Overstrength in seismic design of cold-formed steel framed shear walls
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Date
2020-10-20
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Abstract
The objective of this paper is to re-assess seismic overstrength for cold-formed steel-framed shear walls in light of a recently compiled database of cold-formed steel shear wall tests and consistent with current practice established in ASCE 7 and AISI S400. AISI S400 has established a capacity-based design philosophy whereby elements that are designed to remain elastic, and/or protect the energy dissipating portions of the seismic force resisting system, are designed for an elevated strength using the overstrength multiplier. In AISI S400-15 this multiplier was established using engineering judgment on the expected maximum bounds. We recently compiled a database of over 700 monotonic and cyclic cold-formed steel framed shear wall tests, spanning walls with wood structural panel sheathing, steel sheet sheathing, and strap bracing. These tests form the primary basis for the re-evaluation of the overstrength multiplier known as the expected strength factor. Care must be taken to insure that the selected specimens from the database are consistent with the detailing requirements of AISI S400. For those tests that are valid, direct comparison of the strength in the shear wall tests versus that specified in AISI S400 are used to examine the ratio of the mean tested strength over the nominal strength from AISI S400. Finish systems on the wall can greatly influence this strength ratio and the existing data is utilized to justify a simple additive model for the impact of finish systems. Finally, an expected strength factor for CFS frames, including the effect of finished systems, is formalized and recommended for use in seismic design of CFS framed shear walls.