There’s one thing that they do particularly well at the Ecole des Ponts et Chaussées (School of bridges and roads), and that is teaching how to build bridges and roads. I had my last lecture today in a course called construction métallique. It has been one of the hardest courses I have taken here but has also been one of the most enjoyable. As well as covering all the theory of how metal structures ‘work’ we have also looked at a range of specific examples, including the Stade de France, the pyramid at the Louvre and the Milau Viaduct. Today’s final class was given by an expert on metallic bridge construction and he had the following to say about America’s brutal motorway bridge design.
In France they keep it simple: put two metal beams across and then fill the gap between the two beams with a concrete deck. I say simple because it is very easy to work out how strong each beam needs to be, and that, after all, is what we engineers are paid for, right? In the USA however, things are not quite so straight forward. Famously low budgets for construction have lead to the use of lower quality materials and so there is a greater chance these bridge beams could fail. Each state has it’s own set of rules (which must make for nightmares when trying to build an interstate highway) but in all states, they are so worried about the strength of their beams that instead of allowing just two, they require five.
When I was living in the States, I noticed how brutal the motorway architecture could be. And now I think I know why. It is very difficult to make five enormous steel beams under a bridge look elegant. There is also no architectural budget, so all you get is the bare minimum. This five beam system also makes it very difficult for the engineer to work out what is going on. For reasons that I won’t go into here, when you have five beams under a bridge, it is complicated to calculate which is supporting the car and which is sitting there looking ugly. The ultimate irony is that if they spent a little more on materials, they would of higher quality and so they could use much less.
A ‘brutal’ New Jersey bridge across an esturary close to New York
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