Drive across a long bridge and you may hear a familiar sound:

Thump… thump.

It happens when your vehicle passes over the small gaps built into the bridge deck.

They are called expansion joints.

At first, deliberately leaving gaps in a massive structure might sound like bad engineering.

But those gaps are there for exactly the opposite reason.

They help prevent the bridge from damaging itself.

A bridge may look solid and motionless, but it is constantly moving.

Not enough for you to notice, but enough for engineers to worry about.

A bridge can move because of:

One of the biggest everyday causes is temperature.

Most bridges contain huge amounts of steel and concrete.

Like many materials, they expand when heated and contract when cooled.

Imagine a steel bridge on a cold winter morning.

Now imagine the same bridge after hours under a hot summer sun.

Its components can become measurably longer.

On a bridge hundreds of metres long, even a tiny percentage of expansion can translate into several centimetres of movement.

Imagine fixing both ends of a long metal structure so tightly that it has nowhere to expand.

As its temperature rises, enormous internal stresses can develop.

For a bridge, uncontrolled movement can contribute to problems such as:

Engineers therefore design bridges so certain parts can move safely.

That's where expansion joints come in.

An expansion joint creates a controlled separation between sections of the bridge.

As temperatures rise:

The gap can become smaller.

As temperatures fall:

The gap can become larger.

The bridge is therefore allowed to expand and contract instead of fighting against itself.

Think of an expansion joint as giving a bridge breathing room.

Expansion joints have to maintain a usable road surface while still allowing movement between bridge sections.

Different bridges use different joint designs.

Some are relatively simple gaps with seals.

Large bridges may use sophisticated modular expansion joints made from multiple steel elements that can accommodate much greater movement.

When your tyres cross these components, you may feel or hear the familiar:

Thump.

You're literally driving across a part of the bridge designed to move.

Expansion joints aren't working alone.

Many bridges also sit on special bearings between the bridge deck and its supporting piers.

These bearings can allow controlled:

Together, bearings and joints let a massive structure respond to changing forces without losing its structural integrity.

Expansion-joint requirements depend on the bridge.

Engineers consider factors such as:

Some modern bridges are even designed as integral bridges, eliminating conventional expansion joints over certain lengths by allowing movement to be accommodated elsewhere in the structure.

Why avoid joints when possible?

Because expansion joints themselves require inspection and maintenance and can be vulnerable to wear, water intrusion, debris, and traffic loads.

The next time you drive across a bridge and hear that distinctive thump, remember what's happening beneath your tyres.

The bridge isn't one perfectly rigid object.

It's an enormous structure that expands, contracts, bends, and moves throughout its life.

Those seemingly insignificant gaps give that movement somewhere to go.

Without them, forces created by temperature and structural movement could cause serious damage.

So sometimes, one of the smartest ways to make a structure stronger is surprisingly simple:

Leave a little room for it to move.

The gaps accommodate movement caused by temperature changes and other structural forces.

Yes. Materials such as steel and concrete expand as their temperature rises and contract when it falls.

Your vehicle's tyres are passing over the joint between movable sections of the bridge deck.

No. The number and type depend on the bridge design. Some modern integral bridges are designed to accommodate movement without conventional deck expansion joints.

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