January 21, 2003

An "utter revolution" in bridge technology (but what's a cable stayed bridge?)

Brian Micklethwait | Other

Michael Jennings has a long response today to a samizdata piece by Paul Marks which claimed that technology, despite the journalistic chatter of our time, is not actually advancing very fast. Buried in this piece are comments on bridge-building technology, something which apparently has been advancing quite fast recently:

… As for bridges, in the last 15 years we have seen an utter revolution. More advanced materials science means that cable stayed bridges have become practical where suspension bridges were needed before. Cable stayed bridges can be built for a fraction of the price, so we have a golden age of bridge building. In December I visited the Pont de Normandie, the second longest cable stayed bridge in the world (856 metres). 20 years ago a suspension bridge which would have cost several times as much to build would have been required.

The longest bridge in the world was 1410 metres in 1997. It is now 1992 metres: the longest bridge being a particularly economically pointless bridge in Japan. It seems likely that a bridge connecting Italy and Sicily will soon be built, with a span of approximately 3000 metres. …

So, all I now need to know is what a "cable stayed" bridge is. Is it perchance one of those bridges where lots of wires attached to the poles holding the thing up radiate down, sometimes at varying angles (but sometimes at an angle in parallel), to all the different points along the side of the road, railway or walkway?

I find Jennings to be one of the few techno-bloggers whom I enjoy reading and from whom I actually learn things. And in addition to writing so clearly (mostly), he understands that there are more things going on in the world than just technology. (See also, e.g., his piece on Detroit, 8-Mile, etc.) But like all super-knowledgeable super-geeks, he sometimes explains absolutely everything about what he's talking about, except what he's talking about.

Cable stayed bridges? But everyone knows what they are. Don't they? Not me.

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There are some pictures in this earlier post of mine. When I was in Normandy over Christmas, I went and saw the Pont de Normandie across the Seine near 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 my article, but it wasn't a very good photo so I refrained. I should have found a photo somewhere else but it was time for bed.

This is one of those situations where a picture tells a thousand words, but where I didn't provide a picture. A suspension bridge (the second picture in the posting, of the first Severn Crossing, is one of those) is a bridge where the towards 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 (the first picture in the posting, of the Second Severn Crossing) 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. 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 the 1980s, and they only really got going in the 1990s. However, you now see them everywhere. For the very largest bridges (longer than 1000m) cable stayed bridges are still impractical.

Posted by Michael Jennings on January 21, 2003

Well Brian, you've got the idea. The main difference between cable-stay and suspension is that the suspension bridge deck is supported ENTIRELY by the cables, hanging straight down from a main cable slung from counterweights, over the towers. We are all familiar with sagging cable that denotes a suspension bridge. A cable-stayed bridge is supported partially by the piers along its length, but the cables coming down from the pylons (radially or in parallel as you described) hold up the deck in the middle to allow for longer and longer spans. As this type of bridge has no massive main calbe like a suspension bridge, they are significantly less expensive and I find the design more esthetically pleasing as well.
One last thing, Michael referred to "the longest bridges" a few times. I'm not sure if he meant longest suspension bridge because the longest bridge is the COnfederation bridge from New Brunswick to PEI in Canada at 22km. It is a cantilevered concrete box-girder bridge. There is a plan for the longest suspension bridge with a centre span of >3km and a total span of ~5km from Italy to Messina in Sicily and I believe construction is just beginning there or will soon. Don't quote me on that one.

Hope that cleared a few things up for you...

Graham

Posted by Graham Cranston on October 29, 2004

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