A bridge bearing is a structural device that helps transmit the load from the superstructure, e.g. the bridge deck, to the substructure, e.g. the piers or abutments. Bearings in bridges are designed to control movements and/or rotations and to reduce stresses on structural members.
Thermal expansion and contraction, creep, shrinkage, and fatigue, among others, can cause movement on the bridge. Additionally, external factors, such as ground settlement and seismic activity increase the displacements and the rotations. Thus, during the designing process of the structural members, they should be controlled.
There are several types of bridge bearings which are commonly used. The decision of the appropriate type is based on different factors including the bridge span, loading conditions, and performance specifications. The oldest bridge bearing is simple and consists of two plates resting on top of each other. Modern bridge bearings are primarily elastomeric.
Another category of bridge bearings is the mechanical bridge bearings. There are several variations of mechanical bridge bearings, e.g. the pinned bearings (which include the rocker bearing and the roller bearing) and the fixed bearings, which allow rotation, but not other forms of movement.
Types of bridge bearings:
Sliding bearings have a flat sliding surface that allows horizontal movement and a spherical surface that allows rotation. While they were traditionally made of metal, nowadays, sliding bearings are more commonly made of materials such as Teflon.
The use of sliding bearings on bridges allows for the bridge components to move relative to each other without causing damage or failure to the structure. This can help to reduce the stresses and strains that occur in the bridge components, thereby prolonging the life of the bridge.
Rocker and pin bearings
Rocker bearings have curved surfaces that allow rocking and they are designed to support both vertical and horizontal loads. The use of a pin at the top is an effective way to enable rotation. As the bridge expands, the bearing follows a rocking motion in order to allow the movement in the horizontal direction. Rocker bearings are primarily used for highway bridges, while they are primarily made of steel.
Pin bearing is a fixed bearing that enables rotation movement through the use of a steel pin and can be assumed similar with the rocker bearing. The difference is based on the bottom of the pin bearing which is flat and fixed to the concrete pier.
Roller bearings are a type of bearing that can be utilized in the construction of steel and reinforced concrete bridges. They come in two primary configurations: single and multiple roller bearings. A single roller bearing is composed of a single roller situated between two plates, enabling both longitudinal translational and rotational movement. Although it is affordable to manufacture, its vertical load capacity is limited. Conversely, a multiple roller bearing can only accommodate translational movement, but can include rotational movement if combined with a pin bearing. Multiple roller bearings are more expensive but they can support larger vertical loads. Regular inspections and maintenance are crucial since roller bearings are susceptible to damage and corrosion.
The most commonly used type of bridge bearing today is the elastomeric bridge bearing. It is made of rubber and does not include any moving components, since the rubber material allows for movement within the bridge without requiring any mechanical parts. These bearings are cost-effective to manufacture and do not require regular maintenance, unlike other types of bearings that incorporate moving parts. Elastomeric bearings can be further strengthened with steel reinforcements, if necessary, to increase their load-bearing capacity.
A curved bearing typically consists of two plates that are curved to match each other. If the bearing is cylindrical in shape, it can only accommodate rotational movements, while if the curved bearing is spherical, it can handle both rotational and translational movements. Since both gravity loads and the curved geometry of the bearing generate lateral resistance, limiting lateral movement, a polytetrafluoroethylene (PTFE) slider is often attached to the bearings to allow for lateral movement.
A pot bearing consists of an elastomeric disk that is contained within a pot structure, along with a steel piston that is fitted into the pot wall, and flat sealing rings to secure the elastomeric disk in place. Pot bearings can support significant vertical loads, which are typically transferred through the steel piston to the nearly incompressible elastomeric disk. When it comes to lateral loads, they are transferred as the steel piston moves toward the pot wall. However, pot bearings have limited translational movement, that is why PTFE surfaces are often introduced to the sliding surface to allow for greater translational movement.
Disk bearings, known as thrust bearings, are a type of bearing that is designed to mainly support axial loads. The rotational movement in a disk bearing is accommodated through the deformation of the elastomer, while translational movement is facilitated through the use of a PTFE slider. The elastomer used must be sufficiently hard to support vertical loads without experiencing significant deformations, yet flexible enough to allow for rotational movement. The elastomeric disk supports vertical loads, while the metal ring at the center of the bearing supports lateral loads.