Seismic Retrofit of an Existing 10-Story Chevron-Braced Steel-Frame

Abstract

A seismically deficient 1-story chevron-braced frame located on the west coast of Canada is retrofitted using different schemes. The building was designed in accordance with the Canadian codes applicable in the mid-198s, prior to the implementation of the special seismic design provisions in the steel design standard. The ASCE 41 nonlinear dynamic procedure (NDP) was used to develop and validate the retrofit schemes. The performance objective was collapse prevention under seismic hazard with probability of exceedance of 2% in 5 years. Three retrofit solutions developed to satisfy the ASCE 41 component-based criteria and achieve uniform demand-to-capacity ratios over the structure height exhibited inadequate global inelastic seismic response characterized by soft-story mechanisms and structural collapse. The same behavior was observed after having increased the lateral strength of the system by 8%, clearly indicating that ASCE 41 should include acceptance criteria aimed at verifying overall seismic response. Story drift concentration and structural collapse could be avoided by combining the retrofitted braced frame with a moment frame or elastic vertical truss installed from outside along the perimeter walls. A vertical truss with a flexural hinge at midheight was found to be the most cost-effective option. The exterior frames can serve as temporary lateral system during the repair of the existing braced frame.