Thermal expansion, and the gaps in bridges

How much a structure grows when it warms, and what happens if it cannot.

Bridges carry deliberate gaps built into their decks because the metal and concrete they are made from grow slightly longer on a hot day than on a cold one, and without somewhere for that growth to go, the structure would be pushing against itself with enormous force every single summer.

Warmer atoms take up more room

Almost every solid material undergoes thermal expansion as it warms, because heat makes the atoms inside it vibrate more vigorously, and vibrating atoms take up slightly more room on average than calmer ones, so the whole material grows a little in every direction as its temperature rises. For a small object the growth is too tiny to notice in daily life, but for anything built on the scale of a bridge, spanning a long distance in a material that heats up substantially between a winter night and a summer afternoon, the total growth along its length becomes large enough to matter structurally.

Steel grows by about twelve millionths of its length for every degree it warms, which sounds like nothing until it is multiplied out. The steel in a bridge deck can easily swing through sixty degrees between a frosty night and a sunny afternoon, since dark steel in direct sun runs well above the air temperature, and across that range a three hundred metre span changes length by around twenty centimetres, about the width of a sheet of writing paper.

Loosening a jam jar lid under the hot tap

A metal lid on a jam jar fresh from the fridge often grips tightly enough to resist a firm twist, but running hot tap water over the lid alone for a few seconds is usually enough to loosen it, sometimes enough to turn it with barely any effort. The thin metal lid heats up quickly and expands more than glass does for the same warming, so the metal band grows slightly larger in diameter while the thick glass underneath barely changes in the same short time, opening just enough of a gap for the grip to loosen. The seal gives way without any extra force, simply because the metal has grown away from the glass by a tiny amount. The jar shows on a scale small enough to feel in a few seconds what the bridge shows on a scale large enough to need a purpose-built gap, and the cause in both is warmed material taking up more room than it did when cold.

Giving the growth somewhere to go

A steel bridge deck locked rigidly at both ends would try to expand anyway as the day warmed, and being prevented from doing so would build up enormous internal force, with nowhere to go except into bending, buckling, or cracking whatever held it in place. Since no building material is free of expansion, engineers give it somewhere harmless to happen, usually a toothed expansion joint at one or both ends of a span: a gap that closes slightly on hot days and opens on cold ones, absorbing the structure's growth and shrinkage without passing damaging force into the rest of the bridge. Twenty centimetres of movement is far too much for any rigid joint to absorb by flexing, and it is exactly what a properly sized expansion gap is built to swallow, closing through the summer and opening again through the winter. The joint only works if the deck is free to slide towards it, so the deck usually rests on bearings at the piers that let it move lengthways, often a pad of rubber that shears slightly or a plate that slides on a low-friction surface, while one end is held fixed so the whole span grows in a known direction. Different materials expand by different amounts for the same rise in temperature, so a bridge combining steel and concrete has to be designed with each material's behaviour in mind, the two parts shifting at their own rates and relying on the joints between them to take up the difference.

Because every construction material expands and contracts by a predictable amount for a given change in temperature, designing around thermal expansion comes down to deciding in advance where its effects will be allowed to show up. Railway tracks were historically laid with small gaps between each rail section for the same reason bridges carry expansion joints, though modern welded track is laid under carefully controlled tension so that the stresses of expansion and contraction stay within what the rail can safely carry without a visible gap at all. Long pipelines carrying hot fluids are often laid with a deliberate zigzag or loop in an otherwise straight run, giving the pipe room to bow outward as it grows instead of pushing against whatever holds each end fixed.

A bridge inspector checking an expansion joint in the depths of winter expects to find it open near its widest setting. Finding it already fully closed on a cold morning is treated as a real warning sign, evidence that something else has eaten into the room the joint was supposed to keep free for the coming summer.