Allegheny River interstate highway bridge crack

Tugboat captain noticed a crack in an interstate highway bridge over the Allegheny in Pittsburgh growing between trips. By the time inspectors reached it, the crack was nine feet long and visible from 20-30 feet. Used to illustrate: (1) why pressure vessel safety factors exceed bridge/building safety factors (consequences of failure), (2) redundancy in bridge structures.

Yes, the consequences of failure of a pressure vessel are usually much more severe than the cost of a crack in a bridge or a building. When I first started, young faculty member here, there was a big brouhaha in the Federal Highway Administration because some tugboat captain who was going up and down the Allegheny River in Pittsburgh noticed that, when he went down the river, the crack in the interstate highway bridge he was going under was shorter than when he came back up. It was getting longer, noticeably longer. And when they got to it, it was like nine feet long. He could see it from 20, 30 feet away. This was not a small crack. They ended up closing the interstate highway until they got out there to fix it. It created a lot of consternation. But fortunately bridges tend to be redundant structures.