On December 13th, 2000, a section of the Daniel Hoan Memorial Bridge through downtown Milwaukee buckled and sagged about 4 feet. No one one was injured.
For details on the initial incident… Wisconsin Winter Wages War on Hoan Bridge. Cold weather was a factor. Outdated construction methods were a factor.
At approximately 7 a.m. on the morning of Wednesday, Dec. 13, 2000, northbound commuters got a rude surprise when a 217-ft. long by 55-ft. wide (66 by 17 m) portion of Milwaukee, WI’s, Hoan Bridge buckled and sagged.
The bridge had been inspected a few weeks earlier, but nothing to indicate it would be unsafe was detected.
Weight-bearing steel beams on bridges are usually flexible and behave much like a rubber band, bending and moving with the wind and traffic. In the case of the Hoan’s two failed beams, the girders probably experienced what is known as a brittle failure.
Two weeks after the deformation the offending section was removed with explosive demolition and repairs begun. It took just under a year to complete repairs.
Initial thoughts were that all the salt trucks which had left the Port of Milwaukee, fully loaded to combat the weather, were at fault. But the forensic analysis points to a failed weld and related design issues, with the cold weather contributing.
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.
The details of that joint design, coupled with the cold weather caused the bridge to be susceptible to brittle fractures, which is apparently what happened on that day. The forensic analysis was detailed, right down to testing of the steel.
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).
So here’s a question: If they knew there were problems with that joint design, to the point that they don’t use it anymore, shouldn’t every bridge built using that design be retrofitted or reinforced? And temperatures in Milwaukee are frequently -20°F, so shouldn’t the steel be able to handle even much colder weather? (There was one day when I was in college where the mercury in Chicago showed -30°F. I doubt Milwaukee was warmer.)
The complete forensic report is at this link: HOAN BRIDGE FORENSIC INVESTIGATION FAILURE ANALYSIS FINAL REPORT. It is long and written by engineers who work for the government.