The Gaps in a Bridge That Let It Move
You've almost certainly felt one, even if you've never looked at it. You're driving over a long bridge, and somewhere along the way there's a ba-dum under the tyres. Look down if you're walking and you might see it: a row of interlocking metal fingers, like two combs pushed into each other, or a dark rubber strip running across the road.
It looks like a join. Something that holds two bits of bridge together.
In a sense it is. But its real job is the opposite. It's there to let the two bits move apart, and back together again, every single day.
Bridges breathe
Here's the thing most of us never think about. A bridge is not a fixed length.
Like almost everything, the steel and concrete in a bridge expand as they warm up and contract as they cool. On a single beam, the change is tiny. But a bridge deck can run for hundreds of metres, and a tiny change in every metre adds up. Measurements on real bridges have found movements ranging from under a centimetre to around six centimetres.
So between a cold winter night and a hot summer afternoon, a big bridge can be noticeably longer.
That might not sound like much. A few centimetres, on something the length of several football fields. The problem isn't the size of the movement. It's what happens if you try to stop it.
Don't fight the expansion
If a bridge isn't allowed to expand, the expansion doesn't simply disappear. The material still "wants" to grow, and if it can't, it pushes. Extremely large forces build up when that movement is restrained, and those forces can cause severe damage if the movement isn't properly allowed for.
Think of a door that's swollen in damp weather. It hasn't grown much, a millimetre or two perhaps, but it jams solid in the frame. Now imagine that same jamming force, but from hundreds of metres of steel and concrete, pressing against the land at each end.
So engineers don't fight it. They let it happen, and they design the bridge to cope. The two main tools are expansion joints and bearings. An expansion joint is the gap you drive over, which opens and closes as the deck moves. A bearing is what the deck sits on at its supports. Some bearings are designed to let the deck slide. For larger movements, bridge engineers commonly use sliding bearings once the expected movement goes beyond about five centimetres either way.
Setting the gap on the day
There's a lovely practical detail in all this. How wide should the gap be?
It depends on the weather on the day it's built.
If you install an expansion joint on a hot day, the bridge is already at the long end of its range, so the gap needs to be on the narrow side. It'll open up when winter comes. Install it on a cold day and the opposite applies. The average temperature of the bridge when it's being built controls the gap for the expansion joint, and joint openings are set according to the air temperature at the time the joint is permanently fixed in place.
Bearings get the same treatment. A sliding bearing has to be positioned with an offset, based on how far the temperature on the day differs from the temperature it was designed around. It's a bit like setting a watch for the time zone you're about to fly into, not the one you're standing in.
The trouble with gaps
Now for the twist. Those gaps solve one problem and create another.
A gap in a road is a gap for water. And on many roads, that water carries salt, spread in winter to melt ice. Expansion joints are supposed to stop water, de-icing chemicals and debris getting through to the structure below. In practice, they don't always manage it. A British study in 1989 found that most of the expansion joints surveyed on existing bridges had failed, letting in water and salt. Once salt water gets through, it reaches the supports and the underside of the deck, where it can attack the steel inside the concrete. In the worst cases, that kind of attack can eventually lead to a span failing.
So the gap that protects the bridge from heat can let in the thing that slowly eats it.
That's why there's now a push to avoid joints altogether on shorter bridges, by building what are called integral bridges. An integral bridge has no expansion joints at all. Instead, the movement from heat and cold is taken up by the end walls at each end of the bridge.
Some road authorities now prefer this wherever they can, purely because of the maintenance headaches joints cause. In the UK, for example, guidance says bridges with a deck shorter than 60 metres, and not too sharply angled, should be designed as integral structures unless there's a compelling reason not to.
Gaps everywhere
Once you know to look for them, you'll find these deliberate gaps all over the place.
Pipework uses flexible sections, sometimes metal bellows, to soak up the movement, and bends in pipework can do the same job. Concrete footpaths and pavements have lines cut into them at regular intervals. Those aren't for expansion as such. They're there so that when the concrete does crack, as concrete tends to, it cracks along the neat line rather than wandering across the slab. And where a railway runs across a bridge with a big joint, the track itself needs special expansion joints, with the rails sliding past each other at a very shallow angle.
Not every bridge can go joint-free
There's a limit to the joint-free approach, though. Integral bridges suit short to medium spans. On a very long bridge, the total movement is simply too big for the end walls to absorb. Those bridges still need large expansion joints, sometimes built from several gaps side by side, each one sealed with a watertight rubber strip.
So the next time you feel that ba-dum under your tyres, you'll know you're crossing a long bridge doing exactly what it was designed to do. Moving.
Built to move
There's a broader lesson in that strip of metal fingers. We tend to think of strong structures as rigid ones. Unbending. Fixed.
But good engineering is often less about stopping things from moving, and more about letting them move safely. A bridge with nowhere for that movement to go can build up forces big enough to do serious damage. A bridge that's allowed to stretch and shrink can stand for a very long time.
Sources
Transportation Research Board, "Thermal Movement Design Procedure for Steel and Concrete Bridges" (NCHRP 20-07(106) final report). https://onlinepubs.trb.org/onlinepubs/archive/NotesDocs/20-07(106)_FR.pdf (accessed 25 Oct 2026)
Tennessee Department of Transportation, Structures Design Guidelines, Section 9, "Joints and Bearings" (2023). https://tn.gov/content/dam/tn/tdot/structures/SDG-9-Joints_and_Bearings-V10112023.pdf (accessed 25 Oct 2026)
Wikipedia, "Expansion joint." https://en.wikipedia.org/wiki/Expansion_joint (accessed 25 Oct 2026)
Wikipedia, "Integral bridge." https://en.wikipedia.org/wiki/Integral_bridge (accessed 25 Oct 2026)
