An unnatural disaster in Minneapolis

Wednesday, August 1st, at 6:05 PM, the bridge that carried Interstate 35W across the Mississippi River just east of downtown Minneapolis, completely collapsed, dropping an unknown number of vehicles and passengers some 60 feet into the river and onto surrounding banks. At this time of writing, 4 deaths are confirmed, but that number is certain to rise since some cars were submerged, 20-30 people are missing, and many others are in critical condition.

This major disaster occurred during rush-hour traffic on a major thoroughfare that carried 140,000 vehicles/day. Citizens nearby jumped into action, even before emergency crews, and very quickly the scene was filled with rescuers. I must say that the local responders reacted quickly and effectively, and with considerable coordination. Within a short time, fire, police, and medical forces were in place and organized, along with structural engineers, the Red Cross, and other emergency responders. Homeland Security and the FBI were soon on site, trying to detect possible terrorist causes, and inspection of other bridges began. Alternate traffic routes were quickly designated to replace this key route.

Although construction workers were active on the bridge, it sounds as if their work was only surface, not structural, and that they did not contribute to the failure. No evidence has been found indicating foul play. At this point the collapse seems to have been caused by structural failure.

Reports are that the bridge has been repeatedly inspected, as late as a year before collapse. I've examined a research paper (pdf file) about extensive load testing of the bridge in 2001. Although some problems were identified, fixes were applied. This is a quote from that report:

The bridge, known officially as Bridge 9340, was constructed in 1967, is 581 meters long with 14 spans. While there have been no structural problems with the deck truss, there have been recent problems with the approach spans on both ends of the bridge. In 1997, cracks were discovered in the cross girder at the end of the approach spans.

Reports from people who were near the bridge when it collapsed say that it started buckling from the approach, and recent video camera coverage has revealed that the collapse started from the south end.

While watching coverage of the collapsed bridge, I was intrigued by the twisted steel support structure, covered by mostly intact sections of the concrete roadbed. I was also interested by the nearby 10th Avenue bridge, still standing. I accessed Microsoft's Live Search to get a good look at the two bridges before the collapse.

That's the 8 lanes of 35W on the front bridge, with the 4-lane Cedar Avenue bridge behind it. I'm no engineer, much less a bridge designer, but doesn't the supporting structure of the 35W bridge look positively flimsy? The piers are small, and the steel spans seem relatively frail for the load above.

I decided to pull together similar photos of other bridges across the Mississippi. Of the 16 bridges I looked at, the 35W bridge stands out as the most fragile in appearance. I have to say that, looking at aerial views of all those bridges, I can't be surprised that it was the one bridge doomed to collapse.

It also occurred to me that there are probably more films of the collapse. The local freeway system is covered by a large number of cameras. I'll guess that those films, and/or structural inspections, will decide that the steel members supporting the deck plates tore apart... that the cracks noted and patched for years finally gave way.

Reconstruction may take two years. For many years, and for many reasons, maintenance of highways has suffered here. Gas tax revenue that should have been spent on such maintenance, has been diverted for other projects, such as the ridiculous and horribly expensive resurrection of fixed-rail trains... the so-called light rail.

It may be impolitic to talk of responsibility immediately after a tragedy, but it is not unreasonable. This bridge, the longest in the Twin Cities, the widest river bridge here, carried heavy traffic for 40 years. It collapsed under loads and conditions that were not unusual... in reasonable weather conditions. That naturally leads one to think that this collapse was avoidable. I'll wager that there are current and former MNDOT employees who are furious today, having warned about potential problems and been ignored. Government systems are notorious for subduing uncomfortable truths and silencing employees trying to get the truth aired. There are whistleblower laws in place to protect such employees, but government agencies can bring a lot of pressure to bear.

We're going to hear a lot of political rhetoric, expressing sympathy, praising the systems that respond after an emergency. Politicians at every level will make appearances and pronouncements. The Federal Department of Transportation has already said that the state has responsibility for correcting any problems. There were clearly deficiencies revealed by inspections. Will we ever get a clear picture of why those problems were not sufficiently corrected? Knowing government, I really doubt it. Covering up is one of the few things such systems do very well.