Tuesday, January 21, 2003

Suspension bridges, and cable stayed bridges

I have discussed this at least in passing before .

A suspension bridge such as the first Severn Crossing, above) is a bridge where the towers are connected to each other by cables. (The shape of the arc of the cables is a shape called a "catenary", by the way). The deck is then held up by vertical cables that connect to the main catenary shaped cables. The key point is that the only stresses on the deck are vertical.

A cable stayed bridge (such as the Second Severn Crossing, above) is a bridge where the cables are connected directly from the towers to the deck. If you build a bridge this way, the towers have to hold a lot less weight, and the bridge can be much less massive, and therefore much cheaper to build. However, the stresses on the deck are horizontal as well as vertical, and therefore the deck has to be made out of something stronger than is the case for a suspension bridge. For this reason, it was not practical to make large cable stayed bridges until materials that could withstand greater lateral stress than could traditional materials were developed in the1980s, and this sort of bridge only really became a big deal in the 1990s. However, you now see them everywhere. For the very largest bridges (longer than 1000m) cable stayed bridges are still impractical.

When I was in Normandy over Christmas, I went and saw the Pont de Normandie across the mouth of the Seine connecting Le Havre and Honfleur. I took some photographs, but unfortunately it was an overcast day and the light coloured bridge didn't show up very well. I scanned one of these and was going to post it with yesterday's article, but it wasn't a very good photo so I refrained from doing so. I should have found a photo somewhere else but it was time for bed.

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