Seismic Fragility Analysis of Double-Column Bridge Piers Under Freeze–Thaw Cycles

This study investigates the influence of freeze–thaw (F–T) cycles on the seismic fragility of double-column bridge piers. Mechanical tests were conducted on standard concrete specimens subjected to 0, 25, 50, 75, and 100 F–T cycles using an HC-HDK9/F rapid freeze–thaw testing machine. The experimental results were used to calibrate and validate the applicability of the selected concrete constitutive model. A nonlinear finite element model of a double-column bridge pier was developed in the OpenSees platform, incorporating material degradation parameters corresponding to varying F–T cycles. Incremental dynamic analysis (IDA) was performed to derive seismic demand curves and quantify fragility corresponding to multiple damage states. The results indicate that the failure probability of the piers increases significantly with the number of F–T cycles, particularly for slight and moderate damage levels. In the low to moderate peak ground acceleration (PGA) range, the exceedance probabilities for slight and moderate damage states show a sharp rise, highlighting the sensitivity of early-stage damage to F–T degradation. It is worth noting that under the extreme condition of PGA = 1.0 g and 100 freeze–thaw cycles, the piers still exhibit a certain degree of redundancy against severe and complete damage, which to some extent reflects the certain rationality of the current seismic design in freeze–thaw environments. These findings underscore the robustness of current seismic design provisions in cold regions and provide theoretical and data-driven support for performance assessment, service life prediction, and maintenance planning of bridges exposed to freeze–thaw environments.