Fragility Analysis of Cold-Formed Steel Structures Under Combined Wind and Earthquake Hazards

Cold-formed steel (CFS) structures are lightweight, flexible and lightly damped, making them sensitive to both wind and seismic actions. Although wind and earthquakes can occur concurrently, the nonlinear response and fragility of CFS structures under combined wind–earthquake hazards remain insufficiently understood, and existing fragility models generally adopt a single intensity measure. This study develops refined nonlinear models of low-rise CFS structures in OpenSees using the Pinching4 hysteretic material to capture the pinching, stiffness degradation and strength deterioration of shear walls. Fluctuating wind velocity histories are generated with an autoregressive model using the Davenport spectrum as the target spectrum. Eighty natural ground-motion records are selected from the PEER database and randomly paired with 80 wind velocity histories. Nonlinear dynamic time-history and multi-hazard fragility analyses are then performed. The results show that seismic action causes marked redistribution of interstory drift along the building height, with the bottom floor governing the nonlinear response under strong ground motions. Wind–earthquake coupling produces a consistent but moderate amplification of displacement demand: the maximum displacement coupling amplification factor (CAF_disp) reaches 1.276, with a mean of about 1.08, whereas most CAF_accel values are below 1.0 (maximum 1.158). Fragility surfaces conditioned on PGA and wind speed are developed using an unconstrained second-order probabilistic demand model selected through five-fold cross-validation. Demand and exceedance probability generally increase with both intensity measures over most of the sampled domain, although localized nonmonotonicity remains near the high-PGA–high-wind boundary. Sheathing type and number of stories significantly affect structural fragility. The single-hazard envelope method is unconservative for 98.75% of the examined wind–earthquake combinations. Wind–earthquake coupling analysis is therefore recommended when PGA is moderate or high, when the basic wind speed at the site is low to moderate, or when deformation governs and the interstory drift ratio approaches its limit. The findings provide a basis for multi-hazard resilience design and fragility assessment of CFS structures.