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AWC’s 2008 Special Design Provisions for Wind and Seismic ASD/LRFD – Part 1: Overview and Changes from Previous Editions
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Copyright Materials
This presentation is protected by US and
International Copyright laws.
Reproduction, distribution, display and use
of the presentation without written
permission of AWC prohibited.
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Learning Objectives
On completion of this course, participants will:
1. Be able to understand load path basics and how it applies to wood structural design.
2. Be familiar with the significant changes between the 2005 and 2008 SDPWS.
3. Be able to identify the lateral resisting systems and understand where to obtain design specifications for these systems
4. Be able to analyze format and content within the 2008 SDPWS.
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Outline
Lateral load basics
Code acceptance of Standard
2005/2008 Wind & Seismic
Standard Overview
2008 Wind & Seismic
Standard
Frequently Asked Questions
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Load Path
 Wind
 Seismic (Earthquake) Motion
 IBC Section 1604.4 requires complete load path
Point of origin
Resisting element
Image courtesy APA – The Engineered Wood Association 2003
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Lateral Loads
 Wind Load Path
 Diaphragm SUPPORTS out‐of‐
plane walls
 In‐plane walls SUPPORT diaphragm
Windward wall
Leeward wall
Image courtesy APA – The Engineered Wood Association 2003
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Building Response Motions
Horizontal force transfer  in‐plane shear
Reaction
Vmax at edges
diaphragm
Diaphragm
load, w
Reaction
shear wall
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Building Response Motions
torsion mass center  stiffness center
mass
center
stiffness
center
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eccentricity
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Lateral Structural Elements
 Shear wall
 Drag strut (or collector)
Vertical plane
 Diaphragm
Horizontal plane
 Collector beam (or drag strut)
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Shear Wall ‐ Parts
Five parts of a shear wall
2 wood
structural
panels
wood frame
1
3
nails
5
hold downs
4
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plate
anchors
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Shear Wall ‐ Types
1. Individual Full‐Height Wall Segments
Ignore all but full‐height segments
2. Perforated Shear Walls
3. Force Transfer Shear Walls
Moves hold downs to corners
With or without force transfer around openings
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Diaphragm ‐ Types
 Rigid
 Diaphragm behaves as a fully rigid body
 diaphragm < 2shearwalls
 Inherent and accidental torsion considered in design  Flexible
 Diaphragm behaves as a series of simple beams
 diaphragm > 2shearwalls
 No torsion
 Tributary loading to vertical resisting elements (shear walls)
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Drag Struts and Collectors
“collects” diaphragm load and “drags” it back to a shear wall
occur most frequently at junction of diaphragm and shear wall Collector beams (floor plane)
Drag struts (wall plane)
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Collector Beam in Floor
For “wall continuity”
Collector beam
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Boundary nail (B.N.)
diaphragm
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Drag Strut in Wall
Usually the top plate serves as collector and diaphragm chord
Might be window or door header
Drag strut elements
Diaphragm shear
Shear wall panel
Wall shear
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Code Acceptance of Standard
 2006 IBC
 Permitted alternative to
Section 2305
 References 2005 SDPWS
 2009 IBC
 Mandatory
 References 2008 SDPWS
in Section 2305 for lateral
design and construction
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General Overview
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Scope:
“The provisions of this document cover materials, design and construction of wood members, fasteners, and assemblies to resist wind and seismic forces.“
ASD and LRFD
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General Overview
Outline
 Chapter 1: Flowchart
 Chapter 2: General Design Requirements
 Chapter 3: Members and Connections
 Chapter 4: Lateral Force Resisting Systems
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Chapter 1 – Designer Flowchart
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Chapter 2 – General Requirements
 General
 Terminology
 Notation
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Chapter 2 – Design Methodologies
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Chapter 3 ‐ Members and Connections
 Framing
 Sheathing
 Connections
 Covers out‐of‐plane wind load resistance of shear walls and diaphragms
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Chapter 3 ‐ Members and Connections
Framing – walls
 Accounts for composite action
Extension of 1.15 repetitive member factor, Cr
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Chapter 3 ‐ Members and Connections
Sheathing ‐ walls
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Chapter 3 ‐ Members and Connections
Sheathing – roof
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Chapter 4 ‐ Lateral Force‐Resisting Systems
 General
 Wood Diaphragms
 Wood Shear Walls
 Covers in‐plane wind and seismic load resistance of shear walls and diaphragms
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Wood Diaphragms
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Wood Diaphragms
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Chapter 4 ‐ Lateral Force‐Resisting Systems
• Wood Diaphragms •
•
Deflection example
2008 SDPWS Commentary
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Chapter 4 – Design Value Format
Nominal design values tabulated for diaphragms:

ASD
reduction factor (2.0)

LRFD
resistance factor  (0.80)
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Wood Diaphragms
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Chapter 4 – Nominal Design Value
Wind nominal unit shear capacity vw

IBC allowable stress design value x 2.8 Seismic nominal unit shear capacity vs

vs = vw / 1.4
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Wood Shear Walls
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Chapter 4 – Design Value Format
Nominal design values tabulated for shear walls:

ASD
reduction factor (2.0)

LRFD
resistance factor  (0.80)
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Wood Perforated Shear Walls
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Adhesive Attachment of Shear Wall Sheathing
SDPWS 4.3.6.3.1 Exception…
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Wood Shear Walls
2:1 unless va = 2(bs/h)
2:1
3:1
3½:1
1.00
0.67
0.57
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Panel Widths for Shear Walls

4.3.7.1…Panels shall not be less than 4′×8′, except at boundaries and changes in framing where 24″ is allowed unless framing members or blocking is provided at the edges of all panels.
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Chapter 4 – Nominal Design Value
Wind nominal unit shear capacity vw

IBC allowable stress design value x 2.8 Seismic nominal unit shear capacity vs

vs = vw / 1.4
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Chapter 4 – Nominal Unit Shear Capacities
ASD


Reduction factor = 2.0 Wind
Seismic
 = 0.8
LRFD



vw / 2.0 vs / 2.0
Wind
Seismic
vw x 0.8 vs x 0.8 = vw x 0.8 / 1.4
= vw x 0.57
LRFD has up to 12% strength benefit for seismic design for shear when using the following IBC load factors:


LRFD: 1.0E or 1.6W
ASD: 0.7E or 1.0W
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Chapter 4 ‐ Lateral Force‐Resisting Systems
Summing Shear Capacities

4.3.3.2 Summing Shear Capacities… For shear walls sheathed with dissimilar materials on opposite sides, the combined nominal unit shear capacity, (νsc) or (νwc), shall be either two times the smaller nominal unit shear capacity or the larger nominal unit shear capacity, whichever is greater.
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Appendix A
Nominal Shear Capacity Tables



Wood‐Frame Plywood Blocked Wood Structural Panel Diaphragms
Wood‐Frame Plywood Unblocked Wood Structural Panel Diaphragms
Wood‐Frame Plywood Shear Walls
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2008 Wind & Seismic Standard
Top Ten Changes! 1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
High load diaphragms
WSP ‐ combined uplift and shear
Unblocked shear walls
Shear wall anchorage provisions - 3x square
WSP over gypsum shear walls
Shear walls sheathed on 2 sides
Fiberboard shear wall aspect ratio adjustment
Increased strength limit for PSW
PSW shear strength equation
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High Load Diaphragms (4.2.7.1.2)
Consistent with IBC 2006
Includes apparent shear stiffness (Ga)
Table 4.2B Nominal Unit Shear Capacities for Blocked Wood-Frame Diaphragms
Utilizing Multiple Rows of Fasteners (High Load Diaphragms)
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High Load Diaphragms
4.2.7.1.2 rules for building them
Need 3" or greater nominal members
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High Load Diaphragms
Boundary and panel edge nailing details
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Combined Wind Uplift & Shear ‐ WSP
Wood structural panels (WSP)  Resist combined wind uplift and shear
 Resist tension only from wind uplift
 Alternate to metal straps
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Combined Wind Uplift & Shear ‐ WSP
Table 4.4.1 Shear & Uplift
 amount of available uplift capacity beyond shear capacity
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Combined Wind Uplift & Shear ‐ WSP
Minimum panel thickness = 7/16“
Vertical sheathing
Minimum spacing of fasteners in a row = 3“
Horizontal blocking
All shear wall types
 Individual full‐height
 Perforated
 Force‐transfer
Photo: APA
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Combined Wind Uplift & Shear ‐ WSP
Critical details
 Note minimum edge distance is 1/2"
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Combined Wind Uplift & Shear ‐ WSP
Sheathing tension splice vs. shear blocking
(for shear)
(deals with tension perp)
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Combined Wind Uplift & Shear ‐ WSP
Sheathing Splices per 4.4.1.7
Number of Stories
No Horizontal Sheathing Joint over Studs
Horizontal Sheathing Joint over Studs
Sheathing Tension Splice over Studs
One‐Story
No splice required
•Blocking to resist Resists both shear shear (4.4.1.3)
and uplift •Stud nailing to resist (4.4.1.7 Exception)
uplift (4.4.1.7(2))
Multi‐Story
Nail spacing at common horizontal framing > 3″ single row or > 6″ double row (4.4.1.7 (1) and Fig. 4H)
•Blocking to resist Resists both shear shear (4.4.1.3)
and uplift •Stud nailing to resist (4.4.1.7 Exception)
uplift (4.4.1.7(2))
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Combined Wind Uplift & Shear ‐ WSP
Multi‐story – sheets splice at band joist
Nail spacing limited to address tension perp
Combined
Uplift
Shear
6″ o.c. minimum
3″ o.c. minimum
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Combined Wind Uplift & Shear ‐ WSP
Multi‐story – sheets splice at stud mid‐height
Combined
Uplift
Shear
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Combined Wind Uplift & Shear ‐ WSP
Example: Splice over studs
 110 MPH Exposure B
 Uplift to resist: 277 plf
• Use (2.0 x 277 = 554 plf) and Table 4.4.1 below
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Combined Wind Uplift & Shear ‐ WSP
Example: Splice over studs
 Uplift too high to allow multi‐story splice to occur over band joist, so design splice over studs
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Combined Wind Uplift & Shear ‐ WSP
Example: Splice over studs
Double row 8d nails @ 4″ o.c.
4 x 12
4x8
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Combined Wind Uplift & Shear ‐ WSP
Example: Splice over studs
Single row 8d nails @ 4″ o.c.
See 4.4.1.7 (1)
4 x 12
4x8
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Combined Wind Uplift & Shear ‐ WSP
Example: Splice over studs
Additional nails (n) required above and below joint studs to resist uplift capacity (4.4.1.7(2)):
in For 8d @ 4” & 12” ASD Capacity = 324 plf > 277 plf OK
n = [324 x 16/12]/Z x C_D
= [324 x 16/12]/[67 x 1.6]
4 x 12
= 4.03 use 4 nails per stud
4x8
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Combined Wind Uplift & Shear ‐ WSP
Example: Splice over studs
Check field nailing requirements for stud supporting 4 x 12 panel below the rim joist:
Min. stud length on 4 x 12 panel below rim = 31.875”
#nails/stud = 31.875”/12” + 1 = 3.65 nails = 4 nails
Total # nails required/stud = 4 + 4 = 8 nails
Spacing: 31.875”/(8‐1) = 4.55”
4 x 12
Specify 4” o.c. in field 4x8
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Combined Wind Uplift & Shear ‐ WSP
Uplift only case
 Single or double row of fasteners
 Tension not shear  Test verified
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Combined Wind Uplift & Shear ‐ WSP
Default anchorage (4.4.1.6)
 Anchor bolts at 16" o.c. with 3" x 3" x 0.229" steel plate washer
 Plate washer within ½" of sheathing face that provides uplift
 Resist cross grain bending of bottom plate
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Shear Wall Anchorage – 3" x 3" Default
New shear wall anchorage provisions at foundation – Section 4.3.6.4.3
 3" x 3" x 0.229" steel
 slotted hole permitted
 placed within ½" of sheathing material
 automatically satisfied for 2x4 plate
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Shear Wall Anchorage – 3" x 3" Default
New shear wall anchorage provisions at foundation–
Section 4.3.6.4.3
 Exception: round washers permitted
• provided hold down appropriately sized for overturning
• Only applies to Individual Full‐
Height Wall Segments
• Limited shear wall capacities
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Unblocked Shear Walls
Nails 6" panel edge spacing Up to 2:1 aspect ratio
16' height limit
Based on cyclic testing
Shear capacity reduction
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Unblocked Shear Walls
Deflection (4.3.2.2)
 Less stiffness
 Deflection component amplified by:
v
Cub
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WSP over Gypsum Shear Walls
Section 4.3.7.2 and Table 4.3B
Consistent with IBC 2006
Shear wall with fire resistance
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WSP over Gypsum Shear Walls
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Shear Walls Sheathed on 2 Sides
IBC 2006 provisions for shear walls sheathed on two sides ‐ Table 4.3A Footnote 6
6. Where panels are applied on both faces of a
shear wall and nail spacing is less than 6" on center
on either side, panel joints shall be offset to fall on
different framing members. Alternatively, the width
of the nailed face of framing members shall be 3"
nominal or greater at adjoining panel edges and
nails at all panel edges shall be staggered.
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Shear Walls Sheathed on 2 Sides
Adjoining Panel Edge Details
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Structural Fiberboard Shear Walls
Aspect ratio adjustment for structural fiberboard shear walls  Table 4.3.4 Footnote 3
 Now permitted up to 3.5:1 based on cyclic testing
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Structural Fiberboard Shear Walls
Aspect ratio adjustment
 If < 1:1 reduce design unit shear
 Wind factor tied to strength
• at 3.5:1 wind factor = 0.78
 Seismic factor tied to equivalent stiffness for same seismic drift level
• at 3.5:1 seismic factor = 0.36 - Structural
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Structural Fiberboard Shear Walls
Structural fiberboard shear walls – Table 4.3A
 Nail size recommendation • 8d common is dropped
 4.3.7.4 increased minimum distance from panel edge at top and bottom plates to 3/4"
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PSW Increased Strength Limit
Increased strength limit for perforated shear walls ‐ Section 4.3.5.3 (3)
 1740 plf nominal seismic shear capacity
(980 plf nominal in 2005 SDPWS)  2435 plf nominal wind shear capacity
(1370 plf nominal in 2005 SDPWS)
 Tests with10d nails @ 2" o.c. edge
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PSW Shear Strength Equation
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PSW Shear Strength Equation
Section 4.3.3.5
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PSW Shear Strength Equation
Alternative to tabulated values – Section 4.3.3.5
 Allows more efficient designs • Actual area of openings
 Table requires maximum opening size
• Example: 1 door & 2 windows
• Table assumes windows are same height as door • Takes away panel shear area
Table
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Standard Nails and Cut Washers
Appendix A
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Wind & Seismic Standards
More details on changes
Wood Design Focus papers
 2005 Special Design Provisions for Wind and Seismic (SDPWS)
 2008 Special Design Provisions for Wind and Seismic
 Use of Wood Structural Panels to Resist Combined Shear and Uplift from Wind
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Frequently Asked Questions
Q: What about pneumatic nails or staples?
A: Design capacities are provided in a National Evaluation Report held by International Staple and Nail Tool Association: www.isanta.org
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Questions?
 www.awc.org
 Online eCourses
 FAQ’s
 Calculators
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