Time-varying seismic vulnerability of precast column-to-pile shaft pier assemblies

In precast column-to-pile shaft pier assemblies, it is essential to confine inelastic deformations and damage to the easily inspectable precast columns, while ensuring the pile shaft foundation remains a capacity-protected element within its elastic range. Achieving this at the initial design stage is feasible. However, corrosion and climate change may introduce progressive deterioration, potentially shifting the failure mode over time. This study develops a framework for assessing the lifetime seismic vulnerability of precast column-to-pile shaft assemblies, accounting for site-specific chloride exposure and environmental factors. A benchmark assembly was designed per current seismic standards, ensuring plastic hinge formation at the column base. Subsequently, chloride-induced corrosion damage was characterized for the column and pile shaft under historical and future climate scenarios. A seismic fragility assessment was then conducted to evaluate time-dependent damage and failure risks for the column, pile shaft, and entire pier assembly. The findings indicate that corrosion, particularly when intensified by climate change, significantly increases damage risks to pile shafts relative to precast columns, leading to a shift in the plastic hinge region from the column to the pile shaft—contrary to the original design intent. This study provides a methodological framework to address the progressive shift in failure mode from the column to the initially capacity-protected pile shaft in aging bridges.