Bridge expansion joints are designed to allow for continuous traffic between structures while accommodating movement, shrinkage, and temperature variations on reinforced and prestressed concrete, composite, and steel structures. They stop the bridge from bending out of place in extreme conditions, and also allow enough vertical movement to permit bearing replacement without the need to dismantle the bridge expansion joint.
Expansion joints come in all shapes and sizes, from clay bricks to molded rubber. Some forms of construction, like railways or air ducts, have specialized breather joints meant to deal with their unique thermal stresses. Copper is often seen as a good one-size-fits-all solution for an expansion joint, given its durability and ability to take a wide variety of forms.