How engineers handle heat expansion in steel structures

HostI was walking across a big bridge last weekend and I kept noticing these weird metal teeth in the sidewalk. They look like two giant combs pushed together, and you can see right down through the gaps to the water way below. It got me thinking about how much we just trust that these massive piles of steel are going to stay put. But do they actually stay put when the weather gets hot?

GuestNot at all. In fact, if that bridge couldn't move, it would probably tear itself apart. Most people think of steel as this stiff, unmoving thing, but it's actually pretty restless. When the sun beats down on a bridge all day, the steel gets longer. It can grow by inches, or even feet if the bridge is long enough. If there wasn't a gap for it to grow into, it would push against the concrete at the ends and start to buckle or crack the whole thing.

HostThat feels hard to wrap my head around. Steel is what we use because it's so tough and strong. How does just a little bit of warmth from the sun have the power to bend a massive bridge beam?

GuestWell, it's all about what's happening at the tiniest level. Think of the bits that make up the steel like people standing in a crowd. When it's cold, they're all standing still, huddled close together. But as they get warmer, they get more energy. They start to wiggle and jiggle around. To have room to move, they need more space. So every single tiny bit pushes its neighbor just a little bit. When you have billions of them doing that across a mile-long bridge, those tiny nudges add up to a lot of extra length. It's a huge amount of force. You can't stop it, so you have to give it somewhere to go.

HostSo those metal comb teeth I saw are just giving the bridge room to breathe?

GuestYeah, those are what we call expansion joints. They let the bridge sections slide back and forth without leaving a giant hole in the road. But bridges aren't the only problem. Think about train tracks. They're just miles and miles of steel bars pinned to the ground. In the old days, you would hear that click-clack sound because they left gaps between the rails. But modern rails are often welded together into one long piece. If it gets too hot and the steel has nowhere to grow, the whole track will suddenly snap sideways in a big curve. Engineers call it a sun kink. It looks like a giant steel snake has been dropped on the ground. It's enough to knock a whole train off the tracks.

HostIf steel is such a headache, why do we even use it? There has to be some kind of metal out there that doesn't grow when it gets hot.

GuestAlmost everything grows when it gets warm, but steel is actually a great choice for a reason most people don't realize. It plays very well with concrete. It turns out they grow and shrink at almost the exact same rate. If you used a metal that grew much faster than the concrete it was buried in, it would shatter a building from the inside out every time the sun came up. We're kind of stuck with the growing, so we just have to be clever about where we put the gaps.

HostWhat about things that don't have gaps? I'm thinking of those long pipes that carry oil or water for hundreds of miles. You can't just leave a hole in a pipe for it to grow into or the oil would leak everywhere.

GuestThat's where you get to see some really cool shapes. If you ever drive past a big refinery or a long pipeline, you'll notice they're not usually a straight line. They'll have these big U-shapes or zig-zags every once in a while. Those are expansion loops. Instead of the pipe trying to push straight ahead and bursting its own welds, the U-shape just flexes a little bit. It's like a spring. When the pipe gets hot and grows, the U-shape gets a little bit wider. When it cools down, it pulls back. It keeps the pipe from snapping under its own weight.

HostHmm. It still feels like we're just leaving things loose. If I build a table at home, I want the legs to be tight. If the bridge is just sliding around on top of its pillars, what stops it from just sliding right off during a storm?

GuestWell, it's not just loose and rattling around. One end of a bridge section might be bolted down tight, but the other end sits on something like a rocker or a big rubber pad. Think of it like a skateboard. The board can tilt and move a bit, but it's still locked to the wheels. On a bridge, that one end is sitting on a set of rollers. So when the sun hits and the steel grows, that one end just rolls out an inch or two. It stays safe because we planned for the movement instead of trying to fight it.

HostSo the whole world is basically breathing in and out as the sun goes up and down.

GuestIt really is. Engineers in some hot cities have to check the train tracks every single hour during a heat wave because the steel can move so fast it'll bend right before your eyes.

HostThose metal teeth on the bridge sidewalk are just a way of letting the giant wake up and stretch its limbs without breaking the road.