A Pilot Research on Seismic Performance Factors for CFS Shear Walls
Andrew C. Hedric1, Noritsugu Yanagi2, Zhenhua Huang3, and Cheng Yu4
ABSTRACT
Cold-formed steel (CFS) framed shear walls with steel sheet sheathing form a codeapproved lateral force resisting system. AISI S213 and ASCE 7-05 specify that the Seismic
Performance Factors (SPFs), R and Ω, for the bearing wall building systems using light framed
walls with steel sheets be equal to 6.5 and 3, respectively. These recommended SPFs are based
upon engineering experience and comparison with other materials. This paper presents an
exploratory analysis of the SPFs using the FEMA P695 iteration procedure. The analytical model
for the CFS shear walls is based on a series of cyclic tests recently completed at the University of
North Texas. The results indicate that the existing code-recommended SPFs for the CFS shear
walls are not conservative.
INTRODUCTION
Cold-formed steel (CFS) has become an attractive construction material for multi-family
homes, mid-rise hotels, and office buildings because of its advantages of light weight, high
durability, incombustibility, and relatively lower costs compared with hot-rolled steel and
concrete masonry. The lateral force resisting system in cold-formed steel construction may use
CFS stud framed shear walls with wood structural panels or sheet steel sheathing. The North
American Standard for Cold-Formed Steel Framing - Lateral Design (AISI S213-07) provides
shear strength values for CFS framed shear walls with three type of sheathing materials: 15/32 in.
Structural 1 plywood sheathing, 7/16 in. oriented strand board (OSB), and 0.018 in. and 0.027 in.
flat steel sheet. Those published values were based on Serrette et al. (1996, 1997, and 2002).
A research project on CFS shear walls using thicker sheet steel sheathing was recently
conducted at the University of North Texas (Yu 2008). The test program at UNT investigated the
static and cyclic performances of 8 ft. × 4 ft. and 8 ft. × 2 ft. CFS framed shear walls using
single-sided 0.033 in., 0.030 in., and 0.027 in. steel sheathing; and nominal shear strength was
established for those tested shear wall configurations. The finish and insulation materials were
not installed on the test specimens.
To utilize the CFS framed shear walls in seismic zones, the seismic performance factors
including the response modification factor (R) and the over-strength (Ω) must be applied. ASCE
7-05 (2005) specified that the R and Ω for the bearing wall building systems using light framed
walls with steel sheets be equal to 6.5 and 3, respectively. These values are based upon
1
Research Assistant, Department of Engineering Technology, University of North Texas
Research Assistant, Department of Engineering Technology, University of North Texas
3
Corresponding author, Assistant Professor, Department of Engineering Technology, University of North Texas,
3940 N. Elm St. F115M, Denton, TX 76207; Email: zhenhua.huang@unt.edu; Telephone: 940-369-8148
4
Assistant Professor, Department of Engineering Technology, University of North Texas
2
1
engineering experience and comparison with other materials. This paper presents exploratory
analyses of the seismic performance factors of CFS framed shear walls with steel sheet sheathing.
The FEMA P695 (2009) iteration procedure is adopted. Four building archetypes are designed
using the UNT-tested 0.033 in. steel sheathing CFS shear walls. More analyses with 0.027 in.
and 0.030 in. thickness steel sheathing shear walls and the FEMA required peer reviews will be
performed as future researches.
SHEAR WALL TESTS AT UNT
Test Setup
The shear wall tests were conducted on a 16 ft. span 12 ft. high adaptable testing frame at
UNT. Figure 1 shows the front view of the test setup with an 8 ft.  4 ft. CFS framed shear wall.
The shear wall was bolted to the base beam and loaded horizontally on the top. The out-of-plane
displacement of the shear wall was prevented by a series of steel rollers on the front side and four
individual rollers on the back side of the wall top. The rollers also worked as a guide for the load
spread T-shape. The T-shape was attached to the top track of the wall by No. 121-½ in. hex
washer head self-drilling screws installed one pair every 3 in. The horizontal force was applied to
the T-shape by a hydraulic actuator through a lever made of structural steel tube. The testing
frame was equipped with one 35 kip hydraulic actuator with 5 in. stroke.
Figure 2 shows the dimensions of the sheathed steel framed shear wall. The framing
members were assembled using No. 8×18-1/2 in. modified truss head self-drilling screws.
Double C-shaped studs (back-to-back) were used for both boundary studs and a single C stud
was used at the center of the wall. The steel sheathing was installed on one side. Blocking,
bracing, and finish and insulation materials were not installed. A Simpson Strong-Tie S/HD10S
hold-down was attached to the tension boundary studs. For all specimens, the hold-down was
raised 1.5 in. above the flange of the bottom track.
Lateral support
Load cell
Position transducer
Hydraulic
actuator
Lever
Position transducer
Figure 1 Front view of the test setup
2
Figure 2 Dimensions of 8-ft. × 4-ft. wall assemblies
Test Method
Both the monotonic and the cyclic tests were conducted in a displacement control mode.
The procedure of the monotonic pushover tests was in accordance with ASTM E564 (2006)
“Standard Practice for Static Load Test for Shear Resistance of Framed Walls for Buildings.” A
preload of approximately 10% of estimated ultimate load was applied first to the specimen and
held for five minutes to seat all connections. After the preload was removed, an incremental
loading procedure started until failure; the load increment was approximately one-third of the
estimated ultimate load. The CUREE (Krawinkler et al. 2000) protocol, in accordance with the
method C in ASTM E2126 (2005) was chosen for the cyclic tests. The reference displacement
for CUREE protocol in this test program was chosen to be 2.5% of the wall height (2.4 in. for an
8 ft. high wall). A constant cycling frequency of 0.2 Hz was used for all the cyclic tests.
Test Results
Figure 3 shows the failure mode and the load-displacement relationship of the 8 ft. × 4 ft.
framed shear wall with 0.033 in. steel sheathing. It was found that screw pull-out was the
primary failure mode for CFS framed shear walls with steel sheathing using large screw spacing
at panel edges. The screw pull-out failure commonly occurred at the peak load, which resulted in
instant loss of shear resistance in the post-peak region.
The mechanical properties of the tested shear walls are summarized in Table 1. The
notations in the table are defined as: Vy - the yielding shear strength; Vn - the nominal shear
strength; k - the stiffness; and  - the inter-story drift limits defined as the ratio of the maximum
lateral displacement to the shear wall height. The complete test results are provided in Yu (2008).
Shear Wall
Configuration
4’×8’×0.033”-6”
Vy
(kips)
3.827
Vn
(kips)
4.224
k
(kips/in)
4.74

0.020
Hysteresis
KD+SD
85%
Table 1 Test results of the 4 ft. × 8 ft. CFS framed shear wall with 0.033 in. steel sheathing
3
.
Test 4x8x43x33-6/12-C1
1000
Applied load (plf)
500
0
-500
-1000
-3
-2
-1
0
1
Lateral displacement at top of wall (in.)
2
3
Figure 3 Failure mode and load-displacement relationship of the tested shear wall
SEISMIC PERFORMANCE FACTORS
This study analyzes the SPFs (R and Ω) of the CFS framed shear wall system with 0.033 in.
steel sheet sheathing using the FEMA P695 (2009) iteration procedure. The iteration begins with
an initial value of R = 1.5 and checks whether such designs pass the acceptance criteria. The
value R = 1.2 is finally suggested by this study. This value is much lower than the current value
of R = 6.5 in ASCE/SEI 7-05, which indicates that the current ASCE specified R-value for CFS
framed shear wall system with steel sheathing is not conservative. More analyses are required to
propose a reasonable R factor for this system.
Archetype
The first step of this iteration procedure is to develop structural system archetypes. Typical
2-story residential and 5-story commercial buildings are used to build the archetypes.
Dimensions of the typical buildings are illustrated in Figure 4. Each building is designed for both
seismic design category D upper (Dmax) and lower (Dmin) bound. Total 4 archetypes as shown in
Table 1 are utilized.
Figure 4 2-story residential and 5 story commercial buildings (FEMA P695)
Building
SDC D
Archetype
4
Configuration
2-Story Residential
5-Story Commercial
Dmin
Dmax
Dmin
Dmax
1
2
3
4
Table 2 Archetypes Used in Evaluation
Analytical Model
The second step of the iteration procedure is to develop analytical models for each of the
four archetypes. Two-dimensional nonlinear finite element models (FEM) are built using the
DRAIN-2DX (Prakash et al 1993) to simulate the structural collapse.
Figure 5 illustrates the FEM model for the two-story residential building archetypes
(archetypes 1 and 2). The CFS shear wall is modeled as a column member with a horizontal
plastic spring in the middle (Kim, LaFave, and Foutch 2005). The horizontal springs (shown as
k1 and k2 in Figure 5) are used to represent the shear resistant properties of the shear wall, which
include the yielding shear strength, the stiffness, the inter-story drift limits, and the hysteresis
behavior. Those properties in the model are decided by the UNT test results and the number of
shear wall pieces needed. The number of 4 ft. × 8 ft. CFS shear walls needed at each story for the
four designed archetypes are calculated using AISI S213-07 (2007) and ASCE/SEI 7-05 (2005)
design provisions. Table 4 shows the results for R = 1.5 and 1.2.
The m1 and m2 in the model represent the masses at the second floor and roof level, which
are caused by the seismic weights of the building. Table 3 lists the seismic weights at each story
for building archetypes 1 through 4. They are calculated using the dead load plus 25% of the live
load. The seismic weights are also assigned as the gravity loads p1 and p2 in the model to include
the P-delta effects.
Figure 5 CFS framed shear wall analytical model
Story
1
Archetype
2
3
4
5
5th
18
50
4th
36
100
3rd
2nd
1st
49
138
58
62
162
175
7
11
13
21
Table 3 4 ft. × 8 ft. CFS shear walls needed
Archetypes 1 &
2
Roof
5th Floor
4th Floor
3rd Floor
2nd Floor
Total
Archetypes 3
&4
87.36
110.72
110.72
110.72
110.72
530.24
27.3
34.6
61.9
Table 4 Design seismic weight (kips)
Nonlinear Dynamic Analyses
The third step of the iteration procedure is to perform dynamic analyses of the established
FEM models to predict the collapse capacities. Three different dynamic analyses are conducted,
including modal analysis, incremental dynamic analysis (IDA) (Luco and Cornell 1998), and
monotonic pushover analysis.
The modal analyses are performed to identify the natural frequencies, damping ratios, and
mode shapes of the building archetypes, which are needed to scale the earthquake records for the
incremental dynamic analysis. The first periods of the building archetypes with R = 1.5 and 1.2
are listed in Table 5. For the first and second nodes, 5% viscous damping values are included.
Archetype
1
2
3
4
R=1.5
R=1.2
0.4286
0.1840
0.312
0.134
0.844
0.323
0.504
0.193
Table 5 Natural periods of the building
The IDA is completed to compute the collapse capacity of each archetype, with 44 far-field
earthquake records as shown in Figure 6. FEMA355f describes the IDA procedure in-depth so it
will not be described in detail herein. Figure 7 shows the example IDA curves for the archetype 1
with R = 1.2. The spectral acceleration at collapse (defined as the spectral acceleration when the
story drift is equal to 0.02 based on test results) is computed for each of the 44 curves. Then the
collapse fragility curve is constructed, as shown in Figure 8. The median collapse intensity, SCT,
is defined as the spectral acceleration which causes 50% collapse. The ratio between the median
collapse intensity (SCT) and the Maximum Considered Earthquake intensity (SMT) (ASCE 7-05)
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is the collapse margin ratio (CMR). CMR is the primary parameter used to characterize the
collapse safety of the structures.
Monotonic pushover analyses are also performed with an inverted triangular lateral load
pattern. The system over-strength (Ω) and the global ductility capacity (μc) of each archetype are
obtained from the pushover curves as illustrated in Figure 9. The system over-strength is defined
as the ratio of the maximum base shear resistance to the design base shear. The global ductility
capacity is defined as the maximum roof drift ratio over the yielding roof drift ratio.
Figure 6 Response spectra of the 44 far-field earthquake records set
Figure 7 Example Incremental Dynamic Analyses curves
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Figure 8 Example collapse fragility curve
Figure 9 Example pushover curve
Performance Evaluation
The previous section discusses how to simulate structural collapse, compute median
collapse intensity (SCT), and calculate the collapse margin ratio (CMR). A simplified spectral
shape factor (SSF) is considered to adjust SCT to account for the spectral shape effects. The SSF
values are selected based on the building natural period (T) and the global ductility capacity (μ c).
The adjusted collapse margin ratio (ACMR) is computed by multiplying SSF and collapse
margin ratio (CMR). The ACMR is the value used to judge the performance (Pass or Fail) of a
building archetype by comparing it with an acceptable value.
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Acceptable values of adjusted collapse margin ratio (AACMR) are selected based on total
system collapse uncertainty, βTOT, and established values of acceptable probabilities of collapse.
The total uncertainties include the uncertainty in the nonlinear structural modeling, the quality of
the test data used to calibrate the element models, and the quality of the structural system design
requirements. For this project, the total uncertainty is based on the upper limit conditions, which
include an “A-Superior” quality for structural models, “A-Superior” quality of design
requirements, and “A-Superior” quality of test data. Thus, βΤΟΤ equals 0.275 for all the building
archetypes. Assuming a collapse probability of 20% for all individual archetypes, and a collapse
probability of 10% for the average of the archetype group, the AACMR of 1.26 for an individual
archetype and1.42 for the average of archetype group are used for this study.
Acceptable performance results are shown in Table 6. Based on the authors’ best
knowledge, R = 1.2 and Ω = 1.4 are suggested for the CFS framed shear wall systems with steel
sheet sheathing. The ASCE 7-05 tabular value of R = 6.5 is non-conservative. Complete FEMA
P695 analyses with 0.027 in. and 0.030 in. thickness steel sheathing shear walls, including peer
reviews, will be performed as future researches.
Arche-type ID
1
2
3
4
R
1.2
T
Stati
c
CMR
SSF
AC
MR
1.56
1.48
1.25
1.31
2.38 2.82 1.08 2.57
1.95 2.82 1.13 1.74
0.92 2.42 1.07 0.98
1.55 2.38 1.15 1.78
Table 6 Analyses results
Accep.
ACMR
10%
1.42
1.42
1.42
1.42
Pass/
Fail
Pass
Pass
Fail
Pass
CONCLUSION AND FUTURE RESEARCH
An exploratory investigation is performed to verify the SPFs of light framed building using CFS
stud framed shear walls with steel sheet sheathing. Four archetypes were analyzed following the
FEMA P695 iteration procedure. The analytical models were based on the test results of CFS
framed shear walls with 33 mil steel sheathing and No. 8 screws. The screw spacing is 6 inches
at the perimeter and 12 inches in the field of sheathing. The results indicate that the existing
SPFs in ASCE 7-05 are non-conservative for the tested CFS shear walls. Complete FEMA P695
analyses are needed to propose more realistic SPFs of the CFS shear wall structure systems.
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9
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