Multimodal Capacity Design of Strongback Braced Frames: Method Development and Application

Abstract Strongback braced frames (SBFs) supplement a classic braced frame with a stiff and strong vertical steel truss, or strongback. The strongback is designed to remain essentially elastic during strong ground shaking, providing an elastic load path to more uniformly distribute drifts with building height. Although SBFs can provide enhanced performance for drift-sensitive components, their adoption in practice has been limited in part by the lack of an illustrative load path demonstrating higher-mode effects, limiting design intuition about the force demands needed to proportion the strongback components. This study proposes a multimodal capacity design (MMCD) method that uses a series of free-body diagrams to illustrate the characteristic behavior of SBFs, including estimates of the expected dynamic contributions from multiple modes. To illustrate the MMCD method, an eight-story SBF archetype with buckling-restrained braces (BRBs) was designed so that the strongback remains elastic for ground motions consistent with the maximum considered earthquake (MCE). Results illustrate the influence of the mode shape, modal combination rule, and number of modes on the ability of MMCD to estimate demands from modal pushover analysis (MPA) and nonlinear response history analysis (NRHA), both of which explicitly include higher-mode demands. Depending on the assumed mode shape, force demands estimated using MMCD were on average 0.9 to 1.7 times the median NRHA demands, providing credible force estimates comparable to those obtained from more computationally demanding methods. The free-body diagrams presented as part of MMCD can be extended to other strongback geometries and bracing configurations, providing designers with improved intuition of the force demands in SBFs and supporting the future integration of SBFs into practical design guidelines.