Seismic performance of round-ended solid bridge piers considering variable axial force

ObjectiveThis study aims to precisely quantify the influence mechanism of significant axial force variations induced by near-fault ground motions on the seismic performance of solid bridge piers with round-ended cross-sections. It seeks to address the shortcomings in current specifications and assessment methods in this field, providing a theoretical basis for performance-based design and post-earthquake evaluation. Three approaches are adopted, i.e., quasi-static test, refined numerical simulation, and theoretical analysis.MethodFirst, two specimens of round-ended piers with different shear-span ratios and reinforcement ratios were designed and fabricated. A hybrid loading protocol, which dynamically adjusted the axial force on piers according to the horizontal displacement, was constructed and implemented during test to simulate the most unfavorable load combination under earthquakes. Second, a nonlinear time-history analysis model for near-fault bridges was established on the platform OpenSees. The composite element, i.e., fiber-layered shell, was used to accurately simulate the flexural-shear deformation of piers. Seven sets of near-fault pulse-type ground motion records were input for analysis. Finally, Park-Ang damage index applicable to solid round-ended piers was modified based on experimental and numerical results. A calculation method for seismic performance indicators that considering variable axial force was proposed.ResultThe variable axial force leads to highly asymmetric performance responses in piers. The peak strength in axial force increasing direction (positive direction) is 150% of that in axial force decreasing direction (negative direction) for both specimens. Furthermore, the damage development, stiffness degradation, and residual deformation differ significantly between positive and negative directions. The numerical simulation further reveals that the energy dissipation of key hysteretic loops in piers accounts for 40%-60% of the single-cycle total, playing a dominant role in the whole-process damage. The proposed calculation method for seismic performance indicators considering variable axial force achieves a prediction accuracy of 75.6% for the pier performance level, representing 6% improvement in accuracy compared with traditional methods that ignore axial force variations.ConclusionThe axial force has a notable influence on solid round-ended piers. It is recommended to incorporate the factor of variable axial force in performance evaluation of such piers.