August 2, 2007 at 12:33 AM by Dr. Drang
It is, of course, too early to know what caused the I-35W bridge to collapse. Based on the traffic density in the approach spans, it seem unlikely that the bridge was heavily loaded at the time of the collapse. I’ve heard that the bridge is 40 years old—unless the photos and videos on the news are very misleading, it has almost certainly had higher loads earlier in its life.
There’s been some attention given to the construction work being done on the bridge, because a failure during or shortly after repair work is always makes the repair work suspect. But repairs seem to be limited to the bridge deck, not the steel structure under deck, which is what collapsed. I’m pretty sure this document (PDF) describes the work that was being done. It’s described on the first page as “CONC CRACK & JT REPAIR, CONC PLANING, SURFACING, BRIDGES ETC.” “CONC” is “concrete;” “JT” is “joint,” which refers to the expansion and control joints in the concrete bridge deck, not the bolted and welded joints in the supporting steel structure; concrete planing is a grinding operation that removes the high spots in a concrete slab.
Although the overall impression is that the bridge went straight down, some of the views suggest a sideways or twisting motion of the deck at one of the banks of the river. My initial guess is that that will be important in determining the cause.
Again, everything is a guess right now, but those are my initial impressions. And this is a topic I deal with in my non-blogging life.