Thursday, August 2, 2007

The bottom dropped out

During the rush hour of August 1, 2007, what began with a rumbling on the Interstate 35W bridge between St Paul and Minneapolis MN, ended in catastophe. The 40 year-old bridge, which was in the middle of having it's surfaces repaired as part of the improvements along that stretch of the interstate, collapsed into the Mississippi River.

Russ Knocke, spokesman for the Homeland Security Department, stated that they had received no indications that the collapse was related to any act of terrorism. An estimated 60 vehicles fell into the water and Mayor R T Rybak announced later in the evening that seven people had been killed and an unknown number had been injured. Dr Joseph Clinton, emergency medical chief at Hennepin County Medical Center, said that the hospital had treated 28 injured people with six of them in critical condition. Other area hospitals were also treating victims and by 11 pm est, 38 were reported injured but rescue teams are still working and the numbers are expected to rise.

"There were two lanes of traffic, bumper to bumper, at the point of collapse. Those cars did go into the river. At this point there is nothing to suggest that this was anything other than a structural collapse," stated Minneapolis police Lt Amelia Huffman.

A school bus with children aboard had just crossed the bridge and was stuck one slab, along with a burning truck, without falling in. News reports state that the children had been safely exited from the rear of the bus. At least eight cars and a truck were submerged in the river. Dozens of vehicles could be seen stacked on top of each other and scattered throughout the rubble. Some people were stranded on parts of the bridge that weren't underwater.

The arched bridge that had been built in 1967 and rose approximately 64 feet above the water. Sen Norm Coleman, R-Minn, stated that while the focus right now is on the emergency response, authorities will also have to set up an alternate transportation system. According to him, the bridge had been inspected in 2004 and that sometime in the near future, a forensic analysis will have to be performed to find the cause of the collapse.

This bridge collapse is rare but not unheard of in recent history. June 28 1983, in the darkness of the midnight hours, a 100 foot deck of the Mianus River Bridge on Interstate 95 in Cos Cob CT, collapsed. Three people were killed and three injured when their vehicles fell 70 feet into the Mianus River. Some vehicles fell with the span, while others drove off into the river, unable to see the missing span until it was too late.

The collapse was later found to be caused by the failure of two pin-and-hanger assemblies that held the bridge in place on the outer side of the bridge. The combined weight of two heavy trucks and a vehicle on that section cused a remaining expansion joint to fail and the deck plummeted to the river. It was found that highway drains had been deliberately blocked during previous roadmending and that led to the corrosion of the pins. The bridge collapse had occured even after the national inspection procedures that had been brought into place by the December 1967 collapse of the Silver Bridge in West Virginia.

Interstate 95 was not fully reopened for use for another six months and that was due to the installation of a "temporary span." The final repairs with a permanent span cost approximately $20 million. Mayor R T Rybak is facing a much larger problem than was faced in CT with their bridge collapse, the sections that collapsed span a much larger area. After the rescues have ended, he will have to find a way to rebuild not only the bridge, re-route traffic and try to begin the healing within his city.

"This will be a very tragic night when this night is over," stated Mayor R T Rybak.

Update: posted Aug 2

The number of fatalities in the Interstate 35W bridge collapse has been lowered to 4 as of the morning. The initial reports of seven had been based on the best estimates that authorities had made late Wednesday night although they expect that number to change throughout the following days. There are reports that many of the 79 people that have been treated at area hospitals for injuries arrived there two and three to an ambulence. There are an estimated 20 to 30 people still missing according to Police Chief Tim Dolan.

Gov Tim Pawlenty stated that the bridge had been inspected in 2005 and 2006 and that only minor things needed attention, there were no major structural problems noted then. He also stated that it might need to be replaced or rehabilitated in 2020 or beyond. The Star Tribune has reported that the U S Department of Transportation's National Bridge database listed the bridge as "structurally deficient" two years ago and in need of possible replacement. Jeanne Aamodt, spokeswoman for the Minnesota Department of Transportation said that there are many other bridges around the country that carry that same designation.

Road crews had been working on the bridge's joints, guardrails and lights this week. There were 18 construction workers on the bridge at the time of the collapse and at this time, only one is unaccounted for. Tom Sloan, head of the bridge division for Progress Contractors Inc. in St Michael said his workers described a horrific scene as the bridge collapsed.

"They said that they basically rode the bridge down into the water. They were sliding into cars and cars were sliding into them," he stated.

The collapse of the Silver Bridge WV, on Dec 18 1967 was supposed to be the wake-up call on the condition of America's bridges. Until then, very little had been done to inspect the condition of them, once they had been completed. After years of corrosion and neglect, a crucial joint in the 30 year-old bridges suspension system snapped and normal vibrations from heavy rush-hour traffic shook it apart. Dozens of cars and trucks followed the structure into the river. A simple monument stands today, near the West Virginia approach to the bridge, inscribed with the names of the 46 people who died in the collapse.

In April 1987, the Scholarie Creek Bridge on the New York Thruway near Albany collapsed after a flood undermined the bridge supports in what is known as "scour." Ten people died in that collapse and while it wasn't due to neglect, it alerted officials to the dangers a natural disaster can bring to bridges as well.

Connecticut suffered the collapse of two buildings as well, the L' Ambiance Plaza that collapsed during construction in Bridgeport and the Hartford Civic Center. On April 23, 1987, the planned sixteen-story building in Bridgeport collapsed halfway through construction due to defiecient temporary connections and killed 28 workers as the floors pancaked onto each other.

Late in the night of January 18 1978, after the crowds from a sporting event had long left the Civic Center and following a heavy snowfall, the roof the Hartford Civic Center collapsed. At the time, the construction of the area roof had been innovative, being that it was constructed completely on ground level and then raised into position. During construction, deflections of the struts were observed and in 1974, one year after completion, a citizen expressed concerns to engineers about an obvious downward deflection of a large section of the roof. Engineers assured authorities in Hartford that the roof was fine....... that was until it collapsed.

Probably the most horrific collapse of a bridge in this country's recent history didn't involve the vibrations due to vehicular traffic but instead, human traffic. In the evening of July 17, 1981, between 1500 and 2000 people filled the atrium and walkways of the Hyatt Regency MO to watch a dance compitition. At 7:05 pm, a loud crack was heard to echo throughout the building and the 2nd and 4th floor walkways collapsed to the ground. A lack of redundancy in the supporting rods was found to be the cause of the collapse that killed 114 people and injured over 200 that day.

Despite the increases in safety inspections since the years of our early bridge construction in this country, it seems as though history is proving we may need to make more indepth and serious changes to how we identify and fix what look to be "safe" bridges.

Update: posted Aug 9

U S Transportation Secretary Mary Peters has issued a statement that cautions states to consider the additional weight placed on America's aging bridges during construction or repair projects. The word of caution was raised by a new report from the NTSB that raised concern about the gusset plates that tied the steel beams together. Ferderal officials are working on verifying the loads and stresses that were placed on those beams before the bridge failure. The report also raises suspicions over whether the material was too weak or that the loads had been miscalculated during the bridge's construction.

Officials from the Minnesota Department of Transportation wouldn't comment on the significance of the gusset plates in the bridge collapse. Gary Peterson, the state's assistant bridge engineer had commented to the New York Times that inspections in the previous decades had raised red flags about the gusset plates on the Interstate 35W bridge.

It has been shown in previous collapses that the incorrect loading or initial failures of materials have led to failures months and years into the future. If it is found that there was little incorrect in the initial construction but that time and increased load weights on the bridge led to it's collapse, then the U S certainly has the large task of inspecting similar bridges to prevent another collapse.

For my post detailing the I-35W bridge inspections, go here.

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