During the 2011 Great East Japan Earthquake and Tsunami, numerous bridge structures were damage or destroyed. The damage to bridge systems was caused by long duration strong ground shaking, tsunami inundation forces, or both. Long duration strong ground shaking from subduction zone earthquakes and the multi-hazard scenario of combined earthquake and tsunami attack are particularly important in the Pacific Northwest (PNW) where the Cascadia Subduction Zone is located. The main objective of this thesis is to evaluate the safety and resilience of a typical PNW coastal bridge to solely long duration strong ground shaking and to the combined multi-hazard of a tsunami following an earthquake.
Bridge system design and research has predominantly focused on short duration shallow crustal earthquakes, different from the long duration subduction zone earthquake expected in the PNW. Although the peak design demands (i.e., deck drift ratio) are typically the same for crustal and subduction zone motions, the duration and number of loading cycles is not. By not considering the effects of earthquake motion duration, the potential damage to a bridge system solely from a subduction zone earthquake motion may be under-predicted. To examine the effects of earthquake motion duration, a soil-bridge model was developed in the Open Systems for Earthquake Engineering Simulation (OpenSees) framework and the soil-bridge system response was evaluated for a suite of shallow crustal and subduction zone earthquake motions. The crustal and subduction zone motions were linearly scaled to pronounce ground motion duration and minimize amplitudinal differences. It was determined that earthquake intensity parameters that do not account for earthquake motion duration are poor predictors of potential damage to soil-bridge systems when subjected to subduction zone earthquake motions.
The combined multi-hazard scenario of long duration strong ground shaking and tsunami attack extends two novel methods for determining hydrodynamic loading of the soil-bridge model. The Particle Finite Element Method (PFEM) was used to simulate impact of an idealized tsunami bore and the FEMA P-646 method was used to simulate quasi-steady state long duration tsunami loading. Originally designed for seismically induced lateral forces, the fluid-soil-bridge model was not able to resist tsunami lateral loading. Furthermore, the preceding earthquake motion had little effect on the fluid-soil-bridge models ability to resist hydrodynamic tsunami forces.