Room for a Changing Bridge
An explanation of how small gaps and sliding covers let bridges change size as metal warms and cools.
Play the quiz as a gameOn a warm day, a metal bridge can get a little longer. But the road at each end does not move along with it. How can the bridge change size without pushing hard against the road? Small gaps leave room for movement.
Metal expands when it gets warmer. That means it takes up a little more space. When metal cools, it contracts, or gets a little smaller. These changes are often too small to notice, but bridge builders plan for them.
Builders put special gaps at the ends of many bridges. These spaces are called expansion joints. As the bridge warms, the gaps become narrower. As it cools, they become wider. Some joints have metal cover pieces that can slide past one another. The covers help keep the road surface smooth while the bridge moves.
A bridge diagram can show this change. One drawing labeled “Warm Day” shows a narrow gap. Another labeled “Cool Day” shows a wider gap. The caption under the drawings says, “Spaces give a bridge room to change size.”
A gap might look like a missing piece. Yet it has an important job. By leaving space and using moving covers, builders make room for tiny changes. A bridge does not need to stay exactly the same size to do its work.