Pigeon Dung helped Minneapolis Bridge Collapse

ST. PAUL, Minn. — Pounded and strained by heavy traffic and weakened by missing bolts and cracking steel, the failed Interstate 35W bridge over the Mississippi River also faced a less obvious enemy: Birds, specifically pigeons.

Inspectors began documenting the buildup of pigeon dung on the span near downtown Minneapolis two decades ago.

Experts say the corrosive guano deposited all over the span's framework helped the steel beams rust faster. . . . .