Bridge Bearings: Types, Their Function and Applications

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The bridge represents a remarkable example of intricate civil engineering, characterized by its intricate design and assembly of numerous components that seamlessly unite to form a cohesive structure. It stands as a testament to human ingenuity and engineering prowess. Within this vast array of components, there exists a pivotal element known as a bearing, which holds significant importance. In the forthcoming article, we will delve into a comprehensive exploration of this critical component, examining its functionality, significance, and role within the broader context of bridge construction.

Bridge Bearing

Bridge bearings are mechanical components that are provided to the bridge between the substructure (abutment) and superstructure. They help in transmitting the live load from the superstructure, dead load, wind load, earthquake load impact load, etc. The main function of the bearing is: –

• To transmit the vertical load from the superstructure to the substructure
• To facilitate the movement of the superstructure due to thermal expansion and contraction
• Also to provide rotational movement of the girders

Force on Bearing

Bearings transmit the force from the superstructure to the substructure. They also sustain the translational and rotational movements of the bridge structure. The four major forces acted upon the bearing are: –

• Longitudinal force
• Transverse force
• Reactive force
• Uplifting force

Due to earthquake load, thermal expansion and contraction, and breaking effect, the resultant force is Longitudinal and Transverse force. The resultant force developed at the end of the support generates reactive force.

Also, read:Types of Bridges based on Superstructure.

Function of Bearings

The primary functions of bridge bearings include:

1. Load Transmission: Bridge bearings transfer loads from the bridge superstructure (the part that carries the traffic) to the substructure (supporting piers, abutments, or other foundations).
2. Movement Accommodation: Bridges are subjected to dynamic forces and deformations due to factors such as temperature changes, wind, and traffic loads. Bridge bearings are designed to allow controlled movement and rotation, preventing the transmission of excessive forces to the structure.
3. Rotation Allowance: Bridge bearings allow rotational movement, which is essential for accommodating changes in alignment and dealing with different forces acting on the bridge.
4. Vibration Isolation: In some cases, bridge bearings may also serve to dampen or isolate vibrations caused by dynamic loads such as traffic.

Type of Bridge Bearings

There are many types of bearings used in the bridge structure. They are classified into two categories namely fixed abearing and rotational bearings. Fixed bearing allows only rotation whereas rotational bearing allows both rotation and translation. The cause of the moments is due to the creeping of concrete, shrinkage of concrete, settlement, uplift force and thermal force, also due to the sudden breaking of the vehicle.

1. Rocker pin bearings
2. Rocker and roller bearings
3. Sliding plate bearings
4. Elastomeric bearings

Rocker Pin Bearings

Rocker pin bearings are used in the straight steel bridge. This type of bearing is made of steel and is connected to the substructure through the steel plate. The assembly of the rocker pin bearing is shown in Figure 2. The bearing consists of a pin to accommodate the large live load and vertical load as well. This type of bearing is used for the bridge spanning more than 15m.

IRC 83 has stipulated the following specifications for rocker pin bearings.

• The diameter of the pin shall not be less than 16mm.
• The pin shall be fitted to the groove at a depth of 0.5d.
• The minimum clearance above the top surface of the rocker pin shall be 2.5mm.

Rocker and Roller Bridge Bearings

This type of bearing consists of two different types of assembly. One with a single roller as shown in figure-3 and the other with numerous rollers beneath the rocker pin bearing as shown in figure-4. Such kinds of assembly are known as rocker and roller bearings. The translation movement is achieved by the rollers and rockers that facilitate the rotational movement. The disadvantage of such assembly of bearing is an accumulation of dust on the rollers and it gets gam or freezes. Thus, timely inspection and rehabilitation of the roller are required for the functioning of the bearing. This type of bearing is ideal for the intermediate length of the bridge.

The code requirements for this type of bearing are as follows: –

• The diameter of the roller should be a minimum of 75mm.
• The ratio of the length of the roller to its diameter shall not be more than 6. However, in some cases, it shall not be more than 10.
• The gap between the rollers shall not be more than 50mm when there are multiple rollers.

Sliding Plate Bearings

The design of this specific type of bearing aligns with its name, utilizing plates that smoothly glide over one another to enable translational movement. These plates are typically constructed from steel, although alternative materials such as Teflon and bronze can also be employed. These plate bearings are particularly effective for bridges with spans of up to 15 meters and are highly recommended for use in straight steel bridges. However, they are not suitable for bridges that experience lateral forces or require rotational movement.

Sliding plates are employed to mitigate the frictional forces that occur between the superstructure and substructure. To achieve this reduction in friction, a lubricant is applied between the plates. One commonly used lubricant for this purpose is polytetrafluoroethylene (PTFE), known for its resistance to friction and ability to withstand high temperatures (up to 350 degrees Celsius). Figure 5 presents an illustration showcasing the assembly of sliding plate bearings.

Elastomeric Bearings (IRC 83 part II)

The elastomer is a polymer obtained from natural rubber by the processes of vulcanization or synthesized from polychloroprene. Thus, the elastomer used is natural or synthetic. Elastomeric bearings can be of reinforced bearing or unreinforced pad. Reinforced bearings are fabricated with steel-reinforced, fibreglass-reinforced or cotton duck-reinforced bearing pads. The steel-reinforced elastomeric bearing has the highest strength among the four types and the unreinforced elastomeric bearing is the weakest of all.

The special feature of this type of bearing is that it takes the direct compression loads, shear force and moment by undergoing deformation by itself to the above force.

Advantages of Elastomeric Bearing

The advantages of elastomeric bearing as mentioned below: –

• They require no maintenance
• It can be easily accessed and replaced during the event of damage to it.
• Due to less height, it requires less headroom and thus reduces the cost of approaching the road.
• This type of bearing is not costly and does not require frequent inspection.

Selection of Bridge Bearings

The successful bridge structure is greatly dependent on the functioning of the bearing capacity and capability as anticipated in the design. Therefore, when selecting the bearing for the bridge, the following factors should be considered: –

• High vehicle load-taking capacity.
• Movement capability.
• Rotational capability.
• Capability to resist external horizontal forces like wind, etc.
• Should have good seismic resistance.
• The overall cost should be low.
• Environmental condition. It should not be used in the vicinity of water. Roller bearings and sliding bearings are not suitable for dusty places.

Also, read: Difference Between Bridge and Culvert

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