Quantification of seismic performance factors of mixed concrete/steel buildings using the FEMA P695 methodology

Abstract

The concurrent utilization of both concrete and steel stories within buildings results in the establishment of a hybrid lateral load-resistant system throughout its height. Different seismic codes do not provide specific values for the seismic factors to be used in the design of mixed concrete/steel buildings. Through these parameters, the nonlinear responses of the structure are estimated. In this study, an investigation was conducted into the seismic design parameters' quantification, encompassing the response modification factor (R), the system over-strength factor (Ωo), and the deflection amplification factor (Cd). Whether to apply the transition story or not in the transition zone between the concrete and steel sections along with considering the different ratios between the number of concrete-based stories and steel-based stories, 18 archetypes in the form of three performance groups of 7, 13, and 19 stories were studied. Pushover curves of all archetypes were achieved. Then, to evaluate the system's behavior and measure the adequacy of the assumed R factor, Incremental Dynamic Analysis (IDA) was used and fragility curves were also extracted to investigate the collapse probability of different archetypes. The results showed that the values of R = 8, Ω0 = 3, and Cd = 5.5 were suitable for the mixed concrete/steel buildings studied. Therefore, a unique R can be assigned to archetypes in different concrete-to-steel floor ratios and the two transition methods. Taking overall uncertainty into account, R= 8 was entirely adequate to satisfy the acceptance criteria of the FEMA P695 methodology.