Dissertation Defence: A performance-based framework for seismic design of circular reinforced concrete bridge columns under mainshock-aftershock scenarios

Saqib Ahmed Khan, supervised by Dr. Shahria Alam, will defend their dissertation titled “A performance-based framework for seismic design of circular reinforced concrete bridge columns under mainshock-aftershock scenarios” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Civil Engineering.

An abstract for Saqib Ahmed Khan’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Please email shahria.alam@ubc.ca to receive the Zoom link for this exam.

Abstract

Large-magnitude earthquakes are frequently accompanied by strong aftershocks that can significantly degrade the seismic performance of structures designed considering only the mainshock event. The Canadian Highway Bridge Design Code is currently the only known design standard requiring consideration of aftershock effects for seismic bridge design. However, neither the Code nor its Commentary provides a practical methodology for incorporating these effects. Since reinforced concrete (RC) bridge columns are the primary components of the seismic force-resisting system, this research develops a performance-based framework for designing circular RC bridge columns subjected to mainshock-aftershock earthquake sequences. A design of experiments comprising 144 bridge column configurations was established by varying column diameter, longitudinal reinforcement ratio, axial load ratio, and aspect ratio. Fibre-based OpenSees models were calibrated against 17 quasi-static and 5 shake-table tests before nonlinear time-history analyses using 35 mainshock-aftershock ground-motion sequences representing crustal and subduction hazards, scaled by factors of 1, 2, and 3. Fragility relationships were developed using multistripe analysis and maximum likelihood estimation. Damage characterization employed code strain limit-derived drift ratios and the modified Park and Ang damage index, while candidate intensity measures included peak ground acceleration, peak ground velocity, spectral acceleration, cumulative absolute velocity, specific energy density, and Arias Intensity. Cumulative absolute velocity and specific energy density were selected as the preferred intensity measures for crustal and subduction hazards, respectively. Mean Loss Ratio (MLR) and a weighted damage state index were evaluated as consequence measures, with MLR providing the preferred design basis. The results demonstrated that all damage states must be considered to quantify aftershock-induced performance degradation. Response Surface Methodology developed surrogate models, while Gaussian Process Kriging confirmed their predictive accuracy. A generalized consequence-equalization framework transformed conventional mainshock designs into equivalent mainshock-aftershock designs through longitudinal reinforcement modification or diameter-assisted design. An independent resilience assessment verified that the equalized designs restored and modestly improved resilience lost due to aftershock effects. This thesis provides a practical, generalized framework for the seismic design of circular RC bridge columns that explicitly accounts for mainshock-aftershock effects, establishing a rational basis for future performance-based bridge design procedures and for potential implementation in design standards.