Pounded and strained by heavy traffic and weakened by missing bolts and cracking steel, the failed interstate bridge over the Mississippi River also faced a less obvious enemy: Our friend the pigeon.
Yahoo reports that inspectors began documenting the buildup of pigeon dung on the span near downtown Minneapolis about two decades ago. Experts are saying that the corrosive guano deposited all over the Interstate 35W span's framework helped the steel beams erode faster.
They have not decided what caused the collapse, but pigeon dung is one of many factors that played a part.
"There is a coating of pigeon dung on steel with nest and heavy buildup on the inside hollow box sections," inspectors wrote in a 1987-1989 report.
They installed screens in the hollow sections of the beams in 1996 in hopes of keeping the pigeons out. Unfortunately that did not stop the dropping from building up everywhere.
The droppings contain ammonia's and acids. If it isn't washed away it dries and becomes a concentrated salt. If water combines with that salt and ammonia, it creates electrochemical reactions that rust the steel.
"Every time you get a little bit of moisture there, you wind up having a little bit of electrochemistry occurring and you wind up with corrosion," said Langerman. "Over a long term, it might in fact cause structural weaknesses."
But he is not ready to say that pigeon droppings caused the collapse.
This is a common problem with bridges. For instance, The Colorado Department of Transportation spent so much time cleaning pigeon droppings off of bridges that they are beginning a two-year research project looking for ways to keep pigeons away from its spans.
"It can be damaging to our structures because it's slightly acidic and it has other compounds in it that can dissolve especially things like concrete," said Patricia Martinek, the agency's environmental research manager.
In Denver, the DOT pays environmental specialists to clean their bridges wearing biohazard suits with respirators due to fears about bird flu and other diseases.
The art of keeping pigeons off bridges can often include netting to block holes, spikes so they can't land, poisoning, or even shooting.
Meanwhile, the NTSB has issued an update on their findings about the bridge collapse. They are investigating whether chemicals used in an automated de-icing system had any corrosive properties.
Oh my gosh, that is so fucked up! Fucking pidgeons! See, they have no other purpose on earth except to destroy homes, bridges and other bird's habitats! Anyone need a pidgeon exterminator?? Hire me! I've currently been able to exterminate most of the large population of pidgeons that tried to make new homes under our houses and they aren't trying to come back! Heh! I hate pidgeons!
