Performance-Based Seismic Retrofit of Soft-Story Woodframe Buildings

Abstract

As early as 1970, the structural engineering and building safety community recognized that a large number of two-, three, and four story woodframe buildings, designed with the first floor used either for parking or commercial space, were built with readily identifiable structural deficiencies, referred to as a “soft story”. Often these buildings also have a strength deficiency when compared to the stories above, in which case they are also classified as “weak”. The majority of these multi-story woodframe buildings have large openings and few partition walls at the ground level. This open space condition results in the earthquake resistance of the first story being significantly lower than the upper stories. Thus, many of these multi-story woodframe buildings are susceptible to collapse at the first story during earthquakes. Furthermore, in-plane torsional moments and consequently rotational displacements, can be induced when the center of rigidity (i.e. the point where seismic force is resisted) of a story does not coincide with the center of mass (i.e. the point where seismic force is applied). In this case, the building experiences additional displacement due to torsional moment, which causes more damage and increases the chances of collapse.

Citation

Bahmani, Pouria; Lindt, John W. van de; Pryor, Steven E.; Mochizuki, Gary L.;Gershfeld, Mikhail; Rammer, Douglas; Tian, Jingjing; Symans, Michael D. 2014. Performance-Based Seismic Retrofit of Soft-Story Woodframe Buildings. Structure Magazine, June 2014, pp. 24-27.