Sunday, August 05, 2007

Ask a Civil Engineer

Like most people, I'm fascinated by the collapse of this Mississippi River bridge in Minneapolis.

Looking at this image, I can pretend to understand the forces involved. But to understand what really happened, I asked an expert bridge builder, my brother John:

"In 90% of cases like this the collapse is due to fatigue. I got a report today on this bridge that was done in 2001. The bridge was designed to old AASHTO standards that are now known to be very non-conservative for fatigue. That's why the 2001 study was done. The result of the study was that the bridge detailing (welding etc) is very vulnerable to fatigue, but that the stresses are low. They concluded that fatigue "shouldn't" be a problem.

"I wonder? I note from photos that only half the bridge lanes were open. So loading shouldn't have been a factor. I also noted that the deck repairs seem to involve a deck overlay (ie thickening of the deck). This would add a huge extra weight to the bridge. The other problem is that there are only 2 trusses carrying 8 lanes and the trusses have absolutely no redundancy. So any member failure could be catastrophic.

"The good thing, is that like all disasters, there will be no trouble getting funds for bridge maintenance etc for a few years until memories fade."

Photo by Mordac

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