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Effect of trapped air on wave forces on coastal bridge superstructures
J. Ocean Eng. Mar. Energy
Effect of trapped air on wave forces on coastal bridge superstructures
Mohsen Azadbakht 0 1
Solomon C. Yim 0 1
B Solomon C. Yim 0 1
0 School of Civil and Construction Engineering, Oregon State University , Corvallis, OR , USA
1 Mohsen Azadbakht
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.
Bridge superstructures; Wave; Forces; Height; Period; Trapped air effect; Uplift force
1 Introduction
Recent hurricanes have caused significant damage to coastal
bridges in the southern US coastal areas along the Gulf of
Mexico
(Padgett et al. 2008)
. Figure 1 shows the extent of
the damage to US 90 Biloxi Bay Bridge after Hurricane
Katrina in 2005. After the hurricane, extensive research studies
were conducted in recent years to examine wave forces on
coastal bridges both experimentally and numerically
(e.g.,
Douglass et al. 2006; Bradner 2008; Bradner et al. 2011;
Robertson et al. 2007a, b, 2011; Jin and Meng 2011)
. A
review of previous studies of wave forces on coastal highway
bridges indicated the importance of the trapped air between
the bridge girders on the resultant wave force on bridges
(Sheppard and Marin 2009b; Bozorgnia et al. 2010; Cuomo
et al. 2009)
. While there is a body of literature on wave forces
on bridge superstructures, few studies to date have focused
on the evaluation of the effect of trapped air.
A literature review of studies investigating different
aspects of the wave loading on coastal bridges is presented
in this section. Failure modes of the coastal bridges under
wave loading and numerical and experimental investigations
on wave loads on bridge superstructures as well as current
methods of predicting the wave loads on coastal bridges are
also examined.
Robertson et al. (2007a, b) reported inundation due to
storm surge and wind-induced wave actions as the primary
cause of damage to coastal infrastructure. The space bounded
between girders and diaphragms was defined as the potential
trapped air volume as the water level reaches the bottom of
the bridge girders.
Okeil and Cai (2008)
recommended using
partially grated deck as a possible countermeasure to reduce
loads on deck, because it would permit the trapped air to
escape.
Cuomo et al. (2009)
performed a set of experimental
studies to measure wave loading on the bridge
superstructures. They found that the quasi-static upward loads on bridge
deck were lower when decks had air vents. These air vents
were believed to be effective in reducing the load on the
bridge deck. These air vents led to higher quasi-static
downward loads on the bridge decks.
Cuomo et al. (2009)
also
reported that the compressed air pocket trapped between
girders and diaphragms could decrease the wave impact pressure,
as part of the wave energy was dissipated in compressing the
air.
Douglass et al. (2006)
provided a set of equations to
estimate horizontal and vertical wave loads on bridge decks
where each load component consists of two parts, a slowly
varying load and an impact load. Sheppard and Marin (2009a)
reported the drag, inertia, buoyancy, and slamming forces
as forces acting on a structure due to the presence of an
accelerating fluid with a free surface. Sheppard and Marin
(2009b) reported that the air trapped between girders acted
as the spring in a spring–mass system. These air pockets
reduced the magnitude of slamming force and increased
the duration of the wave force. In the experimental study,
they obtained slamming forces (...truncated)