Response modification factor of concentrically braced frames with hexagonal pattern of braces

Abstract

This paper presents the response modification factor ‘R’ of the new bracing configuration of concentrically braced frames (CBFs). This innovative bracing system called “hexa-braced frame” is composed of hexagonal pattern of braces in which vertical structural elements connect the V and inverted-V bracings over three stories to form the hexagonal bracing configuration. FEMA P695 provides a global methodology to quantify seismic performance factor for new structural systems. Following this methodology, a set of 4-, 10- and 20-story archetypes representing low-, mid- and high-rise buildings, respectively, were used to evaluate the R factor of the hexa-braced frame. Trial values of R factor were examined through nonlinear static and dynamic analyses to satisfy acceptance criteria of the P695 methodology. The results were compared with the responses of similar X-braced frame models as the benchmark. The iterative process to determine R factor for the hexa-braced frame was performed using values of R factor, 6 and 7. Based on the performance evaluation of hexa- braced frame archetypes by measuring their collapse fragility, the value of R factor, 7 achieved the safety margin against collapse during the earthquakes. As expected, the analysis results confirmed the given value of R factor 6 for X-braced frame system in design codes (ASCE 7).