Tuesday’s Star Tribune carried a story about how problems with pigeons and spiders complicated bridge inspections over the past 13 years. How could those creatures play a part in a bridge going down? Read on.
Pigeon poop is a nuisance in all urban areas and was chronicled in this post to Science Buzz a few months ago. And evidently at the I-35W bridge, pigeon droppings were a big problem. Large numbers of pigeons were nesting in the box beam sections of the bridge structure from as far back as 1994. The box beam is vertical support beam between the bridge deck and the supporting floor beam below the bridge. The box beams had holes in them for inspectors to look inside, but that was also the access that pigeons were using to get inside and build nests.
With large numbers of pigeons in the bridge came heavy amounts of pigeon droppings. And the waste matter in those droppings can be very corrosive to metal. The solution taken in 1999 to solve that problem was to put plastic covers over the box beam holes. And those areas were some of the most critical areas for fatigue cracking that was occurring in the bridge. Some are now wondering if those plastic covers limited inspectors’ views of these critical areas of the bridge.
As for the spiders, inspectors said that the huge number of spider webs in and under the bridge could often be confused for bridge structure cracks.
The story also mentioned one other species that made inspections more challenging to engineers: humans. While the inspection work would be targeted to non-rush hour times from 9 a.m. to 3 p.m., inspectors said they still were often the targets of road rage from passing motorist who felt inconvenienced by having one lane of the bridge shut down during the inspection. Inspectors said they even had object thrown by passing drivers as they were trying to do an inspection.