State to closely examine bridge infrastructure

by Michael Evert

Tuesday, February 12, 2008 22:26

After inspecting more than a dozen Wisconsin bridges in response to a collapse in Minnesota last August, state officials and experts will scrutinize a total of 127 bridges’ design in the upcoming weeks.

The additional bridges will be examined next month, as a group led by the Wisconsin Department of Transportation and the University of Wisconsin will hold a workshop to analyze the so-called “truss” bridges, all of which have a design structure similar to the one that collapsed.

The decision to evaluate these particular bridges stems from new information about a critical safety element they share. The strength of the gusset plates (the one-half- to one-inch-thick load-bearing devices that are one of the most important structural elements of truss bridges) are being evaluated.

Initially, only the 14 Wisconsin bridges with the “deck-truss” design most similar to the collapsed I-35 bridge in Minneapolis were inspected, but the state now plans to implement a design check to all 127 truss bridges in Wisconsin. Beth Cannestra, director of DOT’s Bureau of Structures, said the design checks are mainly a precautionary measure.

“We don’t anticipate changing the gusset plates,” Cannestra said, adding these design checks, which constitute a systematic appraisal of how the bridge is built, will be limited to high-risk bridges. “We are keeping the design check within fracture-critical bridges, consistent with [National Transportation Safety Board] recommendations.”

Cannestra also emphasized the difference between inspections and design check. “An inspection is a physical visit to a site in which engineers analyze the integrity of the structure, whereas design checks take place away from the actual site of the structure and are intended to evaluate and analyze the design of the structure,” she said.

The design check will take place during a workshop held next month. This workshop will include the Wisconsin DOT and staff from UW-Madison, UW-Milwaukee and Marquette University.

Gary Whited, program manager for the Construction and Materials Support Center at UW-Madison, said in the second workshop, the group will use the data taken from the first set of bridges inspected to apply to the rest of the truss bridges.

Noting the I-35 collapse was in many ways unique, Whited also said these precautions were little cause for alarm.

“The I-35 bridge was built with only half-inch gusset plates and had been structurally modified over the years. These factors contributed to its collapse,” Whited added.                                                             

UW professor Jeffrey Russell, chair of the civil and environmental engineering department, said the evaluation is little cause for concern.

“Wisconsin tends to build bridges that are built with tried-and-true approaches rather than riskier techniques that have not been implemented on a full, real-life scale,” Russell said. “We may build bridges even safer than they need to be.”     

According to DOT, the closest truss bridge to UW is on East Dyerson Road, near Stoughton over the Yahara River. Another similar bridge runs over the Rock River on U.S. Highway 51 in Rock County.