Seismic Vulnerability and Rehabilitation of Older Concentrically Braced Frames

Abstract eng:
Concentrically braced frames (CBFs) are one of the most common lateral-force-resisting systems in steel construction. In the US, they have been used to resist earthquake loading extensively but many were built prior to 1988, before the codification of capacity-based design and other provisions which ensure ductility. These older CBFs are considered nonductile CBFs (NCBFs), and an infrastructure review of 12 NCBFs from the US was conducted to quantify the frequency and severity of global and local deficiencies relative to special CBF (SCBF) requirements, but the consequences of these deficiencies are unclear. An experimental research program was devised to advance the understanding of NCBF brace and connection deficiencies and potential retrofit schemes. This paper presents a case study of three (3) of the 18 singlediagonal-brace NCBF specimens tested; the case study specimens all had double-angle gusset-plate connections. The results demonstrate the poor behavior of a typical NCBF and two (2) partial retrofit schemes which enhanced the deformation capacity of the system. The exceptional performance of the retrofits highlights the importance of designing schemes which do not punish yield mechanisms while mitigating vulnerable failure modes.

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