Bridge bearings play a crucial role in the structural integrity of a bridge, functioning as an intermediary between the bridge’s substructure and superstructure. These components transfer the various loads acting on the bridge, such as traffic loads, wind loads, earthquake forces, and the weight of the superstructure itself. Beyond load transfer, bridge bearings also allow for necessary movements between the substructure and superstructure, such as rotational and translational movements. This article explores the different types of bridge bearings and their specific functions in ensuring a bridge’s stability and flexibility.
I. Introduction
Bridges, by nature, are subjected to a variety of external forces and environmental conditions that can cause them to shift, expand, or contract. Bridge bearings are structural devices installed at the interface between the substructure (usually piers or abutments) and the superstructure (the bridge deck). These bearings facilitate necessary movements, allowing the bridge to accommodate temperature changes, traffic loads, and seismic events. The primary role of bridge bearings is to transfer loads and ensure that the bridge can move in a controlled and safe manner.
There are two main categories of bridge bearings: expansion bearings and fixed bearings. Expansion bearings allow both translational and rotational movements, while fixed bearings allow rotational movement and limit translational movement.
II. Types of Bridge Bearings
1. Sliding Bearings
Sliding bearings are one of the simplest and most commonly used types in bridge construction. These bearings consist of two metal plates, typically stainless steel, that slide relative to one another. This sliding mechanism accommodates translational movement, with lubricating materials such as polytetrafluoroethylene (PTFE) often added between the plates to reduce friction.
However, sliding bearings have a limitation: they are only effective for spans of less than 15 meters as specified by the American Association of State Highway and Transportation Officials (AASHTO). This is because sliding bearings are not ideal when a bridge experiences significant rotational movement. However, their span limitation can be overcome when combined with other types of bearings.
Image Reference: Sliding Bridge Bearing
2. Rocker and Pin Bearings
Rocker bearings are expansion bearings with a curved surface at the bottom, allowing for translational movement. A pin at the top accommodates rotational movement. This setup is commonly used in bridges with steel structures. On the other hand, pin bearings are fixed bearings that only allow for rotational movement. The pin is steel and is typically fixed to a concrete pier, with a flat base rather than a curved one.
Rocker and pin bearings allow both translational and rotational movements, but they are limited to only one direction. They are best used in situations where the bridge’s movement is well understood. These bearings, though durable, are prone to corrosion and require regular inspection and maintenance.
Image References: Rocker and Pin Bearings
3. Roller Bearings
Roller bearings are commonly used in both reinforced concrete and steel bridge structures. These bearings come in two main configurations: single roller bearings and multiple roller bearings. Single roller bearings consist of one roller placed between two plates, while multiple roller bearings use several rollers in the same arrangement.
Single roller bearings accommodate both rotational and translational movement in the longitudinal direction, making them a cost-effective solution. However, their vertical load capacity is limited. In contrast, multiple roller bearings are more expensive but support larger vertical loads and only accommodate translational movement. These bearings, however, are susceptible to corrosion, so regular inspection and maintenance are necessary.
Image References: Single and Multiple Roller Bearings
4. Elastomeric Bearings
Elastomeric bearings are made from synthetic or natural rubber and can accommodate both rotational and translational movements through the deformation of the elastomer. These bearings are highly effective in carrying large vertical loads due to the reinforcement that prevents lateral bulging of the elastomer. Different types of elastomeric bearing pads include steel-reinforced, plain, fiberglass-reinforced, and cotton duck-reinforced pads. Steel-reinforced elastomeric bearings are the strongest, while plain elastomeric pads are the weakest.
Elastomeric bearings are affordable, low-maintenance, and widely used due to their ability to perform well under a variety of load conditions. They are the preferred choice for many bridge designs.
Image Reference: Elastomeric Bridge Bearing
5. Curved Bearings
Curved bearings are composed of two plates that match each other in a curved shape. There are two types of curved bearings: cylindrical and spherical. Cylindrical bearings allow only rotational movements, while spherical bearings can accommodate both rotational and translational movements.
Due to the geometry of the bearing, lateral movement can be resisted by gravity loads. To facilitate lateral movements, a PTFE slider is often incorporated to reduce resistance. These bearings are particularly useful when precise control over movements is necessary, such as in bridges exposed to complex forces.
Image Reference: Spherical Bearing
6. Pot Bearings
Pot bearings consist of an elastomeric disk confined within a steel pot. The pot also contains a steel piston, which is carefully designed to fit into the pot wall. The elastomeric disk within the pot can withstand significant vertical loads. These loads are transferred through the steel piston to the elastomeric disk, which is almost incompressible. For lateral loads, the steel piston moves toward the pot wall, transferring the lateral forces.
While pot bearings are excellent for handling vertical loads, their translational movement is limited. To allow translation, PTFE is used as a sliding surface. Pot bearings are commonly used in larger bridge structures where substantial load support is required.
Image Reference: Pot Bridge Bearing
7. Disk Bearings
Disk bearings consist of an elastomeric disk and a metal ring at the center. The elastomeric disk is responsible for accommodating vertical loads, while the metal ring supports lateral loads. The elastomer must be sufficiently hard to bear vertical loads without excessive deformation and flexible enough to allow rotational movement.
Similar to pot bearings, disk bearings utilize PTFE sliders to allow for translational movements. These bearings are especially useful for bridges that require both vertical and lateral load support while allowing rotational flexibility.
Image Reference: Disk Bearing
III. Conclusion
Selecting the appropriate bridge bearing is crucial for the safety and durability of a bridge. Each type of bearing has specific advantages and limitations, which must be considered based on the type of bridge, expected load conditions, and movement requirements. Regular inspection and maintenance are essential to ensure the continued functionality of bridge bearings, especially those prone to corrosion or wear over time. By understanding the different types of bearings and their functions, engineers can design bridges that are not only structurally sound but also able to accommodate the movements required by both static and dynamic forces.