Seismic Vulnerability of Older Braced Frames

Abstract

Concentrically braced frames (CBFs) are broadly used as lateral-load resisting systems in buildings throughout the United States. Current state-of-the-practice is the use of special concentrically braced frames (SCBFs) where ductility under seismic loading is necessary. Prior to modern seismic codes, braced frames were designed without ductile detailing. Here these systems are termed nonseismic braced frames (NCBFs), which are essentially CBFs designed with no special detailing requirements. These may comply with older code requirements in high-seismicity regions or current code requirements in low-seismicity regions. Many are still in service throughout the United States. Prior research has focused on SCBFs, which has improved their seismic performance. In comparison, there is significant uncertainty regarding the seismic performance of NCBFs and they may be vulnerable to collapse. A study was conducted to evaluate this vulnerability. At the start, a pilot experimental test of NCBFs was conducted. These results were combined with prior analytical models developed for SCBFs to develop a preliminary model of NCBFs. The model was then used to evaluate seismic performance of an NCBF system and compare these results to an SCBF system. A 3-story prototype building was the vehicle for this comparison. The results show there is a significant seismic vulnerability and uncertainty associated with these structures, and they represent an unstudied significant seismic hazard in the United States.