Effects of Bridge Work Weren’t Analyzed
Author / Coordinator: Pat Doyle and Dan BrowningStar Tribune
August 2007
As federal investigators continued to pursue evidence Thursday in the Interstate 35W bridge collapse, state officials said they had no reason to analyze the potential impact of resurfacing the bridge before authorizing that work.
The Minnesota Department of Transportation (MnDOT) said it didn’t need an analysis beforehand because the resurfacing work wouldn’t add weight to the bridge.
Construction officials and some engineers deny or doubt that the resurfacing contributed to the collapse of the steel deck truss bridge. However, an unusual federal alert was put out Wednesday to other states that have similar bridge repair projects underway.
U.S. Transportation Secretary Mary Peters said the weight of construction equipment or materials on the 35W bridge could have contributed stress to gussets, or connecting steel plates. Peters, who is scheduled to return to Minneapolis today to give an update on the federal response to the collapse, told bridge engineers nationwide to "carefully consider" the additional weight during construction projects.
Decisions on whether to conduct risk analysis of future construction vary from project to project, according to authorities familiar with the process.
Earlier work on the Intercity Bridge, which connects St. Paul and Minneapolis across the Mississippi River on Ford Parkway/46th Street, involved not only a complete redecking but also the replacement of some underlying components.
County engineers, a consultant and the construction company evaluated construction weight before beginning work on that concrete arch bridge.
On the 35W project, crews had been working on a $9 million resurfacing job for six weeks before the bridge collapsed Aug. 1. Neither the firm doing the work, Progressive Contractors Inc. (PCI), nor MnDOT studied the potential impact of the work on the integrity of the bridge, said Donna Lee Lindberg, a MnDOT spokeswoman.
"There was no analysis done on that," she said. "Since the contractor doesn’t bring onto the bridge any more material than is needed to be replaced, we don’t see a need to analyze the structure."
A PCI spokesman declined to comment Thursday. The firm has repeatedly denied that its work contributed to the collapse.
A top official with the National Transportation Safety Board (NTSB) said the collapse of a large bridge without an obvious cause is so rare that investigators must explore all possible angles.
Bruce Magladry, the NTSB’s highway safety director, said he knew of no case where resurfacing was found to be a precipitating factor in a bridge collapse, but that everything must be considered.
"It’s unlikely there was a single thing that led to this bridge collapse," Magladry said.
While there was a water truck, cement truck and several other large vehicles on the bridge, experts said the span should have been able to handle the load.
Dan Dorgan, MnDOT’s chief bridge engineer, said the bridge was designed to meet military specifications.
Kent Harries, an assistant professor of structural engineering and mechanics at the University of Pittsburgh, said that means the bridge should have been able to support a traffic jam of flatbed trucks loaded with M1 Abrams tanks.
"The loading that that bridge was seeing at the time it collapsed was considerably lower than its design loads," Harries said.
He agreed with Magladry that the cause of the collapse would probably come down to a combination of factors, such as corrosion, weathering, age, vibrations and weight.
Ian Grossman, a spokesman for the Federal Highway Administration, said all states generally follow guidelines set by the American Association of State Highway and Transportation Officials regarding construction weight on existing bridges.
"Basically, it says if it’s beyond the load you need to either … do work to the bridge beforehand so that it can hold the load … or not put it on the bridge."
Consultants performed a similar analysis on the Intercity Bridge reconstruction, according to Jim Grube, Hennepin County’s transportation director who helped oversee that project.
A consultant hired by MnDOT to run a parallel investigation of the collapse said Thursday that some of the plates in the bridge’s center span were found to be fractured.
"We’ve been looking at the gussets," said Michael Koob, of Illinois-based Wiss, Janney, Esltner Associates Inc.
But Magladry said the gusset theory may have been overemphasized in the media. "Gussets are simply a design feature we’d like to look at," he said.
Magladry said NTSB investigators will have to determine whether the gusset fractures contributed to the collapse or occurred as the structure fell.
He said the safety board has only just begun working with computer models to determine what caused the disaster. The scenarios will consider various loads on the bridge: the weight of the structure itself, moving vehicles and the placement of construction materials and vehicles.
Magladry said it could take thousands of simulations. The investigation is complicated because it is "very, very, very rare" for bridges to collapse for no apparent reason. Usually they fall after an impact from a barge, ship or vehicle, Magladry said.
Scouring of underwater piers has also taken bridges down, but both scouring and lateral stress from wind have been virtually ruled out in the I-35W case, he said.