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Making Bridges Safer

Making Bridges SaferIn the wake of the bridge collapse in Minneapolis, a Tufts engineering professor discusses the causes of the tragedy and how to prevent similar events in the future.

Medford/Somerville, Mass. [08.13.07] On Aug. 1, an eight-lane highway bridge spanning the Mississippi River in Minneapolis buckled and collapsed, killing eight people and leaving many others wondering how such a disaster could happen. According to Tufts Engineering Professor Masoud Sanayei, a leading researcher into the condition assessment of structural systems, the bridge's design as well as everyday wear-and-tear may have played a role in the tragedy. Advances in structural engineering, however, are making modern bridges safer, he said.

"There are different types of bridges and some of them benefit from modern design techniques and some of them have older design features," Sanayei, who will take over as chair of the department of civil and environmental engineering at Tufts in Sept., told WBZ News Radio 1030 AM. The 40-year-old, arch-shaped steel truss structure spanned the deep waters of the river with supports on each bank.

Sanayei, who is also coordinator of the structural engineering graduate program and director of the structures laboratory at Tufts School of Engineering, explained to WBZ News Radio that when the waters are deep and it's difficult to construct piers beneath bridges, "structural engineers use arch-shaped bridges that can distribute the loads long distances into the piers [on the sides] and into abutments." He added, "We are kind of moving away from these systems."

Newer bridge designs focus more on higher degrees of redundancy, which can help to prevent a sudden or "progressive collapse," according to Sanayei.

"As structural engineers, we want to build systems that in such situations, if they collapse, they collapse slowly," he told WBZ News Radio. "There should be early signs of collapse so people know something's happening and they can close the bridge or evacuate the building or any other type of a structure. In this sudden collapse, it takes a split second for the whole bridge to come down, and there is no way that you can save lives."

He cited Boston's Zakim Bridge as an example of the way engineers today are using different systems to better ensure bridge safety.

"We try to use high strength cables and build suspension-type bridges and cable-stayed bridges," he told WBZ News Radio. "These cables have great ductility in them, meaning that they can stretch and have some give in them. Also, because these bridges have so many cables in them, they create more or less a fail-safe system, meaning that if one cable pops or fails or if one bridge segment fails, you may be able to keep the rest of the bridge together and not have a catastrophic failure."

But in the case of the disaster in Minneapolis, factors besides design may have been at play, according to Sanayei. Everyday wear and tear caused by increased traffic is something to consider, he said.

"Demand on these bridges is increasing. Every 10 years, demand on our highway system goes up about one third," Sanayei said on the radio program. "These cyclic loads can cause what we call fatigue cracks and fatigue cracks can propagate quickly, causing member failure."

There are other factors that contribute to the weakening of bridges, including chemical deterioration caused by road salts used during the winter.

"[The de-icers] can deteriorate concrete, and as chemicals penetrate into concrete, it makes the reinforcing bars corrode," Sanayei explained. "As they rust, they expand several times in volume, and concrete cracks and falls off. Concrete can lose its strength and supporting steel members can corrode and become weaker and possibly fail."

According to Sanayei, the interstate highway system comprises approximately 600,000 bridges, 27 percent of which are structurally deficient or functionally obsolete. While the rate of deterioration still outpaces the rate of repairs, he says the government is making progress, with a goal to reduce that number to 15 percent by 2010.

"These bridges that were built in the '50s and '60s are now 40, 50 years old and they need attention," Sanayei told WBZ News Radio. He further explained to The Boston Globe that repairing old bridges will take a government investment, as will equipping new bridges with cutting edge technology to make them safer.

"We have the knowledge today where we can embed sensors into the construction, such as accelerometers, tilt meters, and strain gauges, and hook them up to real-time monitors to see if the bridge is behaving the way we think it should," he told the Globe. "These newer technologies are just emerging, but they're worth investing in."

The question is whether or not that will happen, according to Sanayei.

"This is not an engineering problem;it's a policy problem," he told the Globe. "We have the way, but it costs money. Are we, as a nation, willing to invest in our infrastructure?"

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