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An architecture lesson from the Brooklyn Bridge

29 April 201506:14PMusa-2015travel

So we were walking across the Brooklyn Bridge today - as you do, when staying with a cousin who lives in Brooklyn - and Grace asks a very interesting question.

"What is all this stuff for? I mean, is it all just decorative?"

Which by no means is a stupid question. Mostly because the Brooklyn Bridge is frickin' gorgeous.

mmmm yes bridge.

Anyway. This question precipitated a pretty interesting 20-minute discussion of physics, which I'm going to transcribe and extend on here, largely for my own amusement, because that's the kind of thing I find interesting and I will enjoy the cringes it produces on the faces of every engineer.

The immediate follow up to that question was thus: "Why can't you just build a bridge straight across?"

Why you can't just build it straight across.

If we started with something conveniently flat and roadlike and just tried to extend it across the East River to Brooklyn, we'd quickly run into a problem. That problem would be that our lovely flat roadlike thing would pretty quickly break into bits and fall into the river. While eventually, if you try this enough times, you would be able to cross to Brooklyn, we're trying for a bridge, not a dam, so something a little more elegant is in order.

The problem here is a thing called torques. Or moments. I never really figured out the difference. Basically, whenever a force is acting at a distance from somewhere else, that distance has a multiplier effect. That's not a video game metaphor. The equation really is a multiplication. The result is this. Normally, if we take our one tonne car and put it on our nice road on a nice river bank, then that road and that bank are holding up a tonne of force, straight down through the road. This is what roads and the ground are good at. If we put it a metre away from the river bank, then it's still about a tonne of force, but now it's sort of trying to bend the road over the bank, and roads don't particularly like that. Our straight-up-and-down force has become a rotational torque.

Here's the thing, though. If we extend that road to the middle of the East River, between Brooklyn and Manhattan, then suddenly our one tonne car is several hundred metres out and exerting several hundred times the torque on our road, which is probably not even supporting its own torque, and things get super messy.

What all this stuff is for.

A bridge of any design, then, is essentially a structure for taking all of these torques and forces and redistributing them and balancing them against each other and changing which direction they run through which materials in just such a way that nothing breaks and we don't end up getting washed out into the Atlantic Ocean.

The Brooklyn Bridge is a particularly fine example of a type of bridge called a suspension bridge. Before writing this up, that was all I knew about suspension bridges, but walking across a thing has a way of making you curious about how it works, so here we are.

Suspension bridges solve our problem of things breaking when stretched across a river by having loads of cables reaching up into the sky to hold the road up across its entire length, each one taking a little bit of that weight - and because they're closer to where the weight is happening, reducing the multiplier effect of the bridge's distance on that weight. Unfortunately you can't just suspend things from mid-air by cables - so these wires are suspended from something as well. It's more cables.

Can you tell that the guy who designed the Brooklyn Bridge got his starts as a cable manufacturer?

The useful thing about cables is that unlike roadbase, which is only really good under compression, cables are good under tension as well. They're really good at pulling, which is sort of what makes this kind of bridge design feasible. You're offloading a lot of weight into pulling upwards, instead of pushing downwards, and you've got to do it using something that can take that kind of force.

This second set of wires runs in a swooping, downwards arc, which is shaped such that the weight on each vertical cable is more-or-less evenly distributed and balanced. I can't say for certain, but my gut feeling is that if you had your cable stretched tight horizontally, you'd probably end up with some sections bearing more of the brunt of the bridge than others, which sort of sends you right back to square one. I'm sure some engineers I know would happily clarify this one for me though.

So what are these cables attached to? Well, because they're horizontal rather than vertical, we can run them over the top of something tall but good-under-compression, like a big tower of bricks and stuff, and they'll hold it up in the air quite nicely, and without having to take too much pesky torque. These are, in turn, held up by the ground, and if you want to know what holds that up then you're probably going to have to find another blog post, I'm sorry.

What this tells us about architecture.

So this is why the Brooklyn Bridge looks the way it does. It's a product of the maths needed to hold it up, and the materials available at the time. I'm guessing steel cable was pretty amazing stuff for then, given that. It was not designed to look pretty - or if it was, that was largely secondary. It was designed to be a weight-redistributing apparatus, with the more abstract purpose of allowing people to cross a fairly wide river without getting wet.

This is what architecture, traditionally, is all about. It's the limitations of your mathematical knowledge, and of your materials, and how you best make those work for you. Flying buttresses and Gothic arches aren't used in Gothic architecture (just) because they look cool. It's because if you want to make a bloody great church out of nothing but rock and without a copy of AutoCAD, then that's the kind of structure you need to build.

The idea of architecture becoming divorced from that utter practicality, and of being about appearance and human interactions and the environment - has only really come about really recently, because our materials and mathematics are pretty much actually magic at this point, and we can sort of do what we want. Which is pretty dang incredible.

But I still really like the Brooklyn Bridge.

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