Elastomeric Bridge Bearings

Posted by admin@Ataya No Comments on Elastomeric Bridge Bearings Construction

Elastomeric Bridge Bearings

Like all reinforced concrete metallic structures, bridges are also subject to certain movements. They are caused by temperature variations, wind load, traffic movements, or seismic activities. Modern bridges, unlike conventional bridges, have longer spans and are lighter in construction and hence have high chances for a wider range of movements.

If the relative movement between the bridge superstructure and substructure is restricted, a large number of forces may develop in the girder or the substructure. It can push its supports thereby causing damage to the bridge. Elastomeric bearings are hence used to allow necessary relative movement between the girders and the substructure. They are placed on the bottom of the substructure, i.e., below the girder and above the pier cap, over a pedestal. It is an interface between the superstructure and the substructure.

Generally, elastomeric bearings work in three broad areas as below:

• They allow the ‘permitted’ longitudinal and rotational movements.
• Prevent those ‘not permitted’ undesirable movements.
• Transfer load from the superstructure to the substructure.

Interestingly elastomeric bearings have no moveable parts and hence the movements are absorbed by the bearings in the form of shear. There is no sliding motion that occurs between the pad and the beam or between the pad and abutment unless some sliding elements are already designed into the surface of the bearing.

Role of an Elastomer

Engineers understand elastomers less than they do concrete or steel because the elastomers do not obey Hooke’s Law. They are very flexible in shear but very stiff in bulk compression. The simple theory of mechanics characterizing the behavior of rubber is quite different from that used for conventional materials. Some of those striking features of elastomers are as follows:

• Since elastomers do not follow Hooke’s Law, the modulus of elasticity-E is not constant.
• The shear modulus-G is fairly constant and hence it becomes more relevant during elastomeric bearings design.
• The coefficient of friction between the elastomer and the base material is fairly unaffected by the nature of the contact surfaces whether it is steel or concrete.
• The coefficient of friction between the elastomer and the base material reduces with an increase in normal load on the bearing
• Except under extremely low temperatures, the performance of the elastomeric bearing is unaffected by temperature variation.
• Under the effect of cyclic loading elastomeric bearings become more flexible.

Elastomer Design

An elastomer is a polymeric material obtained after the vulcanization of rubber. Vulcanization is the process of improving the properties of the rubber by heating it with certain chemicals like Sulphur. This cross-linking makes rubber stronger and it allows the rubber to maintain its shape even when it is stretched over and over.

The formulation of the elastomer is to be designed to give long life and resistance to oils, solvents, ozone, and chemical products contained in the concrete. They need to be resistant to deterioration caused by weather. In general, elastomeric bearings are designed to work well within a temperature range of -30^oC to +50^oC, and even 70^oC for short periods.

Advantages of Elastomeric bearings

The elastomeric bearing could accommodate large movements and at the same time be relatively maintenance-free.

• Requires minimum maintenance compared to all other bearings.
•  Installation is relatively easy.
• It permits movement of the structure in all directions, depending upon the applied forces.
• Occupies small space.
• Serves as a shock absorber due to the anti-vibration properties of the elastomer.

Why Ataya?

Ataya manufactures a wide range of elastomeric bearings to suit most applications. We design our Natural rubber and Neoprene compounds to meet stringent requirements as per EN 1337-3, Table 1 at our Elastomer engineering department. We have a full-fledged R&D facility with most of the test instruments and qualified staff for research and quality control. We maintain strict quality during every stage of manufacturing and do final performance tests at our bearing test rig and ensure our customer gets the best product.