Experimental Seismic Collapse Study of a Full-Scale, 4-Story, Soft-Story, Wood-Frame Building: Fid-Bau Portal

Experimental Seismic Collapse Study of a Full-Scale, 4-Story, Soft-Story, Wood-Frame Building

Bahmani, Pouria / van de Lindt, John W. / Mochizuki, Gary L. / Gershfeld, Mikhail / Pryor, Steven E.

Abstract

In the San Francisco Bay Area and throughout much of California, there are a large number of wood-frame buildings with garage space at ground level, resulting in open fronts on one or two sides. This type of geometry results in a soft and weak first story, and buildings of this archetype are generally referred to as soft-story buildings. During an earthquake, these buildings are susceptible to severe damage and collapse and have been recognized as a disaster-preparedness problem. The five-university Network for Earthquake Engineering Simulation (NEES)-Soft project culminated in a series of full-scale soft-story wood-frame building tests to validate two different retrofit philosophies and included a 2-month test program encompassing four different retrofits. The building had $370 m^{2}$ of living space and was designed to be generally representative of older San Francisco Marina and Mission District construction, circa 1950s. Following the retrofit testing, which only moderately damaged the test building, retrofits were removed, repairs were conducted, and the building was nominally instrumented for testing without retrofits in place. A series of unidirectional shake table tests was conducted, beginning with the Cape Mendocino acceleration record scaled to 0.4g spectral acceleration up to two successive shakes with the Superstition Hills acceleration record scaled to 1.8g spectral acceleration. Little residual lateral displacement was observed until the last two earthquakes. The objectives of the collapse testing phase of the NEES-Soft project were to (1) observe and document the nature of the soft-story collapse mechanism and (2) quantify the collapse drift for these types of soft-story wood-frame buildings. The building collapsed at approximately 19% interstory drift of the soft story (ground floor).