Jan 31 2006
BUFFALO, N.Y. -- An earthquake engineer at the University at Buffalo has developed a new "multi-hazard" design for bridges that will make them more resistant to terrorist attacks and earthquakes.
The new structural design for bridge piers developed by Michel Bruneau, director of the Multidisciplinary Center for Earthquake Engineering Research (MCEER) at UB, will protect bridges from both seismic and blast forces, helping to keep them from collapsing in the event of earthquake or terrorist attack.
"Since many bridges are, or will be, located in areas of moderate or high seismic activity, and because many bridges are potential terrorist targets, there is a need to develop structural systems capable of performing equally well under both events," says Bruneau, a professor in the Department of Civil, Structural and Environmental Engineering in the UB School of Engineering and Applied Sciences.
Bruneau's design for bridge piers -- the columns that support the bridge superstructure -- is intended for small- and medium-sized bridges commonly constructed over major highways or across bodies of water. As targets for terrorist attacks, these bridges may not have the symbolic allure of the Brooklyn Bridge or the Golden Gate Bridge, but they could be targeted because of the potential economic disruption their collapse would cause, Bruneau says.
"There is a lot of interest in protecting large, monumental bridges. However there are other bridges that are extremely important as lifelines to large cities," he says. "Terrorists may not achieve the same symbolic satisfaction taking down one of these smaller bridges as they would a monumental bridge, but if their objective is to disturb the economy, they have more access to these bridges than the monumental bridges."
Bruneau's bridge-pier design uses corrosion-resistant steel tubes filled with concrete, but without reinforcing bars. The steel and concrete bind together, forming a composite structure, which gives the piers superior strength and ductility -- meaning the piers will bend without breaking when subjected to significant blast and seismic forces. For the bridge pier's footing, additional structural shapes are embedded in concrete to resist the large flexural (bending) forces developing at the base of the bridge piers. Most bridges built today are supported by conventional reinforced concrete columns. These columns likely would breach, leading to bridge collapse in the event of a major blast, Bruneau says.
Bruneau's bridge-pier design had been shown previously to provide adequate seismic protection, but had not been tested for blast resistance until recently. In field tests performed at the U.S. Army Corps of Engineers Research Facility in Vicksburg Miss., one-quarter scale prototypes of Bruneau's bridge piers were subjected to blast forces similar to what would occur "if someone packed their trunk with explosives and tried to blow up a bridge," Bruneau says.
Permanent bends, but no significant damage, were experienced by the bridge piers as a result of the test blasts, according to Bruneau. "However, expert opinion and results from software modeling indicate that a comparable concrete pier, reinforced with rebar, would have exhibited significant breaching of the concrete, resulting in failure of the bridge," he says.
Bruneau's bridge-pier design is intended for construction of new bridges, but future research will focus on development of retrofit variations for existing bridges, he says.
The multi-hazard attributes of Bruneau's design, which offers protection against two hazards in one design, should make it attractive to state departments of transportation looking for cost-effective solutions for new bridge construction, Bruneau says.
"There are many similarities between seismic and blast effects on bridges," he explains. "Both are rare events and both induce significant damage in the structural elements of a bridge.
"This is why we approached this design from the multi-hazard perspective. We wanted to develop a design that provides protection against both hazards, at one cost."
The Federal Highway Administration funded the research and testing of Bruneau's bridge-pier concept. Also contributing to the research were UB graduate students Diego Lopez Garcia and Shuichi Fujikura.
The bridge-pier design also could be ideal for accelerated bridge construction, Bruneau says, because a bridge's superstructure can be placed on top of the steel tubes while waiting for the concrete within the tubes to cure and gain strength. In contrast, concrete columns commonly used in new bridge construction must cure for several days before they can support the bridge superstructure.