Edible engineering - how tall can you build?

19 February 2018

Interview with

Allan McRobie - Cambridge University

Chris Smith pitted our experts Anglia Ruskin behavioural ecologist Sophie Mowles, Cambridge University astronomer Matt Bothwell and Cambridge University neurocriminologist Kyle Trieber against each other as, armed with dried spaghetti and marshmallows, they each raced against the clock to build the tallest tower. Cambridge University engineer Allan McRobie was ready to judge...

Chris -  We’ve got a little competition for the panel. We’re going to put Kyle’s, Matt’s and Sophie’s construction skills to the test and engineer Allan is going to act as the judge!

Everyone’s got in front of you some spaghetti - there’s about 15 spaghetti sticks there. We’ve also given you a well known building material which Allan mentioned earlier, which is marshmallows, and you’ve got about 15 marshmallows. If you eat them, which you can do, you’ll just have less to build with.

Because what we want you to do is to build the tallest structure that you possibly can, but crucially it has to be free standing. So we’re looking for the tallest free standing structure.

You’ve got 3 minutes to do it, and then Allan will judge you.

Off you go you three. Kyle, Matt and Sophie, you three get building.

Allan, you and I can have q quick chat between us about what they ought to be doing. We’ll assume they’re not listening!

What would be a good tip for them to make the most stable structure?

Allan - As I said before, wide at the bottom - thin at the top. Triangles are good. Rectangles have the ability to rack and sheer.

Chris - Sort of change into a rhombus?

Allan - Yes, change into a rhombus. If you do see a rectangle you need to put a diagonal across it pretty quick.

Chris - And that stops the length changing?

Allan - I’m talking to Sophie.

Chris - Don’t give them too many tips. But basically, your advice would be: big wide base, and taper to a point. Why is that particular configuration particularly stable though? Why are the buildings you referenced earlier, from an engineering point of view, why are they ideal shapes and structures?

Allan - It puts the centre of gravity lower down so that makes it more stable for a start. The load gets spread out over a wider area at the base.

Chris - They’re using marshmallows and spaghetti. Is that a reasonable model for what an engineer would use in a big building? We don’t use marshmallows and spaghetti obviously - what do you tend to use?

Allan - Steel and concrete mainly. Timber’s beginning to become ever more popular now. People are trying to go higher and higher with timber. Timber’s something of a new material strangely, even though it’s been around for millenia, people are now trying to build skyscrapers out of timber.

Chris - Bamboo?

Allan - Bamboo as well, yes. There’s a lot of work about that in Cambridge. You can build amazing scaffolding out of bamboo as they do in the Far East. It goes right up the side of the skyscraper all out of bamboo.

Chris - Of course, the benefit of using bamboo for your scaffolding is that it’s much lighter than if you use big steel poles. Instead of having to have very big steel poles to support the weight of even more big steel poles, you are able to make the whole structure much lighter.

Allan - Yes, you’re right. Then it’s all about the joints with the bamboo, because the bamboo is incredibly strong but then you have to tie it together somehow.

Chris - Right well, time up. You have to stop building I’m afraid team. Allan, what are your thoughts? Shall we ask each of them? Kyle could you just talk us through your construction?

Kyle - Yeah, sure. It looks something like a drunken camel. I’ve got four big marshmallows on the base and then they’re tapering up to two, so you can imagine the four hooves going up to the front and back of my camel. There’s a body in between which is made out of one of these noodles, and then there’s one single neck going up that didn’t go anywhere because I ran out of time. I would suggest that this is not the best building material.

Chris - Allan, your critique?

Allan -  It’s nice; I like the camel. I think it’s got artistic merit. It’s up to the second layer. There’s some triangulation going on.

Chris - So we’re looking probably about 30 cms of height for Kyle there?

Allan - 50.

Chris - You’re going as high as 50. I think you’re adding a bit on there. Okay, 50.

Matt, what have you got in front of you? Yours is much taller.

Matt - It’s still something of a monstrosity I think. I think it’s something maybe from War of the Worlds.

Chris - Describe this for everyone.

Matt - It’s a couple of tripods which are held together by a kind of crossbeam and a triangle on top. Then I’ve got a completely unnecessary extra couple of centimetres of spaghetti to just try and get some extra height.

Chris - Allan, your verdict?

Allan - It’s very good. The tripods are naturally stable. I’m sure there would have been three tripods had there been more time and more marshmallows. I like the little thing on the top. Lots of real buildings do that, they put a little arial on the top just to get a bit higher in the Guinness Book of Records

Chris - The Shard in London is a classic example isn’t it? The top bit is completely impractical - all aesthetic.

Allan - Most of the buildings do that, yes.

Chris - Sophie?

Sophie - Mine is leaning against my microphone! It looks like the Eiffel Tower has melted. Yeah, I ignored Allan’s advice about the crossbeam and I thought I’d cheat by eating off half the marshmallows to create a stable, sticky structure - but it didn’t work.

Chris - Allan?

Allan - There’s a lovely tetrahedron at the top, which has got all the triangles everywhere so it’s really nice and rigid. But the lower part has got lots of rectangles and they've all just twisted so, sadly, the top is standing up on a twisted, mangled lower storey.

Chris - Yeah. It’s almost like Sophie’s tall building is resting on another tall building next door which wouldn’t have the designers of that building terribly happy, would it?

Who wins in your view Allan?

Allan - I think I have to give it to Matt. He did cheat a little bit by adding the little arial, but that’s how you win this game!

Chris - Well there you go. Matt, your are this week’s Engineer of the Week!

Matt - Thank you so much.

Chris - Very well done!


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