most highway bridges are built with expansion joints that can move back and forth. what is the reason for this?

Most highway bridges have expansion joints so the bridge can safely expand and contract without cracking, warping, or breaking under temperature changes and loads.

Core reason

• Bridge materials like concrete and steel expand in heat and contract in cold, changing the bridge’s length over the day and across seasons.

• Expansion joints create controlled gaps that let sections move back and forth instead of building up internal stress that would cause cracks or structural damage.

What the joints actually do

• They allow longitudinal movement (back-and-forth along the length of the bridge) from temperature changes, shrinkage of concrete, and creep, so the deck does not tear itself apart.

• They also help accommodate other movements from traffic loads, vibration, settlement, and sometimes seismic effects, keeping the bridge alignment and surface usable for vehicles.

Why they are essential for highways

• On long highway bridges, the total expansion can reach centimeters to tens of centimeters; without joints, this would lead to serious cracking, buckling, or “bending out of place” under extreme conditions.

• Expansion joints also help transfer loads between bridge segments while maintaining a relatively smooth driving surface, so traffic can pass continuously even as the structure moves slightly.

Short answer version

Most highway bridges use expansion joints so the structure can “breathe” with temperature and load changes, preventing cracks and distortion and keeping the bridge safe and durable over its lifetime.