The reliability assessment of the seismic demands of beams in chevron-braced frames and the factors affecting them

According to AISC 341-16, beams in chevron-braced frames should be designed for unbalanced forces. Several factors including the yield strength of steel materials, earthquake magnitude, the number of stories, and the details of the member connections affect the force applied to the beam. In this regard, the present study assessed the reliability of the seismic demand of beams in chevron-braced frames by considering the mentioned effective factors. First, 4-, 8-, and 12-story buildings representing low-, medium-, and high-rise buildings, respectively, were designed based on AISC 341-16. The beam-to-column connections and gusset plates were designed and modeled according to the capacity of the members. Then, one frame was selected from each building for stochastic seismic time-history analysis. For the stochastic analysis, 550 earthquake records were first chosen according to the considered characteristics of the site. Then, 100 ground motions were randomly selected and each braced frame was subjected to them. The yield and ultimate strengths of the steel materials were randomly selected based on the probability density function in each analysis. The results showed that a significant amount of the forces was generated in the beam-to-column connections contrary to the design assumptions. The results in the three frames showed that the resistance of the braced beams was on average higher than the demand forces. Moreover, the force distribution did not show an identical trend in different stories and the effect of the number of stories should also be considered in estimating the unbalanced forces of the braced beams.