Performance-based Seismic retrofit methodology of soft-story woodframe buildings with fullscale shake table test validation

Abstract

Soft-story woodframe buildings are recognizable by their large openings at the bottom story which are typically for parking and storage. In soft-story buildings the relative stiffness of the soft-story, typically the bottom story, is significantly less than upper stories due to the presence of large openings which reduce the available space for lateral force resisting system components such as shearwalls. This fact leads to large inter-story drifts and potential collapse, even before the upper stories experience significant drifts. In many cases, the distance between the center of rigidity and center of mass (i.e. eccentricity) of the soft-story is significant which develops a torsional moment in addition to the lateral force caused by the earthquake. In order to improve the performance of these at-risk buildings, two retrofit techniques were applied, namely a new Performance Based Seismic Retrofit (PBSR) method and the FEMA P-807 retrofit procedure. In the PBSR procedure the direct displacement design (DDD) methodology is used to retrofit the building such that it meets the performance criteria defined by current code, whereas, P807 retrofit procedure was applied only to the ground level to prevent collapse during moderate to large earthquakes. In the PBSR, retrofits were installed such that the building meets the performance criteria at the DBE and MCE level and its torsional response reduces to an acceptable range. The seismic performance of the retrofitted building with both PBSR and FEMA P-807 retrofit procedures was evaluated numerically and validated by testing a full-scale four-story wood-frame building that was tested in summer 2013 at NEES at UC San Diego outdoor shake table facility as part of a NEES-Soft project. These types of retrofit techniques do not only improve the performance of the soft-story building but also they can be applied to accommodate the architectural constraints of the building. Steel Special Moment Frames were used at the ground level and wood structural panels were used to retrofit the upper stories in the PBSR methodology; whereas, only steel special moment frames were used to strengthen the ground level, soft-story, of the buildings for the FEMA P-807 approach.