MINNEAPOLIS — A house of cards is how some engineers describe the steel truss system used on the Interstate 35W bridge that collapsed into the Mississippi River, saying that almost any piece of the complex design that failed would have brought down the entire span.
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.
For The Record
Los Angeles Times Saturday, August 25, 2007 Home Edition Main News Part Page News Desk 2 inches; 59 words Type of Material: Correction
Deficient bridges: An Aug. 3 article in Section A about bridge design incorrectly said that 27% of the nation's bridges were classified as structurally deficient by the U.S. Department of Transportation. That statistic includes bridges classified as functionally obsolete. The article also said a growing number of the nation's bridges were being classified as deficient. That number is declining.
The bridge, leading into and out of downtown Minneapolis, was being closely watched by transportation officials after studies warned about its deteriorating condition. Six years ago, significant corrosion and cracking were discovered under the roadway, but a top expert in metal fatigue concluded that they did not pose a safety hazard.
The I-35W collapse also occurred during a program to resurface the roadway, another red flag to many experts who theorized that vibrations or unintended disruptions to the truss structure under the roadway could have weakened the bridge so much that it could not bear the load of evening rush-hour traffic.
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.
"We thought we had done all we could," Dan Dorgan, an engineer for the Minnesota Department of Transportation, said Thursday. "Obviously, something went terribly wrong."
Thousands of steel truss bridges were built in the interstate highway construction boom of the 1950s and '60s 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," said Abolhassan Astaneh-Asl, a UC Berkeley professor who is an expert in such bridges. "They are very vulnerable to instability."
