Noncasing Brace for Seismic Retrofit Using Low Yield Point Steel: Seismic Performance, Feasibility, and Design Procedure

This study explores the possibility of using low yield point steel (LYPS) as the material for seismic retrofitting braces without casing. While buckling restrained brace has commonly been considered for LYPS applications, it is demonstrated that a more effective usage of LYPS can be achieved without the casing, which not only provides economic advantages but also facilitate a more complete utilization of the excellent ductility of LYPS. In developing design procedures for noncasing LYPS braces, calculations for stability, such as those related to compactness requirements and buckling strength, become a major concern, as seemingly too optimistic values are obtained for them when using LYPS with a particularly low yield stress. Therefore, both monotonic and cyclic loading tests were first conducted to extend the current stability-related provisions to LYPS. Cyclic test results confirmed the outstanding hysteretic performance of noncasing LYPS braces compared to those fabricated from ordinary steels. Using the results from the cyclic tests and additional test-validated numerical analyses, a nonlinear static procedure was developed for the seismic evaluation of noncasing LYPS braces. This procedure includes guides for performing a pushover analysis for the retrofitted structure and setting the acceptance criteria. The developed seismic evaluation procedure is demonstrated through a case study, in which a steel moment-resisting frame is retrofitted with noncasing bracing. Noncasing braces made of LYPS are shown to be effective in improving seismic performance, whereas those made of ordinary steel are shown to be inferior for seismic retrofit.