How steel behaves with temperature

Thermal expansion

Steel expands when heated and contracts when cooled. That is not a problem in itself — it becomes one when the movement is restrained: then instead of movement, restraint force appears in the structure.

Where it matters

• Long external members: trims, balustrades, roof components, external structures.
• Structures exposed to large temperature swings: a sunlit facade element, an unheated roof space.
• At the boundary between spaces of different temperature.

Where it does not matter much

Inside the walls of a heated building, where the structure lives in a narrow temperature range, the movement is negligible.

Loss of strength when hot

Above a few hundred degrees the yield strength of steel falls. This is the fire case, and it matters particularly for thin-walled sections because they heat up fast. Section factor.

Behaviour in the cold

At low temperatures steel can become more brittle: it tolerates sudden actions less well. In building structures this rarely governs, but in cold climates or refrigerated spaces it has to be considered.

What this means in design

The junction must allow movement where needed

On long external members the fixing should not restrain both ends rigidly. A slotted hole or a sliding connection here is not sloppiness but a design decision.

A thermal bridge is more than heat loss

Because steel conducts heat well, the colder points of the structure are also a condensation risk. Thermal bridging in steel.

Fire is a separate task

Thermal expansion during a fire creates forces that do not exist in cold design — which is why fire design has to look at the connections too. Connections in fire.

The answer to the layman’s question

“Won’t it creak and crack?” The noise is not caused by expansion itself but by movement with friction: when two members slip tightly against each other. Proper detailing avoids this, and inside a heated building the movement is small in any case.

The point

Steel moves, but predictably. The question is not whether it moves, but whether the junction lets it where it needs to.