Abstract

Conventional braced frames are prone to storey mechanism under earthquake loading. To mitigate this drawback, researchers proposed innovative structural systems that possess an additional vertical force path; hence additional redundancy. Such examples are the Dual system that includes a backup moment resisting frame, the Self-centering rocking braced frames, the Strongback braced frames (SBF), etc. Among them, the SBFs draw researchers’ attention, but the design of strongback (elastic truss) system requires accounting on higher modes effect that demands greater forces at upper floors. Recently, researchers proposed a comprehensive design methodology for SBFs but the applications were only conducted for interior strongback (SB) location. However, the SB can also be displaced exterior to the primary system.
The main objective of this study is to refine the existing design methodology for SBF with focus on determining the optimal location for the exterior SB system. The approach estimates the forces triggered in braces and ties of SB including those generated by higher modes. Herein, the moderately ductile concentrically braced frame (MD-CBF) with asymmetric tension-compression braces was selected as the primary system. Four archetype SBF buildings with asymmetric braces and exterior SB system such as: a) SBF with SB adjacent to long inelastic brace side; b) SBF with SB adjacent to short inelastic brace side; c) SBF with SB adjacent to short and stiff inelastic brace side and d) SBF with reversed exterior SB, were used in analysis. Detailed numerical models were developed in OpenSees. All studied 4-storey SBF buildings with various external SB locations are on Site Class C in Victoria, B.C. Hence, two sets of crustal and subduction ground motions were selected.
From incremental dynamic analysis resulted that the SB location influences the seismic performance of SBF buildings. Overall, the SBF system is able to mitigate the weak-storey response, to distribute the damage uniformly along the building height, to exhibit reduced residual interstorey drift and provide large margin of safety when comparing to the conventional MD-CBF.