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 is form a code -approved 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 on 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 comparing 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 engineering experience and comparison with other materials. This paper presents exploratory 1 Research Assistant, Department of Engineering Technology, University of North Texas2 Research Assistant, Department of Engineering Technology, University of North Texas3 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: 940-369-81484 Assistant Professor, Department of Engineering Technology, University of North Texas

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analyses on 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. 121-½ 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.

Position transducer

Lever

Hydraulicactuator

Lateral support Load cell

Position transducer

Figure 1 Front view of the test setup

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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 5five minutes to seat all connections. After the preload was removed, an incremental loading procedure started until failure; the load increment was approximately 1/3one-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 loseloss 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).

Table 1 Test results of the 4 ft. × 8 ft. CFS framed shear wall with 0.033 in. steel sheathingShear Wall

ConfigurationVy

(kips)Vn

(kips)k

(kips/in)

Hysteresis KD+SD

4’×8’×0.033”-6” 3.827 4.224 4.74 0.020 85%

Table 1 Test results of the 4 ft. × 8 ft. CFS framed shear wall with 0.033 in. steel sheathing

3

.

-3 -2 -1 0 1 2 3

-1000

-500

0

500

1000

Lateral displacement at top of wall (in.)

App

lied

load

(plf)

Test 4x8x43x33-6/12-C1

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 ifwhether 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. It 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)

Table 2 Archetypes Used in Evaluation

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Building Configuration

SDC D Archetype

2-Story Residential Dmin 1Dmax 2

5-Story Commercial Dmin 3Dmax 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. 2Two-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 2two-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 including 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 tested 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

Table 3 4 ft. × 8 ft. CFS shear walls neededStory Archetype

5

1 2 3 4

5th 18 50

4th 36 100

3rd 49 138

2nd 7 13 58 1621st 11 21 62 175

Table 3 4 ft. × 8 ft. CFS shear walls needed

Table 4 Design seismic weight (kips)Archetypes 1 &

2Archetypes 3

& 4Roof 87.36

5th Floor 110.724th Floor 110.723rd Floor 27.3 110.722nd Floor 34.6 110.72

Total 61.9 530.24 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 1st 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. for the first and second modes.

Table 5 Natural periods of the buildingArchetype 1 2 3 4

R=1.5 0.4286 0.312 0.844 0.504R=1.2 0.1840 0.134 0.323 0.193

Table 5 Natural periods of the building

The IDA is completed to compute the collapse capacity of each archetype, with a 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

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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 causing causes 50% collapse. The ratio between the median collapse intensity (SCT) and the Maximum Considered Earthquake intensity (SMT) (ASCE 7-05) 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. And tThe 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 includes 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’s 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. Further cComplete 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.

Table 6 Analyses resultsArche-type ID R Stati

c CMR T SSF AC

MRAccep. ACMR

10%

Pass/Fail

1

1.2

1.56 2.38 2.82 1.08 2.57 1.42 Pass2 1.48 1.95 2.82 1.13 1.74 1.42 Pass3 1.25 0.92 2.42 1.07 0.98 1.42 Fail4 1.31 1.55 2.38 1.15 1.78 1.42 Pass

Table 6 Analyses results

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 analysis results indicate that the existing SPFs in ASCE 7-05 are non-conservative for the tested CFS shear walls. Further cComplete FEMA P695 analyses are needed to propose more realistic SPFs of the CFS shear wall structure systems.

REFERENCES

ASTM E2126 (2007). “ Standard Test Methods for Cyclic (Reversed) Load Test for Shear Resistance of Walls for Buildings,”, American Society for Testing and Materials, West Conshohocken, PA.

AISI S213 (2007). “The North American Standard for Cold-Formed Steel Framing - Lateral Design,”, American Iron and Steel Institute, Washington, DC.

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ASCE/SEI 7-05 (2005). “Minimum Design Loads for Buildings and Other Structures,”, Structural Engineering Institute of American Society for Civil Engineers, Reston, VA.

ASTM E564-06 (2006). “Standard Practice for Static Load Test for Shear Resistance of Framed Walls for Buildings,”, American Society for Testing and Materials, West Conshohocken, PA.

FEMA P695 (2009) “Quantification of Building Seismic Performance Factors,” Federal Emergency Management Agency, Washington, D.C.

Huang, Z., and Foutch, D. “Effect of Hysteresis Type on Drift Limit for Global Collapse of Moment Frame Structures for Seismic Loads,”, Journal of Earthquake Engineering, Vol. 13, No. 7, (2009) 939-964.

Krawinkler, Parisi, Ibarra, Ayoub, and Medina (2000). “Development of a Testing Protocol for Woodframe Structures,”, Report W-02, Woodframe Project. Consortium of Universities for Research in Earthquake Engineering (CUREE), Richmond, CA.

Luco, N, and Cornell, C. A., (2000), “Effects of connection fractures on SMRF seismic drift demands,” ASCE Journal of Structural Engineering 126:127–136.

Prakash, V., Powell, G. H., and Campbell S., (1993). “DRAIN-2DX Element Description and User Guide,” University of California Press, Berkeley, CA.

Serrette, R.L., Nguyen, H., Hall, G. (1996). “Shear wall values for light weight steel framing.” Report No. LGSRG-3-96, Santa Clara University. Santa Clara, CA.

Serrette, R.L. (1997). “Additional Shear Wall Values for Light Weight Steel Framing.” Report No. LGSRG-1-97, Santa Clara University. Santa Clara, CA.

Serrette, R.L. (2002). “Performance of Cold-Formed Steel-Framed Shear Walls: Alternative Configurations,” Final Report: LGSRG-06-02, Santa Clara University. Santa Clara, CA.

Shi, S., and Foutch, D. A. (1997). “Evaluation of Connection Fracture and Hysteresis Type on the Seismic Response of Steel Buildings,”, Ph.D. Thesis, University of Illinois at Urbana-Champaign, Urbana, IL.

Yu, C. (2008). “Shear Resistance of Cold-Formed Steel Framed Shear Wall Assemblies with 0.027-.0.030-,0.033-inch Sheet Steel Sheathing,”, the 19th International Specialty Conference on Cold-Formed Steel Structures, St. Louis, MO.

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