WASHINGTON, 4 August 2007 — Divers at a collapsed interstate bridge anxiously awaited sunlight early yesterday to lend some visibility before resuming their search for victims in the opaque Mississippi River, with as many as 29 people still missing.Police reported that at least five people were killed and 79 injured when the Interstate 35W bridge plummeted more than 18 meters into the river during Wednesday’s rush hour.
The medical examiner’s office confirmed early yesterday that a fifth body had been pulled from the river overnight.The authorities reviewed the safety record of the bridge, which had been designated “structurally deficient” as early as 1990. More than 70,000 bridges across the country are rated structurally deficient like the I-35W bridge, and engineers estimate repairing them all would take at least a generation and cost more than $188 billion.
Authorities cautioned not to read too much into the “structurally deficient” tag. The designation means some portions of the bridge needed to be scheduled for repair or replacement. It was not a candidate for replacement until 2020.
The collapsed bridge is one of 1,160 bridges in that category, which amounts to 8 percent of bridges in the state. Nationally, about 12 percent of bridges are labeled “structurally deficient.” Experts said Thursday that it was far too early to speculate about the specific causes of Wednesday’s bridge collapse, but in the past most bridges over rivers have failed for a handful of known reasons: weak foundations, corrosion, metal fatigue, ship collision or design error.
Catastrophic engineering failures — whether the crash of a space shuttle or the breach of a levee — usually result from a large set of problems that become an accepted risk over time and then reach a crucial point that escapes attention. Although concern was raised about cracks, some experts theorized it was no coincidence the collapse happened when workers and heavy equipment were on the bridge. The construction work involved resurfacing and maintenance on guardrails and lights, among other repairs.
Thousands of steel truss bridges were built in the interstate highway construction boom of the 1950s and 1960s because they could support large traffic loads with minimal amounts of steel. Engineers stopped designing them long ago because they require so much labor to build, require a lot of maintenance and were eclipsed by newer technologies.
The I-35W bridge was entirely supported by two main trusses, composed of many small pieces of steel bolted or welded together like a child’s Erector Set. Though it is possible to design a steel truss bridge with redundancy, the I-35W bridge was supported only by those main trusses.“A truss arch bridge is like a chain — if you try to take out one link, you lose the whole system,” Abolhassan Astaneh, a UC Berkeley professor who is an expert on such bridges, told reporters. “They are very vulnerable to instability.”Astaneh compared a steel truss system to a house of cards, which will quickly collapse if one card is pulled out.The risk was demonstrated in 1967, when a steel truss bridge over the Ohio River collapsed, killing 46 people, said Kent Harries, a civil engineer at the University of Pittsburgh.
Not long afterward, an association of highway engineers banned any new bridges that could collapse if a single element failed, so-called nonredundant structures.The I-35W bridge was completed in 1967, placing it in the last generation before safety rules were upgraded.The bridge collapse was caught on a surveillance video.
It shows the bridge’s 390-foot center span dropping straight down and within seconds the west and east sections buckling and collapsing.
“One of these four arches that they had in this bridge — if one member of one of these four arches cracks, the bridge is gone. So it is not one of the very resilient systems,” said Astaneh.
Another expert says corrosion is also a major problem. Gene Corley is an expert on why bridges and buildings collapse. He was lead investigator in the World Trade Center disaster, the Murrah building collapse in Oklahoma City, and closer to home — the Minneapolis Lake St./Marshall Ave. bridge collapse in 1989. Corley says the bridge design left it susceptible to rusting of the steel in the bridge. Corley says if the bridge did have redundant supports, it likely would not have collapsed. Nevertheless, he says bridges like these that carry some risk are not inherently dangerous as long as there are close inspections. As the nation’s highway system ages, a growing number of bridges are being classified as deficient. A recent US Department of Transportation survey found that 27% of the nation’s bridges are structurally deficient — the I-35W bridge carried that designation for 17 years. A number of bridges collapse each year in the United States, though seldom as dramatically as occurred Wednesday. Of the roughly 500,000 bridges in the nation that span water, about 1,500 collapsed between 1966 and 2005, according to Jean-Louis Briaud, a Texas A&M University civil engineer and expert in bridge failure. The largest portion of those collapses, about 900, occurred when rivers scoured the foundations of support piers, he said. A few dozen deaths occur each year when bridges collapse. Briaud would not rule out scour in the I-35W collapse, even though the piers rest on the river banks and the center span covered the entire width of the river. Scour could still have occurred on the banks and caused the piers to slowly tilt over time, he said. The collapse will most likely trigger a renewed examination of the deteriorating US highway system.
