FHWA
Structures Lab Investigates Milwaukee Bridge Collapse
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:
- Failure Analysis
- Triaxiality and Stress Analysis
- Implications to Other Bridges
- Inspection/Retrofit Options
- Related Case Studies
- Crisis Management and Coordination
- Emergency Retrofit/Permanent Retrofit
- Fractographic and Fracture Analysis
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).
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