This is a shot of the bridge that until yesterday carried I-35W across the Mississippi river in Minneapolis. Built in the sixties, it’s not exactly old, at least not in British terms, and it apparently passed safety inspections last year. There have been works going on to the bridge deck recently, but apparently pretty minor.

However in the height of the rush hour yesterday, the bridge collapsed – not in a minor fashion, but catastrophically. The entire bridge deck plunged down whilst the bridge was busy with traffic. There are a number of sets of pictures appearing on Flickr – this one shows several views of the results.

Quite what caused the collapse obviously remains to be determined. Efforts are currently focusing on the immediate aftermath. However I’m sure before long there will be a full investigation, focusing on whether the safety inspections were carried out correctly, and whether whatever happened should have been spotted. I’m also expecting that there will be a slew of court cases as a result, especially if the maintenance is found to be at fault.

Having said that, looking at the bridge structure in the old picture, and then at the pictures of the collapse it seems that the two points that are designed to hold the bridge up are largely in tact – it’s the structure of the deck that has collapsed – looking at the picture here it looks like the whole structure has tipped backwards, and other shots show the towers holding up parts of the deck. It’s a long time since I did mechanics as part of my A-Level Maths, but it would seem that the structure remains stable due to the horizontal forces across the span, so if something catastrophic happened in for example the centre of the span, the horizontal forces are no longer being balanced and the whole thing collapses – quite how you could build it any other way I’m not sure – some kind of cantilever on the towers maybe, but really questions for bridge engineers I guess.

It does focus your attention onto how important engineering is to our day-to-day lives. People I’m sure drove back and forth over that bridge (about 200,000 cars a day), like so many others all over the world without a thought. Yet despite being regarded as mundane and taken for granted, when it failed it has caused so much damage. Currently the collapse has claimed seven lives, and injured at least sixty others. Certainly a sobering thought.

Old & New Mississippi River Bridges, Minneapolis, Minnesota originally uploaded by bridgepix.

Minneapolis I-35W Bridge Collapse originally uploaded by Diversey.