Evaluation of Methods of Design for Strongback Braced Frames

Abstract

Strongback braced frames (SBFs) are a relatively new structural system intended to reduce structural damage during seismic events and improve resilience. SBFs combine buckling-restrained braces, which provide the primary lateral resistance and energy dissipation, with a stiff elastic spine to distribute demands across the height of the structure and prevent the formation of weak- and soft-story mechanisms. Designing the spine is challenging, as higher mode effects and partial nonlinear mechanisms have been shown to be significant. These effects, and their interaction, are not fully accounted for by standardized design methods. It is also unclear how stiff and strong the spine must be in order to achieve the desired behaviors. There are proposed procedures for designing SBFs; however, they have not been broadly evaluated, and they have not been compared. This work evaluates two proposed design procedures, the simplified modal pushover analysis (SMPA) and generalized modified modal superposition (GMMS), with a “control” procedure based on current standardized capacity design procedures. A total of nine frames were designed for three buildings using the three procedures. Nonlinear response history analyses were performed to evaluate the differences in behavior resulting from the different design methods. To determine the effect of the strength and stiffness of the strongback, the yield strength and elastic modulus of the strongback members were varied and the analyses repeated. The results of this work show that the GMMS and SMPA design procedures are generally well-calibrated and provide benefits over current standardized procedures in several ways: collapse performance is improved, and yielding in the strongback and residual drifts is reduced. The GMMS procedure results in larger members, but provides similar outcomes to the more-complicated-to-implement SMPA. The insights from this work will assist engineers when implementing these design methods and support the codification of strongback braced frames in design standards.