Multihazard fragility assessment of steel-concrete composite frame structures with buckling-restrained braces subjected to combined earthquake and wind

Engineering structures may inevitably be subjected to multiple natural hazards (such as earthquakes and winds) during their life cycles. This paper presents an efficient multihazard fragility methodology based on the structural demand models. The approach is applied to two steel-concrete composite frame structures (SCCFSs), with and without buckling-restrained braces (BRBs), aiming to evaluate the effect of BRBs on controlling the structural responses and fragilities under the combined earthquake and wind loads. In total, 120 earthquake records are selected, and 120 sets of wind drag force time histories are simulated by considering the spatial variation along the height of the exemplar building. The combined "earthquake-wind" events are stochastically assembled, in which the intensities of these two hazards are modeled using the Monte Carlo simulation. The OpenSees platform is employed to calculate the dynamic responses of the SCCFSs with and without BRBs under simultaneous earthquake and wind loads. The goodness of fits of the first-, second-, and third-order polynomial in predicting the structural demand are evaluated, and the optimal polynomial is employed to generate the multihazard fragility surfaces at different damage states. The numerical results indicate that the structural responses and fragilities under the combined earthquake and wind are higher than those under an individual hazard, while the influencing extent varies with the relative intensities of these two hazards. The impact of multiple hazards and the control effect of BRBs on the structural responses and fragilities are systematically quantified and discussed in details.