Bridge Plates Not Cause For Concern, Officials Say
Friday, August 10, 2007
Connectors known as gusset plates, which federal officials are scrutinizing as part of their investigation into Minneapolis's Interstate 35W bridge collapse, are a common feature of steel spans in the Washington region and across the country.
Local engineers said they are redoubling inspections in a small number of deck truss bridges but are not rushing to change regimens until more definitive information emerges. The Minneapolis span was a deck truss bridge, which has a roadway supported by major steel components linked underneath.
"Basically, every steel bridge has gusset plates," said Bob Healy, deputy director of the Maryland State Highway Administration's bridge development office. "That's just the way you connect all parts of the framing together. [They] are really just connections," he said, adding that many gusset plates do not "carry the main load" of a bridge.
"To do a statewide reinspection on all gusset plates is not reasonable based on what they've found so far," he said.
Healy said Maryland inspectors will take a closer look at the plates on a half-dozen bridges that they had decided to reinspect because they were built with a design similar to the Minneapolis span. One such bridge is on the Capital Beltway near the border of Montgomery and Prince George's counties.
Following federal guidance, Virginia officials had planned to look at nearly a dozen similar bridges, including one in the Fredericksburg area. There are no bridges with that design in the District.
"We've directed inspectors out to the 11 deck truss bridges that we have in our inventory, and we will be paying attention based on the guidance and what we're hearing in Minnesota," said Mal Kerley, chief engineer for the Virginia Department of Transportation. "We don't have any reason to expect anything like that, but we're going to be proactive and look at those areas."
Some outside bridge engineering experts say officials would be better off not only inspecting deck truss bridges but also other truss bridges built in the 1950s and '60s.
Abolhassan Astaneh-Asl, a professor of structural engineering at the University of California at Berkeley, said "there's no reason to believe" that a truss bridge with a deck on top of its steel supports -- a deck truss -- has more "critical gusset plates" than a truss bridge in which the deck is below the supports.
National Transportation Safety Board investigators have found what they called "a design issue" with gusset plates in some places on the I-35W bridge.
The 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," the board said in a statement.
Board investigators also said they were analyzing the location and weight of construction materials that were on the Minnesota bridge before its collapse. Given the questions raised by the safety board, "it is vital that states remain mindful of the extra weight construction projects place on bridges," U.S. Transportation Secretary Mary Peters said. Local government bridge engineers said contractors are already subject to clear rules in such cases.
The vague information reflects just how much there is to learn about what caused the collapse, said Kerley, who also heads the bridge subcommittee of the American Association of State Highway and Transportation Officials, which helps determine standards.
"It's too early to tell exactly. That's just an area of interest they are looking at," Kerley said.
Engineers said gusset plates are basic features of many bridges. They serve the same purpose as round blocks in Tinker Toys, said Barry Fuss, a Montgomery bridge engineer. On truss bridges, steel components link up to support the deck.
"They are usually in an X or V shape, and the gusset plate is where those members come together and are attached," Fuss said.
He cited a small historic bridge carrying Montevideo Road over Dry Seneca Creek as an example of a county-maintained structure that uses gusset plates. There is no need to inspect that bridge again now, but depending on the eventual findings, additional inspections could be called for, he said.
Gusset plates would be a routine component in the approximately 150 to 200 steel truss bridges in Virginia, Kerley said.
Healy said bridges are subject to "hands on" inspections in the state, which means an inspector essentially gets "within an arm's length. He can see it and touch it and measure it."