SSI-Bridge 2: Soil-Bridge Interaction During Long-Duration Earthquake Motions

Subduction zone earthquakes are characterized by their large magnitudes, which produce many cycles of strong ground shaking, and thus, long-duration earthquake motions. The Pacific Northwest is prone to subduction zone earthquake motions. Furthermore, many of the coastal bridges in the Pacific Northwest, which will be affected by strong ground shaking, are not designed for seismic loading. The 2011 Great East Japan Earthquake, during which notable damage to coastal bridges occurred, serves as motivation for this study. Herein, a model of a prototypical Pacific Northwest bridge is developed in the finite element framework OpenSees. The modeling considers shaking in two bridge directions, longitudinal and transverse, as well as two site-soil conditions, liquefiable and nonliquefiable. The bridge models are subjected to strong shaking with a suite of 46 subduction zone earthquake motions and 48 shallow crustal earthquake motions. Damage is tracked with two demand parameters: (1) the number of inelastic excursions (NIE), which tracks how many times the bridge column was demanded into its inelastic zone during the strong shaking; and (2) the cumulative plastic rotation (CPR), which accounts for the accumulation of damage in the bridge column during the strong shaking. The results show that subduction zone earthquake motions had much higher NIEs and CPRs compared to the shallow crustal earthquake motions. Furthermore, the earthquake motion intensity parameters that incorporate duration better predicted the NIE and CPR compared to the intensity measures that only incorporated amplitudinal intensity. The prediction of the NIE and CPR for durational dependent earthquake motion intensity parameters was validated with correlation coefficients. The liquefiable site-soil conditions were found to cause a decrease in the NIE and CPR compared with the non-liquefiable site-soil conditions. The difference in damage was attributed to the liquefiable site-soil condition fundamentally changing the earthquake motion and leading to the lengthening of the fundamental period of the soil-bridge systems. The results of the research imply that to increase the safety of our transportation network in the Pacific Northwest, engineers should consider earthquake motion intensity parameters and demand parameters that include the effects of earthquake motion duration when performing seismic design of bridges.Pacific Northwest Transportation Consortiu