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.
Two of the three 10-ft. (3 m) deep steel I-beams that supported the damaged segment of the 3-mi. bridge had major vertical cracks. The cracks allowed a portion of the concrete deck to sag down 4 ft. (1.2 m).
The damaged part of the bridge was located about 200 ft. (61 m) south of the arch. The beams were the center and eastern girders supporting the deck.
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.
Although analysis is not complete, it’s thought that severe cold along with fatigue was the cause of the failure. Just two days before the collapse, the bridge sustained heavy loads when more than 700 trucks hauled nearly 15,000 tons of salt from the Port of Milwaukee over its northbound lanes.
The situation came as a surprise to Wisconsin Department of Transportation (WisDOT) officials and engineers from Graef, Anhalt, Schloemer and Associates, a Milwaukee-area engineering firm that had assisted WisDOT with routine inspections on the bridge since 1992. “We were disappointed that something like this would happen,” said Don Rhodes, communications manager for WisDOT.
Most cracks in steel girders develop slowly and can be monitored. In fact, inspections of the third beam, one as recently as the week before, showed that it too, had some cracking, but not severe enough to warrant immediate repairs.
A repair had been done to a crack on the third beam six months earlier and probably saved the nearly 700-ton (630 t) section from total collapse. Since the accident, inspections found an 18-in. (46 cm) crack in the third beam, which was stressed from holding up the remaining part of the deck.
The damaged section was suspended directly above a portion of the Metropolitan Milwaukee Sewage District’s Jones Island Sewage Treatment Plant. The plant operates at 300-million gal. a day, serving the city of Milwaukee and surrounding suburbs.
The $47-million bridge, which was designed by Kansas City, MO’s, HNTB Corp., has been open since 1977. It has a tied-arch main span and 18 approach spans with a similar three-girder design. Before the accident, it carried approximately 35,000 vehicles a day into, and out of, downtown Milwaukee.
The Hoan Bridge received a major inspection every five years and was current on routine annual inspections. Despite the previous week’s inspection, engineers and WisDOT believe it was unlikely that the failure could have been predicted.
Fears that the section could drop at any time led to the decision to have a 130-ft. (40 m), 500-plus ton (450 t) chunk of the damaged portion brought down by controlled explosive charges. Taking down more of the structure than that would have severely damaged a small building housing key gas and electrical utility services for the sewage treatment plant.
WisDOT called in Controlled Demolition Inc. (CDI), a Phoenix, MD, firm that has leveled more than 7,000 structures, including Seattle’s King Dome and what remained of Oklahoma City’s Alfred P. Murrah federal building after the 1995 bombing.
The firm has been in business since 1948 with brothers Mark and Doug Loizeaux taking over the company from their father in 1976. “It was very difficult for any of those present to cope with that kind of structure that had sustained that amount of damage,” explained Mark Loizeaux, president of Controlled Demolition Inc. “All too often we work on emergency jobs where people have lost their lives. What pleased me most about this job was that no one was injured.”
Working in unseasonably cold weather with temperatures in the single digits and wind chills below zero, 23 crew members from CDI placed lines of charges of RDX demolition explosive on the steel support girders and under the deck. “RDX is the least expensive of the truly high velocity explosives available,” said Loizeaux, explaining his choice of materials.
CDI used RDX as a trigger to dislodge the piece, allowing gravity to take over and bring it down. The job took days longer than expected because of persistent winds and bone-chilling temperatures. “The most memorable aspect of the job was the bitter cold and the time of year, right before Christmas,” Loizeaux noted.
Before the blast, workers used 6,000 tons (5,400 t) of crushed limestone to create 20-ft. (6 m) high berms under the damaged segment to cushion and deflect falling debris as well as minimize the vibrations to the sewage treatment plant structures and buildings in the surrounding area.
Workers from Lunda Construction Company in Black River Falls, WI, and Zenith Tech Inc., in Waukesha, WI, constructed a 100-ft. (30 m) steel tower to shore up a section of the bridge. This precaution helped ensure that the explosion caused no more than the intended section to drop.
At exactly 1:01 p.m. Thursday, Dec. 28, 2000, 500-plus tons (450 t) of steel and concrete fell 100 feet neatly into the ground below. “It fell where it was supposed to and avoided key structures at the sewage treatment plant,” Rhodes said.
The explosion’s velocity was estimated to be 27,000 ft. per second with 3-million lbs. of pressure per square inch being released.
WisDOT officials hope to reopen the bridge to car traffic sometime this month. Rhodes noted that over the past six weeks there have been 70-ton (63 t) cranes on the deck working to repair the damage. “The bridge will be safe beyond a reasonable doubt before we open it,” he added.
The Federal Highway Administration’s National Bridge Inventory ranks Wisconsin among the top 10 states in bridge quality. The state is tied for No. 8 along with California and Delaware. Arizona ranks first with 5 percent of its bridges being rated “deficient” or “obsolete.”
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