Bush Pledges Federal Help for Bridge Repair
As we just heard from the governor, inspectors had found some technical problems with the Interstate 35 bridge, but they did not consider those defects serious enough to justify closing it. Today, highway engineers provided more information about those inspections and the bridge's health.
NPR's Dan Charles reports.
DAN CHARLES: Bridge engineers in Minnesota and Washington, D.C., turned to their files today, studying this bridge's history, looking for clues that might have predicted the disaster.
Mr. DAN DORGAN (Director, Office of Bridges and Structures, Minnesota Department of Transportation): You know, I would say we thought we had done all we could. Obviously, something went terribly wrong.
CHARLES: That's Dan Dorgan from the Minnesota Department of Transportation, speaking at a press conference today in Minneapolis.
Mr. DORGAN: It was our busiest bridge within the state.
CHARLES: And Dorgan said if engineers were designing a bridge today to carry 140,000 cars across the Mississippi each day, they'd do it differently. For one thing, they'd do a better job of welding the steel trusses and girders together to avoid something called fatigue. That's when the pounding of trucks produces tiny cracks in steel, eventually causing a catastrophic collapse.
Mr. DORGAN: The engineering industry knew of fatigue. There were ships in World War II. There was jet aircraft that have had fatigue in the 1950s. But up until the 1960s or late 1960s, it was thought that fatigue was not a phenomenon you would see in bridges. Unfortunately, that was a wrong assumption.
CHARLES: Engineers were looking for signs of fatigue on the I-35 bridge. The state of Minnesota has been inspecting this bridge every year since 1993 instead of every two years, which is standard. And six years ago, the state ordered a special study. A team of engineers from the University of Minnesota looked at what it thought were the weakest points on the bridge and concluded that they were strong enough. Another more in-depth analysis in 2005 agreed. That study recommended that the state do one of two things - either install additional steel to reinforce the bridge or inspect the welding more frequently, at least once a year. Minnesota chose to examine the bridge more frequently.
Inspections for metal fatigue, says Dan Dorgan, are called arm's length inspections.
Mr. DORGAN: Meaning that you get - within an arm's length of the critical details you need to inspect.
CHARLES: But experts say it can be difficult to get to every spot in a bridge that demands close inspection. Jerry Hajjar is a professor of structural engineering at the University of Illinois.
Professor JERRY HAJJAR (Structural Engineering, University of Illinois): The inspection procedures are actually quite good. But, sure, it's possible that something could be missed.
CHARLES: And he says that uncertainty is going to add more pressure to replace thousands of aging bridges around the country because there are half a million bridges in America more than 20 feet long. And they are, on average, just as old as the I-35 bridge in Minneapolis.
Prof. HAJJAR: I certainly know that engineers, in general, are concerned about the old - the ageing infrastructure and, I think, in general, feel that there is much more retrofit that should be going on in the country.
CHARLES: The cost of doing this, though, is staggering. According to the American Society of Civil Engineers, it would cost $9.4 billion each year for 20 years to fix all the bridges currently considered structurally deficient or functionally obsolete.