On Thursday August 9th, 2007, The New York Times reported that investigators are now focusing on the possibility that gusset plates (see graphic at left), the steel parts connecting the girders of a bridge, not only may have failed over a period of time, but may have been incorrectly designed in the first place. Click HERE for full story.
According to Jan Achenbach, an expert in testing metals at the Northwestern University Center for Quality Engineering and Failure Prevention, gusset plates are made of steel, and, in the case of the Minneapolis bridge, shaped like squares, five feet by five feet, and a half inch thick. Such plates are common in bridges as a way to attach several girders together.
However, state transportation department officials seemed surprised by the sudden focus on the bridge’s gusset plates. Officials asserted that there would have been hundreds of these plates on the collapsed bridge, originally built in 1967. Gary Peterson, the state’s assistant bridge engineer, said he knew of no questions that had ever been raised about the gusset plates, no unique qualities to distinguish them from those on other bridges, no inkling of any problem during decades of inspections of the bridge. Ultrasonic testing apparently showed no signs of corrosion or cracking.
“I don’t know what this could be,” Mr. Peterson said. “I’m frankly surprised at this point. I can’t even begin to speculate.”
The possibility that the gusset plates may have been designed wrong has national implications, since other bridges subsequently built may have used the same standards. The dliemma confronting investigators is to determine if either the plates themselves were defectively manufactured, or if the plates themselves were manufactured correctly but may have been insufficient for the job because the engineers who originally designed the bridge in 1964 miscalculated the loads. The original engineers may not have anticipated a future increased volume of daily traffic. However, this theory generates an additional question: Why did the bridge stand for 40 years before collapsing?
One possibility may be that the damage was incrementally progressive over a period of time. Not only has the volume of traffic increased, but deregulation of the trucking industry has resulted in many truckers switching from hauling single trailers to double or even triple-trailers. This would add to the stress.
However, Federal authorities offered another theory. They said that one added stress on the gusset plates may have been the weight of construction equipment and nearly 100 tons of gravel on the bridge, where maintenance work was proceeding when the collapse occurred. A construction crew had removed part of the deck with 45-pound jackhammers, in preparation for replacing the two-inch top layer, and that may also have altered the stresses on the bridge, some experts said.
And the Federal Highway Administration swiftly responded by urging all states to take extra care with how much weight they place on bridges of any design when sending construction crews to work on them. However, they did not immediately issue any broader warnings about gusset plates. Brian Turmail, a spokesman for the Transportation Department, explained that his agency first wanted to conduct "additional analysis to determine whether we need to ask the states to do checks of their designs”. Here's the National Transportation Safety Board's statement:
National Transportation Safety Board
Washington, DC 20594
August 8, 2007
UPDATE ON NTSB INVESTIGATION OF COLLAPSE OF I-35W BRIDGE IN MINNEAPOLIS
Washington, DC -- The National Transportation Safety Board dispatched a Go Team on August 1 to investigate the collapse of the I-35W bridge over the Mississippi River in Minneapolis, Minnesota. The NTSB today provided the following update.
The Safety Board's investigative team of 19 was led by Investigator-in-Charge Gary Van Etten; NTSB Chairman Mark V. Rosenker accompanied the team. The team includes highway engineers, survival factors specialists and the Board's senior metallurgist. Parties to the Board's investigation are Federal Highway Administration, Minnesota Department of Transportation, and Progressive Construction, Inc.
"We are continuing to make progress on this investigation, and each area of inquiry gets us closer to ultimately determining the cause of this tragedy," Chairman Rosenker said.
Recovery divers are working 18 hour shifts from 4 a.m. to 10 p.m. to continue searching for victims. No additional victims have been recovered. They have identified four empty vehicles that can be removed to allow them better access to the wreckage. Those vehicles might be removed later today. NTSB is monitoring this effort so that removal of major pieces of wreckage can commence, which will occur once the Hennepin County Sheriff's office has determined that it has searched all it can under current conditions.
The NTSB is working with the Federal Highway Administration to conduct a structural analysis of the bridge, using computational Finite Element Analysis methods. Within days of the collapse, development of the computer model based upon the original design drawings began at the FHWA's Turner Fairbank Highway Research Center in McLean, Virginia. Data collected at the accident scene, with the help of the Federal Bureau of Investigation's 3-D laser scanning device, will be used in the computer model to further refine the model. This work is expected to take several months.
Since the first day, the NTSB has been conducting inspections of areas of the bridge that are accessible. On Monday, using a gyro stabilizer, high resolution camera mounted on a state police helicopter, the Board looked at the superstructure on the north side of the bridge. Several tensile fractures were observed, but nothing that looked to be the initiating location. NTSB investigators will conduct a closer look at the superstructure on the north end when it becomes more accessible.
Chairman Rosenker said that NTSB investigators have observed a design issue with gusset plates at particular locations (gusset plates are steel plates that tie steel beams together). Safety Board investigators are in the process of verifying the loads and stresses on the gusset plates at these locations, as well as the materials used in constructing the gusset plates. This information has been shared with the other parties to the investigation, including the Federal Highway Administration.
The Safety Board has conducted interviews of eyewitnesses, vehicle occupants and construction employees, as well as with the crew of a dinner cruise ship that was in the lock near the bridge at the time of the collapse. It is reviewing construction records to determine the location of construction equipment and raw materials on the bridge at the time of the collapse, and to verify the weights of those vehicles and materials. The Board has obtained core samples of the bridge deck material to get a better picture of the deck thickness to help make an assessment about the amount of concrete on the bridge at the time of the accident.
The Board has obtained the original security camera video equipment and footage provided by the Army Corps of Engineers that shows a portion of the bridge collapsing. The Board is in the process of reviewing the entire contents of the system in our laboratory, and is providing detailed imagery back to the accident site to help guide investigators in their on-scene activity.
A Safety Board representative from the Office of Transportation Disaster Assistance has been briefing family members each evening about the progress of the NTSB's investigation.
Further information on the progress of the Safety Board's investigation will be released as it develops.
NTSB Media Contact: Terry N. Williams
Another related issue is the use of rivets vs. bolts to attach the gusset plates. When the bridge was built in 1967, its hundreds of gusset plates were attached with rivets, though bridge designers here switched to bolts, a stronger option, in the 1970s. But while Gary Peterson conceded that bolts are better, he doesn't think that there is anything wrong with rivets.
A Fox News Channel report which referenced the New York Times story adds some additional interesting information. They report that Progressive Contractors Inc., the company that was working on the bridge, rejected a report that one of its workers had noticed unusual swaying of the bridge in the days before the collapse. Progressive Contractors Inc. had earlier said that it didn't believe any of its work contributed to the bridge failure, but the company hadn't responded directly to claims of wobbling. "We have now met with every single worker who was on the bridge when it collapsed," Tom Sloan, vice president of the company's bridge division, said in a statement. "None of them observed or reported any unusual swaying".
On the legislative front, Representative Jim Oberstar, the Minnesota Democrat who is chairman of the U.S. House Transportation Committee, has proposed a temporary gas tax increase of 5 cents a gallon. It would pay for a new trust fund to repair and replace structurally deficient bridges on the National Highway System. The new fund would be modeled on the federal Highway Trust Fund, which pays for building and repairing roads and bridges through the gasoline tax. Money in the new trust fund could not be used for any other purpose than bridges.
However, President Bush immediately dismissed raising the federal gasoline tax to repair the nation's bridges, until such time as Congress changes the way it spends highway money. "The way it seems to have worked is that each member on that (Transportation) committee gets to set his or her own priorities first," Bush explained. "That's not the right way to prioritize the people's money. Before we raise taxes, which could affect economic growth, I would strongly urge the Congress to examine how they set priorities."
Fox News has also just reported that a sixth body has been found at the collapse site. If confirmed, this would raise the official death toll to six.