Evaluation and Retrofit of non-Capacity Designed Braced Frames

Abstract

Concentrically braced frames (CBFs) are lateral-load resisting systems that consist of diagonal braces which are framed concentrically with the intersection of either beam-to-column connections or opposing braces. Since the early 1990s, seismic design of special concentrically braced frames (SCBFs) has placed stringent design and detailing requirements on the braces, gusset plates and related connections. By contrast, CBFs designed before the early 1990's were designed to much less restrictive specifications than SCBFs. There are few tests of these older CBFs, resulting in uncertainty about their performance and about viable retrofit options. Older CBFs are common in current infrastructure, so the uncertainty in their performance poses a substantial risk. This research project seeks to address this uncertainty. A series of tests have been undertaken to investigate the response of both existing and retrofitted older CBFs. The experimental data from these tests is used to validate evaluation approaches for older CBFs and can be used for future development of numerical models. The retrofitted systems demonstrate practical methods to increase system ductility and improve seismic performance by mitigating damage. Finally, tools to aid in the seismic evaluation and retrofit of CBFs per ASCE 41 are proposed.