Full-Scale Experimental Investigation of Second-Story Collapse Behavior in a Woodframe Building with an Over-Retrofitted First Story

Abstract

Soft-story woodframe buildings have been identified as a disaster preparedness problem throughout California and are present in many other states of the United States. These buildings can be readily identified by their large openings at the ground floor, often for parking, which results in a soft and weak first story that is prone to collapse in moderate to severe earthquakes. This paper presents the hybrid test results of a full-scale collapse test program that was carried out on a 3-story soft-story woodframe building with an overretrofitted first story. The overretrofitted design was constrained to the soft story only, essentially representing a retrofit that would likely drive the soft-story failure mechanism into the upper stories. The objectives of the collapse testing were to (1) quantify the collapse shift into the upper stories when the first story is overstrengthened, (2) investigate the collapse mechanisms of a woodframe building constructed with archaic building materials and style in the upper level, and (3) investigate the collapse capacity of the unretrofitted structurally deficient upper stories composed of archaic building materials. The overretrofitted design and collapse testing with results are presented in this paper. Even when overstrengthened, a very high-intensity earthquake, approximately 125–150% of the maximum considered earthquake, is required to collapse the archaic assemblies found in the upper stories of soft-story woodframe buildings. This high-intensity earthquake caused a 6.7% residual drift in the upper two stories which resulted in multiple fastener push-outs and dramatic softening (as indicated by the change in fundamental period). The collapse capacity of the archaic and unretrofitted upper two stories was approximately 20-kN lateral strength and 8% lateral drift.