Seismic Evaluation of an Existing Low Ductility Braced Frame Building in California

Abstract

Force-based seismic evaluation techniques for low ductility braced frame buildings generally will classify a braced frame structure as a potential collapse hazard if the brace connections are not capable of developing the force capacity of the associated braces. However, anecdotal evidence from major earthquakes has shown that most steel braced frame buildings have performed to a life safety or better performance level, even when brace connections have fractured. One reason for this lies with the contribution of the gravity framing participating as partially restrained moment frames. This paper discusses the seismic evaluation of a two-story 1980's braced frame building located in California. A nonlinear static analysis in accordance with ASCE 41-06 Seismic Rehabilitation of Existing Buildings was performed, with brace connection fracture and all the gravity frames modeled as partially restrained moment frames. The analysis showed that the building was very stiff, until the brace connections fractured. Following the connection fracture, the building became more flexible, but the gravity frames had sufficient flexibility to accommodate the earthquake induced drifts without compromising the stability of the building. The analysis showed that the building met the Life Safety performance level.