Performance-Based Seismic Retrofit of Soft-Story Woodframe Buildings

Abstract

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 com-ponents such as shearwalls. This leads to large inter-story drifts and potential collapse at the ground floor, be-fore 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.

Citation

Bahmani, P.; Lindt, J.W. van de; Pryor, S.E.; Mochizuki, G.L.; Gershfeld, M.; Rammer, D.; Tian, J.; Symans, D. 2013. Performance-Based Seismic Retrofit of Soft-Story Woodframe Buildings. Wood Design Focus, Volume 23, Number 4, 2013; pp. 3-9.