Reason for collapse not easily apparent

Officials to examine bridge for evidence of fatigue, corrosion

Design

August 03, 2007|By Dennis O'Brien | Dennis O'Brien,Sun reporter

Corrosion, erosion, metal fatigue and ship impacts have all resulted in bridge collapses over the years -- but each disaster is rooted in a different cause.

Experts say there was nothing obviously wrong with the design or the age of the 40-year-old steel deck truss bridge that collapsed Wednesday in Minneapolis. And since the 1980s, major bridge collapses have been rare.

Many newer bridges have redundancies in their design and construction, so that if one support fails, the bridge won't collapse, experts say. While many older truss bridges, including the one in Minnesota, are not designed with such redundancies, that does not mean that they are less safe, experts say.

"Neither design is considered safer than the other," said R. Shankar Nair, a Chicago structural engineer who has designed numerous bridges, including two along interstates that span the Mississippi River.

In fact, the vast majority of the thousands of bridges in the United States do not have redundancies in their design and construction, said Arden Sigl, a civil engineer and professor of structural engineering at South Dakota State University.

"That doesn't mean in any way that these bridges are unsafe. They're designed to a high standard, and this situation is a real puzzle," he said.

"The most common catastrophic failure is when a bridge is hit by a ship, but you don't have that here," said Donald Vannoy, a professor of civil and environmental engineering at the University of Maryland who investigates bridge failures.

There are two basic support systems for major bridges over rivers: a lattice-like truss and a system of steel beams or girders strung under the bridge deck, Nair said. When bridges are meant to span longer distances, designers look at other options, such as suspension bridges.

Erosion and wear from the force of water currents, known as scour, can eat away at supports and cause a bridge to collapse, experts say. Bridges also can collapse from fatigue when steel is repeatedly stressed.

The presence of fatigue cracks was one of the reasons that officials sped up plans to reconstruct the Woodrow Wilson Bridge, a major artery that carries Interstate 95 across the Potomac River south of Washington.

"When you take a piece of steel and stress it repeatedly, it will break more easily. It's just something that's inherent in the material," Nair said.

That's what happened in 1967 when the Silver Bridge connecting Ohio and West Virginia collapsed, killing 46 people whose cars fell into the Ohio River.

But fatigue failure is very rare in large bridges: They're designed to handle heavy loads.

The Minneapolis bridge collapsed at rush hour during a week when road crews had been working on the bridge's joints, guardrails and lights. They had closed lanes overnight Tuesday and Wednesday.

But experts say that there is no way of knowing without a thorough investigation whether the work could have contributed to the collapse.

"I really have no idea what happened here," Nair said.

Investigators will be looking at a number of possible causes.

"Was it a material problem? Was it corrosion. Was there fatigue that caused cracking in some way? Those are the questions people will be asking," said Vannoy, whose investigations have included an incident in which a barge struck an Amtrak bridge in Harford County.

When a 100-foot section of the Mianus River Bridge along I-95 near Greenwich, Conn., collapsed in 1983, killing three people, investigators found corrosion in some holding pins designed to support the bridge.

But when a New York State Thruway bridge over Schoharie Creek collapsed in 1987, it was blamed on scour -- the force of currents and erosion from the creek washing up against footings that supported the bridge.

"Scour can be insidious, because you don't see it," Nair said.

In the years after the Schoharie Creek bridge collapse, states around the country, including Maryland, increased inspection requirements for bridge supports placed in waterways, Vannoy said.

"That sort of raised a red flag that we ought to take a closer look," he said.

When the cause of the Minneapolis bridge collapse is determined, it could lead to safety reforms, he said.

"It could be [that] there was something in the foundation of the structure that was different, that's at the root of this," he said. "Right now, we just don't know."

dennis.obrien@baltsun.com

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