BEARINGS FOR BRIDGES
TYPES OF BEARINGS :
Fixed Bearing :
A Fixed Bearing is designed to transmit vertical and horizontal loads and allow for rotations. Fixed bearings can be made of steel plates - sometimes flat and pinned and sometimes curved to provide for rotation, sometimes of two curved plates with rocking lubricated surfaces and sometimes of elastomeric material - with or without dowel pins through the bearing or preferably keys. Following are the types of fixed bearings:
- Rocker bearing
- Steel hinge
- Steel rocker bearing
- RCC rocker bearing
Moveable or Expansion Bearing :
Moveable or Expansion Bearing allows for horizontal movement of the structure. Moveable bearings may be classified into two categories, namely, Sliding Type and Rolling Type.
Sliding type moveable bearings can be made of steel plates with lubricated surfaces, or a steel plate with special low friction material (e.g. PTFE) to enhance the sliding ability.
Rolling type moveable bearings are usually made of hardened steel curved surfaces which roll on other hardened steel surfaces.
Following are the types of moveable or expansion bearings :
Sliding plate bearing
RCC rocker expansion bearing
From the point of view of the material used, bearings can be classified as :
Metallic rubber bearings
Metallic Bearings :
These are usually made of mild steel and cast steel. Lead sheets, phosphor bronze and stainless steel are used sometimes for their respective special merits in specific cases. Metallic free bearings have the advantage of generating small to very small horizontal force because of low friction coefficient against sliding and very low against rolling surfaces are chemically coated with teflon ( which is a rubber synthetic), the friction coefficient reduces even further. Metallic bearings are made of cast steel of special tensile strength with special resistance to vibration and wear. Armoured steel bearings are basically of mild steel but specially treated so as to harden the surface to a depth of about 10 mm. With this costly treatment, the load carrying capacity of a roller is simultaneously improved.
Metallic Rubber Bearings :
These are of natural rubber or synthetic rubber known as neoprene reinforced by impregnated mild steel plates, and have been widely used as bridge bearings. Elastomer is the trade name of a synthetic rubber (CR or Neoprene), developed by Du-Pont Company. Mere solid rubber blocks cannot be used on account of `squeeze out' effect, shear failure and relatively early aging problems. Interleaved layers of neoprene and steel plates, bonded together, overcome these disadvantages. These bearings can be designed to be sufficiently soft horizontally so as to impose relatively small horizontal forces on the supports ( = shear rating of the bearing x deck movement above it), but sufficiently stiff vertically to prevent appreciable changes in their height variable loads. Rubber (and neoprene) are specified in terms of its `shore' hardness. This is a measure of the penetration into the rubber by an, indenting apparatus under a specified load and is, therefore, related to the elastic modulus of the rubber. These elastomeric bearings are cheap, easy to maintain, and are good in shear under compression. However, on their own, they are not suitable where compressive load is low while rotation is high (e.g. under high torsion conditions) because they accept rotation only under significant load.
Working Principle of Neoprene Bearings Reinforced With Restraining Steel Plates (Laminated Neoprene Bearings )
When subjected to loads and/or displacement, an individual elastomer layer distorts, tangential shear stresses occur in the plane of contact of the layer with its support, which depend not only on the magnitude of the loads applied but also on the shape of the layer - plan surface and lateral exposed surface, on the nature of the elastomer, the rate at which the loads are applied, and on the temperature. These tangential stresses are manifested by a tendency for intergranular sliding, which is opposed by the adherence to the elastomer layer of the steel plates forming the reinforcing laminations. With equal thickness of elastomer, these plates have the further advantage of either reducing the set due to the normal load or increasing the admissible load.
RCC Rocker Bearings :
Cast steel roller bearings prove to be rather expensive. Due to this reason, sometimes, cheaper RCC Rocker and Expansion Bearings are used for concrete bridges. With proper design, these bearings prove to be as effective as steel bearings. Maintenance cost of RCC bearings is negligible in comparison to that of steel bearings. The rocker is made of special quality concrete which is reinforced adequately. Lead sheets are introduced between the contact surfaces of the rocker with the deck and the substructure.