The investigation into the Minneapolis Bridge collapse is focusing on several key areas that could have played a part in the tragedy that left 13 people dead and 100 others injured. Yesterday, the National Transportation Safety Board (NTSB) said that investigators were looking at the corrosive properties of de-icing chemicals used on the I-35 W Bridge. Other news reports said that past inspections had raised concerns about a large build up of pigeon dung on the Minneapolis Bridge that could endanger its structural integrity.
Yesterday, the NTSB issued an update on its investigation into the Minneapolis Bridge collapse. The investigators said that they will be examining the de-icing system used on the bridge to determine if the chemicals that were part of that system might have had corrosive properties. According to the Minneapolis Star Tribune, dangerous black ice was a constant problem on the I-35 W Bridge in the winter because of mist from the nearby St. Anthony Falls. The Minnesota Department of Transportation (MNDOT) had installed temperature and precipitation-activated nozzles on the bridge to keep it free of ice. The nozzles sprayed a chemical called C7 onto the bridge. C7 is a liquid potassium acetate made by Cryotech. C7 contains no nitrogen or chlorides, and is considered safer than other de-icing chemicals.
But according to the Minneapolis Star Tribune, MNDOT discovered that C7 reacted with galvanized metal when some was accidentally spilled onto a grate. In 2005, the manufacturer issued an alert warning that C7 would react with zinc. Zinc is used in galvanized steel. But Cryotech said that the reaction was slow, and did not occur with normal use. According to the Star Tribune, MNDOT initiated an investigation in order to see if C7 had caused any significant deterioration on the Minneapolis Bridge. That investigation concluded that the galvanized steel on the bridge was thick enough to withstand the C7 reaction.
The NTSB is also said to be examining a possible design flaw in the steel gusset plates which bound the I 35-W Bridge’s trusses together. The NTSB is also taking a hard look at the construction that was being done on the Minneapolis Bridge when it collapsed. Yesterday, investigators said that they had determined that the weight from construction on the bridge to be around 575,000 lbs.
Other media outlets reported yesterday that in 1989, MNDOT inspectors had begun to raise concerns about the build up of pigeon droppings on the I-35 W Bridge. Pigeon dung contains large amounts of ammonia and acids, and if not washed away, it can become quite corrosive. Over the long term, it can weaken structures like the Minneapolis Bridge. In 1996, openings on the I-35 W Bridge’s beams were covered with screens in an attempt to keep the birds from nesting there. Pigeon droppings are a common problem on bridges, because the birds are naturally drawn to cliff-like structures.
The I-35 W Bridge collapsed on August 1, at 6:05 p.m. It was the height of Minneapolis’ evening rush hour, and cars were lined up bumper-to- bumper across the span. At least 88 vehicles and hundreds of people fell 60 feet into the Mississippi River below. It would be nearly three weeks before the final victim of the Minneapolis Bridge Collapse was pulled from the river.
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