Pigeon Droppings Damage Bridges
Slow Corrosion Begins When Bird Droppings Build Up On Steel
The collapsed bridge in Minneapolis was used by pigeons and had suffered some corrosion due to the birds, but how significant was it?
In the wake of the devastating collapse of the Interstate 35 W bridge in Minneapolis, a number of news items have mentioned damage to the bridge structures caused by pigeon droppings. The chemistry behind the claims is this: pigeon droppings and the droppings of other birds are an acid mixture with a high nitrogen content in the form of ammonia, a molecule formed from nitrogen and hydrogen. (The nitrogen content is the same thing that makes bird droppings, or guano, popular as an agricultural fertilizer). When pigeon droppings dry out on steel girders the product is a salt—a chemical compound formed when a base, in this case ammonia, reacts with an acid. We all know what happens when water, salt, and steel interact—rust.
Pigeons and some other birds are well known for their preference for bridges as roosting and nesting sites. Those responsible for bridge maintenance have long sought a means to keep the birds from congregating there, with varying degrees of success. Apparently, bird droppings were noted as an issue of concern on the girders of the Minneapolis bridge in the past, but how significant is this? It would take a long time for pigeon droppings to undermine a bridge to the extent that it would collapse. It defies belief that a bridge receiving any degree of competent regular maintenance would collapse for this reason alone. No doubt investigators will eventually establish the cause of this tragedy. I’m willing to bet it wasn’t pigeons.
Read about ways that birds can be pests:
