Parameterized fragility functions for controlled rocking steel braced frames

Parameterized fragility functions are developed for a controlled rocking steel braced frame (CRSBF) system, which link the probability of exceeding system-level limit states to ground motion intensity and design parameters such as frame aspect ratio, the initial post-tensioning force, the yield force in the fuse element and the dead load on the frame. The considered building performance levels include immediate occupancy (IO), repairability (RP), and collapse prevention (CP), which are achieved through non-exceedance of predefined response demand thresholds. Surrogate models are developed to predict the statistical distribution of global (peak transient and residual story drift) and local (posttensioned element strain and fuse deformation) response demand parameters using the ground motion intensity and CRSBF design variables as input parameters. The surrogate models are coupled with Monte Carlo simulations to develop the parameterized fragility functions, while incorporating model parameter uncertainty. The procedure is demonstrated using a 6-story CRSBF building. The results show that the IO and CP performance levels are controlled by the demand thresholds for peak transient story drifts. Among the four design parameters considered in this study, the aspect ratio had the greatest influence on the CRSBF performance. However, when measured in terms of the conditional probability of not meeting or exceeding the performance level at the maximum considered earthquake hazard level, the effect is significantly reduced for higher (better) performance levels.