Monitoring The Deck-Truss Bridges

Story by Mike Larson, Editor | September 28, 2010

The Wisconsin Department of Transportation (WisDOT) is using electronic strain gauges to monitor the state's 14 deck-truss bridges that are similar to the Minnesota bridge that collapsed in August — just to be absolutely positive they are as safe as they seem.

Although all 14 deck-truss bridges in Wisconsin were visually inspected and found sound within two weeks after the Minnesota tragedy, the state is performing the current additional tests to check that the load on the trusses is within the limits the bridges were designed to support.

Finn Hubbard, Wisconsin's state bridge engineer, says, "Public safety is our top priority, so we are taking this extra step to ensure that these bridges are adequately handling their daily traffic loads. We expect to find that these bridges are functioning normally and safely."

The deck-truss bridges are scattered throughout southern and far western Wisconsin, from Janesville, west to La Crosse and north to Eau Claire. They range from 50 to 82 years old, and have consistently passed the normal inspections required every two years by Federal Highway Administration (FHWA) regulations.

Team Of Experts Tackles The Project

WisDOT has assembled a team of experts to help it handle this fast-track project. The work began when WisDOT gathered the team in early October at a "sensor summit" in Madison to evaluate alternatives, plan the project and assign responsibilities. It consists of three engineering consultants, the engineering departments of three universities and a sensor supplier, all selected for their experience with bridges and monitoring.

The consultants include: Fish & Associates, Inc., Middleton, Wis.; Michael Baker Corp., Chicago; and Collins Engineers, Inc., Milwaukee.

The universities are the University of Wisconsin - Madison, the University of Wisconsin - Milwaukee, and Marquette University, Milwaukee.

CTLGroup, Skokie, Ill., is providing, installing and calibrating all the sensors and monitoring equipment.

Each engineering consultant and a corresponding university will monitor four to five of the bridges, in conjunction with WisDOT representatives.

The Project Plan

The plan is to attach strain gauges to the main trusses of the bridges, then collect data up to 100 times per second, 24 hours a day, seven days a week for three months.

The data will then be analyzed to be sure stress levels in the bridge structures are within the levels for which they were designed.

If the data show that the bridges are performing as expected, WisDOT will return them to the normal two-year inspection schedule.

If, however, the data show unexpected stress, WisDOT will take whatever corrective action is required.

Installing And Calibrating

Allan Johnson, P.E., is one of the WisDOT engineers overseeing of a group of bridges in southwestern Wisconsin. He explained the process of installing and calibrating the sensors.

"The number and location of strain gauges for each bridge was determined in the October meeting," he says. "The entire team evaluated the structure of each bridge to determine the best quantity and locations for gauges on that structure. We are now attaching the strain gauges at the pre-determined locations."

Each strain gauge, no larger than a pack of gum, is welded to one of the bridge's trusses. Most of the bridges are equipped with two or three sensors. But two of the bridges that carry interstate highway traffic will have a total of a dozen sensors between them.

Explains Finn Hubbard, "Large trucks are what really can affect bridges. Even a lot of 2,500-pound cars won't affect a bridge the way an 80,000-pound — or heavier — truck does. The two deck-truss bridges that carry the interstate over the Red Cedar River in Dunn County are the most similar in design to the Minneapolis bridge. They also carry far more large trucks than any of the other bridges we are monitoring. That's why they are getting more gauges."

After the strain gauges have been mounted to the bridge, wires are run to a data collection box that records the data continuously on an electronic card, similar to the data collection system used by surveyors. The self-contained collection unit runs on power from a solar panel, backed up by a battery. The data card is collected on a schedule by the consultant engineering firm assigned to each bridge.

Says Johnson, "After the data collection system is installed, we close the bridge to traffic and take an 'at rest' reading to set a baseline when the bridge is seeing no load. We then run a vehicle of known weight across the bridge at specified speeds to establish standardized comparisons. After that, the bridge is opened and we record everything for the next 90 days."

Acting On The Results

The raw data will be processed by the consultants and the universities into a form that the group can use in analyzing the results. When the three months of monitoring are complete, WisDOT and its team will again convene to review the results and recommend further tests or action if any is required.

Says Hubbard, "Advances in engineering, materials and construction equipment have provided more efficient methods of designing and building bridges over the past 50 years, so we no longer build deck-truss bridges. But these 14 bridges were well designed and solidly constructed. I'm confident they are performing as we expect ... but we need to know it for certain. This strain gauge monitoring will help provide that assurance."