How does Tower Bridge work? Part 1. - Tower Bridge
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How does Tower Bridge work? Part 1.

9th November 2018 • Tower Bridge Blog

How does Tower Bridge work? Part 1.

Ask anyone to name one fact about Tower Bridge, and the majority will answer that the road opens up to let boats through. But have you ever wondered how Tower Bridge works?

It is often assumed that Tower Bridge is a drawbridge, as the way the road opens up looks very similar to what you would expect to see on a medieval castle. This was actually how Tower Bridge’s architect, Horace Jones, initially intended the roads to open and was the design submitted to the Tower Bridge design competition. However it was discovered that the roads would be too heavy for the towers to hold so a new solution was needed!

That answer was to make Tower Bridge a bascule bridge. The word “bascule” is a French word, which can be translated as see-saw and describes how the two sides of the road (bascules) of Tower Bridge open. Each bascule moves around an off-centre pivot, meaning they look a bit like a seesaw in a children’s playground, which you can see on the picture above. They are better than a drawbridge because each side of the bascule is evenly balanced, meaning less energy is required to open up the roads and no pressure is put on the tower during opening and closing.

So, instead of chains pulling up the bascules from the towers, the hard work of opening and closing the bridge is left to eight large cogs about 1m in diameter, four on each side, which rotate to open and close the bridge.

The power required to rotate the cogs was initially supplied by steam and then, post 1976, by electricity. Find out more about how the cog was powered before 1976 in part 2 of our blog next month.

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Tower Bridge: History • Engineering • Design