Performance-Based Seismic Retrofit of Soft-Story Woodframe Buildings

Soft-story woodframe buildings are recognizable by their large garage openings at the bottom story which are typically for parking and storage. In soft-story buildings the relative stiffness and strength of the soft-story, usually the bottom story, is significantly less than the upper stories due to the presence of large openings which reduce the available space for lateral force resisting system components such as shearwalls. This leads to large interstory drifts and potential collapse at the ground floor, before the upper stories inter-story experience significant drifts. In many cases the ground floor eccentricity, the distance between the center of rigidity and center of mass, of the soft-story is significant enough to develop considerable in-plane torsional moment in addition to the lateral force caused by the earthquake. A Performance Based Seismic Retrofit (PBSR) procedure can be used to effectively design retrofits that improve the performance of these at-risk buildings. This paper focuses on the PBSR methodology and the application and validation of this retrofit technique to a 4,000 sq. ft. full-scale four-story woodframe building tested at the University of California San Diego (UCSD) Network for Earthquake Engineering Simulation (NEES) outdoor shake table. The structure was retrofitted with various systems including a system that combined wood structural panel sheathing and Simpson Strong-Tie ® Strong Frame ® steel special moment frames. These types of retrofit techniques improve the performance of the soft-story building while accommodating existing architectural constraints of the building.