Wisconsin officials plan to increase bridges under scrutiny

Matthews Bridge in Wisconsin

The Associated Press

2/4/2008

MADISON, Wis. — Wisconsin officials plan to increase by nearly tenfold the number of bridges under added scrutiny since the Interstate 35W bridge collapsed in Minneapolis last summer.

The move follows the Jan. 15 disclosure by federal officials that a bridge element typically assumed to be the strongest link in such structures was actually a source of weakness that contributed to the bridge failure Aug. 1 that killed 13 people and injured over 100.

The problem was in a series of steel "gusset" plates that can be riveted or bolted to the beams and members of a bridge to hold them together to form a larger part called a truss.

After the collapse, Wisconsin put extra scrutiny on 14 bridges with designs most similar to the I-35W bridge. The so-called "deck truss" bridges have trusses running beneath the roadway they support.

Now the state is expanding the extra effort to 127 bridges in all, because all truss bridges have gusset plates. If one truss fails, the whole structure can collapse.

The problem in the I-35W bridge appeared to be an isolated case, but the state isn't taking chances, said Beth Cannestra, director of the Department of Transportation's Bureau of Structures. It oversees the design and inspection of Wisconsin's bridges.

No inspector would have previously looked for the design flaw.

The failure came in partly because decades ago a consultant designed some of its plates to be only a half-inch thick when they should been a full inch thick, said Finn Hubbard, a Wisconsin state bridge engineer.

"The concerning part of it is that what happened there could happen somewhere else," Hubbard said. "This was a bomb sitting in the weeds waiting for somebody to kick it."

The federal National Transportation Safety Board is still investigating what triggered the collapse.

The state's 13,677 bridges include 127 truss bridges, Cannestra said. All of the truss bridges have been inspected within the last two years.

State officials plan t

•Evaluate fully all bridge parts, including gusset plates, before bridge renovations such as a new concrete deck or when considering an increase in traffic or temporary loads on bridges.

• Check the soundness of parts when deciding on posting higher loads on bridges, and

• Evaluate the gusset plates on all bridges as part of a three-year effort to reassess the soundness of all the state's bridges.

Hubbard said that the NTSB and the Federal Highway Administration had recommended the precautions.