I-35 Bridge Collapse.

As most everyone knows by now a large bridge over the Mississippi river on I-35 collapsed a few days ago. Over the past several days TV talking heads have repeatedly opined that a 40 year old steel bridge has outlived its expected life span and should have been replaced. On other Internet forums I’ve seen modern bridges compared to ancient Roman Empire stonework roads and bridges that are still standing, alleging that modern civil engineering is shoddy compared to ancient roads and bridges. Perhaps they are right, but I've done quite a bit of construction work and know a small thing or two about bridge design and construction. I don't know of any reason why most of the bridges constructed in the past century in the US won't last at least a couple of centuries if they have reasonable maintenance, painting, repaving, etc.

One of the problems is the steady parade of 40 ton trucks that roll down our roads day and night. When they go by the ground flexes, the concrete flexes, the steel flexes, and stone flexes. A constant parade of such trucks would hammer the best Roman roads into oblivion in the first year. In cold climates the carbide studs in our tires cut the hardest paving stones into dust in a few years. We won't drive without them because they save our lives in winter, but they are murder on any road surface. The bridge that collapsed carried an estimated 140,000 vehicles per day. None of the Roman roads were built to survive 140,000 vehicles per day, not even ox carts. 140,000 vehicles per year on the Apian Way probably was unthinkable, and none of them weighed 10 tons.

Typical roman engineering also never had to survive chemical de-icers used and the deep freeze/thaw/deep freeze cycles that civil engineering in more northern climates has to live with. A few Minnesota winters with water seeping in between stones and then freezing would soon make a mess out of a roman stone bridge. Nevertheless good bridge engineering should last a couple of centuries even with the constant pounding of heavy trucks. Steel does not have the same kind of fatigue problems that aircraft aluminum designers have to deal with. Keep it painted and keep it from rusting and a well designed steel bridge should last virtually indefinitely.

I noticed that the NTSP has said that they are beginning their inspection at a section that fell 50 feet sideways instead of straight down, and which was apparently the first section to fall. I have a couple of initial questions that I would love to ask someone about the bridge:

1) There are outriggers on the steel structure holding the outer lanes of the roadway. The roadway overhung the structural steel by several feet on both sides. Was that the original structural design, or was the road deck widened and the number of lanes increased at some time after the bridge was constructed?

2) The repaving project closed 4 of the 8 traffic lanes, but not in a balanced mode. Traffic in open and closed lanes at the time of the failure was |HH^^HH^^|. Occupied lanes on one side (but not the other) were out on the outrigger overhang past the edge of the structural steel truss beneath. Did the unbalanced loading of traffic lanes cause a twist force in excess of that for which the structure was designed.

3) Many bridge road decks are built with "lightweight" concrete which saves a lot of weight. More expensive aggregate that feels like foam brick material is used, but its more expensive. Did the repaving project replace lightweight concrete with regular heavy concrete?

These questions should be fairly easy to answer for anyone from the Minnesota Highway department who is involved with bridges. The answers might point to poor decisions by those responsible. I haven't heard anyone ask them from the media, but I suspect that the NTSP will ask them fairly soon.

Kary Witt, the current bridge manager of the 70 year old Golden Gate Bridge, said the life span of that bridge is "more or less infinite'' if the structure is properly maintained. Story here. That really sums up my opinion. There is nothing intrinsic about a steel bridge that wears out other than the road surface. As long as maintenance is done to prevent rust, repair the wear on the surface, etc., a well built bridge should last almost forever. Bridges are not like animals. We get old and die after a while. Steel doesn't.

Last Thursday evening as I watched the initial reports about the bridge collapse I wondered out loud how long it would take the left wingnuts to start blaming Bush for the bridge failure. It didn’t take long. On-line leftists are already blaming “conservatives” and Democratic strategists are discussing how to make political gains out of this human tragedy. Blaming bridge failure on "Conservatives" is the old tired political nonsense that gets nowhere and solves nothing. I've had that kind of partisan politics up to here. Political parties are already using this disaster to blame each other and try to gain votes, and I find that offensive.