Technology article examines seismic design coefficients for steel-clad wood-frame shear walls

A new technology article has been added to the Technical Resources library on ConstructionMagNet.com. The article, “Development of Seismic Design Coefficients for Steel-Clad Wood-Framed Shear Walls” examines the results of a study funded by the National Frame Building Association to test and analyze steel-clad wood-framed shear walls to develop seismic design coefficients needed by designers of post-frame buildings.

Below is a summary of the article.

-By By Khoi D. Mai, PhD, Donald A. Bender, PhD, PE, and J. Daniel Dolan, PhD, PE-

Wind loads typically control the lateral design of post-frame buildings, but in some parts of the country, seismic (or earthquake) loads can control. Steel-clad wood-framed post-frame buildings should perform very well in earthquakes because they are lightweight and ductile (steel panels and screws bend and dissipate the seismic energy). Seismic design coefficients that account for building features such as ductility are published in the code-referenced standard ANSI/ASCE [American National Standards Institute/American Society of Civil Engineers] 7-2010, but unfortunately no seismic design coefficients are published for post-frame buildings. The problem is that no cyclic load tests or analyses have been conducted on SCWF shear walls—until now. In response to this need, the National Frame Building Association funded this study to perform testing and analysis of SCWF shear walls to develop seismic design coefficients needed by designers of post-frame buildings.

Background
The U.S. Federal Emergency Management Agency developed a method to establish equivalency at the component level regarding the seismic performance of components such as shear wall segments. Moreover, International Code Council Acceptance Criterion 322 was developed to establish seismic equivalency for the specific case of nailed wood shear walls in light-frame construction. In this way, post-frame SCWF shear walls can be compared to all-wood shear walls, and if the responses to testing are similar, they can share the same seismic design coefficients that are published in ANSI/ASCE 7-2010.

Materials and Methods
A total of 18 walls of 10 configurations were tested, with three steel panel construction features: unstitched, lightly stitched, and heavily stitched. For unstitched shear wall configurations, no stitch screws were used at the panel lap seams. For lightly stitched and heavily stitched shear walls, stitch screws were used at spacing of 24 inches, and 8 inches at the lap seams, respectively. Figure 1 shows a heavily stitched shear wall with screws spaced 8 inches at the panel seams.

Results and Discussion
The majority of the shear walls tested exhibited extremely ductile behavior, which is a good thing in seismic design. These walls withstood large displacements with minor load degradation. Figure 2 shows a heavily stitched shear wall being subjected to high simulated seismic loads. The ductile behavior absorbs significant energy, thereby making the post-frame building safe in an earthquake. Design shear strengths and shear stiffness are summarized in Table 1.

Figure 1. Shear wall with heavy stitching at the steel panel overlaps. The stitch screws were spaced 8 inches apart.

Figure 1. Shear wall with heavy stitching at the steel panel overlaps. The stitch screws were spaced 8 inches apart.

Figure 2. A heavily stitched shear wall is being subjected to high simulated seismic loads, resulting in buckling. The ductile behavior allows structures to absorb energy from earthquake events, without causing building collapse.

Figure 2. A heavily stitched shear wall is being subjected to high simulated seismic loads, resulting in buckling. The ductile behavior allows structures to absorb energy from earthquake events, without causing building collapse.

The results of the tests against the AC 322 equivalency criteria are shown in Table 2. The majority of the shear walls passed the AC 322 equivalency criteria. Wall Constructions 6, 7, 10 and 14 failed the criterion regarding the displacement at 80 percent post-peak load. The primary reason was that the stitch screws that improved the initial stiffness and strength of the walls were soon ejected after reaching peak load as the holes around the stitch screws enlarged and the panels buckled during cyclic loading. In Wall Construction 11, steel panels were used on one side of the wall with oriented-strand-board panels on the other. The steel added ductility to the wall, enabling it to pass the AC 322 criteria, and the capacities of OSB-sheathed walls and steel-sheathed walls proved to be additive (the combined OSB/steel wall was 5 percent higher in design shear strength than the sum of Wall Constructions 6 and 10). The combined OSB/steel wall system appears to be an excellent choice when high seismic or wind forces must be resisted, or in cases where a shear wall needs to be reinforced to allow a large opening such as an overhead door.

Table1

table2

Conclusions
Tests on 18 walls of 10 configurations were conducted under reverse-cyclic loading to develop seismic design coefficients needed for post-frame shear walls. The test results showed that steel-clad wood-framed shear walls had high ductility, and most compared favorably with all-wood shear walls. Based on our research, we recommend the following:

  • The seismic design coefficients for those walls that passed all the AC 322 criteria can be considered equivalent to wood light-framed shear walls (response modification coefficient R = 6.5, overstrength factor Ωo = 3, and deflection amplification factor Cd = 4).
  • The unstitched constructions (Wall Constructions 1 and 2) had the greatest ductility values and easily passed all three AC 322 criteria. These wall systems would be excellent choices when light seismic or wind loads must be resisted.
  • Wall Constructions 6, 7 and 14 were lightly stitched, and some of these stitch screws were ejected during cyclic testing; therefore, these wall configurations are not recommended for use in high seismic regions but would be excellent choices for high-wind regions.
  • The combined OSB/steel wall system proved to be ductile and high strength and would be an excellent choice when high seismic or wind forces must be resisted, or in cases where a shear wall needs to be reinforced to accommodate a large door opening.

Khoi D. Mai, PhD, is a former graduate student at Washington State University, Pullman, Washington. Donald A. Bender is Weyerhaeuser Professor of Civil and Environmental Engineering, Washington State University, Pullman, Washington, and can be reached at bender@wsu.edu. J. Daniel Dolan, PhD, PE, is professor of civil and environmental engineering, Washington State University, Pullman, Washington.

Acknowledgments
Funding support from the National Frame Building Association and material donations from SFS intec Inc. and Maze Nails are gratefully acknowledged.

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