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Engineers Go Gonzo to Bombproof Bridges
How does disaster strike for the Department of Homeland Security these days? A terrorist drives a car packed with explosives onto a major cable-supported bridge, parks it next to a crucial binding and detonates it. Splat! Paint, of all things, is everywhere -- but the cable remains intact, and the span stays standing.
Sure, stronger steel plates and trusses are primary concerns in the post-Minneapolis world of structural engineering. But as they ramp up high-tech protection against terrorism for hundreds of bridges across the country, government agencies and researchers are turning to more advanced -- and quirky -- solutions, from nanotech coatings to self-healing structures and, yes, blast-absorbing paintballs. In a research facility at Fort Knox in Kentucky, scientists with Homeland's Science and Technology Directorate are blowing up sections of bridge cable to understand how spans might fail -- and be shielded. "These bridges weren't built with the thought of a concerted adversary in mind," says Mary Ellen Hynes, the director of the research. "So we have to work with the existing bridges, and look for solutions that are practical, cheap and not too heavy."
The collapse of the I-35W bridge in Minneapolis may have been the result of maintenance problems rather than sabotage, but it gave a stunning demonstration of the vulnerability of some bridge designs -- and of America's infrastructure. "It was only a single bridge member or connector that cracked," says Abolhassan Astaneh-Asl, a professor of structural engineering at the University of California at Berkeley, "and that collapsed the whole bridge."
Following the disaster, Astaneh-Asl conducted nine days of analysis in Minneapolis as part of his two-year, National Science Foundation-funded research into how bridges respond to bomb attacks, and how they can be retrofitted to withstand the worst. "With buildings, we can protect them from car bombs by having a stand-off distance, like on Pennsylvania Avenue [in Washington, D.C.]," he says. "But bridges are for cars, so we can't do that. Our main line of defense is taken away from us."
A 2003 government study concluded that, among the 600,000-plus bridges in the U.S., about 1000 are vulnerable to attacks that would produce substantial casualties and economic disruption, with the loss of a single critical bridge or tunnel causing an impact upward of $10 billion. What's more, the latest figures from the Federal Highway Administration conclude that over one-quarter of the country's bridges are either structurally deficient or functionally obsolete. So while a successfully detonated car bomb will always cause some damage -- and possibly result in deaths -- Astaneh-Asl insists the job of engineers is to limit the scope of the damage, as opposed to preventing it altogether: "The crucial question is whether the local blast will cause global instability in the structure." There are three main types of bridge structures and, based on their method of construction and level of risk, America's bombproofers are tackling each one:
1. Suspension Bridges Many of the biggest, most inviting terrorist targets are the spans carried by one main cable, such as the Golden Gate Bridge, which packs 70,000 strands of quarter-inch-thick steel into its primary suspension. The good news, other than that the Bay Area wil get a new $6 billion, quakeproof span by 2013? Suspension bridges are already the most difficult to damage with a car bomb, since the primary cable rests so high above the roadway. Plus, the cable's flexibility makes it relatively immune to bomb blasts. "It's like if you try to hammer a rope, you're not going to do much damage," Astaneh-Asl says. -
2. Cable-Stayed Bridges With cables fanning out from a central tower to directly support the roadway, pulling both ends of the bridge deck toward the center, any small portion of a roadway that goes down could wipe out an entire bridge of this kind. Scientists saw this during the Allied bombing of Germany in World War II, where bombers were able to knock down many cable-stayed bridges even with near misses. Now Hynes's team at Fort Knox is studying ways to defend spans with a multilayered approach. They are starting with construction techniques such as an outer layer that could be a simple PVC pipe, a wrap made of Kevlar or some other strong fabric, and potential fillings such as pea gravel, pumice chips, sand or paintballs. "We want a series of layers that each does different things," she says, "and together they absorb or reflect the energy of the blast, and disrupt the coherence of the wave." Astaneh-Asl, meanwhile, sees vulnerabilities in the roadway and bridge tower themselves, and recommends strengthening the bridge tower for 20 to 30 feet above the road and 10 to 20 feet below with a jacket of steel plating -- a technique that has already been implemented for seismic protection on some California bridges.
3. Truss Bridges Scientists have bemoaned these structures since long before the steel trusses on I-35 gave way this summer. "We have hundreds of these bridges built between the 1950s and 1970s," Astaneh-Asl says, "and they're like a house of cards -- remove one piece, and you're in trouble." It is possible to retrofit these bridges, however. One approach is to add cables inside the bridge members to take some of the load; another is to wrap the bridge column in fiber-reinforced polymer composite tape to add strength. Or additional bridge members can simply be added alongside the original sections to create redundancy -- an approach that was recommended for the Minneapolis bridge by external consultants in 2006, but never implemented.
While simple approaches using steel and concrete may offer some protection, they're not always efficient to implement, which is why Hynes plans to start a new research initiative in the Department of Homeland Security next year to explore the uses of nanotechnology for bridge protection. "We're looking for blast resistance, projectile resistance and a bunch of other unique properties," she says.
One of the long-range possibilities is a bridge that can heal itself -- an idea that may sound far-fetched, but is already the focus of widespread research. A major breakthrough in self-healing materials occurred in 2001, when Scott White and his colleagues at the University of Illinois at Urbana-Champaign created a self-healing polymer containing tiny capsules of a simple monomer. When cracks appear in the material, the capsules break open, allowing the monomer to come into contact with a catalyst that turns the monomer into a polymer, filling and repairing its own cracks.
Since then, researchers have developed other self-healing approaches, and roadways and bridges are considered one possible major application. The primary goal is to seal fatigue cracks as they appear, and prevent salt and water from entering cracks and corroding the bridge, with healing timescales ranging from a few minutes to a few hours, White says. In the face of a bomb blast, self-healing action could help stabilize the rest of the structure, sealing cracks and preventing the local damage from taking out the rest of the bridge. But that would require a reaction on the scale of seconds. "There are certainly some chemistries that work ultrafast," White says, "but that's a long time down the road."