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| Tragic: The collapse of the I-35W Bridge in Minneapolis at rush hour Wednesday, killed at least four people and injured dozens
of others. Joshua Lott/Reuters |
Bridge collapse spotlights America's deferred maintenance
About one-quarter of America’s 577,000 bridges were rated deficient in 2004.
from the August 3, 2007 edition
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Most bridge collapses occur from an obvious cause: an earthquake or a barge running into bridge supports. On rare occasions, however, bridges have collapsed for less obvious reasons. In 1983 in Greenwich, Conn., the Mianus River Bridge collapsed, killing three people. A federal investigation blamed excessive accumulation of corrosion on a hangar pin, a key part of the bridge.
In the case of the Minneapolis bridge collapse, the National Transportation Safety Board will lead the investigation. Its investigators were also on the scene to begin to piece together what had caused the collapse.
"It is much too early in this investigation to know what happened," NTSB Chairman Mark Rosenker said Thursday. The first step is to reassemble the pieces of the bridge like a jigsaw puzzle to figure out what triggered the collapse, he said.
State inspection officials had inspected the bridge twice since the federal government rated the bridge "structurally deficient" but concluded the bridge was safe. State officials were in the process of completing a third inspection – interrupted because of construction on the bridge – when the bridge collapsed Wednesday afternoon at the height of rush hour.
As many as 30 people were missing as of press time Thursday.
Concerns about the bridge go back at least six years.
A 2001 report by the University of Minnesota, Department of Civil Engineering, stated, "Concern about fatigue cracking in the deck is heightened by a lack of redundancy in the main truss system. Only two planes of the main trusses support the eight lanes of traffic. The truss is determinate and the joints are theoretically pinned. Therefore, if one member were severed by a fatigue crack, that plane of the main truss would, theoretically, collapse."
This was a steel, arch-truss style bridge with a concrete deck that should have lasted at least 60 years, says P.K. Basu, a civil engineer at Vanderbilt University and an expert on bridge design and failure. Corrosion of rivet connections is a suspect, as are possible cracks around such joints. Some signs of structural failure due to corrosion are subtle, he says, and may only be discernible by experts. Increased weight of trucks in recent years could be another factor.
The bridge was part of Interstate 35, a major transportation link for Minneapolis, and one of the most heavily traveled urban highways in the country. It was also the first of its size in the US to be equipped with an anti-icing system that sprayed a de-icing element on the bridge deck.
President Bush on Thursday promised the federal government would respond "robustly" to help with rescue and recovery and with rebuilding the bridge "as quickly as possible." He also blamed Congress for failing to pass crucial spending bills, including funding for infrastructure.
Rep. Jim Oberstar (D) of Minnesota, chairman of the House Transportation and Infrastructure Committee, said Thursday he would ask for $250 million in emergency funding for Minnesota. Some will be used for alternative ways to move 140,000 vehicles a day that used to cross the bridge. Congress had authorized $283 billion for upgrading the nation's infrastructure over five years. Mr. Natale says ASCE felt the figure should have been $360 billion.
Staff Writer Mark Clayton contributed to this report.
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