The stiffness-weakening strategy for pier base isolation in swivel T-shaped rigid frame bridges: A study on seismic mechanism and system performance

Conventional fixed-base designs for swivel T-shaped rigid-frame bridges typically concentrate seismic damage in the piers, complicating post-earthquake repairs. This study investigates a pier-base isolation strategy that repurposes the cast-in-place closure segment into a stiffness-weakened structural fuse. The structural behavior was evaluated using component-level quasi-static tests and full-bridge macroscopic numerical simulations. Experimental results demonstrate that reducing the closure zone stiffness systematically redirects inelastic damage from the main pier column to the designated sacrificial segment. System-level simulations indicate that the stiffness-weakened boundary elongates the fundamental natural period of the bridge from 0.85 s to 1.96 s, reducing the theoretical resonance risk by over 60%. Compared to a conventional fixed-base design, the proposed isolation system reduces peak pier stresses by over 55%, improves girder stress uniformity by 40%, and dissipates approximately 75% of the input seismic energy through closure zone yielding and spherical hinge friction. Concurrently, the gravity-driven rocking mechanism restricts residual displacements to one-third of those in conventional designs. Crucially, a comparative assessment against a benchmark rocking self-centering (RSC) system reveals that the proposed strategy provides equivalent superstructure capacity protection (girder moment differences < 1%) and superior initial torsional stability (fundamental torsional period of 0.280 s vs. 13.01 s for the RSC configuration). The system also achieves a 77.3% hysteretic damping ratio under longitudinal excitation, avoiding the elastic strain energy constraints inherent to unbonded tendons. By transforming a temporary construction detail into a permanent isolation mechanism, this strategy achieves the seismic performance of advanced RSC systems without introducing specialized mechanical hardware, offering a cost-effective design alternative that eliminates long-term prestress loss and associated maintenance issues.