The tragic rush-hour collapse in Minneapolis of the I-35W Bridge over the Mississippi River is again forcing a reexamination of the nation's approach to maintaining and inspecting critical infrastructure.
According to engineers, the nation is spending only about two-thirds as much as it should be to keep dams, levees, highways, and bridges safe. The situation is more urgent now because many such structures were designed 40 or 50 years ago, before Americans were driving weighty SUVs and truckers were lugging tandem loads.
It all adds up to a poor grade: The American Society of Civil Engineers gave the nation a D in 2005, the latest report available, after assessing 12 categories of infrastructure ranging from rails and roads to wastewater treatment and dams.
"One of America's great assets is its infrastructure, but if you don't invest it deteriorates," says Patrick Natale, executive director of ASCE.
Among scores of recent examples:
•Last month, a 100-year-old steam pipe erupted in midtown Manhattan, killing one man and causing millions of dollars in lost business.
•The inadequacies of levees in New Orleans became horrifyingly clear in the aftermath of hurricane Katrina. The city is still recovering.
•In 2003, the Silver Lake Dam in Michigan failed, causing $100 million in damage.
America's 577,000 bridges are of particular concern because they are subject to corrosion. According to the website of Nondestructive Testing (NDT), which advocates not damaging structures during testing, the average lifespan of a bridge is about 70 years. Bridges are inspected visually every two years. However, NDT notes, "it is not uncommon for a fisherman, canoeist, and other passerby to alert officials to major damage that may have occurred between inspections."
In the federal government's rating system, any bridge that scores less than 80 – on a scale of 1 to 100 – is in need of rehabilitation. A bridge scoring below 50 should undergo reconstruction under federal guidelines. In 2004, 26.7 percent of US bridges, urban and rural, were rated deficient, down from 27.5 percent in 2002, according to the US Department of Transportation (DOT).
Minnesota's record is far better, with only 12.2 percent of its bridges falling into the deficient or obsolete categories.
Federal officials were quick to point out that those designations don't mean the bridges are unsafe.
"None of those ratings said there was any kind of danger," US Transportation Secretary Mary Peters said in Minneapolis on Thursday. The ratings are used to point out deficiencies or overhauls that need to be conducted in the future.
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.