bridge through Mississippi in Minneapolis

Some experts consider that pigeons’ dung caused the bridge through Mississippi in Minneapolis failure. It has speeded up the corrosion of steel beams of the bridge – ammonia and acid containing in pigeons’ dung, made for metal corrosion.
As the representative of American chemical society Neal Langerman has declared: “During long time it could weaken a steel skeleton of the bridge”.
So, the version is good enough… as it has failed for some reason. And it is very convenient – pigeons are guilty, and builders are innocent…