Direct performance-based seismic design of strongback steel braced systems

Abstract

One of the new hybrid bracing systems which are recently proposed as a seismic retrofit of conventional concentrically braced frames (CBFs) is called the strongback system (SBS). In this system, by combining a substantially elastic vertical truss with an inelastic dissipating energy system, the occurrence of the soft-story mechanism is delayed or prevented. However, a generally accepted design method that ensures the elastic behavior of the SBS spine is not available. In fact, conventional code-based design methods underestimate the force demands in the SBS and cannot guarantee to achieve targeted performance. Here, a promising performance-based seismic design (PBSD) is used as a solution that is called yield frequency spectra (YFS). After the design system, its seismic performance is fairly compared to conventional bracing systems by using a series of nonlinear dynamic analysis. To this end, three six-story steel concentric brace frames in different configurations (chevron, Two-Story-X, and SBS) are designed using the YFS method. For performance evaluation purposes, the structures are modeled in OpenSees, and after two iterations, the design process was converged to the 98%-accurate solution. The median collapse intensity value of the YFS-based designed SBS increased by almost 20% compared to the prescriptive code-based designed one. The SBS also was found as the configuration with the highest ductility capacity and the lowest collapse probability.