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Publication Number: FHWA-RD-02-005
Date: December 2001
The Federal Highway Administration's (FHWA) Structures lab became a forensic research center this past year, as FHWA led a team in investigating the partial collapse of a 66-m (72-yd) section of Milwaukee's Hoan Bridge last December.
There was initial suspicion among many citizens and officials in Milwaukee that heavy trucks and low-toughness material could be contributing factors in the bridge failure. The bridge is a common truck route serving a shipping terminal warehouse facility. In addition, an unrelated fracture failure in a new stadium construction project in the city had focused attention on the issue of toughness. But the team of researchers who evaluated the cracked girders has concluded that the problem was much more complex.
"This is the first time we have seen this kind of failure in a bridge," says Bill Wright of FHWA's Turner-Fairbank Highway Research Center (TFHRC), who believes the collapse was the result of a combination of factors aligning at a precise moment. Overweight trucks and subzero temperatures created conditions that probably triggered the failure, but the primary cause was found to be related to the design details used for a welded joint assembly. Wright reports that although the bridge is not old-it was built in 1974-the details of the joint design used for the Hoan structure are no longer used in the construction of new bridges.
Two weeks after the December 13, 2000, collapse, local transportation officials demolished the remaining section of the damaged bridge to prevent it from falling on its own. The broken girders were then transferred to TFHRC's lab in McLean, Virginia, for tests. FHWA's partners in the study were Lehigh University in Bethlehem, Pennsylvania, and Lichtenstein Consulting Engineers of Paramus, New Jersey. The team thoroughly examined the properties of the bridge steel, including its chemical composition, strength, and fracture toughness. The results showed that the steel would have met today's material specification requirements set by the American Association of State Highway and Transportation Officials (AASHTO) for use in bridges down to temperatures as low as -34° C (-30° F). Despite the fact that steel loses ductility at low temperatures, the toughness should have been sufficient to prevent fracture under normal circumstances. The specific details present in the joint detail created an abnormal condition of triaxial constraint that reduced the effective fracture resistance of the bridge.
Without the vulnerable joint design, the other factors present at the time of the collapse would not have resulted in failure to the bridge. A study by the University of Michigan showed a significant number of heavy trucks, some over the normal weight limit of 36,000 kg (80,000 lb), routinely traveled over the bridge. Heavy truck loading can eventually cause fatigue cracking in structures, which reduces the resistance to fracture. However, the investigation found no evidence of fatigue damage at the failure locations in the Hoan bridge. This explains why, despite a recent indepth inspection of the bridge, there was no warning prior to the failure.
Repairs to the Hoan Bridge are scheduled to be completed by this month, with an estimated cost of between $14 million and $16 million. In the interim, the adjacent span of the bridge is open to two-way traffic.
To share the lessons learned from the Hoan bridge investigation, FHWA's Midwestern Resource Center and Wisconsin Division Office, in cooperation with the Wisconsin Department of Transportation (DOT), cohosted a seminar in Milwaukee on September 26-27, 2001. More than 100 bridge design and maintenance experts from 16 States attended the seminar. Among the topics presented were:
A preliminary forensic investigation report was released this past summer by the Wisconsin DOT and FHWA. The report can be found on FHWA's Midwestern Resource Center's Web site (www.mrc.fhwa.dot.gov/team/infra/), as well as information from some of the seminar presentations. A final FHWA publication on the investigation is now being prepared. For more information, contact Bill Wright at 202-493-3053 (email: bill.wright@fhwa.dot.gov).
