Comparing Performance of Strongback and BRB Braced Frames Under Seismic Sequence

One of the most popular lateral load resisting systems is the concentric bracing. However, despite the unique advantages of the system, it has irregular and unstable hysteresis cycles because of differences in compressive and tensile strength. Hence, many studies are devoted to improve these braces to achieve an ideal symmetric elastoplastic behavior which has resulted in construction of Buckling-Restrained Braces (BRB). BRBs, although have a much better seismic performance than ordinary bracing, have a main disadvantage (similar to conventional bracing) of producing large nonlinear displacements due to their low stiffness, and consequently they have potential to form soft story. Recently, Strongback Bracing System (SBS), which is a combination of a zip system and an elastic truss system, has been introduced. SBS actually includes two types of features: a rigid elastic truss to tie story drifts over the height of the structure, and a conventional bracing system to dissipate energy. Therefore, this system can prevent or delay probability of soft story by controlling the distribution of floor displacements and the nonlinear demand of the structure. However, the studies conducted on this system are limited, and to the best of the author’s knowledge performance of this system with BRB configuration under seismic sequences is not yet investigated. In this paper, seismic performance of the SBS system is investigated and compared with the BRB one. First, behavior of these systems are studied under main shock. Next, seismic sequences are applied on the structures to better understand the behavior of SBS frames compared to BRB. For this purpose, three 4- 8- and 12-story frames were designed with two SBS and BRB systems. BRB elements were used as inelastic braces of SBS system. Nonlinear static analysis was conducted to evaluate the seismic parameters of the structures such as response modification factor and overstrength factor. Also, nonlinear time-history analysis was performed to find maximum and residual response of the structures. In the next step, a fragility analysis was conducted using IDA to estimate performance of the structures under mainshock and seismic sequence for different performance levels. 3 performance levels were selected for SBS and 4 performance levels for BRB which show the elastic to global collapse of the structures. The results of static analysis showed that the SBS system has a uniform distribution of displacement in the height of the structure, which prevents the formation of soft story. In all analyses, SBS showed a superior performance, especially in 4 story structure. Also, SBS frames showed higher response modification and overstrength factors. Results of dynamic analysis showed that the 4-story SBS structure was much less vulnerable to seismic sequences compared to the BRB one. However, the performance of SBS system decreases with increase in the height of the structures, such that 12-story frame experienced large deformations and collapsed under lower seismic demands than BRB frame. This was due to buckling of some elements in rigid truss which led to concentration of demands in these elements. Therefore, more stringent provisions are needed for design of taller structures with SBS system.