Effect of trapped air on wave forces on coastal bridge superstructures

Abstract

Recent hurricanes have caused significant damage to coastal bridges of southern US coastal areas along the Gulf of Mexico. Previous studies have identified trapped air between bridge girders as a significant factor in increasing wave uplift loads on coastal bridge superstructures. The objective of this study is to investigate and quantify this effect. A numerical parametric study is conducted to examine the influence of this trapped air effect on resultant wave forces under different wave conditions for a variety of bridge geometry. Numerical results show that the wave loads on a bridge deck superstructure are sensitive to the amount of trapped air between the bridge girders. The wave uplift force is found to be 57–88 %, on average, lower for a wide range of wave periods when the effect of the trapped air is neglected. In addition, the effectiveness of the presence of air vents in reducing the air pressure between girders and the wave forces is evaluated. Numerical results indicate that the vertical (uplift) wave forces acting on the bridge superstructures can be reduced by about 56 % on average using deck air vents. Numerical results of the computational analyses of the wave forces on coastal bridge superstructures are also compared to estimations of the wave force computed from design expressions in AASHTO. It is found that while AASHTO estimations of the horizontal wave force are reasonable, estimations of the vertical forces are less consistent and can vary from overly conservative for larger waves to underpredicting for smaller waves.