Direct displacement-based design and seismic performance evaluation of post-tensioned steel-timber hybrid frames equipped with braces

Post-tensioned (PT) low-damage mass timber frames provide one of the promising lateral load resisting systems (LLRSs) for buildings to obtain both environmental benefits and seismic resilience. As an improved solution to conventional PT timber frames, PT steel-timber hybrid (PTSTH) frames equipped with braces were proposed by the authors and have been examined through experimental testing. This study further develops a seismic design method to implement this new LLRS for mass timber structures. The direct displacement-based design (DDBD) was adopted to fulfill this target. Different key aspects in the DDBD were suggested considering the characteristics of PTSTH frames. Six prototype buildings, having four, six, and eight stories and two types of braces (additional damping and stiffness (ADAS) braces and tension-only braces (TOBs)), were designed using the proposed method. Both pushover and non-linear time history analyses were conducted to examine the performance of the buildings. The results showed that all structures subjected to minor, moderate, or major earthquakes fulfilled the maximum inter-story drift (maxISD) limits for the intermediate occupancy (IO), life safety (LS), and collapse prevention (CP) levels, respectively. Using Sklar's theorem, both maxISD and residual inter-story drift (RISD) were considered when calculating the structural failure probability and reliability index. The calculated annual reliability index for PTSTH frames with braces ranged from 2.6 to 3.1 for IO, 3.3 to 3.6 for LS, and 3.6 to 3.8 for CP.