# Building cantilever bridges

Building cantilever bridges
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A couple of days ago I enjoyed building a model of a cantilever bridge using our home-made set of jenga blocks. It wowed a few kids!

Note that the bridge spans a gap of 3 jenga blocks’ length.

Try building your own bridge, using any modular rectangular blocks. Can you span a larger gap? Can you come up with another design?

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I don’t have such blocks, unfortunately. But do try it again with a central block used as a keystone. I’m curious to know whether you can achieve the same gap using less blocks.

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Hmm I suspect that my blocks do not have a high enough coefficient of friction for this to work. Normally in an arch the blocks are curved so the keystone is held in place that way.

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That’s right. But it ought to be possible to place the centre block transverse, so that it has a diamond shape, seen from the same angle as your pictures. Then the lowest layer of the bridge will be arched. The upper ‘road’ layer can be placed flat, or mostly flat, over that, hopefully. I’m thinking, two layers, max, would be needed, so less blocks overall.

Mostly, the outer ends of the lower layer will need to be buttressed, also, otherwise they’ll just slide away as you load on the upper layer. In a river bridge, that would be done on/against the banks, probably with stone blocks set in the mud.

Sorry I don’t quite understand your suggestion re placing the central block transverse. This is a diagram of the bridge structure (a bit simplified - I’ve taken out 2 layers). Could you draw what you have in mind? Thanks.

Definitely it will work with fewer blocks - the top several layers are unnecessary. As you say, it does work with just a thickness of 2 blocks in the middle

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I’ll give it a try. I don’t have access to a computer for a couple of days, so will see what I can do with the phone, or else sketch it on paper. Whichever works easiest.

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