A comparative performance-based seismic assessment of strongback steel braced frames

Abstract

A recently introduced structural steel braced frame known as the strongback system (SBS) is a hybrid of a conventional bracing system and an essentially elastic vertical truss. This elastic backbone is designed to distribute seismic demands uniformly over the height of the building and consequently prevent damage concentration. Recent studies on developing design procedures for SBSs revealed that prescriptive code-based methods are inadequate to estimate their seismic demands, and they cannot ensure to fulfill the performance objectives properly. Here, to properly evaluate the seismic behavior of a wide variety of the practical configurations of SBS, a comprehensive parametric study is carried out on elastic backbone configuration, rigidity, and height. To this end, a series of different SBS archetypes are designed using a direct performance-based method, and the fulfillment of the performance targets is probabilistically assessed. Their over-strength factors, ductility ratios, and collapse fragility curves are compared together and with the performance of structures using conventional system and design method. Results indicate that the archetypes with the brace intersection point of two-third of the length of the beam, namely X-2/3 configuration, have shown the largest ductility ratio and lowest collapse probability, while the archetypes with chevron configuration have shown the smallest ductility ratio and highest collapse probability. Further, by increasing the rigidity of the vertical backbone, ductility ratio and the median spectral acceleration at collapse level are improved, and SBS archetypes tended to have uniform story drift distribution patterns. Applying the direct performance-based method in six-story structures reduces the story drift ratios of SBS up to 30%.