Blue Crab Boulevard

Winged Disaster

Investigators of the Minneapolis bridge collapse appear to have determined at least a potential contributing factor to the disaster. Pigeons.

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: 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.

Although investigators have yet to identify the cause of the bridge's Aug. 1 collapse, which killed at least 13 people and injured about 100, the pigeon problem is one of many factors that dogged the structure.

The specific problem: pigeon guano.

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 isn't washed away, it dries out 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 wasn't 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 power plants I worked at had active programs to keep pigeons off the structures. It was a continuous battle with the little beasts. But it was bad for the steel and the concrete and also a health hazard for humans. If the pigeons did contribute, watch for the calls to get rid of them mount. Tom Lehrer said it best:

We've gained notoriety,
And caused much anxiety
In the Audubon Society
With our games.
They call it impiety,
And lack of propriety,
And quite a variety
Of unpleasant names.
But it's not against any religion
To want to dispose of a pigeon.

So if Sunday you're free,
Why don't you come with me,
And we'll poison the pigeons in the park.
And maybe we'll do
In a squirrel or two,
While we're poisoning pigeons in the park.
(Tom Lehrer, Poisoning Pigeons in the Park)

Animals | Gaius Wednesday, 22 August , 2007

By Thomas, Wednesday, 22 August , 2007 @ 4:44 pm

There is a statue of Winston Churchill in London that has a low level electrical current running through it to keep birds off of it.

It’s not a strong current, not even enough to make a spark, just enough to tingle if you touch it. Similar devices are used to shore up the fences on horse farms.

Why not install something similar on bridges.

Honestly, though, I’m a bit skeptical. There are older metal bridges in the US that deal with just as many pigeons. Is there a single other incident of a structure collapsing in which this was a major contributing factor?
By feeblemind, Wednesday, 22 August , 2007 @ 5:16 pm

Concur with Thomas. If pigeon guano causes bridge failure, how come we haven’t seen it before?
By Gaius, Wednesday, 22 August , 2007 @ 5:41 pm

It has been well documented - engineers know it is a problem. Here’s the effects on an outdoor sculpture.
By Mwalimu Daudi, Wednesday, 22 August , 2007 @ 5:54 pm

I am waiting for some “Truther” to claim that the pigeons were acting under Karl Rove’s direct orders, and that it was an effort to rub out some critic of the Iraq War.