Short Note on Types of Bearings in Bridge

Types of bearings in bridge

1. Shoe-plates: Shoe plates are attached to the underside girder and these rest directly on masonry. This is an unsatisfactory arrangement, as due to figh action between masonry &steel, expansion is resisted and heavy longitudinal and forces are imparted to the masonry, these type of bearings are gradually being replaced by centralized bearings.

2. Centralized bearings or flat bearings: These are provided for plate girders generally up to a span of 24.4(80ft).In this arrangement, a curved or chamfered bearing plate is fixed to the bottom flange of the plate girder just below the bearing stiffener and the same rests on a bed plate fixed to the heel block. The bed plate at the free end has to be periodically greased to reduce the co-efficient of action. Greasing also prevents rusting of steel surface. This works as preventive maintenance of bearing as well as of bed plates and prolongs their service life. The rocking ‘of the girder is facilitated by the curved shape of the bearing plate.

3. Phosphorous bronze bearings: In this type of bearings the bed plate has a phosphor bronze surface. This is useful for long spans, particularly for spans above 10.5m (100ft.) because of the lower co-efficient of friction of steel on bronze surface(0.15),than that on steel surface(0.25).It has however to be ensured that the hearings are very clean, without any dirt. Phosphor bronze bearings need not be greased as these are corrosion resistant and retain the smooth surface.

4. Rocker and Roller bearings : In this type of the bearings at the fixed end, the girder end is fixed in a position but rocking due to deflection under load is permitted by the curved knuckle pin. At the free end, the arrangement for rocking is similar but the knuckle assembly is mounted on a set of rollers to take linear expansion. It has to be ensured during inspection that the knuckle pin and the rollers are well lubricated.

5. Elastomeric bearings or Neoprene bearings : These are provided for bridges having RCC or prestressed girders and can take deflection as well as movement . Normally, these bearings are provided for Road bridges, but recently Railway bridge are also being provided with this type of bearings.

(i) Elastomeric bearings are made of natural or synthetic rubber if shore hardness of approximately 50 to 70. They are very stiff in resisting volume change but are very flexible when subjected to shear or pure uni-azial tension. They are generally reinforced with steel or concrete girder these permit moderate longitudinal movements ad small rotations at the ends.

(ii) These bearings require periodic cleaning . they may require replacement in service depending on the condition and usage.

6. Oil bath bearings :- this type of bearings is encased in oil baths and does not need periodic oiling and greasing.

7. P.T.F.E. bearings :- this type of bearings consists of a self lubricant ply-tetra- fuoroethylene surface across a smooth stainless steel plate , fixed on a mild-steel plate which slides on it, it makes a non-rotational free end by vulcanizing the P.T.F.E. sheet on an elastomeric bearing. This also gives a very low coefficient of friction in its sliding action. These bearings also require periodic cleaning of the bearing surface. The interface should be protected from dust. Lubricating the mating surface by silicon grease reduces the coefficient of friction and is desirable.

8. Segmental bearings : these are provided in special situations where piers are too slender, as in case of RCC piers.