By now, I assume that everyone reading this blog has heard that the I-35W bridge over the Mississippi in downtown Minneapolis collapsed last night during the evening rush hour.

As rescue operations become recovery operations, I'm thankful that the human toll seems to be remarkably low for a disaster of this magnitude. Everyone I know is fine, though my oldest son was worried last night that some of his school friends were hurt (unlikely, as we live about 20 miles from the bridge in question).

It's going to take years before downtown Minneapolis is back to normal. This bridge is one of five major freeway routes into Downtown, and one of only two freeway bridges over the Mississippi in Downtown (there are also several bridges connected to city streets, but they're not capable of handling anywhere near the 160,000 cars/day which used to use the I-35W bridge). The collapse also cut off river traffic and a major rail line.

I used to live about a block from the bridge--you can see my old apartment building in many of the media photos and videos--and I was never that fond of it. Much of the riverfront of Minneapolis has been revitalized in recent decades, but the 35W bridge has remained an eyesore. I hope they replace it with something more attractive.

There will be a thorough investigation, and whatever the final report says will almost certainly have repercussions. These kinds of events usually have a chain of circumstances which ultimately lead to the disaster, so there usually isn't just one cause.

My totally uninformed and unexpert guess is that the final NTSB report will likely cite some combination of these factors:

1) Bridge design which met 1960's standards, but which isn't sufficient by modern standards (possibly related to traffic load and/or vehicle weight significantly above the original design parameters).

2) Structural weakness which was either undetected in recent inspections (possibly undetectable), or underemphasized in the inspectors' reports.

3) A sustained period of hot weather, and/or construction during the hot weather.

4) Vibration from construction contributing to the actual moment of collapse.

5) (Unlikely but possible) accelerated corrosion from the de-icing chemicals used on the bridge deck in recent years.

The wild card is whether the engineers and inspectors working on the bridge were under any pressure to overlook or de-emphasize known problems. Minnesota's budget for road construction and repair has been very tight the past few years--thanks to a gas tax which hasn't been raised in decades and a governor who refuses to even consider the idea--and replacing or overhauling the I-35W bridge would have been a hugely expensive and disruptive project.

While nobody has suggested this yet, I can certainly see how, in an environment of very tight budgets, it would be tempting to overlook problems with the I-35W bridge (which is, after all, only 40 years old) in favor of other pressing projects which would be cheaper and easier.