Simplified Modal Pushover Analysis to Estimate First- and Higher-Mode Force Demands for Design of Strongback-Braced Frames

Abstract

Strongback-braced frames employ an essentially elastic truss, or strongback, that defines an elastic load path to redistribute seismic demands and mitigate the formation of story mechanisms. However, unlike the forces from traditional capacity design—which assumes that inelastic response limits the earthquake-induced forces—design forces in the strongback arise from a nonlinear first mode and near-elastic higher-mode response, because the strongback is designed to remain elastic in every mode. A design method using modal pushover analysis, which combines the response from pushover analyses in multiple modes, can estimate the magnitude and distribution of the force demands in the first and higher modes, including inelastic response. The existing modal pushover analysis procedure is simplified and applied to the design of the strongback in strongback-braced frames. Accounting for only the first mode of response resulted in estimates below the 16th percentile of force demands extracted from nonlinear dynamic analysis. However, a modal pushover approach using multiple modes resulted in improved estimates able to represent both the distribution and magnitude of the peak force demands from nonlinear dynamic analysis of eight- and four-story strongback-braced frames with buckling-restrained braces, while maintaining the conceptual simplicity of nonlinear static analysis procedures.