Graduate Thesis Or Dissertation
 

Tsunami and hurricane wave loads on bridge superstructures

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  • This dissertation examines tsunami and hurricane wave loads on bridge superstructures. Tsunamis have caused significant damage to coastal communities in recent years. For example, the 2011 Great East Japan Earthquake and resulting Tohoku Tsunami destroyed infrastructure along the east coast of Japan including bridge superstructures. Recent hurricanes have also caused extensive damage to coastal bridges in the southern US coastal areas along the Gulf of Mexico. Several coastal highway bridges were completely destroyed and many more experienced substantial damage during Hurricane Katrina in 2005. The first part of this study examines the tsunami loads on five California and three Oregon coastal bridges. Finite element (FE) models are used to simulate the tsunami loads on these bridges. The FE model includes water and air as a two-phase gravity flow separated by a water free-surface, and a bridge superstructure modeled as a rigid body. The quantities of interest include horizontal and vertical forces and overturning moment. Simulations and analyses are conducted for two tsunami load stages: (1). initial impact and overtopping, and (2). full inundation. The first stage starts from the time when the tsunami water free-surface elevation reaches the low chord of the bridge superstructure, and the water free-surface rises and reaches the top of the bridge barrier where it overtops the bridge and flows on the bridge deck, and until the bridge is totally inundated. The second stage begins when the bridge first becomes fully inundated (i.e. end of first stage) and until all the important events: (a) the maximum tsunami water velocity, (b) the maximum tsunami momentum flux, and (c) the maximum tsunami mass flux, have occurred. In the first part of stage 1, initial impact and overtopping leads to a combination of lateral (horizontal) and uplift (upward vertical) forces. The maximum uplift force during this stage is found to occur when the tsunami water free-surface elevation reaches the top of the bridge barrier right before the water overtopping the bridge and starting to flow onto the bridge deck. The maximum tsunami horizontal and downward vertical loads are found to occur approximately simultaneously when the tsunami flow reaches the landward side of the bridge cross-section and overtops the barrier. It is observed that the time interval representing the initial impact of the tsunami on the bridge superstructure leads to the maximum horizontal force, downward vertical force, and overturning moment. The overall maximum uplift force is found to be in tsunami scenarios where the bridge superstructure is totally inundated, i.e. in stage 2, if total inundation actually occurs. A design procedure is proposed to compute the maximum horizontal and vertical forces on bridge superstructures based on the simulation results. Good agreement between numerical predictions and formula estimations of the tsunami forces is observed. The second part of this study examines the influence of trapped air on resultant wave forces under different wave conditions for a variety of bridge geometries. Both two and three-dimensional model numerical simulations are performed using a validated finite element model in which two different approaches are used to model the air. The first model is a two-phase simulation containing water and air with associated densities and equation of states while in the second model a single-phase (water only) simulation is conducted. The difference in resulting wave forces is totally attributed to presence of the trapped air. Wave uplift forces are found to be 57%-88% smaller for a wide range of wave periods when the effect of the trapped air is neglected. Moreover, the effectiveness of the presence of air vents in reducing the air pressure between girders and the resulting wave forces is evaluated. Numerical results indicate that the uplift wave forces acting on the bridge superstructures can be reduced by about 56% on the average using air vents.
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