Pigeon Poo Blamed for Deadly Minnesota Bridge Collapse
Last updated at 17:13pm on 24th August 2007
Experts have put forward the theory that steel beams rusted faster because of the ammonia and acids in the bird droppings. But there are many theories.
Inspectors began documenting the buildup of pigeon dung on the span near downtown Minneapolis two decades ago. "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.
In 1996, screens were installed over openings in the bridge's beams to keep pigeons from nesting there, but that did not prevent the building up of droppings elsewhere.
Pigeon droppings contain ammonia and acids, said chemist Neal Langerman, an officer with the health and safety division of the American Chemical Society.
If the dung is not washed away, it dries out and turns into a concentrated salt.
"Over a long term, it might in fact cause structural weaknesses," said Langerman. When water gets in and combines with the salt and ammonia, it creates small electrochemical reactions that rust the steel underneath.
However, the exact cause of the collapse has not yet been confirmed.
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