A bridge bearing is a construction method that allows a bridge to transfer forces from its deck down into its foundation without damage. These bearings are used in bridges, but similar layers may be found roads, docks, airports and other structures where large and heavy objects need to operate safely. Basically, the surface of the structure and the foundation of the structure have a layer between them that both absorbs and transfers energy. When the surface is under pressure or struck by a large force, it will transfer the energy into the bridge-bearing layer, which then sends it to the foundation to dissipate.
The bridge bearing is generally easy to find when looking at a bridge’s structure. Most bridges will have a solid road surface and a series of supports underneath it. Between these two sections are the bearings. These bearings will have a wide range of appearances, but they are generally disproportionately small—much smaller than the road deck and usually smaller than the support it sits on.
The bearing’s small size allows forces from the road deck to converge into a very small point. When that energy travels down into the foundation, it travels as a very compact line of force. Since only a small part of the foundation is moving the energy, the rest of the system supports it as the energy moves. This reduces the shearing force on the foundation and makes it less likely to fail during the transference.
There are three basic forms of bridge-bearing but countless variations of each type. Each of these forms works best with a certain type of force, so it isn’t uncommon to find a single structure with more than one style. Typically, the type and size of a bearing are determined by the expected load and usage of the structure.
An elastomeric bearing is essentially a layer of strong elastic polymer. These layers are great for absorbing energy and resisting movement. They are the most general of bridge bearings and are common in nearly every major structure. On the other hand, they negate very little energy.
Sliding bearings are made using two metal plates with a low friction polymer between them. These structures don’t absorb energy very well, but they do nearly negate longitudinal forces applied to them. When the energy hits the polymer layer, the plate slides, transferring the energy into movement. This drastically reduces the energy moved into the lower plate.
Roller bearings are either round balls or rolling pin-like cylinders. These layers negate forces in at least two directions and, due to their thickness, absorb and transfer the remaining energy well. Still, they absorb less than an elastomeric bearing and negate less than a sliding bridge bearing.