Capacity Design Procedure for Rocking Braced Frames Using Modified Modal Superposition Method

Rocking braced frames (RBF) are innovative structural systems that maintain near-zero residual drift through controlled rocking and a self-centering posttensioning system. The braced frame members are designed to remain elastic, with inelasticity concentrated in energy-dissipating elements. Recently developed capacity design procedures are reviewed and compared with a newly proposed method, called the modified modal superposition (MMS) method, for a set of seven RBFs using nonlinear time-history analyses. The proposed design method employs a structural model whose boundary conditions mimic the rocked configuration of the structure and reduction of the forces associated with the first-mode response determined by the base moment demand at maximum considered earthquake (MCE). The proposed MMS procedure is shown to be equally or more accurate than other capacity design procedures in estimating story shear forces and brace and column axial forces in the RBFs. A reliability study supports applying a load amplification factor of 1.3 to the MMS force demands to meet reliability targets for capacity designed force-controlled members under MCE hazard demands.