Pigeons took toll on Minn. bridge
Corrosive waste speeds rusting
ST. PAUL, Minn. -- Pounded and strained by heavy traffic and weakened by missing bolts and cracking steel, the 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.
Specialists say the corrosive guano deposited all over the span's framework helped the steel beams rust faster.
Although investigators have yet to identify the cause of the bridge's collapse on Aug. 1 that killed at least 13 people and injured about 100, the pigeon problem is one of many factors that afflicted the structure.
"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 buildup 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 and turns into a concentrated salt. When water gets in and combines with the salt and ammonia, it creates small electrochemical reactions that rust the steel underneath.
"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."
Langerman emphasized that he was not saying pigeon dung factored into the collapse of the 40-year-old bridge.
"Let's let the highway transportation and safety people do their job," he said.
The problem is familiar to bridge inspectors everywhere.
The Colorado Department of Transportation spent so much time cleaning pigeon manure off bridges that it is embarking on 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.
Pigeon guano is not just a danger to the bridges.
In the Denver area, the Colorado DOT pays outside environmental specialists to clean bridges wearing full biohazard suits with respirators because of heightened fears about bird flu and other diseases, said Rob Haines, who supervises maintenance there.
Keeping pigeons off bridges usually requires a multipronged strategy that can include netting to block holes and surfaces, spikes to keep them from landing, and sometimes poisoning, shooting, or trapping the birds, said John Hart, a Grand Rapids, Minn.-based wildlife biologist with the US Department of Agriculture.
The problem is that pigeons are naturally drawn to bridges and tall buildings since they are descended from cliff-dwellers, said Karen Purcell, who heads Project PigeonWatch at the Cornell Lab of Ornithology.
Bridges offer shelter from predators and flat surfaces for nesting and roosting. "It's a nice fit for them," Purcell said.
Meanwhile, representatives of Minnesota and North Dakota met yesterday to discuss repairs to a highway bridge that was closed Tuesday after a crack was found in a girder support bracket, said Les Noehr, a district engineer for the North Dakota Department of Transportation.
The 53-year-old bridge over the Red River between Robbin, Minn., and Drayton, N.D., would not have collapsed even if the cracked bracket had failed, said Minnesota's state bridge engineer, Dan Dorgan.
The span is scheduled for replacement in 2009. The nearest alternate bridge is 17 miles away.
The National Transportation Safety Board issued an update on its findings in the collapse yesterday, saying investigators are looking at whether chemicals used in an automated deicing system had any corrosive properties.