Engineers to Conduct Collapse-Load Test on 120-Foot Bridge with Fractured Girder

March 12, 2009

Event: Structural engineers at the Ferguson Structural Engineering Laboratory at The University of Texas at Austin will test the load capacity of a full-sized bridge that has previously suffered a complete fracture of a steel girder.

When: Monday, March 16—all day. Beginning in the morning, the bridge will be loaded incrementally with buckets of road base until it collapses, which is most likely to occur between 11 a.m. and 4 p.m.

Media access to the test site after the failure will depend on the condition of the structure and safety precautions, but there should be some dramatic still-photo opportunities to show the failed bridge.

Media credentials will be required to enter the J. J. Pickle Research Campus from the Braker Lane or Burnet Road entrances. Media can notify entrance gate security of their purpose to view the bridge test at Ferguson Lab.

Where: The open storage yard south of the office building of the Ferguson Structural Engineering Laboratory. A map of the campus is online.

Background: For the experiment, a load in excess of 73,000 pounds will be added to the bridge surface in order to test the ultimate capacity of the bridge deck and railing support. Nearly 300 strain gauges and displacement transducers will record how the bridge reacts in this extremely damaged condition to incrementally increasing loads. The data will be used to develop analytical methods that engineers can apply to assess the behavior and safety of the many twin-girder steel bridges in the state's transportation system.

The test is part of a research project to determine the collapse vulnerability of twin box girder bridges in the event of a complete fracture of one of the girders. Structural engineers consider these bridges "fracture critical" because they are expected to collapse if one of their girders fractures. The test team is led by Eric Williamson, associate professor of structural engineering, and Karl Frank, professor of structural engineering. The team is developing analytical methods to determine the vulnerability of these bridges, and the full-size tests of this instrumented bridge will provide valuable data to calibrate their models.

Twin box girder bridges are often used in freeway interchanges when long, curved spans are required, such as the one at Interstate 35 and Highway 290 in Austin. More than two years ago, the laboratory's test bridge, which was removed from the interchange of Interstate 10 and Loop 610 in Houston, was tested to see the effects of a brittle fracture in the bottom flange of one girder, but it suffered only minimal damage.

A second experiment was performed in June 2008 to simulate a full-depth fracture, where a jack system supporting the weight of the fractured girder was dynamically removed by severing its tension ties with high explosives. In response, the bridge deflected about eight inches downward, causing significant damage in the connection between the fractured steel girder and the concrete deck. Before collapse-level damage could be suffered, however, the expansion joints in the railings above the concrete deck closed, acting as additional support beams for the crippled bridge. The test scheduled for March 16 will load the bridge in its current damaged state.

The Texas Department of Transportation and the Federal Highway Administration are funding the research.

For more information, contact: Daniel Vargas;  Eric Williamson, Department of Civil, Architectural and Environmental Engineering, Cockrell School of Engineering College of Engineering, 512-475-6175;  Karl Frank, Department of Civil, Architectural and Environmental Engineering, Cockrell School of Engineering College of Engineering, 512-232-3592.