Elastomeric Bearing Pads: A Practical Guide to Selection and Installation

Elastomeric Bearing Pads are vital structural elements that facilitate a bridge's ability to withstand loads while allowing for linear and rotational movements of the structure.

While selecting a bearing, it is necessary to consider factors such as loads, movement, size, rubber properties, exposure, and design. The proper installation of the bearing is no less important, as even the best choice is rendered useless in an improperly installed location.

In this guide, Elastomeric Bearing Pads will be introduced in terms of their mode of operation, selection criteria, installation practices, and other pertinent information regarding their maintenance and serviceability.

How Do Elastomeric Bearing Pads Work in Bridges?

A good bearing, when selected properly, can allow the following functionality:

• Transfer of normal loads from the supported structure to its foundation
• Restricted linear movement of the upper part of the bearing
• Restricted rotation of the bridge deck
• Absorption of stresses due to restricted movement
• Provision of flexibility at specified locations
• Compensation of thermal expansion and contraction
• Facilitation of the progressive load transfer mechanism

The manner in which the bridge bearing serves the aforementioned purposes is dependent on its type, material, size, loading, and other factors.

Why do Bridges Require Controlled Movement?

A bridge is subjected to continuous stress due to varying traffic and temperature. Traffic places dynamic loads on the structure, as well as imparts impact and rotational forces on the deck.

These loads, in addition to inducing stresses in the superstructure, also cause linear and rotational movements of the deck, which, if restrained, can lead to additional stress. It is the job of the bridge bearings to allow this degree of movement while ensuring that the load is transferred to the foundation.

Factors to Consider When Selecting Elastomeric Bearing Pads

The following factors must be taken into account when selecting a bearing:

• Weathering resistance: governs the effect of exposure on the material
• Ageing resistance: influences the service life of the bearing
• Dimensional stability: ensures compatibility with structural dimensions

The selection of an appropriate material depends on the mechanical properties and exposure of a particular application.

How do Environmental Criteria Influence the Selection of a Bearing?

Since bridge bearings are commonly subject to severe environmental conditions, care must be taken when selecting the materials on the basis of their resistance to sunlight, temperature, humidity, and pollution.

Wherever applicable, consideration must be given to the environmental exposure in addition to the bearing loads. It is therefore pertinent to assess the environmental conditions on the same footing as the loads being carried by the structure.

What are the Advantages of Using Bearing Pads in Precast Construction?

Precast construction requires that the connection between a precast element and its support be capable of carrying load while permitting the degree of movement and rotation envisioned by the structural design.

The bearing allows the transfer of load from the supported structure while undergoing shear deformation and rotation about an axis, as specified by the designer. Due to its simplicity, the bearing can be used in a wide range of precast structures.

How should Elastomeric Bearing Pads be Installed?

A bearing's performance is entirely reliant on its correct installation. As such, before setting the superstructure on the bearing, it is crucial to inspect the surface, bearing, and orientation for flaws, as well as ensuring that the bearing is correctly positioned.

What Pre-installation Checks should be Performed?

Before installation, the bearing should be inspected for defects, and the installation surface should be prepared. Installation checks performed by the personnel must include the following:

• Bearing size and dimensions
• Visual inspection of the bearing
• Correct orientation
• Flatness of the installation surface
• Cleanliness of the surface
• Provision of marking indicating the bearing position
• Compliance of the bearing with drawing requirements

The bearing must not be installed if it is found to possess any defect that may compromise its performance.

What Practices can be followed to monitor the Performance of a Bridge Bearing?

Despite being engineered to provide years of reliable service, a bearing requires regular maintenance and monitoring to ensure that it is performing up to standard. As such, it is the responsibility of the concerned personnel to carry out periodic evaluations while adhering to the codal requirements pertinent to the site conditions.

When should Elastomeric Bearings be Evaluated?

The timing of an inspection depends on the requirements of the structure and the applicable regional regulations. In general, the following aspects must be observed during an inspection:

• Cracking of the bearing
• Checking for splits or bulges in the bearing
• Checking for deformations of the bearing
• Checking for surface irregularities
• Checking for displacement beyond permissible limits
• Checking for unusual rotation of the bearing
• Checking for signs of contamination on the bearing
• Checking for changes in other connected structural components

If any of these conditions are found to exceed permissible limits, the concerned authority must be contacted for further instructions.

How can Environmental Exposure Affect Elastomeric Bearings?

Exposure to sunlight, ozone, moisture, and temperature variations can affect the properties of elastomers. The rate at which these factors contribute to deterioration is dependent on the material of the bearing.

It is therefore recommended that inspection records be maintained in order to distinguish between gradual deterioration and damage due to a specific environmental factor or event.

A bearing cannot be considered in isolation from the rest of the structure, as the performance of the bearing is directly reliant on the performance of the connected components. Any defects in the connected structural elements can adversely affect the performance of the bearing. For example, a settlement in a concrete component can cause unusual stresses in the bearing.

What Attributes should be Evaluated when Reviewing Elastomeric Bearing Pad Manufacturers?

When reviewing the credentials of an Elastomeric Bearing Pad Manufacturer, the following aspects must be taken into account:

• Technical expertise - can the manufacturer understand and address specific technical requirements?
• Material - does the manufacturer provide high-quality materials?
• Dimensions - are the manufactured products within the permissible tolerances?
• Quality management - does the manufacturer possess adequate quality management practices?

For large-scale projects, the reputation of the manufacturer may also be taken into account, since it reflects the level of credibility and trustworthiness.

FAQs

Conclusion

Elastomeric Bearing Pads provide a cost-effective means of load transfer while still allowing the required flexibility to be introduced to the bridge structure. There are several considerations to take into account when selecting a bearing, such as the loads, movement, size, material, exposure, and other design considerations.

Along with an acceptable design, it is also critical to employ the proper installation procedures since they can potentially heavily impact the performance of the bearing.