of wall and the
interstory drift cycle at which the force was recorded.
Additional measurements reported in this chapter for each specimen include uplift forces
in the tie down units and anchor bolts to gain an understanding of the force path at the base of
each specimen. Displacement gages, listed in Chapter 3, are used to measure deformations
related to 1) out-of plane twisting, 2) differential slip between the wood-frame and sheathing in
the horizontal and vertical direction, 3) differential horizontal slip between the bottom sill plate
and test rig, and 4) uplift.
4.2
Planar Control Test: W1
Test specimen W1 is representative of current construction techniques and acts as the
control for the wood-frame suite of experiments.
Additionally, this wall is comparable to
specimens tested by McMullin and Merrick (2001), discussed in Chapter 2. The specimen was
framed with conventional wood 2x4 studs, 5/8 in. gypsum sheathing and mechanical fasteners as
an 8 ft. x 8 ft. planar wall with no door/window openings. Simpson Strong-Tie HDU5 tie-downs
were installed on the inside face of the studs at the ends of the specimen, tightened with a socket
wrench past manufacturer suggested pretensioned load to approximately 3000 lbs. Referring to
Table 4.2, the anchor bolts, which provided the force path for shear forces between the wall and
the test assembly, were tightened to pretension forces of approximately 3000 lbs. with a socket.
The construction framing for this wall is shown in Figure 4.1.
This specimen geometry and type were repeated in test number W1a, W1b and W1c,
where the repetitions occurred to perfect all aspects of the experimental procedure and test setup.
All three specimens had a similar behavior, up to and including failure. Therefore, the results of
70
W1c, which utilized the final test assembly and instrumentation, will be reported as the control
specimen for the wood-framed testing suite.
4.2.1
Summary and overall behavior.
Specimen W1 attained a maximum force capacity of 4760 lb, or 298 lb/ft as listed in
Table 4.1, during the 0.5% interstory drift cycles. The one-sided stiffness of this specimen,
measured at 0.1% interstory drift, was 1361 lb/in/ft. The primary mode of failure for this
specimen was fastener failure at bottom sill plate, which extended upwards such that majority of
the bottom panel fasteners had failed by the 1.0% interstory drift cycles.
The maximum out-of-plane rotation angle of the specimen, as illustrated in Figure 4.2,
was calculated as 0.0015 radians – (ΔEast Pot + ΔWest Pot)/76” – with corresponding displacements
recorded as +0.046” and +0.067” during the 1.0% interstory drift cycles, at the East and West end
of the wall, respectively. This rotation produces a displacement equal to 0.06” and 0.08” at the
East and West ends of the specimen, respectively.
4.2.2
Observed behavior 0-0.5% interstory drift.
The force-deformation behavior of the specimen for the range of cycles up to, and
including, the 0.5% interstory drift is shown in Figure 4.3a. Referring to the figure, the shapes of
the hysteretic loops are indicative of inelastic behavior at deformation magnitudes as small as
0.05% drift cycles. Although no visible damage was apparent, noises began to occur within the
wall during the 0.05% drift cycles. These noises begin as small clicks and progressed into louder
creaks as the first signs of visible damage occurred during the 0.1% interstory drift cycles. This
damage preceded fastener popping, and was characterized by paint flaking at the screw heads
located along the bottom sill plate. An example of this paint cracking at 0.1% is shown in Figure
71
4.4a. During the 0.3% interstory drift cycles, the fastener heads became clearly visible and
defined as screw popping, as shown in Figure 4.4b. This occurred along the bottom sill plate to
wallboard connection, as well as multiple edge screws along the height of the end-stud as
illustrated in Figure 4.5a. In the subsequent group of cycles, 0.4%, the fastener row above the sill
plate experienced screw popping. No further screw popping was observed during the 0.5%
interstory drift cycles.
The maximum axial forces recorded for each tie down and anchor bolt within each of the
0.05% through 0.5% interstory drift cycle sets are shown in Figure 4.6. The forces are measured
above the pretension force and shown as a percentage of the specimen’s corresponding lateral
force. The figure also shows the cumulative change of pretension forces based on the force in the
bolt or tie down rod at the beginning and end of the cycles when the specimen had no lateral
force. Referring to Figure 4.7, which shows the recorded axial force versus time for anchor bolts
#1 and #2, the changes of pretension can be seen between each set of cycles, when the testing was
paused for damage inspection at approximately zero-load in the specimen. In general, and as
expected, the tie downs and anchor bolts experienced a loss of pretension during the experiment,
as the forces of anchor bolt #1 show, which can be contributed to loosening of the bolts, localized
settling, localized damage, or movement of the sill plate. But most specimens have one anchor
bolt which experienced a gain in pretension of the bolt over the experiment, similar to anchor bolt
#2, which may be contributed to a slight elongation of the bolt due to rotation as the specimen
shifted during the experiment and stuck on the edges of the holes in the test rig.
As expected, the maximum axial forces occurred at the tie downs and the minimum
forces occurred in the interior-most bolts. For example, during the 0.5% interstory drift cycles, at
the maximum lateral force capacity of the specimen, the tie-down forces correspond to 72-78% of
the theoretical uplift force – equal to the lateral load for a square specimen – at the tie downs,
72
18% at the exterior-most anchor bolt, and 1-2% at the interior anchor bolts, accounting for the
entire uplift force in the sill.
4.2.3
Observed behavior post 0.5% interstory drift.
Referring to Figure 4.8a, which shows the force-deformation behavior of Specimen W1
for the entire test, the strength of the wall reached its maximum capacity during the negative
displacement of the 0.5% interstory drift, but decreased in the positive and negative
displacements of the subsequent cycles. The final damage of this specimen, displayed in Figure
4.5b, consisted of screws popping and/or complete fracture for several screws, which began at the
bottom row of screws and progressed towards the top of the wall as the applied displacements
increased.
After the screws popped/sheared, the wallboards pulled away from the frame and were
able to move separately as demonstrated by the images of the lower east corner of the specimen
presented in Figure 4.9. The left image demonstrates how the wallboards displaced past the end
of the frame when a negative displacement was applied to the specimen. The right image shows
the same corner when a positive displacement is applied to the specimen, where once again the
separation between frame and wallboard allows the panel to displace further than the frame.
Referring to Figure 4.10, the specimen maintained the distribution of axial forces to the
anchor bolts and tie downs during the larger displacements. The largest forces occurred in the
tie-downs and exterior anchor bolts while the lowest forces occurred in the interior anchor bolts.
However, due to the separation between frame and wallboard, the forces became concentrated in
the end studs rather than being distributed across the wall. Therefore, with the exception of the
edge bolts, the forces in the tie downs increased while forces in the anchor bolts decreased.
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4.3
Planar Wall Tests With Unibody Enhancements: W2 Through W6
Specimens W2 through W6 have a similar framing geometry and construction details to
the W1 specimen, explained previously and shown in Figure 4.1, with the addition of
construction adhesive between the wallboard and framing members.
This group of tests
represented an iterative process to improve the strength, stiffness and damage performance of the
8 ft. x 8 ft. planar specimens. These improvements included the use of various uplift constraints,
flexible matting at the anchor bolt connections, and mid-height blocking.
As mentioned previously, walls W2 through W6 featured iterative improvements to the
construction details, which increased the strength by 37% and stiffness by 32% of the planar
walls, between walls W2 and W6. The final unibody enhancements for a planar wall (W6),
improved the strength and stiffness by 110% and 164%, respectively, over the control test (W1).
Figure 4.11 shows the cyclic backbone curves for wall specimens W1 through W6,
generated from the one-sided strength force-deformation response by plotting peaks of leading
cycles for each group. Referring to the figure, note the conventional construction test reached its
largest load at 0.5% interstory drift, while the unibody improvements resulted in a less ductile
response with maximum loads at 0.2-0.4% interstory drift, with the exception of the more flexible
W3 which reached the maximum load at 0.75%. However, following the maximum load, the
effect of the construction adhesive decreases and specimens W2-W6 behave similar to the
traditionally constructed specimen, W1, albeit with a higher residual strength which
asymptotically approaches 175 lb/ft.
In addition to the stiffness and strength, the progression and propagation of damage was
considered for each iteration of improvements. Initially, the inclusion of construction adhesive
causes the wallboards and studs to act as a single unit, but the first set (W2, W3 and W4) of tests
revealed a concentrated tension field location at the horizontal plane between the studs and sill
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plate and failed by the propagation of a crack across the bottom wallboard. Each step of
improvements from W2 to W6, attempted to improve the unity of the wall, redistributing the
location of the weak link in the specimen and thus relocated the primary damage. In the final
iteration (W6), the weak link was the gypsum board such that the mode of failure between the
gypsum infill material and the paper, illustrated by Figure 4.12 where the paper backing remained
attached to the studs while the rest of the gypsum had pulled away. After the sheathing failure at
the edges, the specimen behaves similar to the damage progression described previously for W1,
albeit not a severe due to the presence of the adhesive pockets throughout the specimen still
attached after the initial failure.
4.3.1
W2: Characteristics.
The first iteration of unibody improvements, specimen W2, is identical to the framing
and construction details of W1, discussed previously, in that the wall is 8 ft. x 8 ft. planar, with no
openings and Simpson Strong-Tie HDU5 tie-downs at the inside of the end studs. However,
construction adhesive was used in addition to standard drywall fasteners to attach the wallboard
to the frame. The tie downs and anchor bolts were tightened to approximately 3000 lbs, as listed
in Tables 4.2 and 4.3.
4.3.1.1 W2: Summary and overall behavior.
This specimen attained its maximum force capacity of 7310 lb, or a one-sided strength of
457 lb/ft, during the 0.3% interstory drift. The one-sided stiffness of the specimen, measured at
0.1% interstory drift was 2712 lb/in/ft. The primary failure of the specimen was a crack that
propagated across the wallboard, which separated the main panel from the bottom edge screws.
Following the primary damage, adhesive and fastener failures occurred at the bottom of the wall
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with damages progressing upwards such that approximately half of the screws on the specimen
had failed.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0037
radians with corresponding displacements recorded as +0.136” and +0.148” during the 0.3%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to +0.17” and +0.19” at the East and West end of the specimen, respectively.
4.3.1.2 W2: Observed behavior 0-0.5% interstory drift.
Figure 4.3b shows the force-deformation behavior for the specimen during the initial
range of 0 to 0.5% interstory drift. Compared to the hysteresis loops in Figure 4.3a for specimen
W1 which exhibited inelastic action as early as 0.05%, W2 is shown to be elastic up to, and
including, the 0.1% interstory drift cycles.
Similar to W1, noises from within the wall preceded any visible damage. These noises
were quiet pops at the peak displacements of the 0.075% interstory drift cycles. The initial
visible damage occurred during the 0.2% interstory drift cycles when a crack formed on the
wallboard approximately 3 in. from the bottom of the wall. This crack corresponds with the
horizontal plane between the top of the sill plate and bottom of the studs as shown in Figure 4.13.
During the 0.3% interstory drift cycles, the crack propagated across both sides of the wall and
screws along the bottom edge of the wall popped. Additionally, the maximum strength capacity
of the wall was reached within the 0.3% interstory drift cycles. The opening of the crack that
occurred during the 0.4% cycles coincided with uplift of the end stud. During the 0.5% cycles, the
edge screws on the bottom wall board and first row of screws above the cracked section of the
wall popped, resulting in the damage shown in Figure 4.14a.
Further confirmation of the adhesive failure and crack formation is demonstrated by an
increase in the differential movement of the wallboard uplift and horizontal displacements at the
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bottom of the wall. Analysis of the time displacement plots in Figure 4.15 shows the effect of the
adhesive on the wallboard behavior. Referring to Figure 4.15a, prior to the adhesive failure
during the last cycle of the 0.1% interstory drift, at time  2500 seconds, the horizontal
displacement at the bottom of the wallboard was approximately 4% of the displacement at the top
of the wall, demonstrating that the wallboard was attached to the frame. As the adhesive began to
fail in the 0.2% through 0.4% interstory drift, the horizontal wallboard displacement increased to
51% of the specimen displacement, demonstrating that the bottom of the wallboard began to
move with the top of the wall. Similar effects can be seen when comparing the uplift of the stud
and wallboard in Figure 4.15b before, and after, a time test of 2500 seconds. Prior to 0.2%
interstory drift, the wallboard and studs moved together with uplift values within 5% of each
other. However, the damages sustained through the 0.4% interstory drift caused the differential
wallboard uplift to increase to 53% larger than the differential stud uplift.
The maximum axial forces recorded by the anchor bolts and West tie down are shown in
Figure 4.16. The forces of the East tie down were not used for analysis of this specimen due to
off-scale readings of the instrument, which were addressed following the test. The maximum
force occurred in the tie down, with an axial force of 74% of the theoretical uplift force applied to
the specimen. When compared to the distribution of forces in W1, the adhesive caused the
anchor bolts to have a slightly different distribution through the 0.4% interstory drift where
anchor bolt #3 has a larger force than the neighboring bolts. Following the propagation of the
crack, the forces redistributed similar to W1, and all of the anchor bolts experienced reduced
forces while the force in the tie down increased.
4.3.1.3 W2: Observed behavior post 0.5% interstory drift.
The final damages to the specimen, displayed in Figure 4.14b, consisted of approximately
half of the drywall fasteners popping and/or shearing through the 1.0% interstory drift. No
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additional damage occurred during the larger drifts. After the crack formed along the top of
bottom sill plate and the adhesive failed, the wallboards behaved similar to the wallboards of the
control test as they pulled away from studs and displace parallel to the applied displacement
similar to the wallboard as shown in Figure 4.13c.
Similar to the behavior of the anchor bolts in W1, the forces in the anchor bolts reduced
during the post 0.5% interstory drift cycles while the force in the tie down increased. The forces
recorded during the larger displacements are shown in Figure 4.17.
4.3.2
W3: Characteristics.
To allow the wall to accommodate rigid body in-plane rotations (i.e., rocking), flexible
matting was added at the anchor bolts of Specimen W3, the second iteration of planar walls with
unibody improvements. Polychloroprene (PCP) pads, often referred to by their patented name
“neoprene” pads, were installed at the anchor bolts between the washer plate and sill plate, as
shown in Figure 4.18. To ensure the pads were not compressed before the test began and to allow
the wall the ability to rotate, the anchor bolts were only tightened to approximately 500 lbs, as
opposed to the 3000 lbs of previous tests. The tie downs located at the inside face of the end
studs, were increased to improve the force transfer of tension from the wall to the test assembly.
The tie downs were tightened with a socket wrench to approximately 3000 lbs.
These adjustments result in an 8 ft. x 8 ft. planar wall, with no openings, Simpson StrongTie HDU8 tie-downs, gypsum wallboard installed with construction adhesive and mechanical
fasteners, and anchor bolts installed with PCP pads.
4.3.2.1 W3: Summary and overall behavior.
The wall experienced a maximum force of 7919 lb, or one-sided strength of 495 lb/ft as
listed in Table 4.1, during the 0.75% drift. The one-sided stiffness of the specimen, measured at
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the 0.1% interstory drift, was 1856 lb/in/ft, a 32% decrease from specimen W2. The primary
damage for this specimen included cracks in the wallboard at the bottom of the wall and between
the top and bottom panels, similar to those discussed for W2. As a result, adhesive and fastener
failures occurred at the bottom of all panels, and progressed upwards such that the majority of
fasteners on the wall had failed.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0029
radians with corresponding displacements recorded as +0.153” and +0.067” during the 0.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.18” and +0.10” at the East and West end of the specimen, respectively.
The stiffness and strength of this specimen, while improved from the control test, are
decreased from the results of W2. Furthermore, the cracking and damage that occurred on the top
panels at lower interstory drift levels is undesirable; therefore none of the remaining tests use
PCP pads.
4.3.2.2 W3: Observed behavior 0-0.5% interstory drift.
Similar to the previous tests, noises within the wall began during the early cycles before
visible damage occurred in the 0.2% interstory drift.
However, the deformation behavior,
depicted in Figure 4.3c, shows that the specimen began exhibiting inelastic behavior at 0.05%
interstory drift. The first damage presented as a crack that formed 3 in. from the bottom of the
wall. Next, a crack formed along the joint between the top and bottom wallboard panels during
the 0.3% interstory drift cycles. As the width of the crack expanded through subsequent cycles,
the panels no longer moved together as an 8 ft. x 8 ft. unit, allowing equal damage to occur on the
top and bottom panels during the 0.5% interstory drift cycles. The damages that occurred on the
specimen through the 0.5% interstory drift are shown in Figure 4.19a.
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Due to the lower clamping forces of the anchor bolts at the sill plate, analog gages
measured sill plate displacements as large as 0.13” in the direction of loading, starting at the 0.2%
interstory drift group of cycles, and continuing through the end of the test. In contrast to
Specimen W2, the horizontal and vertical movements of the wallboard behavior relative to the
framing members is noticeable even at smaller drift levels due to the slip (horizontal) and rocking
(vertical) motion of the specimen from the flexible sill plate condition. Referring to Figure 4.20a,
the horizontal displacement of the wallboard at the bottom of the wall is 30-40% of the applied
specimen displacement at the top of the wall through the 0.5% interstory drift. Similarly, the stud
and wallboard uplift, shown in Figure 4.20b, remain within 75% to 80% of each other.
Figure 4.21, which reports the maximum axial forces for the anchor bolts and tie downs
during the 0-0.5% interstory drift, shows that the distribution of axial forces follows the same
pattern as the previous specimens, although the anchor bolts have a higher percentage of the uplift
force due to the flexibility at the sill plate. The tie downs receive the largest forces while anchor
bolts #2 and #4 receive the smallest. Through the applied displacements of these cycles, the
forces generally increased through the 0.4% cycles, after which the forces in anchor bolt #1 and
the West tie down increase while the forces in anchor bolts #2 through #5 and the East tie down
decrease. The local decrease in forces in the East tie down and anchor bolt #1 during the 0.3%
interstory drift correspond to the crack that formed at the bottom of the East side of the wall,
depicted in Figure 4.19a. Similarly, the decrease in forces in the interior anchor bolts, #2 through
#5, during the 0.5% interstory drift corresponds to the increasing crack width at the joint between
wallboard panels.
4.3.2.3 W3: Observed behavior post 0.5% interstory drift.
As a result of the flexible construction details, the strength capacity of the specimen was
delayed until the 0.75% interstory drift, as displayed in Figure 4.8c. Screw popping progressed
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upwards through the larger drifts causing the majority of fasteners connecting to the interior studs
to fail on the top and bottom panels. Additionally, the crack at the bottom of the wall propagated
during these larger displacements, as shown in Figure 4.19b. The stronger tie down may have
affected the shape of this crack which propagated up and around the tie down. Due to an increase
in crack width and the failure of all adhesive and screws within the wallboard section, the lower
East corner of wallboard fell off during the 2.0% interstory drift cycles.
The distribution of axial forces to the anchor bolts and tie downs, shown in Figure 4.22,
follows the distribution determined in the smaller displacements where the tie downs record the
largest forces while anchor bolts #2 and #4 record the smallest.
4.3.3
W4: Characteristics.
Specimen W4 returns to the construction details and procedures used for W2, with
modifications to address the undesirable damages that occurred on Specimens W2 and W3.
According to the technical specifications for the tie downs used in these tests, the tie downs
exhibit a deflection of approximately 0.1 in. during the maximum load. This deflection may
allow the uplift of the end studs which causes a crack to form in the gypsum wallboard at the
bottom of the wall. To reduce this deformation and provide a higher stiffness to the wall, new tie
downs, shown in Figure 4.23, were manufactured and installed in W4 with similar procedures to
the tie downs of previous tests.
In summary, Specimen W4 is an 8 ft. x 8 ft. planar wall with no openings, with two-sided
gypsum wallboard, construction adhesive and drywall screws, and stiffness enhanced tie downs
on the inside of the end studs. The recorded values of axial pretension in the anchor bolts and tie
downs at the beginning of the test, shown in Tables 4.2 and 4.3, range from 1500 lbs to 2500 lbs.
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4.3.3.1 W4: Summary and overall behavior.
The stiffness enhanced tie downs proved to be successful for the initial cycles as the
specimen had a one-sided stiffness of 3186 lb/in/ft, an increase of 17% from W2. The wall
experienced a maximum force of 7436 lb, or 465 lb/ft as listed in Table 4.1, during the 0.2%
interstory drift. The primary failure for this specimen is a crack formed due to the large tension
field action at the base of the wallboard along the top of the sill plate due to uplift of the end
studs. Additionally, fastener failure of five of the six rows of screws on the bottom wallboard
panel occurred.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0005
radians with corresponding displacements recorded as +0.033” and +0.006” during the 0.75%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to +0.04” and +0.01” at the East and West end of the specimen, respectively.
Separation between the bottom panel and the frame, caused by the aforementioned crack
and fastener failures, allowed for a portion of the wallboard to be removed without affecting the
strength of the wall. This allowed for a better observation of the double end stud and tie-down,
revealing that the eccentricity created by the distance between the tie down to the center of the
end studs allows for a chord rotation of the end stud. This rotation caused uplift at the exterior
edge of the end studs, creating large tensile stresses which fractured the brittle gyp board and led
to crack propagation at the plane of the sill plate.
4.3.3.2 W4: Observed behavior 0-0.5% interstory drift.
The first internal noises and damage occurred during the 0.1% interstory drift cycles.
This damage consisted of fastener popping of the majority of screws in the bottom wallboard
panel, as shown in Figure 4.24a. In the next set of cycles (0.2% interstory drift) a crack formed in
the wallboard approximately 3 in. above the bottom of the panel on the East side of the wall.
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Additional cracking occurred at the same height across the middle of the wall during the 0.3%
interstory drift cycles. Referring to Figure 4.3d, the force-deformation behavior plot, the strength
capacity of the wall was noticeably diminished in the cycles following these damages. No
additional damage occurred during the 0.4% and 0.5% cycles.
The graphs of wallboard and frame displacement, in Figure 4.25, emphasize that
wallboard separation began occurring during the 0.2% interstory drift cycles. Within the first
three sets of cycles, 0.05% through 0.1% interstory drift, the wallboard clearly remained attached
to the frame with horizontal displacements at the bottom of the wall less than 0.02 in. and a
vertical displacement that matched the uplift recorded at the studs. However, as the crack formed
across the bottom of the wall, the wallboard behavior changed. The horizontal displacement at
the base of the wallboard increased from 15% of the specimen displacement during the 0.2%
interstory drift cycles, to 59% by the end of the 0.5% cycles. For the same reason, the wallboard
uplift decreased from 80% of the end stud uplift at 0.2% interstory drift to 51% at 0.5% drift.
The distribution of maximum axial forces in the anchor bolts and tie downs is shown in
Figure 4.26. The largest forces occur in the tie downs while each of the anchor bolts receives
approximately 7% or less of the theoretical uplift. The chart shows that the axial forces in the
anchor bolts decrease at the exterior-most bolt during the 0.2% cycles and progressed inwards
during the larger drifts.
These decreases correspond to the observed separation between
wallboard and frame.
4.3.3.3 W4: Observed behavior post 0.5% interstory drift.
As displayed in Figure 4.24b, the only damage that occurred during the larger drift cycles
is the buckling of the wallboard located between the cracked sections at the bottom of the wall.
Following the fastener failures and crack propagation during the smaller drift cycles, the
wallboards separated from the frame causing the panels to cease their contributions to the strength
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and stiffness of the specimen. A portion of panel was cut away to study the behavior of the frame
and tie down. Figure 4.27, shows the East side of the specimen at peak displacements of positive
and negative drift, suggesting that the external uplift of the stud is caused by rotation of the end
studs due to the eccentricity of the tie down to the center of the end stud. The application of
negative displacements to the wall causes compression in the end studs and no uplift occurs. On
the other hand, positive displacements cause tension in the end studs which causes an uplift of the
studs that the tie-down acts to reduce. As a result of the eccentricity of the tie down to the center
of the studs, uplift is reduced at the inside plane of the end studs but not exterior face, allowing a
rotation of the end studs and a gap to form between the sill plate and stud. This gap causes the
propagation of the crack at the top of the sill plate.
While specifically noted within the
observations of this specimen, this behavior is most likely the cause of the cracking along the sill
plate of the previous specimens (W2 and W3).
Figure 4.28 shows that the decrease in axial forces in the anchor bolts observed during
the smaller displacement cycles continued such that the forces in anchor bolts during the larger
cycles is negligible. Through these larger displacements, the force in the tie down at the east side
of the wall increased while the force in the tie down at the west side of the wall decreased.
During the breakdown of the specimen, it was noted that the adhesive was still pliable;
perhaps suggesting that the amount of adhesive applied is possibly excessive for cure time.
4.3.4
W5: Characteristics.
After considering the rotation of the double end-stud, shown in Figure 4.27, and the
undesirable failure mechanism from the horizontal crack at the top of the sill plate, specimen W5
was designed to limit the uplift of the stud on the exterior of the double end-stud. Considering
the limited availability of the stiffness enhanced tie downs, it was determined that, while
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improving the specimen stiffness, the new tie downs may present no advantages over the easily
available Simpson Strong-Tie HDU8 tie-downs. Thus, as shown in Figure 4.29, HDU8 tie-downs
were used in conjunction with bent straps on the outside of the end studs, wrapped below the sill
plate. The bottom strap was attached to the bottom of the sill plate with countersunk screws and
to the double end-studs with Simpson lag-bolts.
To ensure the strap did not affect the
pretensioning of the tie down, the specimen was installed onto the test rig and the anchor bolts
and tie downs were pretensioned to 2600 to 4200 lbs, prior to attaching the vertical legs of the
bent straps. After the straps were secured onto the end studs, the wallboards were installed in the
same manner as specimens W2 through W4. The pliability of adhesive noted during the tear
down of Specimen W4 was addressed by reducing the amount of adhesive applied to the frame to
a one 3/8 in. bead on each stud and two 3/8 in. beads on the top and bottom plates and exterior
double studs.
In summary, Specimen W5 is an 8 ft. x 8 ft. planar wall with no openings, gypsum
wallboard installed with construction adhesive and mechanical fasteners, and uplift constraints of
Simpson Strong-tie HDU8 tie-downs and bent straps at the end studs.
4.3.4.1 W5: Summary and overall behavior.
Specimen W5 attained a maximum force capacity of 9348 lb, or 584 lb/ft/side as listed in
Table 4.1, during the 0.4% interstory drift cycles. The one-sided stiffness of this specimen,
measured at the 0.1% interstory drift, was 3560 lb/in/ft. The primary mode of failure for this
specimen was the formation of a crack between the panels of wallboard resulting in fastener
failures on the top and bottom panels.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0021
radians with corresponding displacements recorded as -0.062” and -0.100” during the 1.75%
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interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.08” and -0.12” at the East and West end of the specimen, respectively.
The bent strap, in conjunction with the tie downs, led to increased stiffness and strength
compared to W2-W4 as the large tension field action, and subsequent crack formation, at the base
of the wall was avoided.
However, the failure of the seam between the top and bottom
wallboards led to the eventual adhesive failure at the studs.
4.3.4.2 W5: Observed behavior 0-0.5% interstory drift.
The first sign of visual damage occurred during the 0.1% interstory drift cycles as the
mud and paint cracked at the bottom row of screws on each side of the specimen. This behavior
is reinforced by the shape of the force-deformation plots shown in Figure 4.3e, which shows that
the specimen remained relatively elastic and damage-free through the 0.1% displacement cycles.
During the 0.2% interstory drift cycles, screws began popping at the bottom of the wall and a
crack formed at the horizontal joint between top and bottom sheathing boards. The maximum
strength capacity of the wall was achieved during the 0.4% drift but decreased when additional
screws at the bottom of the wall popped. The increase of the width of the crack between
wallboard panels caused fastener popping at 0.5% interstory drift cycles along the edges of the
top wallboard. An interesting note is that on the North face of the wall, this damage occurred at
the top of the panel and progressed downwards, while on the South face the damage occurred at
the bottom of the panel and progressed upwards. Damages through 0.5% interstory drift are
shown in Figure 4.30a.
Since the damage mostly occurred to the top wallboard, the graphs comparing wallboard
and frame displacements in Figure 4.31 do not provide the similar evidence of adhesive failure of
previously discussed specimens. Instead, Figure 4.31a illustrates that the panel remains secured
to the frame as demonstrated by the relatively small horizontal deformation (less than 0.01”)
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recorded at the bottom of the gyp board. While fastener failures have occurred on the bottom
wallboard panel, the panel remains attached to the frame through adhesives or screws which
remained engaged as shown by the wallboard uplift remaining 60-70% of the recorded stud uplift.
The maximum axial forces in the anchor bolts and tie downs recorded during the 0-0.5%
interstory drift cycles are shown in Figure 4.32. Similar to the previous tests, the largest forces
were observed in the tie downs. However, a new force distribution was observed where the
largest axial anchor bolt force occurred in one of the interior-most anchor bolts, #3. During the
0.4% interstory drift, when the wall achieved its maximum strength, the tie downs presented axial
forces equivalent to 44-54% of the lateral force in the wall, while anchor bolt #3 presented 10%,
and the other bolts presented 2-4% of the lateral force. Following the peak strength, the forces in
the anchor bolts generally decreased while the forces in the tie downs generally increased.
4.3.4.3 W5: Observed behavior post 0.5% interstory drift.
As a result of the crack between the wallboards, all further fastener failures, which
occurred during the 0.75% and 1.5% interstory drift cycles, were located on the top panel, as
depicted in Figure 4.30b. Progression of the damage at the top of the wall caused the top panels to
become separated from the frame and appeared to be hazardous should the panels fall off.
The axial forces in the tie downs and anchor bolts, shown in Figure 4.33, follow the
trends observed in the smaller displacement cycles.
In addition to the reduction of forces
distributed to the anchor bolts, the forces in the tie downs begin to decrease during the larger
displacements, which may be attributed to the limitations of uplift of the end studs provided by
the combination of tie down and strap.
The adjustments that were made to the construction procedure for this test, were
successful in improving the performance of a unibody planar wall. The new uplift constraint
assembly successfully prevented the crack at the bottom of the wall. Therefore, the remaining
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wood-framed walls with free-standing ends or openings in the direction of the applied
displacement feature this uplift constraint assembly. The second feature of this test is the lesser
amount of adhesive used for wallboard installation. During specimen tear-down, it was found
that the adhesive developed the full strength of the gyp board as evidenced by residual paper
backing on the studs and top/bottom plates. Thus, all subsequent tests feature 3/8 in. beads of
construction adhesive on the frame.
4.3.5
W6: Characteristics.
Specimen W6 is the final iteration of the planar 8 ft. x 8 ft. wall for the wood-framed
suite of tests. Mid-height 2x4 blocking was added between the studs to improve wallboard
behavior by providing four sides of edge screws for each wall panel. Specimens W1 through W5
all featured a joint compound containing a quick-cure concrete substrate to improve the unity of
the horizontal wallboard panels. However, the behavior of specimens W3 and W5 showed that
the shear forces at the panel joints can exceed the strength of the tape and enhanced compound.
With the addition of the blocking, the enhanced joint compound was replaced with an all-purpose,
more traditional, pre-mixed joint compound. While the addition of blocking increases the time
required for construction of the frame, it significantly reduces the time required for the mixing
and application of the quick-cure joint compound while also eliminating the use of a possibly new
material on typical construction sites.
In summary, W6 is an 8 ft. x 8 ft. planar wall with no openings, 2x4 blocking at midheight of the frame, gypsum wallboard installed with construction adhesive and mechanical
fasteners, and a tie down assembly of HDU8’s and bent straps on the end studs. The anchor bolts
and tie downs were tightened with a socket wrench to provide axial pretension forces of 11004700 lbs, as listed in Tables 4.2 and 4.3.
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4.3.5.1 W6: Summary and overall behavior.
During the 0.3% interstory drift cycles, Specimen W6 attained a maximum force capacity
of 9986 lb, or 624 lb/ft as listed in Table 4.1. The one-sided stiffness of this specimen, measured
at 0.1% interstory drift, was 3589 lb/in/ft. The primary mode of failure for this specimen was
adhesive and fastener failure at bottom sill plate, extending upwards such that majority of the
bottom panel fasteners had failed.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0045
radians with corresponding displacements recorded as -0.252” and -0.087” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.29” and -0.14” at the East and West end of the specimen, respectively.
The modifications to the construction procedure used for this specimen resulted in a 32%
increase in stiffness and 37% increase in strength over the first iteration, W2, and 164% and
109%, respectively, over typical construction, W1. The failure of this specimen produces no
cracks within the wallboard and is similar to the typical construction, except that the progression
of damage is delayed and restricted to the bottom wallboard panel.
4.3.5.2 W6: Observed behavior 0-0.5% interstory drift.
As illustrated in the force-deformation behavior, shown in Figure 4.3f, the wall remains
elastic through the 0.1% interstory drift cycles. The first inelastic behavior, occurred during the
0.2% cycles, and was accompanied by noises from within the wall. The first visible damage
occurred in the next set of cycles (0.3%) when the bottom edge screws began bulging and the
maximum strength capacity of the wall was achieved.
The location of popped fasteners
progressed upwards to include the vertical edges of the bottom gyp panel during the 0.4%
interstory drift and bottom row of interior screws during the 0.5% interstory drift cycles. The
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locations of all of the popped screws during the range of 0-0.5% interstory drift are shown in
Figure 4.34a.
The time-history graphs in Figure 4.35 show the recorded wallboard and frame
displacement through the 0.5% interstory drift and provide confirmation that the adhesive
successfully attaches the wallboard to the frame and that separation began to occur during the
0.2% interstory drift cycles. Prior to failure, the horizontal wallboard displacements suggest that
the panel was firmly attached to the frame with displacements of less than 0.007 in. and an uplift
that was 91-95% of the recorded stud uplift. After separation, the horizontal displacement
increased to 34% of the specimen displacement and the uplift reduced to 88% similarity. At the
end of the 0.5% cycles, the horizontal wallboard displacement had increased to 74% and the
uplift had reduced to 52% similarity.
Figure 4.36 depicts the maximum axial forces recorded at the tie downs and anchor bolts
within the 0-0.5% interstory drift cycles. Corresponding to the peak strength of the wall, the
anchor bolts experienced peak axial forces ranging between 3% in the interior anchor bolt #4 and
11% in the exterior most anchor bolt #1. Similar to many of the Specimen W5, the interior
anchor bolt #3 experienced larger forces than the neighboring bolts during these smaller
displacements.
4.3.5.3 W6: Observed behavior post 0.5% interstory drift.
During the 0.75% interstory drift cycles, the field screws in the third and fourth rows of
the bottom panel popped as shown in Figure 4.4b. No additional damage occurred to the
specimen during the larger displacements.
The maximum recorded tie down and anchor bolt axial forces are shown in Figure 4.37.
The tie forces continue to increase during the larger displacements while the interior anchor bolts
decrease to axial forces equivalent to 3% or less of the lateral force within the wall. In these
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larger displacements, the exterior most anchor bolt, #1, increased to the equivalent of 28% of the
lateral force within the wall. This unique increase is most likely caused by the bent strap that was
installed at exterior corners of the wall which extends past the location of the anchor bolt.
The addition of blocking at the mid-height of this specimen proved to be successful as the
detail prevented a crack from forming on the face or at the joints of the gypsum wallboard during
the test with damages limited to the bottom wallboard panels.
4.4
Planar Wall Tests With End Returns and Unibody Enhancements: W7 And W8
To examine the interaction between a planar wall in the direction of an applied
displacement and orthogonal walls in a unibody system, five walls were tested with 4 ft. T-shaped
return walls. The first two of these walls featured 8 ft. x 8 ft. shear walls with no openings.
Similar to the process used for specimens W2 through W6, improvements were made to the
construction techniques of the second test to increase the stiffness, strength, and damageability
performance. These walls use the features of the previously described W6 adjusted to reflect the
addition of orthogonal walls using the details of returns and corners as previously discussed in
Chapter 3. The construction plans for these specimens are shown in Figure 4.38.
With details that allow for a continuous load path between the planar and orthogonal
walls, the stiffness and strength values for W8 listed in Table 4.1 increased by 54% and 16%,
respectively, as compared to the planar W6 specimen. The cyclic backbone curves of specimens
W7 and W8, illustrated in Figure 4.39, show that in addition to the increase in stiffness and
strength, the walls with returns retain a higher residual strength value than the free-standing walls,
ranging between 300 lb/ft and 530 lb/ft after the peak strength. Comparisons of the force
deformation plots for these two specimens, Figures 4.3f and 4.41b, illustrate that the attached
walls also improve the elastic behavior and ductility of the specimen.
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A notable difference in the damageability characteristics of the specimens with return
walls is vertical cracking in the compound between the planar and return wall, as well as buckling
at the bottom corner of the planar wall. Referring to the later damage state, after the planar gyp
board separated from the frame, and unlike the wallboards of W1 through W6 which were
uninhibited from sliding, the wallboards in W7 and W8 bear against the return walls and buckle
at large interstory drift excursions, beginning at 0.75%.
4.4.1
W7: Characteristics.
Specimen W7 was the first wall to feature the orthogonal returns with an 8 ft. x 8 ft.
planar wall with no openings and 4 ft. return walls at each end. The wall featured gypsum
wallboard installed with construction adhesive and mechanical fasteners, mid-height blocking on
the framing, and Simpson Strong-tie HDU8 tie-downs at the ends of each return wall. By placing
the tie downs at the ends of the returns, and not at the ends of the main wall, the test investigates
if the forces developed within the walls could be transferred around the T-joint or if an additional
tie down would be required at the end of each main wall. For this specimen, the axial pretension
in the anchor bolts and tie downs at the beginning of the test ranged from 4000 lbs to 13000 lbs,
as depicted in Tables 4.2 and 4.3.
4.4.1.1 W7: Summary and overall behavior.
The maximum force capacity of this specimen was achieved during the 0.4% interstory
drift cycles at 8411 lbs, or 526 lb/ft as listed in Table 4.1. The one-sided stiffness of this
specimen, measured at 0.1% interstory drift, was 3795 lb/in/ft. Similar to previous tests, the
primary mode of failure was fastener and adhesive failures at the bottom sill plate, which
extended upwards such that half of the screws connecting the bottom wallboard panels to the
interior studs had failed. Interaction between the main and return walls resulted in a crack at the
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joint between walls, the failure of the majority of vertical edge screws, and bucking of the bottom
corners of wallboard. During tear-down of the specimen, the connection at the T-joint between
the planar wall and orthogonal end returns had failed, significantly bending the 16d nails (12 in.
o.c.) used to join the 2x4 studs of the adjoining wall sections. This failure is shown in Figure
4.40d.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0010
radians with corresponding displacements recorded as -0.039” and -0.035” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.05” and -0.05” at the East and West end of the specimen, respectively.
While the performance of this wall is better than the control specimen (W1), the strength,
stiffness, and elastic behavior decreased as compared to W6.
4.4.1.2 W7: Observed behavior 0-0.5% interstory drift.
The force-deformation behavior plot in Figure 4.41a shows that the specimen began
exhibiting inelastic behavior during the initial, 0.05% interstory drift, cycles. This behavior
increased in the 0.2% cycles, as the first visible damage appeared as a hairline crack at the joint
between the main and return walls (Figure 4.40a). Then, as shown in Figure 4.42a, the fasteners
at the bottom edge of the wall popped in the 0.3% and 0.4% interstory drift cycles. The
maximum capacity was reached during the 0.4% cycles and is decreased in all subsequent cycles.
The wallboard and frame displacement graphs in Figure 4.43 show that the addition of
return walls do little to influence the relative horizontal movement between the sheathing material
and the frame. Compared to similar plots from W6, for example, the horizontal movement of the
sheathing is shown to increase when the adhesive fails at a specimen interstory drift of about
0.2%. Furthermore, the vertical motion of the end stud and wallboard uplift are identical to
adhesive failure (at a test time of approximately 3000 seconds). However, unlike the planar-only
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wall specimens where the stud would lift more than the gyp sheathing, the returns tend to have
the effect of decreasing the stud uplift relative to the vertical wallboard motion.
As shown in Figure 4.44, the instrumented tie downs were moved to the ends of the
return walls to determine how forces are much the return walls contribute to resisting the uplift of
specimens with returns.
The anchor bolts in the return walls were not instrumented, but
presumably the bolts took a share of the uplift forces similar to the forces in the tie downs.
Referring to the chart, during the 0.4% cycles in which the wall experienced its maximum
strength, the tie downs recorded maximum axial forces equivalent to 9% to 12% of the lateral
force within the wall. In contrast to the previous tests, where the axial forces in the anchor bolts
increased through the displacement cycles in which the wall experienced its maximum strength,
the forces of the anchor bolts of this specimen only increased through the 0.075% or 0.1%
interstory drift and generally decreased afterwards.
4.4.1.3 W7: Observed behavior post 0.5% interstory drift.
The damages that occurred within the smaller displacements progressed upwards during
the larger drift cycles. Fastener popping included the bottom edge screws and approximately half
of the screws connecting the interior studs and bottom wallboard panels. The cracks at the corner
joints grew throughout the larger displacements causing the vertical edge screws to pop.
The return wall interrupted the established behavior of the wallboard after separation in
which the board slides past the edge of the frame during the larger displacements. This caused
the lower corners of wallboard to buckle during the 0.75% and 1.25% interstory drifts, as shown
in Figure 4.40b. Increased damage at the ends of the main wall caused the buckled wallboard to
crumble off, resulting in the shapes shown in Figure 4.42b.
A crack and bulge, displayed in Figure 4.40c, formed on the exterior of the return wall
during the 1.5% interstory drift. Removal of the wallboard after the test’s completion showed
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that this damage was caused by failures of the built up corner stud assembly. Figure 4.40d shows
that these failures included both the connections between the studs of the assembly and the
connection between the studs and sill plate.
As depicted in Figure 4.46, the axial forces in the tie downs at the ends of the returns
continued to increase to 29-33% of the lateral force of the wall through the larger displacements.
The axial forces in the anchor bolts continued to decrease through the larger cycles to forces of
less than 2% of the lateral force within the wall.
4.4.2
W8: Characteristics.
From the behavior illustrated in specimen W7, seismically enhanced partition walls with
returns require sufficient load-transfer connections at the corner assemblies. In addition, the lack
of tie-downs on the ends of the planar wall were likely to have had a negative effect on the
stiffness and strength characteristics of W7. Therefore, two improvements were made to the
construction details in the construction of Specimen W8. First, to improve the transfer of forces
around T-corners, the fasteners connecting the corner assemblies were changed from 16d nails at
12 in. on center to 3 in. screws spaced at 4 in. on center. Second, HDU8 tie-downs were added at
the ends of the main wall allowing the tie downs at the ends of the returns to be exchanged for
smaller ones.
In summary, Specimen W8 was an 8 ft. x 8 ft. planar wall with no openings with 4 ft.
wide return walls at each end. The frame, which featured mid-height blocking, was sheathed with
gypsum wallboard installed with construction adhesive and mechanical fasteners. The tie downs
of the specimen were Simpson Strong-tie HDU8 tie-downs at the ends of the main wall and
HDU5 tie-downs at the ends of the return walls. The anchor bolts and tie downs began the test
with 5400-11500 lbs of axial pretension.
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4.4.2.1 W8: Summary and overall behavior.
Specimen W8 experienced a maximum force capacity of 11560 lb, or a one-sided
strength capacity of 723 lb/ft, during the 0.4% interstory drift cycles. The one-sided stiffness
measured at the 0.1% interstory drift for this specimen was 5534 lb/in/ft. The primary mode of
failure for this specimen was adhesive failure, followed by fastener failure at the bottom sill plate,
extending upwards such that the majority of fasteners connecting the bottom panel to the interior
studs had failed. The strength, stiffness and elastic behavior of the specimen are improved over
the behaviors of both Specimens W6 and W7.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0021
radians with corresponding displacements recorded as -0.058” and -0.098” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.08” and -0.12” at the East and West end of the specimen, respectively.
4.4.2.2 W8: Observed behavior 0-0.5% interstory drift.
Referring to Figure 4.41b, which shows the force-deformation behavior for the range of
0-0.5% interstory drift, the specimen retains a generally elastic behavior through the 0.1%
interstory drift. The first visible damage occurred during the 0.1% and 0.2% interstory drift
cycles when hairline cracks formed at the vertical joints between the main and return walls.
Increase elastic behavior, shown in the figure, corresponds to the popping of screws at the bottom
of the wall in the 0.2% and 0.3% cycles. The locations of the damages that occurred on the
specimen during the range of 0-0.5% interstory drift are shown in Figure 4.47a.
A new wallboard to frame behavior is shown by the displacement graphs in Figure 4.48.
Similar to the free-standing unibody walls, the horizontal displacement at the bottom of the wall
remains 0.1 in. or less prior to adhesive failures.
Following the failure, the horizontal
displacement increases from 13% to 72% of the applied specimen displacement between the 0.2%
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and 0.5% interstory drift cycles. The difference occurs through the recorded stud and wallboard
uplifts, as the wallboard begins the test with a different amount of uplift than the studs and
becomes more similar through the larger displacements.
The maximum recorded axial forces of the anchor bolts and tie downs for this specimen
are shown in Figure 4.49. During the 0.4% interstory drift cycles, when the wall experienced its
maximum strength, the tie downs recorded forces equivalent to 8% and 21% of the lateral force
within the wall. At the same time, the anchor bolts recorded forces equivalent to 0.1-0.5% of the
lateral force. This distribution of forces to the anchor bolts, while being low, returns to the
expected distribution where the outer-most bolts maintain the larger forces and the interior bolts
maintain the smallest. The difference in the recorded tie down forces shows that the enhanced
corner stud assembly, while improving the behavior of the main wall, does not evenly transfer the
forces to the ends of the return.
4.4.2.3 W8: Observed behavior post 0.5% interstory drift.
The locations of the popped screws progressed upwards during the larger displacements
to include the majority of the screws connecting the bottom wallboard panels to the interior
screws as depicted in Figure 4.47b. Similar to the behavior of specimen W7, the bottom corners
of the wallboard panels began to buckle during the 0.75% interstory drift cycles. However, as a
result of the improved details of the corner assemblies, the corners of this specimen do not
crumble off during the higher drift cycles of the test. Additionally, these details successfully
delayed the crack and bulge on the exterior of the return wall until the 2.0% interstory drift.
Figure 4.50, which was taken after the wallboard was removed after the test’s completion, shows
that the resulting bulge was caused only by failure of the end nailing of the assembly.
Figure 4.51 shows the maximum axial forces for the anchor bolts and tie downs for the
post 0.5% interstory drift cycles. As shown in the figure, the forces in the anchor bolts decreased
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to negligible during these displacements. Additionally, the figure shows that the forces in the tie
downs increased and the imbalance of forces transferred around the corners to the end walls
became more apparent.
4.5
Planar Wall Tests With Openings, Varying Aspect Ratios and Unibody
Enhancements: W9 Through W11
As previously discussed, five wood-framed walls were tested to determine the effects of
orthogonal walls in the unibody system. Walls W7 and W8, discussed above, were 8 ft. x 8 ft.
walls with no openings and T-shaped returns. Specimens W9 through W11 explore the effects of
door openings and differing aspect ratios upon the strength, stiffness, and resulting damages of
the unibody construction.
For the walls with door openings, the strength and stiffness are calculated per foot of full
height wall. Referring to Figure 4.52, which presents the backbone curves for walls W6 and W8
through W11, the walls with openings and differing aspect ratios behave similar to W8 in that
after the peak strength is reached, the wall retains a residual strength between 300 and 520 lb/ft.
The one-sided stiffness and maximum strength of all the wood-framed the specimens W9
through W11 can be seen in Table 4.1. The results of Specimen W9 show that stiffness and
strength characteristics of a short wall with a door opening are similar to a free standing wall.
The longer, 16 ft., specimens with returns – even W11 with a door opening – have similar but
improved stiffness and strength characteristics to the square walls with returns. An accurate
curve fit line (R2 = 0.99), shown in Figure 4.53a, captured the ultimate strength capacity, Vmax, of
the experimental specimens well with Vmax = 11.6(H/L)-1.1 where H/L is the aspect ratio of the
wall. Similarly, a linear line captured the stiffness, K,of the specimens with K = -112(H/L)+205.
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The damages of the specimens with door openings shows that racking of the wall causes
changes in the angle of the top corners of the door openings, causing cracks to propagate from the
corner of the opening towards the upper corner of the wall.
The differing aspect ratios showed that when the main wall is longer than the length of
wallboard being used, cracks will form at the vertical and horizontal joints around 0.1% to 0.2%
interstory drift. However, since the wallboards all have four sides of edge screws, the width of
the crack only increases to approximately 1/16 in. and does not cause fastener damage around the
locations.
4.5.1
W9: Characteristics.
Specimen W9, the first specimen to feature an opening, features a main wall that is 8 ft. x
8 ft. with a 32 in. wide door opening with four foot wide return walls at each end. The
construction plans for this specimen are shown in Figure 4.54. Taking construction details from
Specimen W8, the frame features mid-height blocking, improved corner stud assemblies, and was
sheathed with gypsum wallboard installed with construction adhesive and drywall fasteners.
Similarly, Simpson Strong-tie HDU8 tie-downs were installed at the ends of the main wall and
HDU5 tie-downs were installed at the ends of the return walls. Bent straps were installed on the
outside faces of the door opening to prevent the stud uplift and wallboard cracking observed in
the free standing walls. An additional tie down was installed on the inside face of the end stud on
the east side of the doorway which corresponds to the end of the larger pier. To ensure that the
smaller pier had enough anchorage an additional shear bolt was located in the sill plate near the
inside face of the end stud on the West side of the doorway. The anchor bolts and tie downs were
tightened to provide an axial pretension of 1700-2600 lbs as noted in Tables 4.2 and 4.3.
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An important note for this specimen is that while this geometry is fairly common in
residential structures, the height to width ratio of the wall piers (2.4 for the large pier and 4.0 for
the small pier) exceed the 2.0 limit for gypsum sheathed shear walls with blocking as set by the
International Residential Building Code (2009). Therefore, according to the current building
standards, walls with this geometry would not be considered to be a part of the lateral force
resisting system.
4.5.1.1 W9: Summary and Overall Behavior.
Specimen W9 attained a maximum force capacity of 7349 lb, or 689 lb/ft as listed in
Table 4.1, during the 0.4% interstory drift cycles. The one-sided stiffness of this specimen,
measured at 0.1% interstory drift, was 3615 lb/in/ft. The primary failure for this specimen
consisted of cracking of the wallboard at the top corners of the door opening extending to the top
corners of the wall. Additionally, fastener and adhesive failures occurred at the vertical edges of
the doorway and bottom edges of the wall biers, and a hairline crack began to form at the joint
between wallboard panels.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0003
radians with corresponding displacements recorded as -0.002” and +0.024” during the 0.4%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to 0.00” and 0.03” at the East and West end of the specimen, respectively.
4.5.1.2 W9: Observed behavior 0-0.5% interstory drift.
Similar to Specimens W6 and W8, this specimen retained a generally elastic behavior
through the 0.1% interstory drift as shown in the force-deformation behavior plot in Figure 4.55a.
The initial damage occurred during the 0.1% interstory drift cycles when hairline cracks formed
at the joints between the main wall and returns. The primary damage began during the 0.2%
interstory drift cycles when cracks formed at the top corners of the doorway as a result of changes
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to the angles of the opening during wall racking. During each of the subsequent cycles, through
0.5% interstory drift, these cracks propagated approximately 2 ft. towards the top corners of the
wall where the main and return walls meet the ceiling. Following the damages of the 0.4%
interstory drift cycles, which consisted of crack propagation and the popping of screws at the
bottom of the 40 in. wide pier, the maximum strength capacity of the specimen was reached and
is decreased in all subsequent cycles. The locations of damages through the 0.5% interstory drift
cycles are shown in Figure 4.56a.
The plots of horizontal and vertical displacements of the large pier of the specimen,
presented in Figure 4.57, show that adhesive failures at the beginning of the wall were delayed
until 0.4% interstory drift. Prior to the adhesive failure, the bottom of the wallboard moved less
than 20% of the displacement applied to the top of the wall and maintained an uplift that was 80%
similar to the stud uplift. However, through the progression of adhesive failures in the 0.4% and
0.5% interstory drifts, the horizontal displacement increased to 30% of the applied specimen
displacement and the uplift of the wallboard reduced to 55% similarity of the stud uplift.
Figure 4.58 shows the distributions of axial forces to the tie downs and anchor bolts of
this specimen. As shown in the figure, the locations of instrumented tie downs were adjusted to
observe the uplift forces at the ends of the large wall pier and the north end of the east return.
Through these, it can be determined that during the 0.4% interstory drift, when the wall reached
its maximum strength, the ends of the large pier experience axial forces of approximately 70% of
the lateral force within the specimen.
The anchor bolts located closer to the door opening
recorded larger axial forces than those located closer to the return walls.
4.5.1.3 W9: Observed behavior post 0.5% interstory drift.
Propagation of the cracks over the doorway finished during the 0.75% interstory drift
cycles after both cracks had reached the ends of the main wall. The damages that occurred during
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the larger drifts include the popping of screws at the bottom of the wall and along the vertical
edges of the doorway. Additionally, hairline cracks began to form at the joint between the top
and bottom wallboard panels as shown in Figure 4.56b.
Figure 4.60 shows that the distribution of axial forces to the anchor bolts observed within
the smaller displacement cycles continued through the larger displacements. Following the
behavior of previous tests, the forces in the interior anchor bolts decreased after separation of the
wallboard occurred while the forces in the tie downs increased.
4.5.2
W10: Characteristics.
Specimens W1 through W9 explored the effects of planar wall aspect ratio on the
strength, stiffness and damage states of square walls in the unibody system. Specimen W10
determines how these characteristics change for a long rectangular wall with 4 foot return walls.
The main wall of the specimen is a 16 ft. long wall and 8 ft. tall planar wall with no openings
featuring mid-height blocking, improved corner stud assemblies and gypsum wallboards were
installed upon the wood frame with construction adhesive and mechanical fasteners with
staggered joints. Due to the expected values of tension at the end studs, Simpson Strong-tie
HDU8 tie-downs were installed at the ends of the main and return walls. The construction plans
for this specimen are shown in Figure 4.61.
The anchor bolts and tie downs provided a
pretension force of 2000 lbs to 4400 lbs at the beginning of the test.
4.5.2.1 W10: Summary and overall behavior.
A maximum force capacity for the specimen W10 of 26060 lb, or 814 lb/ft as listed in
Table 4.1, was attained during the 0.3% interstory drift cycles. The one-sided stiffness of the
specimen, measured at 0.1% interstory drift, was 5426 lb/in/ft. The primary mode of failure for
this specimen was adhesive failure which allowed the horizontal and vertical joints at the edges of
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the wall panels to crack. Following the observed behavior of previous specimens, additional
failures for this specimen included fastener and adhesive failure along the bottom of the wall and
progressed upwards to include half of the fasteners on the bottom panels.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0010
radians with corresponding displacements recorded as -0.020” and -0.057” during the 2.0%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.03” and -0.07” at the East and West end of the specimen, respectively.
4.5.2.2 W10: Observed behavior 0-0.5% interstory drift.
Figure 4.55b shows the force-deformation behavior for the range of 0-0.5% interstory
drift. The first visible damage occurred during the 0.1% interstory drift cycles when hairline
cracks formed at the vertical joints between wallboards. During the next set of cycles, 0.2%,
hairline cracks also formed at the horizontal joint between wallboards. The joints in the main and
return walls continue to increase through the larger displacements while the joints between the
panels of the main wall only increase to an approximate width of 1/16 in. Though no new visible
damage occurred, the maximum strength capacity of the wall was reached during the first cycles
of the 0.3% interstory drift and decreases in subsequent cycles. The first screws popped at the
bottom edge of the wall during the 0.4% interstory drift cycles and progressed upwards during the
0.5% cycles to include the damages shown in Figure 4.62.
The recorded horizontal displacement at the base of the wallboard sheathing, shown in
Figure 4.63a, provides further confirmation of when the separation of wallboard and frame began
to occur. Prior to the 0.2% cycles, the wallboard displaces less than 0.1 in. Then, corresponding
to the formation of cracks at the wallboard joints during the 0.2% interstory drift, the
displacements increased to 0.2 in. The displacement of the bottom of the wallboard continued to
increase through the larger applied displacements. Following the behavior of Specimen W8, the
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wallboard uplift prior to adhesive failure is dissimilar to the recorded stud uplift, as shown in
Figure 4.62b. The wallboard uplift continues to increase through the larger displacements.
Similar to previous tests, the forces in the tie downs generally increase throughout the test
and the transfer of forces to the tie downs at the ends of the wall is unequal as shown in Figure
4.64. During the 0.3% interstory drift, when the wall reached its maximum capacity, the axial
forces in the anchor bolts were 0.1% to 0.4% of the lateral force in the wall, and the forces in the
tie downs were 2-4% and 10% of the lateral force at the ends of the returns and main wall,
respectively.
4.5.2.3 W10: Observed behavior post 0.5% interstory drift.
The locations of the popped screws progressed upwards during the larger displacements
to include approximately half of the screws in the bottom panel as shown in Figure 4.65. Similar
to W8, the disengaged wallboards were unable to displace past the edges of the frame, resulting in
buckling at the bottom corners of wallboard during the 0.75% interstory drift.
Additional
buckling occurred at the bottom corners of the interior wallboard panels during the 1.25%
interstory drift.
The axial forces of the anchor bolts and tie downs follow the same observed the same
observed distributions during the larger displacements, as shown in Figure 4.66.
4.5.3
W11: Characteristics.
The third and final wall of the specimens with openings and differing aspect ratios is
Specimen W11 which features a 16 foot long wall with a doorway and 4 foot returns. As shown
in Figure 4.67, this specimen combines the construction details of Specimens W9 and W10. The
frame features mid-height blocking, a 32 in. wide door opening, improved corner assemblies and
gypsum wallboard installed horizontally with construction adhesive and mechanical fasteners.
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Uplift constraints for the specimen include Simpson Strong-tie HDU8 tie-downs at the ends of the
main wall and East side of the doorway, which is the end of the large pier, HDU5 tie-downs at the
ends of the return walls, and bent straps on the outside faces of the door opening. The pretension
forces in the anchor bolts and tie downs at the beginning of the test ranged from 550lbs to 2450
lbs, as noted in Tables 4.2 and 4.3.
4.5.3.1 W11: Summary and overall behavior.
Specimen W11 experienced a maximum force of 20450 lb during 0.3% drift, equivalent
to a one-sided strength of 767 lb/ft as listed in Table 4.1. The secant stiffness, measured at 0.1%
interstory drift, was 4541 lb/in/ft for this specimen. The primary mode of failure for W11
resulted from the combination of adhesive and fastener failures at the bottom of the wall and
cracking at the horizontal and vertical wallboard joints and at the top corners of the door opening.
These failures progressed upwards such that the fastener failures included half of the screws in
the bottom wallboard panels and along the vertical edges of the doorway and the crack above the
doorway had propagated 2 ft. on the smaller pier and 6 in. on the larger pier.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0014
radians with corresponding displacements recorded as -0.047” and -0.063” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.06” and -0.08” at the East and West end of the specimen, respectively.
4.5.3.2 W11: Observed behavior 0-0.5% interstory drift.
The force-deformation plots for this specimen are shown in Figure 4.55c. The first
visible damage occurred during the 0.075% interstory drift when hairline cracks formed at the
joint between the East end of the main wall and attached return wall. During the 0.2% interstory
drift, when the wall began exhibiting a more inelastic behavior, damages to the wall included
screws popping at the bottom of the wall and the formation of cracks between the wallboard
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panels and at the top of the doorway. Similar to W9, the cracks above the doorway propagated
toward the corners during the subsequent applied displacements.
However, they did not
propagate as far on the larger pier side, as shown by the damages presented in Figure 4.68.
The observed wallboard uplift and horizontal displacements of the large wall pier, shown
in Figure 4.69, shows that wallboard follows the behavior of previous specimens. Prior to the
0.2% cycles, when the screw popping and crack propagation occurred, the horizontal and vertical
displacements were less than 0.3” and 0.003”, respectively. Following the damages that occurred
through the 0.5% interstory drifts, the vertical and horizontal displacements increase to 0.48” and
0.025”, respectively.
Figure 4.70 shows the distribution of axial forces to the anchor bolts and tie downs for
this specimen. As expected, it follows a combination of the behaviors of Specimens W9 and
W10. During the 0.3% interstory drift, when the specimen experienced the maximum strength,
the tie downs recorded forces equivalent to 23%, 13%, and 6% of the lateral force within the wall
at the West end of the large wall pier, East end of the large pier, and North end of the East return,
respectively. The anchor bolts recorded forces between 2% and 9% of the lateral force.
4.5.3.3 W11: Observed behavior post 0.5% interstory drift.
Following the behavior of Specimen W9, the damages during the larger displacements
included the propagation of the crack above the doorway, buckling of the bottom corners of the
wall, and the popping of screws along the vertical edges of the doorway. In addition, the wall
follows the behavior of Specimen W10 during the larger displacements through the upwards
progression of popped screws, which include half of the screws in the bottom panels of the larger
pier and the buckling of corners between the interior panels. The one notable difference is that
the joint between the west end of the main wall and the return did not begin to crack until the
106
0.75% interstory drift cycles. The damages sustained through the end of the test are shown
Figure 4.71.
The maximum recorded axial forces for the anchor bolts and tie downs, shown in Figure
4.72, follow the expected trends of increased forces in the tie downs and decreased forces in the
anchor bolts during the larger displacements.
107
Table 4.1 One-sided stiffness and strength of interior wood-framed wall specimens
Comments
Current construction (control)
Iterative test series to improve
performance of 8 ft. x 8 ft.
planar walls with no end
returns
Specimen
W1
W2
W3
W4
W5
W6
Iterative test series on 8 ft. x 8
W7
ft. walls with returns
W8
Test series with varying
W9
aspect ratios
W10
W11
*Secant stiffness at 0.1% interstory drift
Stiffness
(lb/in/ft)
Strength
(lb/ft)
Interstory Drift at
Max Strength (%)
1362
2712
1856
3186
3560
3589
3795
5534
3615
5426
4541
298
457
495
465
584
624
526
723
689
814
767
0.5
0.3
0.75
0.2
0.4
0.3
0.4
0.2
0.4
0.3
0.3
108
Table 4.2: Anchor bolt pretension forces at beginning of test (lbs)
1
W1
W2
W3
W4
W5
W6
W7
W8
W9
W10
W11
1731
2607
1354
10120
9760
2175
3051
554
Anchor Bolt Number
2
3
4
UNKNOWN (Approx. 3000)
UNKNOWN (Approx. 3000)
UNKNOWN (Approx. 500)
2225
1866
1803
3269
3413
3481
1304
1128
3271
5306
4144
5699
6074
5416
7165
2728
N/A
2564
3491
4404
3915
697
552
627
5
1584
3053
1500
6881
7498
2155
3424
1055
Table 4.3: Tie down pretension forces at beginning of test (lbs)
W1
W2
W3
W4
W5
W6
W7
W8
W9
W10
W11
Main Wall:
West
UNKNOWN
Approx. 3000
UNKNOWN
Approx. 3000
UNKNOWN
Approx. 3000
2130
4124
4641
Not
Instrumented
Tie Down Location
Door Opening:
Main Wall:
East
East
UNKNOWN
Approx. 3000
UNKNOWN
Approx. 3000
UNKNOWN
Approx. 3000
N/A
2440
3403
3112
Not
Instrumented
East Return:
North
East Return:
South
N/A
N/A
13110
10930
10420
11470
Not
Instrumented
2445
Not
Instrumented
2097
2307
1767
N/A
2019
3040
796
1164
2441
109
Figure 4.1 South elevation construction framing and details for specimens W1 - W6.
110
(a)
(b)
Figure 4.2 Out-of-plane measurements for specimen W1;
(a) Locations of the measurements and
(b) Measured and calculated displacements.
111
8
3
6
2
4
Applied Force (kips)
Applied Force (kips)
4
1
0
-1
-2
2
0
-2
-4
-3
-6
-4
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
-5
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(b) W2
8
8
6
6
4
4
Applied Force (kips)
Applied Force (kips)
(a) W1
2
0
-2
-4
-6
2
0
-2
-4
-6
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(c) W3
(d) W4
10.0
10.0
7.5
5.0
Applied Force (kips)
Applied Force (kips)
7.5
2.5
0
-2.5
-5.0
-7.5
5.0
2.5
0
-2.5
-5.0
-7.5
-10.0
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(e) W5
-10.0
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(f) W6
Figure 4.3 Force-deformation response for 0-0.5% interstory drift cycles
for specimens W1 – W6.
112
(a)
(b)
Figure 4.4 Fastener damage states;
(a) Paint and mud cracking over screw at 0.1% and
(b) Popped screw head at 0.3%.
113
(a)
(b)
Figure 4.5 Specimen W1 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
Figure 4.6 Axial forces in anchor bolts and tie downs for Specimen W1, 0-0.5% interstory drift cycles.
114
115
600
Bolt #1
Axial Force (lbs)
400
200
0
-200
-400
-600
Axial Force (lbs)
(a)
300
200
Bolt #2
100
0
0
1000
2000
3000
Time (s)
4000
5000
6000
(b)
Figure 4.7 Force time-history for Specimen W1 in anchor bolts (a) #1 and
(b) #2 over the 0-0.5% interstory drift cycles.
116
8
4
Applied Force (kips)
6
Applied Force (kips)
2
0
-2
4
2
0
-2
-4
-6
-4
-8
-2
-1
0
1
Interstory Drift (%)
2
-2
-1
0
1
Interstory Drift (%)
(b) W2
8
8
6
6
4
4
Applied Force (kips)
Applied Force (kips)
(a) W1
2
0
-2
-4
2
0
-2
-4
-6
-6
-8
2
-2
-1
0
1
Interstory Drift (%)
-8
2
-2
-1
0
1
Interstory Drift (%)
(c) W3
2
(d) W4
10
10
6
Applied Force (kips)
Applied Force (kips)
8
4
2
0
-2
-4
5
0
-5
-6
-8
-2
-1
0
Interstory Drift (%)
(e) W5
1
2
-2
-1
0
1
Interstory Drift (%)
(f) W6
Figure 4.8 Force-deformation response for 0-2.5% interstory drift cycles
for specimens W1 – W6.
2
117
(a)
(b)
Figure 4.9 Wallboard separation and sliding in W1 at (a) negative and
(b) positive specimen deformations.
Figure 4.10 Axial forces in anchor bolts and tie downs for Specimen W1, post 0.5% interstory drift.
118
119
Figure 4.11 Cyclic backbone curve comparisons for specimens W1 - W6.
120
(a)
(b)
Figure 4.12 Images demonstrating gypsum wallboard failures as evidenced by residual paper
backing on (a) double end stud and (b) interior stud.
121
(a)
(b)
(c)
Figure 4.13 Images showing damages at bottom of sill plate of W2;
(a) Crack and screws popping on South face at 0.3% interstory drift,
(b) Crack location compared to top of sill plate (0.3%), and
(c) Wallboard disengaged from studs at 2.0%.
122
(a)
(b)
Figure 4.14 Specimen W2 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
123
0.5
Bottom Wallboard Displacement
Top Specimen Displacement
0.4
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
Time (s)
4000
5000
6000
4000
5000
6000
(a)
0.2
0.15
Stud Uplift
Wallboard Uplift
Displacement (in)
0.1
0.05
0
-0.05
-0.1
-0.15
0
1000
2000
3000
Time (s)
(b)
Figure 4.15 Displacement time history of wallboard sheathing and frame members
for W2 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.16 Axial forces in anchor bolts and tie downs for Specimen W2, 0-0.5% interstory drift cycles.
124
Figure 4.17 Axial forces in anchor bolts and tie downs for Specimen W2, post 0.5% interstory drift.
125
126
Figure 4.18 Neoprene pads at anchor bolts for specimen W3.
127
(a)
(b)
Figure 4.19 Specimen W3 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
128
0.5
0.4
Horizontal Specimen Displacement
Horizontal Wallboard Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
Time (s)
5000
6000
5000
6000
(a)
0.08
0.06
Stud Uplift
Wallboard Uplift
Displacement (in)
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
0
1000
2000
3000
4000
Time (s)
(b)
Figure 4.20 Displacement time history of wallboard sheathing and frame members
for W3 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.21 Axial forces in anchor bolts and tie downs for Specimen W3, 0-0.5% interstory drift cycles.
129
Figure 4.22 Axial forces in anchor bolts and tie downs for Specimen W3, post 0.5% interstory drift.
130
131
Figure 4.23 Stiffness enhanced tie down used for specimen W4 prior to installation.
132
(a)
(b)
Figure 4.24 Specimen W4 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
133
0.5
0.4
Wallboard Horizontal Displacement
Specimen Displacement
Displacement (in)
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
Time (s)
6000
7000
8000
(a)
0.1
0.05
Wallboard Uplift
Stud Uplift
Displacement (in)
0
-0.05
-0.1
-0.15
-0.2
-0.25
0
1000
2000
3000
4000
5000
Time (s)
6000
7000
8000
(b)
Figure 4.25 Displacement time history of wallboard sheathing and frame members
for W4 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.26 Axial forces in anchor bolts and tie downs for Specimen W4, 0-0.5% interstory drift cycles.
134
135
(a)
(b)
Figure 4.27 End stud and tie down behavior in W4 at (a) positive and
(b) negative specimen deformations.
Figure 4.28 Axial forces in anchor bolts and tie downs for Specimen W4, post 0.5% interstory drift.
136
137
Figure 4.29 Uplift constraint assembly consisting of tie down and bent strap for W5.
138
(a)
(b)
Figure 4.30 Specimen W5 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
139
0.5
Specimen Displacement
Wallboard Horizontal Displacement
0.4
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
Time (s)
6000
7000
8000
6000
7000
8000
(a)
0.03
Stud Uplift
Wallboard Uplift
0.02
Displacement (in)
0.01
0
-0.01
-0.02
-0.03
-0.04
0
1000
2000
3000
4000
5000
Time (s)
(b)
Figure 4.31 Displacement time history of wallboard sheathing and frame members
for W5 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.32 Axial forces in anchor bolts and tie downs for Specimen W5, 0-0.5% interstory drift cycles.
140
Figure 4.33 Axial forces in anchor bolts and tie downs for Specimen W5, post 0.5% interstory drift.
141
142
(a)
(b)
Figure 4.34 Specimen W6 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
143
0.5
0.4
Horizontal Wallboard Displacement
Specimen Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
Time (s)
5000
6000
5000
6000
(a)
0.1
0.08
Wallboard Uplift
Stud Uplift
0.06
Displacement (in)
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
0
1000
2000
3000
4000
Time (s)
(b)
Figure 4.35 Displacement time history of wallboard sheathing and frame members
for W6 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.36 Axial forces in anchor bolts and tie downs for Specimen W6, 0-0.5% interstory drift cycles.
144
Figure 4.37 Axial forces in anchor bolts and tie downs for Specimen W6, post 0.5% interstory drift.
145
146
(a)
(b)
(c)
Figure 4.38 Construction framing for Specimens W7 and W8;
(a) East elevation, (b) South elevation, and (c) Plan view.
147
Figure 4.39 Cyclic backbone curve comparisons for specimens W1 and W6 - W8.
148
(a)
(b)
(c)
(d)
Figure 4.40 Observed damages of specimen W7;
(a) Hairline crack formed at corner between main wall and return wall,
(b) Buckling of wallboard at corner, (c) Crack on return wall caused by
failure of corner stud assembly, and (d) Failure of stud assembly.
149
8
Applied Force (kips)
6
4
2
0
-2
-4
-6
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(a) W7
12.5
Applied Force (kips)
10.0
7.5
5.0
2.5
0
-2.5
-5.0
-7.5
-10.0
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2
Interstory Drift (%)
0.3 0.4
0.5
(b) W8
Figure 4.41 Force-deformation response for 0-0.5% interstory drift cycles
for specimens W7 – W8.
150
(a)
(b)
Figure 4.42 Specimen W7 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
151
0.5
0.4
Specimen Displacement
Horizontal Wallboard Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
Time (s)
5000
6000
7000
8000
5000
6000
7000
8000
(a)
0.12
Wallboard Uplift
Stud Uplift
0.1
Displacement (in)
0.08
0.06
0.04
0.02
0
-0.02
-0.04
0
1000
2000
3000
4000
Time (s)
(b)
Figure 4.43 Displacement time history of wallboard sheathing and frame members
for W7 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.44 Axial forces in anchor bolts and tie downs for Specimen W7, 0-0.5% interstory drift cycles.
152
153
8
Applied Force (kips)
6
4
2
0
-2
-4
-6
-8
-2
-1
0
1
Interstory Drift (%)
2
(a) W7
Applied Force (kips)
10
5
0
-5
-10
-2
-1
0
1
Interstory Drift (%)
2
(b) W8
Figure 4.45 Force-deformation response for 0-2.5% interstory drift cycles
for specimens W7 – W8.
Figure 4.46 Axial forces in anchor bolts and tie downs for Specimen W7, post 0.5% interstory drift.
154
155
(a)
(b)
Figure 4.47 Specimen W8 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
156
0.5
0.4
Wallboard Horizontal Displacement
Specimen Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
Time (s)
5000
6000
7000
8000
(a)
0.06
Wallboard Uplift
Stud Uplift
0.04
Displacement (in)
0.02
0
-0.02
-0.04
-0.06
0
1000
2000
3000
4000
Time (s)
5000
6000
7000
8000
(b)
Figure 4.48 Displacement time history of wallboard sheathing and frame members
for W8 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.49 Axial forces in anchor bolts and tie downs for Specimen W8, 0-0.5% interstory drift cycles.
157
158
Figure 4.50 Improved corner stud assembly failure on specimen W8.
Figure 4.51 Axial forces in anchor bolts and tie downs for Specimen W8, post 0.5% interstory drift.
159
160
Figure 4.52 Cyclic backbone curve comparisons for specimens W6 and W8 – W11.
Strength Capacity, Vmax (kips)
161
30
Vmax = 11.6(H/L)-1.1
W10
25
W11
20
15
W8
10
W9
H
5
L
0
0
0.5
1
1.5
Aspect Ratio, H/L
2
(a)
160
W11
Stiffness, k (k/in)
140
W10
k = -112(H/L) + 205
120
100
80
W8
60
40
W9
20
0
0
0.5
1
1.5
Aspect Ratio, H/L
(b)
Figure 4.53 Curve fits capturing the behavior for varying aspect ratios;
(a) Strength capacity and (b) stiffness.
2
162
(a)
(b)
(c)
Figure 4.54 Construction framing for Specimen W9;
(a) East elevation, (b) South elevation, and (c) Plan view.
163
8
Applied Force (kips)
6
4
2
0
-2
-4
-6
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(a) W9
30
Applied Force (kips)
20
10
0
-10
-20
-30
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(b) W10
20
Applied Force (kips)
15
10
5
0
-5
-10
-15
-20
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(c) W11
Figure 4.55 Force-deformation response for 0-0.5% interstory drift cycles
for specimens W9 – W11.
164
(a)
(b)
Figure 4.56 Specimen W9 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
165
0.5
0.4
Specimen Displacement
Horizontal Wallboard Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
Time (s)
6000
7000
8000
6000
7000
8000
(a)
0.1
0.08
Stud Uplift
Wallboard Uplift
0.06
Displacement (in)
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
0
1000
2000
3000
4000
5000
Time (s)
(b)
Figure 4.57 Displacement time history of wallboard sheathing and frame members
for W9 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.58 Axial forces in anchor bolts and tie downs for Specimen W9, 0-0.5% interstory drift cycles.
166
167
8
Applied Force (kips)
6
4
2
0
-2
-4
-6
-8
-2
-1
0
Interstory Drift (%)
1
2
1
2
(a) W9
Applied Force (kips)
20
10
0
-10
-20
-2
-1
0
Interstory Drift (%)
(b) W10
20
Applied Force (kips)
15
10
5
0
-5
-10
-15
-2
-1
0
1
Interstory Drift (%)
2
(c) W11
Figure 4.59 Force-deformation response for 0-2.5% interstory drift cycles
for specimens W9 – W11.
Figure 4.60 Axial forces in anchor bolts and tie downs for Specimen W9, post 0.5% interstory drift.
168
Figure 4.61 South elevation construction framing for Specimen W10.
169
Figure 4.62 Specimen W10 damage illustration at 0.5% interstory drift.
170
171
0.5
0.4
Horizontal Wallboard Displacement
Specimen Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
6000
Time (s)
7000
8000
7000
8000
9000
10000
(a)
0.06
0.05
Wallboard Uplift
Stud Uplift
0.04
Displacement (in)
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
-0.04
0
1000
2000
3000
4000
5000
6000
Time (s)
9000
10000
(b)
Figure 4.63 Displacement time history of wallboard sheathing and frame members
for W10 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.64 Axial forces in anchor bolts and tie downs for Specimen W10, 0-0.5% interstory drift cycles.
172
Figure 4.65 Specimen W10 damage illustration at end of test.
173
Figure 4.66 Axial forces in anchor bolts and tie downs for Specimen W10, post 0.5% interstory drift.
174
Figure 4.67 South elevation construction framing for Specimen W11.
175
Figure 4.68 Specimen W11 damage illustration at 0.5% interstory drift.
176
177
0.5
0.4
Horizontal Wallboard Displacement
Specimen Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
2000
4000
6000
Time (s)
8000
10000
12000
(a)
0.06
Wallboard Uplift
Stud Uplift
0.04
Displacement (in)
0.02
0
-0.02
-0.04
-0.06
0
2000
4000
6000
Time (s)
8000
10000
12000
(b)
Figure 4.69 Displacement time history of wallboard sheathing and frame members
for W11 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 4.70 Axial forces in anchor bolts and tie downs for Specimen W11, 0-0.5% interstory drift cycles.
178
Figure 4.71 Specimen W10 damage illustration at end of test.
179
Figure 4.72 Axial forces in anchor bolts and tie downs for Specimen W11, post 0.5% interstory drift.
180
181
CHAPTER 5
EXPERIMENTAL RESULTS OF CYCLIC TESTED PLANAR STEEL-FRAMED WALLS
5.1
Introduction
This chapter discusses the results of the interior steel-framed planar walls which will
contribute to the lateral force resisting system of residential buildings using unibody construction
techniques. This series consisted of four planar walls with frames made of cold formed light gage
steel, listed as S1-S4 in Table 3.1, which were tested to determine the construction details that
would be suggested for this system.
Similar to the results presented for the interior wood-framed suite, the geometry,
construction details, and behaviors will be presented for each specimen during the 0-0.5% and
post 0.5% interstory drifts. The effects of the construction details are investigated through the
stiffness, strength, and damage progression of each specimen.
The reported stiffness in Table 5.1 is a secant stiffness calculated with the maximum
forces sustained during the first cycle to +/- 0.1% interstory drift. Also listed in Table 5.1 is the
one-sided maximum strength capacity in pounds per linear foot of wall and the interstory drift
cycle at which the force was recorded.
Additional measurements reported in this chapter for each specimen include uplift forces
in the tie down units and anchor bolts to gain an understanding of the force path at the base of
each specimen. Displacement gages, listed in Chapter 3, are used to measure deformations
related to 1) out-of-plane twisting, 2) differential slip between the wood-frame and sheathing in
the horizontal and vertical direction, 3) differential horizontal slip between the bottom sill plate
and test rig, 4) uplift.
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5.2
Planar Control Test: S1
Test specimen S1 is representative of the current construction techniques for walls built
with light-gage steel and acts as the control specimen for the steel-frame suite of experiments.
The specimen was an 8 ft. x 8 ft. planar wall with no door/window openings that was framed with
1-5/8 in. x 3-5/8 in. interior cold formed studs with a thickness equivalent to 20 gage, and 1-1/2
in. x 3-5/8 in. cold formed track with a thickness of 18 gage. Both faces of the frame were
sheathed with 5/8 in. thick Type X gypsum wallboards, installed with 1-5/8 in. long drywall
screws. Simpson Strong-tie S\HDU6 tie-downs were installed on the inside web of the king studs
at the ends of the specimen, tightened with a socket wrench to a force of approximately 3200 lbs.
The anchor bolts were tightened to pretension forces which range from 1800 lbs to 2000 lbs. The
construction framing for this wall is shown in Figure 5.1
5.2.1
Summary and overall behavior.
Specimen S1 attained a maximum force capacity of 4259 lbs, or 266 lb/ft as listed in
Table 5.1, during the 0.75% interstory drift cycles. The one-sided stiffness of this specimen,
measured at the 0.1% interstory drift, was 1249 lb/in/ft. The primary mode of failure was
fastener failure at the bottom sill plate, extending upwards such that the majority of the bottom
panel fasteners had failed.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0022
radians with corresponding displacements recorded as -0.095” and -0.072” during the 1.0%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.11” and -0.10” at the East and West end of the specimen, respectively.
5.2.2
Observed behavior 0-0.5% interstory drift.
The force-deformation behavior plots in Figure 5.2a, shows that the specimen exhibited
inelastic behavior throughout the test. This behavior increased during the 0.2% interstory drift
183
cycles when edge screws along the bottom of the wall popped. These damages progressed
upwards during the 0.4% and 0.5% cycles to include all of the edge screws on the bottom and
East side of the wall, half of the edge screws on the West side of the wall, and half of the interior
screws of the bottom panel as shown in Figure 5.3a.
Figure 5.4 shows how the popped screws of the specimen were identified. As a result of
the wallboard pulling away from the frame and screw, the initial damage appears as a divot in the
wall (Figure 5.4a). Additionally, as shown in the figure, the divot characterizing the location of a
popped screw is not always accompanied by the cracking of the mud and paint covering the
screw. These observed divots are unique to the steel-framed wall because the fine threaded
screws do not withdraw or shear from the studs as occurred in the wood-framed tests. In many
cases however, the visible damage at the popped screws increases to the state shown in Figure
5.4b, where the mud and paint have clearly cracked and the wallboard has become fully
disengaged from the stud.
Figure 5.5 shows the maximum recorded axial forces for the anchor bolts and tie downs
as a percentage of the lateral forces within the specimen. The chart shows that the largest forces
were recorded at the tie downs while the anchor bolts each recorded maximum forces equivalent
to 4% or less of the theoretical uplift force – equal to the lateral load for a square specimen. The
chart also shows that the maximum recorded force in each of the anchor bolts and tie downs
generally increased through the 0.5% interstory drift.
5.2.3
Observed behavior post 0.5% interstory drift.
The force deformation plot for the entire specimen, shown in Figure 5.6a, gives two
important insights for the response of this specimen to the applied displacements. First, it shows
a notable difference in strength between the positive and negative displacements, which was most
likely caused by the asymmetry of the c-shaped studs. Second, the plot shows that the specimen
184
achieved its maximum strength capacity during the negative excursion of the 0.75% interstory
drift.
During the larger interstory drift cycles, the locations of visible damages progressed
upwards from the bottom of the wall to include the popping of all of the screws in the bottom
wallboard panels and approximately one third of the screws in the top panels. Similar to the
behavior of the planar wood-framed walls, after the screws failed and the wallboards pulled away
from the studs, the wallboards were able to move separately from the studs with the applied
displacements. The locations of the final damages to the wall are shown in Figure 5.3b.
Figure 5.7 shows that the maximum recorded forces in the anchor bolt and tie downs
during the larger displacements maintain a similar distribution as during the smaller
displacements. During the 0.75% interstory drift, when the specimen achieved its maximum
strength capacity, the tie downs recorded forces equivalent to 34-37% of the lateral force within
the specimen and the anchor bolts recorded forces equivalent to 0-4% of the lateral force.
Following the peak, the forces in the East tie down and anchor bolts generally decreased and the
force in the West tie down increased. The difference in tie down forces is most likely caused by
the asymmetry that causes the difference in specimen strength between the “positive” and
“negative” displacements.
5.3
Planar Wall Tests With Unibody Enhancements: S2 – S4
Three tests were performed that featured unibody enhancements in the steel-framed tests,
consisting of one free-standing wall and two walls that featured T-shaped returns.
These
enhancements consisted of the details which produced the best performing wood-framed walls
(W6 and W8) and included the installation of wallboards with construction adhesive and
mechanical fasteners, properly sized tie downs at the ends of wall segments, mid-height blocking
185
on the frame, and improved built-up stud assemblies at the ends of the wall. Following the
construction techniques for steel-framed walls, the blocking used for these specimens was 1.5 in.
wide, 18 gage cold-formed steel straps installed on the faces of the frame.
As mentioned in Chapter 3, the studs used in this study featured flanges with dimples,
shown in Figure 5.8a, which was intended to improve the bond between the construction adhesive
and framing members. These grooves were proven to be successful when the wallboard was
removed after the completion of a test and the majority of the adhesive remained bonded to the
studs while less adhesive remained on the smooth track, as shown in Figure 5.8b.
Figure 5.9 shows the cyclic backbone curves for all of the steel-framed specimens,
generated from the one-sided strength-deformation response by plotting peaks of leading cycles
for each group. Referring to the figure, note the conventional construction test reached its largest
load in 0.75% interstory drift, while the optimized unibody improvements of specimens S2 and
S4 resulted in a more limited ductility system with its maximum load occurring in the range of
0.3% to 0.4% interstory drift. The adhesive affects the curves by increasing the while reducing
the differences of strength between the positive and negative displacements which were observed
on specimen S1.
The unibody enhancements for a free-standing planar wall, featured in specimen S2,
improved the strength and stiffness by 82% and 87%, respectively, over the control test (S1).
Following the maximum load, the effect of the construction adhesive decreases and the specimen
behaved similar to the traditionally constructed specimen by asymptotically approaching a
residual strength value of 60 lb/ft in the “negative” displacement and 130 lb/ft in the “positive”
displacement.
Similar to the results of the wood-framed walls, the stiffness and strength of walls
constructed with unibody enhancements increased further when applied to planar steel-framed
186
walls with orthogonal end returns. Specimen S4, which featured the optimized construction
procedures for a planar wall with end returns, improved the strength and stiffness by 29% and
74%, respectively, over the unibody planar wall (S2), and by 134% and 225% over the typical
construction (S1). Following the maximum load, the residual strength of the specimen quickly
reduced from 625 lb/ft to approximately 430 lb/ft, before asymptotically approaching a residual
strength value of 230 lb/ft in the “negative” displacement and 315 lb/ft in the “positive”
displacement.
5.3.1
S2: Characteristics.
The first wall which featured the unibody construction techniques, specimen S2, was a
free-standing 8 ft. x 8 ft. planar wall with no openings. This specimen featured Simpson Strongtie S\HDU9 tie-downs on the inside web of the king studs, blocking straps at the mid-height of
the frame, and horizontal gypsum wallboards installed with construction adhesive and mechanical
fasteners.
The anchor bolts and tie downs were pretensioned to 2500 - 3200 lbs and
approximately 1000 lbs respectively.
5.3.1.1 S2: Summary and overall behavior.
Specimen S2 experienced a maximum force capacity of 7736 lb, or a maximum onesided strength capacity of 484 lb/ft, during the 0.4% interstory drift cycles. The one-sided
stiffness of the specimen was calculated to be 2332 lb/in/ft during the 0.1% interstory drift. It is
interesting to note that the specimen was weaker and less stiff as compared to its wood-framed
counterpart, W6, which had a one-sided strength and stiffness of 624 lb/ft and 3589 lb/in/ft,
respectively. The primary reason for the difference in performance can be attributed to the weaker
bond between the Loctite adhesive and steel studs, as compared to the stronger Liquid nails and
187
wood stud bond. The primary damage for this specimen consists of adhesive and mechanical
fastener failures at the bottom sill of the wall.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0024
radians with corresponding displacements recorded as -0.152” and -0.029” during the 2.0%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.17” and -0.06” at the East and West end of the specimen, respectively.
5.3.1.2 S2: Observed behavior 0-0.5% interstory drift.
The force-deformation behavior for the range of 0 to 0.5% interstory drift of specimen S2
in Figure 5.2b shows that the unibody construction details change the behavior of the specimen in
a few ways. First, the response is relatively elastic behavior through the 0.1% interstory drift,
improving the secant stiffness of the specimen. Second, it reduces the difference in strength
capacity between the positive and negative applied displacements through the 0.5% interstory
drift. And third, the construction details allow the specimen to have an improved strength
capacity.
The first visible damage for the test occurred during the 0.1% interstory drift cycles when
approximately half of the screws connecting the bottom edge of the wallboard and the sill plate
popped. This damage was noticeable in the force-deformation response of the specimen through
larger inelastic hysteretic loops after the 0.075% cycle group.
Additional damage occurred
during the 0.2% through 0.5% interstory drift cycles as the bottom row of screws connecting to
the studs began to pop. After all of the screws connecting the wallboard to the sill plate had
popped during the 0.4% interstory drift cycles, the strength capacity of the wall decreases. As
shown in Figure 5.10a, the damage of the specimen through the 0.5% interstory drift cycles was
limited to the popping of the screws in approximately the bottom 6 in. of the wall.
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The recorded horizontal and vertical displacements of the frame and wallboard, in Figure
5.11, show that the wallboard separation was delayed until 0.2% interstory drift when screws
connected to the studs began to pop. Prior to the separation, the horizontal displacement of the
bottom of the wall was less than 0.015”, but increased from 0.026” to 0.22” during the 0.2%
through 0.5% interstory drift cycles. Similarly, the wallboard had less than 0.015” of uplift
through the 0.1% interstory drift, but increased to 0.08” through the 0.05% interstory drift.
Figure 5.12 shows the maximum recorded values for the anchor bolts and tie downs
during the 0-0.5% interstory drift cycles as an equivalent to the percentage of lateral force within
the specimen. As expected, largest forces occurred at the exterior of the specimen and reduced as
it approached the inner-most anchor bolts. During the 0.4% interstory drift cycles, when the
specimen attained its maximum strength capacity, the tie downs recorded forces equivalent to
56%-82% of the lateral force, the exterior most anchor bolt recorded 7% equivalent force and the
interior anchor bolts recorded 1-3% equivalent force. The difference in recorded forces for the
East and West tie-downs that was noted in the control specimen increased within this specimen
and ranges between 10% and 25% during the 0-0.5% interstory drift cycles.
5.3.1.3 S2: Observed behavior post 0.5% interstory drift.
In the larger displacements, the adhesive has failed and the specimen begins acting
similar to Specimen S1 in that the residual strength continues to decrease and shows a difference
between the positive and negative displacements. The visible damage progressed upwards from
the bottom of the wall during the 1.0% and 1.5% interstory drift cycles. However, only half of
the screws in the bottom panel popped as opposed to the majority of all screws on the wall as
occurred in Specimen S1. The locations of all of the damages that appeared on the wall are
displayed in Figure 5.6b.
189
The distribution of axial forces to the tie downs and anchor bolts remained the same
through the larger displacements, as shown in Figure 5.13. The difference between the forces in
the East and West tie downs increased to a 73% by the end of the test.
5.3.2
S3: Characteristics.
Specimen S3 is the second steel-framed specimen that featured the unibody
enhancements and the first as such to feature the orthogonal return walls. The main wall of the
specimen is an 8 ft. x 8 ft. planar wall with mid-height blocking on the frame, gypsum wallboards
installed with construction adhesive and mechanical fasteners, and Simpson Strong-tie S/HDU9
tie-downs. The return walls are 4 ft. wide returns with S/HDU6 tie-downs at the ends. The
construction framing for this specimen is shown in Figure 5.14. The anchor bolts and tie downs
were maintained pretension force between 1150 lbs and 3100 lbs, as listed in Tables 5.2 and 5.3.
5.3.2.1 S3: Summary and overall behavior.
During the 0.75% interstory drift cycles, the specimen achieved a maximum one-sided
strength capacity of 473 lb/ft, or a maximum applied force of 7571 lbs. The one-sided stiffness,
as calculated during the 0.1% interstory drift cycles, was 3202 lb/in/ft. The primary damage
presented by this specimen was adhesive and fastener connection failure at the bottom of the wall
which progressed upwards to include the bottom foot of the wall. Additionally a crack formed at
the joint between wallboard panels, which allowed fastener connection failures around the joint.
While the stiffness of the specimen increases 27% over the free standing wall, the strength
decreased 2%.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0017
radians with corresponding displacements recorded as -0.049” and -0.081” during the 2.5%
190
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.06” and -0.10” at the East and West end of the specimen, respectively.
5.3.2.2 S3: Observed behavior 0-0.5% interstory drift.
The first visible damage occurred during the 0.1% interstory drift cycles, when the cracks
begin forming at the corners between the main and return walls. During the next set of cycles
(0.2%), the screws connecting the bottom of the wallboard to the sill plate and studs popped. A
crack forms at the joint between the top and bottom wallboards during the 0.3% interstory drift
cycles. No additional visible damage occurs during the 0.4% and 0.5% cycles. The damage that
occurred through the 0.5% interstory drift is shown in Figure 5.15a.
Referring to Figure 5.2c, which shows the force-deformation behavior of the specimen
for the range of 0-0.5% interstory drift, a local peak strength was achieved during the first cycle
of the 0.1% interstory drifty as the damages of the specimen caused the wall to begin displaying
an inelastic behavior. However, after a decrease in residual strength during the 0.2% cycles, the
strength of the specimen maintained its residual strength of approximately +841lb/ft and -786
lb/ft in the positive and negative displacements, respectively.
The recorded horizontal and vertical displacements of the frame and wallboard, depicted
in Figure 5.16, confirm that wallboard separation began during the 0.2% interstory drift. Prior to
this failure, the wallboard had horizontal and vertical displacements of 0.02” or less, and the
wallboard uplift was similar to the uplift in the studs. In the following cycles through 0.5%
interstory drift, the horizontal displacements increased to 0.17” and the vertical displacements
increased to 0.08”. Additionally, the recorded uplift values show that the corner stud assembly
restricts the movement of the end stud within the specimen.
Figure 5.17, which show the axial forces in the anchor bolts and tie downs through the
0.5% interstory drift cycles shows that the axial forces increase in all of the bolts and tie downs
191
through the 0.1% drift to match the local peak strength of the specimen. After the local peak, the
forces in the tie downs at the ends of the main wall began to decrease while the forces in the
anchor bolts in the main wall and tie down in the return wall continue to increase.
5.3.2.3 S3: Observed behavior post 0.5% interstory drift.
The true peak strength of the wall was achieved during the 0.75% interstory drift cycles,
as displayed in Figure 5.6c. Similar to the behavior of the wood-framed specimens, the bottom
corners of the main wall began to buckle during the 0.75% interstory drift cycles as a result of
being unable to slide past the frame. The additional damages occurred during the larger interstory
drift, shown in Figure 5.15b, consists of two to three screws popping in the top and bottom
wallboard panels.
Figure 5.18 shows the maximum recorded forces in the anchor bolts and tie downs during
the larger displacements.
The forces in the anchor bolts increased during these larger
displacements showing the difference in forces at the East and West ends of the wall that was
noticed in the previous specimens.
5.3.3
S4: Characteristics.
To improve the behavior of a specimen featuring return walls, including strength,
stiffness, and damage states, improvements were made to the construction procedures for
Specimen S4. To improve the unibody performance, an adjustment was made to the blocking to
prevent out of plane movement of the blocking. A spacer made of stud material was added
between the blocking straps at the ends of the wall, as shown in Figure 5.19. The improved
specimen experienced a strength increase of 29% and stiffness increase of 74% over the unibody
planar wall (S2).
192
The main wall of specimen S4 is an 8 ft. x 8 ft. planar wall with mid-height blocking on
the frame, gypsum wallboards installed with construction adhesive and mechanical fasteners, and
Simpson Strong-tie S/HDU9 tie-downs. The anchor bolts and tie downs held pretension forces of
approximately 3000 lbs and 2000 lbs, respectively, at the beginning of the test.
5.3.3.1 S4: Summary and overall behavior.
Specimen S4 experienced a maximum force of 9996 lbs, or 625 lb/ft as listed in Table
5.3, during the 0.3% interstory drift cycles.
The one-sided stiffness for the specimen, as
measured at 0.1% interstory drift, was 4069 lb/in/ft. The primary damage for this specimen was
mechanical fastener and adhesive failure at the bottom of the wall that progressed upwards to
include all of the screws within the bottom foot of the specimen.
The maximum out-of-plane rotation angle of the specimen, was calculated as 4E-5
radians with corresponding displacements recorded as -0.072” and +0.076” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.07” and +0.08” at the East and West end of the specimen, respectively.
5.3.3.2 S4: Observed behavior 0-0.5% interstory drift.
Figure 5.20a shows the force-deformation behavior of the specimen. The first visible
damage occurred as cracking at the corners between the main and return walls during the 0.2%
interstory drift cycles. Though no new visible damage occurred, the peak strength occurred
during the 0.3% cycles.
During the next set of cycles (0.4%) the bottom row of screws,
connecting the wallboard and sill plate, popped. Figure 5.2d shows the damages that occurred on
the specimen through the 0.5% interstory drift.
The recorded horizontal and vertical displacements of the frame and wallboard, shown in
Figure 5.21, confirm that, while no new visible damage occurred, wallboard separation began
during the 0.3% interstory drift cycles. In the first four sets of cycles, 0.05% through 0.2%
193
interstory drift, the bottom of the wallboard recorded horizontal displacements of less than 0.02”
and uplift similar to the recorded values of the end stud. However, during the 0.3% interstory
drift and later cycles, the horizontal displacement began to increase and the uplift became less
similar.
Figure 5.22 shows the maximum recorded forces for the anchor bolts and tie downs
during the 0-0.5% interstory drift cycles. The anchor bolts and tie downs at the ends of the wall
experienced local peak forces during the 0.2-0.4% cycles corresponding to the peak strength of
the wall that occurred in the 0.3% interstory drift. The chart shows that the largest forces of this
specimen occur in the tie-downs at the East end of the wall, while the anchor bolts and West tiedown record axial forces of less than 5% of the theoretical uplift.
5.3.3.3 S4: Observed behavior post 0.5% interstory drift.
As expected, the bottom corners of the wall began to buckle during the 0.75% interstory
drift cycles. During the higher interstory drift cycles, the screws which that had visibly popped
progress upwards such that the bottom two rows of screws connecting the wallboard to the
interior studs had popped as shown in Figure 5.20b.
Similar to the previous specimens, the forces in the tie downs continue to increase during
the larger displacements as shown in Figure 5.23. However, unlike the previous specimen, the
force in the tie down at the end of the wall is greater than the tie down in the end return.
194
Table 5.1 One-sided stiffness and strength of steel walls
Comments
Specimen
Current construction (control)
Planar 8 ft. x 8 ft. wall with no
returns
Iterative test series on 8 ft. x 8
ft. walls with returns
*Secant stiffness at 0.1% interstory drift
S1
S2
S3
S4
Stiffness
(lb/in/ft)
Strength
(lb/ft)
Interstory Drift at
Max Strength (%)
1249
2332
266
484
0.75
0.4
3202
4069
473
625
0.75
0.3
Table 5.2 Anchor bolt pretension forces at beginning of test (lbs)
Specimen
S1
S2
S3
S4
1
1995
2810
2886
2
1832
3036
1912
Anchor Bolt Number
3
1805
3122
2026
Approximately 3000
4
1882
2645
3072
5
1864
2553
2275
Table 5.3 Tie down pretension forces at beginning of test (lbs)
Specimen
S1
S2
S3
S4
West End
Main Wall
3180
983
2336
Tie Down Location
East End
North East
Main Wall
End Return
3270
N/A
1048
1231
1182
Approximately 2000
South East
End Return
N/A
Not
Instrumented
195
Figure 5.1 South elevation construction framing and details for specimens S1 and S2.
4
8
3
6
2
4
Applied Force (kips)
Applied Force (kips)
196
1
0
-1
-2
2
0
-2
-4
-6
-3
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(a) S1
(b) S2
10.0
7.5
Applied Force (kips)
Applied Force (kips)
6
4
2
0
-2
-4
5.0
2.5
0
-2.5
-5.0
-7.5
-6
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(c) S3
-10.0
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(d) S4
Figure 5.2 Force-deformation response for 0-0.5% interstory drift cycles for S1 – S4.
197
(a)
(b)
Figure 5.3 Specimen S1 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
198
(a)
(b)
Figure 5.4 Fastener damage states of screws popping showing (a) only a visible divot and (b)
cracked mud with screw fully disengaged from wallboard.
Figure 5.5 Axial forces in anchor bolts and tie downs for specimen S1, 0-0.5% interstory drift cycles.
199
200
8
4
6
Applied Force (kips)
Applied Force (kips)
3
2
1
0
-1
-2
4
2
0
-2
-4
-6
-3
-8
-2
-1
0
1
Interstory Drift (%)
2
-2
-1
(a) S1
0
1
Interstory Drift (%)
2
(b) S2
8
10
Applied Force (kips)
Applied Force (kips)
6
4
2
0
-2
-4
5
0
-5
-6
-8
-10
-2
-1
0
1
Interstory Drift (%)
(c) S3
2
-2
-1
0
1
Interstory Drift (%)
(d) S4
Figure 5.6 Force-deformation response for 0-2.5% interstory drift cycles for S1 – S4.
2
Figure 5.7 Axial forces in anchor bolts and tie downs for specimen S1, post 0-0.5% interstory drift.
201
202
(a)
(b)
Figure 5.8 Steel framing chosen to improve unibody enhancements features
(a) grooved flanges of studs improve bonding of construction adhesive as shown when (b)
wallboard was removed after test.
203
Figure 5.9 Cyclic backbone curve comparisons for specimens S1 – S4.
204
(a)
(b)
Figure 5.10 Specimen S2 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
205
0.5
0.4
Specimen Displacement
Wallboard Horizontal Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
Time (s)
5000
6000
5000
6000
(a)
0.1
Displacement (in)
Stud Uplift
Wallboard Uplift
0
-0.1
-0.2
0
1000
2000
3000
4000
Time (s)
(b)
Figure 5.11 Displacement time history of wallboard sheathing and frame members
for S2 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 5.12 Axial forces in anchor bolts and tie downs for Specimen S2, 0-0.5% interstory drift cycles.
206
Figure 5.13 Axial forces in anchor bolts and tie downs for Specimen S2, post 0.5% interstory drift.
207
208
(a)
(b)
(c)
Figure 5.14 Construction framing for Specimens S3 and S4
(a) East elevation, (b) South elevation, and (c) Plan view.
209
(a)
(b)
Figure 5.15 Specimen S3 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
210
0.5
0.4
Wallboard Horizontal Displacement
Specimen Displacement
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
6000
4000
5000
6000
(a)
0.1
0.08
Wallboard Uplift
Stud Uplift
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
0
1000
2000
3000
(b)
Figure 5.16 Displacement time history of wallboard sheathing and frame members
for S3 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 5.17 Axial forces in anchor bolts and tie downs for Specimen S3, 0-0.5% interstory drift cycles.
211
Figure 5.18 Axial forces in anchor bolts and tie downs for Specimen S3, post 0.5% interstory drift.
212
213
(a)
(b)
Figure 5.19 Spacers added to blocking of specimen W4;
(a) Image of blocking spacer material and
(b) Locations of blocking in plan view of specimen.
214
(a)
(b)
Figure 5.20 Specimen S4 damage illustration at
(a) 0.5% interstory drift and (b) End of test.
215
0.5
0.4
Wallboard Horizontal Displacement
Specimen Displacement
0.3
Displacement (in)
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
Time (s)
4000
5000
6000
4000
5000
6000
(a)
0.08
0.06
Wallboard Uplift
Stud Uplift
Displacement (in)
0.04
0.02
0
-0.02
-0.04
-0.06
-0.08
-0.1
0
1000
2000
3000
Time (s)
(b)
Figure 5.21 Displacement time history of wallboard sheathing and frame members
for S4 measuring (a) the horizontal displacement at bottom of wallboard
and top of frame and (b) vertical uplift of wallboard and end stud.
Figure 5.22 Axial forces in anchor bolts and tie downs for Specimen S4, 0-0.5% interstory drift cycles.
216
Figure 5.23 Axial forces in anchor bolts and tie downs for Specimen S4, post 0.5% interstory drift.
217
218
CHAPTER 6
EXPERIMENTAL RESULTS OF WOOD-FRAMED WALLS WITH EXTERIOR
SHEATHING CONDITIONS
6.1
Introduction
This chapter discusses the results of the exterior wood-framed planar walls built with
unibody construction techniques. This series consists of three planar walls with end returns and
varying exterior sheathing, listed as W-DG, W-PLY, and W-STU in Table 3.1, which were tested
to investigate the effects of applying the unibody construction procedures to an exterior wall.
Similar to Chapters 4 and 5, this chapter will present the specimen characteristics, overall
behavior, and observed behaviors during the 0-0.5% and post 0.5% interstory drift cycles.
6.2
Planar Wall Tests With Unibody Enhancements and Exterior Sheathing
To simulate an exterior wall of a small building, the specimens had an 8 ft. x 8 ft. main
wall with no openings and 2 ft. long orthogonal walls connected in an L-shape at the ends,
creating a large C-shaped specimen as shown in the construction framing plans in Figure 6.1.
The interior of the “C” was sheathed with 5/8 in. Type X gypsum wallboards using the techniques
of the best performing wood-framed interior wall with returns (W8). The exterior sheathing
varied for each test and included fiberglass mat gypsum sheathing wallboards, plywood, and
fiberglass mat gypsum sheathing wallboards plus 7/8 in. three-coat stucco. The details and
procedures for these specimens were influenced by specimen W8 and typical construction
practices for the applicable exterior sheathing materials.
Figure 6.2 illustrates the cyclic backbone curves for the exterior wall specimens,
generated from the force-deformation plots using the peaks of the leading cycle from each group
of displacements. Due to the asymmetry caused by the locations of the return walls, the results of
219
these tests are compared to the best performing interior free-standing wall (W6). The figure
shows that the stiffness of the fiberglass and plywood sheathed specimens was similar to the
stiffness of the interior wall, while the stucco sheathed specimen had an increased stiffness. The
figure also shows that the fiberglass sheathed specimen had a lower strength than the interior wall
while the plywood and stucco sheathed specimens had a higher strength. The recorded two-sided
stiffness and strengths of the specimens, shown in Table 6.1 confirm these visual observations.
6.2.1
W-DG: Characteristics.
The first exterior wall, specimen W-DG, featured 5/8 in. (DensGlass®) fiberglass mat
gypsum sheathing which was chosen to behave similar to the interior gypsum wallboards while
providing additional moisture resistance.
The exterior wallboards were installed using the same adhesive and mechanical fastener
details as the interior specimens. According to typical construction techniques for using this
exterior sheathing material, joint tape and mud specific to the wallboard material would be used
at the horizontal and vertical joints between the wallboard panels. These were not used for the
specimen under the assumption that the wallboards would be used under stucco or an
architectural finish causing the aesthetics of the joints to be unnecessary as they would not be
visible.
In summary, this specimen is an 8 ft. x 8 ft. wall with 2 ft. wide L-shaped returns,
featuring an exterior sheathing of 5/8 in. DensGlass® wallboards and an interior of 5/8 in. gypsum
wallboards installed with construction adhesive and 1-5/8 in. drywall screws. The wood-framing
of this specimen features mid-height blocking and Simpson Strong-tie HDU8 tie-downs at the
ends of the main and return walls. The axial forces in the anchor bolts and tie downs at the
beginning of the test ranged from 3800 lbs to 11540 lbs as listed in Tables 6.2 and 6.3.
220
6.2.1.1 W-DG: Summary and overall behavior.
This specimen attained its maximum force capacity of 8682 lbs, or strength of 1085 lb/ft,
during the 0.4% interstory drift; a 13% decrease when compared to the two-sided strength of W6
– the interior wall specimen with gyp board sheathing. The stiffness of the specimen, measured
at the 0.1% interstory drift, was 7474 lb/in/ft, a 4% increase over the stiffness of the interior wall.
The primary failure of the specimen was adhesive and fastener failures on both faces of the
specimen to include the popping of the bottom screws on both sides of the wall and at the vertical
edges of wallboard on the exterior side.
6.2.1.2 W-DG: Observed behavior 0-0.5% interstory drift.
The force-deformation behavior plot for the specimen, in Figure 6.3a, shows that the
specimen exhibited a relatively elastic behavior through the 0.1% interstory drift. The inelastic
shape of the figure indicates adhesive failures probably began during the 0.2% interstory drift
cycles even through no visible damages occurred.
The strength capacity of the specimen
plateaued after the 0.2% drift cycles, which may have been caused by the fibrous material of the
exterior wallboard which extended the ductility of the specimen and allowed the specimen to
maintain the strength capacity. The force-deformation plot for the entire test, in Figure 6.4a,
shows that this plateau continued through the 0.75% interstory drift cycles.
The first visible damage occurred during the 0.075% interstory drift when cracks began
to form on the interior face of the wall at the corners between the main and return walls. Edge
screws at the bottom of both faces of the wall and along the vertical edges of the exterior
wallboards began to pop during the 0.3% interstory drift cycles, as shown in Figure 6.4. In
addition, a crack propagated approximately halfway across the wall at the horizontal joint
between the wallboard panels on the interior face. The crack propagated across three-quarters of
the wall during the 0.4% cycles. The locations of popped screws also progressed upwards during
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these cycles to include the bottom foot of interior screws on the gypsum side and approximately
half of the interior screws on the larger bottom panel of the DensGlass® side.
Within the 0-0.5% interstory drift cycles, the maximum out-of-plane rotation angle of the
specimen, was calculated as 0.0053 radians with corresponding displacements recorded as -0.27”
and -0.13” during the 0.5% interstory drift cycles, at the East and West end of the wall,
respectively. This rotation produces a displacement equal to -0.31” and -0.19” at the East and
West end of the specimen, respectively.
Figure 6.6 shows the recorded horizontal displacements for the frame and sheathing of
this specimen through the 0.5% interstory drift. The figure confirms that adhesive failures began
during the 0.2% interstory drift cycles (approximately 2800s) on both faces of the wall. Prior to
this failure, the wallboards moved 0.01” or less on both faces. The displacement increased
through the 0.5% interstory drift when both faces moved approximately 0.26”. Through these
graphs, it can be seen that the interior and exterior faces behaved similarly during the test.
However, the larger displacements of the DensGlass® sheathing prior to the 0.2% cycles indicate
that failures may have begun on the exterior face first.
Figure 6.7 shows maximum recorded axial forces in the anchor bolt and tie downs during
the 0-0.5% range of interstory drift cycles. Following the behavior of the force-deformation plot,
the forces in the anchor bolts and tie downs increase through the 0.2%-0.3% interstory drift
cycles, corresponding to when inelastic damage began to occur. In the subsequent cycles, as
visible damage occurs to the specimen, the axial forces in the anchor bolts become negligible and
the forces in the tie down plateau and/or reduce.
6.2.1.2 W-DG: Observed behavior post 0.5% interstory drift.
As expected, the interior face acted similar to the interior wood-framed walls discussed in
Chapter 4 during the larger interstory drift cycles. This included the buckling of the lower
222
corners of wallboard and an upwards progression of damages to include half of the screws on the
bottom wallboard as shown in Figure 6.8. No additional damages occurred on the exterior face of
the wall during the larger interstory drift cycles.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0064
radians with corresponding displacements recorded as -0.32” and -0.16” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to -0.38” and -0.24” at the East and West end of the specimen, respectively.
Figure 6.9 shows the axial forces in the anchor bolts and tie downs during the larger
displacement cycles. The distribution of forces remains similar during these larger displacements
as the forces in the tie downs increase and the forces in the anchor bolts remain negligible.
6.2.2
W-PLY: Characteristics.
The second exterior wall is an 8 ft. x 8 ft. wall with 2 ft. wide L-shaped returns which
features 15/32 in. Structural I plywood on the exterior face and 5/8 in. gypsum wallboards on the
interior face installed with construction adhesive and mechanical fasteners. Guided by typical
construction practices for installing plywood sheathing, the mechanical fasteners were 10d nails
spaced at 6 in. on center along the edges and 12 in. on center in the field. The frame featured
mid-height blocking and Simpson Strong-tie HDU8 tie-downs at the ends of the main and return
walls. At the beginning of the test, the anchor bolts and tie downs contained 1500 lbs to 2600 lbs
of pretension, as listed in Tables 6.2 and 6.3.
6.2.2.1 W-PLY: Summary and overall behavior.
This specimen attained its maximum force capacity of 15120 lbs, or 1890 lb/ft as listed in
Table 6.1, during the 1.0% interstory drift. This strength is 51% higher than the interior planar
wall W6 and 31% higher than W8 - the interior wall with 4 ft. orthogonal end returns. The
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stiffness of the specimen, measured at 0.1% interstory drift was 6930 lb/in/ft. This stiffness is 3%
decreased from the stiffness of specimen W8. The primary failure of the specimen was the
adhesive and mechanical fastener failure along the bottom of the gypsum wallboard on the
interior of the wall.
6.2.2.2 W-PLY: Observed behavior 0-0.5% interstory drift.
Similar to the previous specimens, Specimen W-PLY maintained a relatively elastic
behavior through the 0.1% interstory drift as shown in the force-deformation behavior plot in
Figure 6.3b. An increase in the inelastic behavior began during the 0.2% interstory drift cycles
when the first visible damage occurred as a crack between the main and return walls on the
interior face of the wall. Fastener failures occurred on the interior face at 0.2% and progressed
upwards along the second interior stud on the East side of the wall during the 0.4% and 0.5%
interstory drift cycles as shown in Figure 6.10. This damage coincided with the vertical joint of
the exterior side. No visible damage occurred on the exterior face of the wall during the 0-0.5%
interstory drift cycles.
Within the 0-0.5% interstory drift cycles, the maximum out-of-plane rotation angle of the
specimen, was calculated as 0.0055 radians with corresponding displacements recorded as +0.25”
and +0.17” during the 0.5% interstory drift cycles, at the East and West end of the wall,
respectively. This rotation produces a displacement equal to +0.30” and +0.24” at the East and
West end of the specimen, respectively.
Figure 6.11 shows the horizontal displacements for the bottom of the wallboards of both
faces of the wall as compared to the specimen displacement. These graphs illustrate that the
inelastic behavior observed in the force-deformation plot during the 0.2% interstory drift cycles
was caused by adhesive failures on both sides of the wall. Prior to the adhesive failure, the
wallboards moved 0.01” or less. Following the failure, the gypsum wallboard displacement
224
increased from 0.02” to 0.15” between the 0.2% and 0.5% interstory drift cycles, while the
plywood wallboard displacement only increased from 0.02” to 0.07” within these cycles. This
difference in displacement reflects the observed damages on the interior wallboard while no
damage was observed on the exterior wallboard.
The distribution of forces to the anchor bolts and tie-downs, shown in Figure 6.12, shows
that the largest forces in occurred at the ends of the main wall in the tie downs and generally
decreased towards the inner-most anchor bolts. However, the low values of the recorded forces
in the tie down at the end of the return wall suggest that the forces did not transfer around the
corner of the wall as well as other specimens. Additionally, the figure shows that forces in the
anchor bolts increased through the 0.5% cycles even though damages occurred at the bottom of
the interior wallboard during the 0.3% interstory drift.
6.2.2.2 W-PLY: Observed behavior post 0.5% interstory drift.
During the higher interstory drift cycles, additional screws popped on the bottom panel of
the interior face of the wall and the bottom corners of the wallboard buckled. Visible damage
also occurred on the exterior side after the peak strength occurred when nails began to withdraw
from the wallboard and frame. The final damages to the specimen are shown in Figure 6.13.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0204
radians with corresponding displacements recorded as +0.80” and +0.76” during the 2.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to +0.99” and +0.97” at the East and West end of the specimen, respectively.
Figure 6.14, which shows the maximum recorded axial forces in the anchor bolts and tiedowns during the larger displacements, suggests that the forces in the anchor bolts and tie downs
increased through the 1.0% interstory drift cycles when the specimen attained its maximum
strength capacity. In the subsequent cycles, the forces in the tie downs increased through the
225
1.25% cycles where the maximum recorded forces equivalent to 42.6%, and 62.8% at the West
and East ends of the main wall, respectively, occurred. Similar to the previous cycles, the tie
down at the end of the return wall recorded low forces suggesting that the uplift forces did not
transfer around the corner.
6.2.3
W-STU: Characteristics.
Specimen W-STU is the third and final exterior wall specimen. It features exterior
sheathing of 7/8 in. three coat stucco over 5/8 in. DensGlass® wallboard and interior sheathing of
5/8 in. Type X gypsum wallboard.
The construction procedure from W-DG was used for the installation of the frame and
wallboard for this specimen. Then, following the guidelines of typical construction for stucco,
two layers of building paper, and wire lath were installed on the specimen between the wallboard
and stucco. To improve the engagement of the stucco to the frame, 3 in. long #14 hex-washerhead screws with rubber washers were installed at 4 in. on center along the edges of the wall and
7 in. on center along the studs. The chosen screw and washer combination allowed the wire lath
to sit approximately 1/8 in. away from the wallboard while the top of the screw sat approximately
3/8 in. above the wallboard. Three coats of stucco, consisting of the 3/8 in. thick scratch coat, 3/8
in. thick brown coat and 1/8 in. thick top coat, were then applied, as shown in Figure 6.15. The
scratch and brown coats were given 11 and 9 days, respectively, to cure before the next coat wall
applied. The finishing coat was given 5 days to cure before testing.
In summary, Specimen W-STU is an 8 ft. x 8 ft. wood-framed wall with no openings and
two foot L-shaped returns. The frame featured mid height blocking and Simpson Strong-tie
HDU8 tie-downs at the external ends of the main and return walls. The faces of the specimen
feature interior sheathing of gypsum wallboard and exterior sheathing of stucco applied over
226
DensGlass wallboard.
The anchor bolts and tie downs held 780 lbs to 3425 lbs of axial
pretension at the beginning of the test.
6.2.3.1 W-STU: Summary and overall behavior.
This specimen attained its maximum force capacity of 16280 lbs, or 2035 lb/ft, during the
1.0% interstory drift. The stiffness of the specimen, measured at 0.1% interstory drift was 8418
lb/in/ft. The primary failure of the specimen was adhesive and fastener failure at the bottom of
the interior face and cracking at the lower East corner of the exterior face. These damages
progressed upwards during the larger interstory drifts to include damages on the bottom foot of
the interior face and a 45 degree shear crack across the main and East return walls.
6.2.3.2 W-STU: Observed behavior 0-0.5% interstory drift.
Figure 6.3c shows the force-deformation behavior plots for the specimen. The first
visible damage occurred during the 0.2% interstory drift cycles when a hairline crack formed at
the horizontal joint between the top and bottom wallboard panels on the interior face. During the
next set of cycles (0.3%), screws along the bottom edge of the interior face began to pop and
hairline cracks formed at the joints between the main and return walls. On the exterior face, a 4
in. long crack formed at the bottom of the wall along the corner between the main and return wall.
This crack propagated an additional 5 in. during the 0.4% interstory drift cycles. However, even
with these damages, shown in Figure 6.16, the strength of the specimen had not been reached by
the end of the 0.5% interstory drift cycles.
Figure 6.17 shows the horizontal displacement time history for both faces of the wall
during the 0-0.5% interstory drift cycles. Referring to Figure 6.17a, adhesive failure occurs on
the interior face during the 0.2% interstory drift (approximately 3600s). Prior to this failure, the
gypsum wallboard moved less than 0.004”. Following the failure, the wallboard displacement
increased to match the recorded displacement of the stucco finish. Between the 0.2% and 0.5%
227
interstory drifts, the displacement of the sheathing on both faces of the wall increased from 0.03”
to 0.13”.
Figure 6.18 shows that the axial forces in the anchor bolts and tie-downs are distributed
such that the forces are the largest at the ends of the main wall and decrease as progress towards
inner-most anchor bolts. Referring to the figure, local peaks in the recorded forces occur in the
anchor bolts within the 0.2% to 0.4% cycles, which corresponds to the adhesive failures and
visible damages that occurred on the interior face of the wall during those cycles.
Within the 0-0.5% interstory drift cycles, the maximum out-of-plane rotation angle of the
specimen, was calculated as 0.0077 radians with corresponding displacements recorded as +0.32”
and +0.26” during the 0.5% interstory drift cycles, at the East and West end of the wall,
respectively. This rotation produces a displacement equal to +0.39” and +0.35” at the East and
West end of the specimen, respectively.
6.2.3.2 W-STU: Observed behavior post 0.5% interstory drift.
During the larger displacements, the damages at the bottom of the wall progressed
upwards on the interior face to include the typical buckling of the bottom corners of wallboard
and the popping of screws in the bottom foot of the wall as shown in Figure 6.19. On the exterior
face, the crack propagated into a 45 degree shear crack that spanned across the main and East
return wall, as depicted in the figure. A similar crack began forming on the West end of the main
wall but did not finish its propagation before the test was completed. Referring to Figure 6.5c,
which shows the force-deformation response plot, the maximum strength capacity of the
specimen was reached at 1.0% interstory drift.
Figure 6.20 shows that the distribution of axial forces to the anchor bolts and tie-downs
remained the same during the larger displacements. The largest forces occurred in the tie-downs
at the end of the main wall and decreased as progress towards the inner-most anchor bolts.
228
The asymmetry of the wall geometry and strength in the wall faces caused large out of
plane rotations at the top of the wall which caused cracking at the top of the interior face of the
wall. The test was ended when these forces overcame the out of plane restrictions of the test
assembly during the 1.5% interstory drift cycles.
The maximum out-of-plane rotation angle of the specimen, was calculated as 0.0259
radians with corresponding displacements recorded as +0.66” and +1.31” during the 1.5%
interstory drift cycles, at the East and West end of the wall, respectively. This rotation produces a
displacement equal to +0.88” and +1.61” at the East and West end of the specimen, respectively.
229
Table 6.1: Stiffness and strength of exterior wood-framed walls
Test Name
W1
(Typ. Construction)
W6
(Free standing)
W8
(T-shaped Returns)
W-DG
W-PLY
W-STU
Secant Stiffness
at 0.1% Drift
(lb/in/ft)
Strength
(lb/ft)
% Interstory Drift
Where Max.
Occurred
2724
520
0.5
7178
1248
0.3
11068
1446
0.4
7474
6930
9867
1085
1890
2020
0.4
1.0
1.0
Table 6.2: Anchor bolt pretension forces at beginning of test (lbs)
Specimen
W-DG
W-PLY
W-STU
1
10620
2563
1966
2
4258
1838
2128
Anchor Bolt Number
3
3802
2179
2519
4
6539
2550
2213
Table 6.3: Tie down pretension forces at beginning of test (lbs)
Specimen
W-DG
W-PLY
W-STU
East Return
North End
11530
1590
2086
Tie down Locations
Main Wall
East End
9914
2830
785
Main Wall
West End
9310
2420
1000
5
5444
2119
3421
230
(a)
(b)
(c)
Figure 6.1 Construction plans for exterior wall specimens featuring (a) the East elevation, (b) the
South elevation, and (c) plan view.
231
Figure 6.2 Cyclic backbone curve comparisons for exterior specimens.
232
Applied Force (kips)
8
4
0
-4
-8
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(a) W-DG
Applied Force (kips)
10
5
0
-5
-10
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(b) W-PLY
Applied Force (kips)
10
5
0
-5
-10
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5
Interstory Drift (%)
(c) W-STU
Figure 6.3 Force-deformation response for 0-0.5% interstory drift cycles
for exterior walls.
233
Applied Force (kips)
10
5
0
-5
-10
-2
-1
0
1
Interstory Drift (%)
2
(a) W-DG
15
Applied Force (kips)
10
5
0
-5
-10
-15
-2
-1
0
1
Interstory Drift (%)
2
(b) W-PLY
15
Applied Force (kips)
10
5
0
-5
-10
-15
-1.5
-1
-0.5
0
0.5
Interstory Drift (%)
1
1.5
(c) W-STU
Figure 6.4 Force-deformation response for 0-2.5% interstory drift cycles
for exterior walls.
234
(a)
(b)
Figure 6.5 Specimen W-DG damage illustration at 0.5% interstory drift
on (a) North (Gypsum Wallboard) face and (b) South (DensGlass® Wallboard) face.
Displacement (in)
Displacement (in)
235
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
Specimen Displacement
Gypsum Horizontal Displacement
1000
2000
3000
4000
Time (s)
(a)
5000
6000
7000
5000
6000
7000
Specimen Displacement
DensGlass Horizontal Displacement
1000
2000
3000
4000
Time (s)
(b)
Figure 6.6 Displacement time history of wallboard as compared to the frame for W-DG
(a) interior and (b) exterior wallboard horizontal displacements.
Figure 6.7 Axial forces in anchor bolts and tie downs for specimen W-DG for 0-0.5% interstory drift cycles.
236
237
(a)
(b)
Figure 6.8 Specimen W-DG damage illustration at end of test
on (a) North (Gypsum Wallboard) face and (b) South (DensGlass® Wallboard) face.
Figure 6.9 Axial forces in anchor bolts and tie downs for specimen W-DG for post 0.5% interstory drift cycles.
238
239
(a)
(b)
Figure 6.10 Specimen W-PLY damage illustration at 0.5% interstory drift
on (a) North (Gypsum Wallboard) face and (b) South (Plywood) face.
Displacement (in)
Displacement (in)
240
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
Specimen Displacement
Gypsum Horizontal Displacement
1000
2000
3000
4000
5000
Time (s)
(a)
6000
7000
8000
6000
7000
8000
Specimen Displacement
Plywood Horizontal Displacement
1000
2000
3000
4000
5000
Time (s)
(b)
Figure 6.11 Displacement time history of wallboard as compared to the frame for D-PLY
(a) interior and (b) exterior wallboard horizontal displacements.
Figure 6.12 Axial forces in anchor bolts and tie downs for specimen W-PLY for 0-0.5% interstory drift cycles.
241
242
(a)
(b)
Figure 6.13 Specimen W-PLY damage illustration at end of test
on (a) North (Gypsum Wallboard) face and (b) South (Plywood) face.
Figure 6.14 Axial forces in anchor bolts and tie downs for specimen W-PLY for post 0.5% interstory drift cycles.
243
244
(a)
(b)
(c)
(d)
(e)
Figure 6.15 Construction of W-STU (a) after Densglass® sheathing is installed,
(b) after building paper and wire lath are installed, (c) after scratch coat,
(d) after brown coat, (e) after finish coat.
245
(a)
(b)
Figure 6.16 Specimen W-STU damage illustration at 0.5% interstory drift
on (a) North (Gypsum Wallboard) face and (b) South (Stucco) face.
246
0.5
Specimen Displacement
Gypsum Horizontal Displacement
0.4
Displacement (in)
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
Time (s)
6000
7000
8000
6000
7000
8000
(a)
0.5
0.4
Displacement (in)
0.3
Specimen Displacement
Stucco Horizontal Displacement
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
1000
2000
3000
4000
5000
Time (s)
(b)
Figure 6.17 Displacement time history of wallboard as compared to the frame for
W-STU (a) interior and (b) exterior wallboard horizontal displacements.
Figure 6.18 Axial forces in anchor bolts and tie downs for specimen W-STU for 0-0.5% interstory drift cycles.
247
248
(a)
(b)
Figure 6.19 Specimen W-STU damage illustration at end of test
on (a) North (Gypsum Wallboard) face and (b) South (Stucco) face.
Figure 6.20 Axial forces in anchor bolts and tie downs for specimen W-STU for post 0.5% interstory drift.
249
250
CHAPTER 7
CONCLUSIONS AND FUTURE WORK
7.1
Summary
This thesis summarizes experimental results from twenty light-frame shear wall
specimens tested as part of the first phase of a 3-year NSF (NEES) project to determine the
required construction details to improve the stiffness, strength, and damage resistance of the
components. The specimens are representative of partition walls in residential buildings with
wood or steel studs, spaced at 16 in. on center, and gypsum sheathing to resist lateral loading.
Enhancements had the effect of increasing the stiffness and shear capacity of the specimens, most
notably by using construction adhesive between the studs and sheathing. The seismic
enhancements are part of a proposed design methodology for limited ductility residential
structures in an effort to build more damage resistant homes, reduce expensive repairs and
minimize other negative societal effects of large earthquake events.
The first eleven specimens were representative of interior walls with wood framing
members featuring variations of wall length, openings, and the inclusion of attached orthogonal
walls. The next four specimens tested represented interior walls with light-gage steel frame
featuring full-scale walls with and without attached orthogonal walls. The final three specimens
tested represented exterior wood-framed walls with varying external sheathing.
This chapter summarizes key results from this phase of testing, highlights additional
areas of work to achieve a limited ductility system, and discusses future testing plans of room
assembly tests and a full-scale two-story home at the NEES-Berkeley and San Diego testing
facilities, respectively.
251
7.1.1
Interior Wood-framed Specimens.
Table 7.1 and Figure 7.1a illustrates the stiffness, strength, and damageability results of
the interior wood-framed specimens.
The first specimen, W1, was a planar wall representative of current construction
techniques. The primary mode of failure for this specimen was fastener failure at the bottom sill
plate, which extended upwards such that majority of the bottom panel fasteners had failed. The
specimen achieved a strength of 298 lb/ft and a stiffness of 1362 lb/in/ft.
The unibody modifications to the construction procedure of the specimens – primarily the
addition of construction adhesive between the sheathing and framing members – resulted in a
32% increase in stiffness (3589 lb/in/ft) and 37% increase in strength (624 lb/ft) when compared
to the final test, W6, of an iterative series aimed to improve the racking behavior of an 8 ft. x 8 ft.
specimen. Initial damages to the seismically enhanced specimen, including the primary mode of
failure, were similar to the control specimen (W1), but propagation of the damages were delayed
until larger interstory drift demands.
Orthogonal end returns increased the stiffness by 54% (5534 lb/in/ft) and the strength by
16% (723 lb/ft) when comparing W8 (with returns) to W6 (without returns). The primary mode
of failure for the specimen was adhesive failure, followed by fastener failure at the bottom sill
plate, extending upwards such that the majority of fasteners connecting the bottom panel to the
interior studs had failed. The attached orthogonal wall led to buckling at the bottom corners of
the planar wallboard panels for high deformation levels.
Specimens W9, W10 and W11 investigated the effect of openings and aspect ratios on
the stiffness and strength of the enhanced construction partition walls, as well as the
damageability performance. Referring to Chapter 4, an accurate curve fit line (R2 = 0.99)
captured the ultimate strength capacity, Vmax, of the experimental specimens well with Vmax =
252
11.6(H/L)-1.1 where H/L is the aspect ratio of the wall. The damage of the specimens with door
openings showed that racking of the wall causes cracks to propagate from the corner of the
opening towards the upper corners of the wall. The results from the specimens with differing
aspect ratios illustrated that multiple sheathing panels along the length of the wall initiate cracks
at the vertical and horizontal joints around 0.1% to 0.2% interstory drift. However, the width of
the crack only increases to approximately 1/16 in. and does not cause fastener damage around the
locations.
7.1.2
Interior Steel-framed Specimens.
Table 7.1 and Figure 7.1b illustrates the stiffness, strength and damageability results of
the interior light-gage steel-framed specimens.
The first specimen tested with steel framing members, S1, was similar to W1 in that
adhesive was not used and the specimen was designed to demonstrate the performance of
common construction practices without seismic enhancements. Similar to the wood-framed
specimens, the primary mode of failure for S1 was fastener failure at the bottom sill plate,
extending upwards such that the majority of the bottom panel fasteners had failed. Referring to
Table 7.1, the maximum stiffness and strength for S1 was 1249 lb/in/ft and 266 lb/ft, respectively.
Referring to Figure 7.2a, while the specimen performs similar to the wood-framed control
specimen, the asymmetry of the c-shaped studs within the frame caused a notable difference in
strength between the positive and negative displacements.
The unibody enhancements for a planar wall, featured in specimen S2, improved the
strength by 82%, by achieving 484 lb/ft, and the stiffness by 87%, with 2332 lb/in/ft, over the
control test, S1. Prior to adhesive failure, the adhesive increased the elastic behavior and strength
capacity of the specimens while reducing the differences of strength between the positive and
253
negative displacements which were observed on specimen S1. Following the maximum load, the
effect of the construction adhesive decreases and the specimen behaved similar to the
traditionally constructed specimen. Interestingly, the specimens with steel framing members had
lowers strength and stiffness capacities as compared to their wood-framed counterparts, as shown
in Figure 7.2b, presumably due to the adhesive de-bonding action between the relatively smooth
steel studs and the gyp board. Thus, unlike the wood-framed specimens, less adhesive was
observed on the metal studs during the post-test inspection.
Similar to the results of the wood-framed walls, the stiffness and strength of walls
constructed with unibody enhancements increased further when applied to planar steel-framed
walls with orthogonal end returns. Specimen S4, which featured the optimized construction
procedures for a planar wall with end returns, improved the strength by 29%, with 625 lb/ft, and
the stiffness by 74%, with 4069 lb/in/ft, over the unibody planar wall. The primary damage for
the specimen was mechanical fastener and adhesive failure at the bottom of the wall that
progressed upwards to include all of the screws within the bottom foot of the specimen.
7.1.3
Exterior Wood-framed Specimens.
Table 7.1 and Figure 7.1c illustrates the stiffness, strength, and damageability results of
the exterior wood-framed specimens.
The first exterior wall, specimen W-DG, featured fiberglass mat gypsum sheathing
installed with the same procedures as the interior gypsum wallboards. This specimen attained a
maximum strength of 1085 lb/ft, a 13% decrease when compared to the planar interior wall W6.
The stiffness of the specimen was 7474 lb/in/ft, a 4% increase over the stiffness of the interior
wall. The primary failure of the specimen was adhesive and fastener failures on both faces of the
254
specimen to include the popping of the bottom screws on both sides of the wall and at the vertical
edges of wallboard on the exterior side.
The second exterior wall, specimen W-PLY, which featured plywood sheathing,
experienced a maximum strength capacity of 1890 lb/ft, which was 51% higher than the freestanding interior wall, W6, and 31% higher than interior wall with returns, W8. The stiffness of
the specimen, 6930 lb/in/ft, was 3% decreased from the stiffness of Specimen W6. The primary
failure of the specimen was the adhesive and mechanical fastener failure along the bottom of the
gypsum wallboard on the interior of the wall. Visible damage did not occur on the exterior
sheathing until the displacements after the maximum strength capacity was achieved.
The third and final exterior wall specimen, W-STU, featured an exterior sheathing of 7/8
in. thick – three-coat stucco applied over fiberglass mat gypsum sheathing. The specimen
attained a strength capacity of 2020 lb/ft, which was 62% greater than the interior planar
specimen, W6, and a stiffness of 9867 lb/in/ft which was 38% greater. The primary failure of the
specimen was adhesive and fastener failure at the bottom of the interior face and cracking at the
lower East corner of the exterior face. These damages progressed upwards during the larger
interstory drifts to include damages on the bottom foot of the interior face and a 45 degree shear
crack across the main and East return walls.
7.2
Conclusions for Limited Ductility/Unibody Construction Techniques
This research demonstrated that the strength and stiffness of planar light-framed walls
improved through the addition of construction adhesive and other construction details. To ensure
that the primary failure of the wall is adhesive and fastener failure at the bottom of the wall, these
details include blocking at the mid-height of the frame – either with 2x4 blocks or a metal strap –
and properly sized shear and uplift constraints on the ends of the wall. For wood-framed
255
specimens, the uplift constraints should include a tie down on the inside of the end studs and
straps on the outside to prevent chord rotation of the end studs. The stiffness and strength of walls
increase further when attached to intersecting (return) walls. To ensure an efficient transfer of
forces around corners, the wall intersections require corner assembly studs attached with screws
at 4 in. on center and properly sized tie downs at the ends of the main and return walls.
Through these improvements to the construction details commonly used in partition
walls, the interior walls of residences may contribute significant capacity to the lateral force
resisting system. The improved strength and stiffness of these walls will be effective within the
“unibody” construction methodology to decrease the deformation demands, displacementsensitive damage and repair cost/time.
However, the new system will present more challenges over typical construction. While
the methodology has been developed using enhanced, inexpensive procedures, these procedures
will increase the costs of materials, labor, and inspection for the construction of residences within
this methodology. For effective implementation of the limited ductility or “unibody” system,
contractors and homeowners will need to be educated on the methodology and buy into the
advantages and limitations of the system.
7.3
Future Work and Recommendations
As discussed in previous chapters, this research is the first phase of a multi-phase project
to develop efficient low-cost construction details to improve the seismic performance of lightframe residential buildings.
The results of this research will influence the geometries and
construction details of three-dimensional room assemblies for the next phase of testing on the
interaction between the shear walls and floor diaphragms. In turn, the results of room tests will
256
influence the design and construction details of the shake table testing of a two-story building in
summer of 2014.
Due to time and funding limitations, several experimental variables were not included in
the testing. These variables include variations of the specimen geometry, investigation of the
long-term effects of the proposed construction details, and the testing of more efficient materials
and procedures for interior and exterior walls. Recommendations for future studies of unibody
planar walls include:
1- Variations of the door and window openings, aspect ratios, and return wall
configurations
2- Exploration of ways to prevent or reduce cracking above openings and at the
wallboard panel joints
3- Investigate more adhesive products for bonding between the wallboards and framing;
including the gypsum wallboard to steel-framing connections and DensGlass® to
wood-framing connections
4- Further investigate construction details for stucco specimens to improve the
efficiency of the construction procedures, including the wire lath and installation
screws
5- Further exploration of the life-cycle benefits of these construction techniques
6- Determination of wall behavior after extended adhesive curing
7- The effects of building settlement on adhesive integrity
8- Determination of wall behavior after repairs have been made; including common
household repairs and repairs following a large seismic event
257
Table 7.1 Summary of interior specimen behaviors
Secant
Stiffness at
0.1% Drift
(lb/in/ft)*
Specimen
W1
Wood, Planar,
Typical Const.
W6
Wood, Planar,
Adhesive
W8
Wood, Returns,
Adhesive
S1
Steel, Planar,
Typical Const.
S2
Steel, Planar,
Adhesive
S4
Steel, Returns,
Adhesive
Strength
(lb/ft)*
% Interstory
Drift at
Maximum
Strength
% Interstory Drift at
Visible Damage (Type)
1362
298
0.5
0.1 (Screws pop)
3589
624
0.3
0.3 (Screws pop)
5534
723
0.2
0.1 (Corner joints crack)
0.2 (Screws pop)
1249
266
0.75
0.2 (Screws pop)
2332
484
0.4
0.1 (Screws pop)
4069
625
0.3
0.1 (Corner joints crack)
0.4 (Screws pop)
*Note: 1-sided stiffness and strength
Table 7.2 Summary of exterior specimen behaviors
Specimen
Secant
Stiffness at
0.1% Drift
(lb/in/ft)*
W-DG
DensGlass®
7474
Strength
(lb/ft)*
1085
% Interstory
Drift at
Maximum
Strength
% Interstory Drift at Visible
Damage (type)
0.4
0.075 (Crack at corners- Int. face)
0.3 (Screws pop- Both faces)
W-PLY
Plywood
6930
1890
1.0
0.2 (Crack at corners- Int. face)
0.3 (Screws pop- Int. face)
1.75 (Nails withdraw- Ext. face)
W-STU
Stucco +
DensGlass®
9867
2020
1.0
0.2 (Crack at corners- Int. face)
0.3 (Screws pop- Int. face)
0.3 (Crack at corners- Ext. face)
*Note: 2-sided stiffness and strength
258
(a)
(b)
(c)
Figure 7.1 Cyclic backbone curves for wood and steel specimens;
(a) Interior wood specimens, (b) interior steel specimens, and
(c) exterior wood specimens.
259
(a)
(b)
(c)
Figure 7.2 Cyclic backbone curves for interior wood and steel specimens;
(a) Typical construction specimens, (b) planar unibody specimens, and
(c) unibody specimens with returns.
260
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