Bearings for Bridges

Why Provide Bearings?

To allow relative movement between superstructure and sub-structure of a bridge.

To transfer the forces/loads to the foundation.



Basic Definitions

1.Fixed Bearings: A fixed bearing is a point of connection between a structure and its support, designed to transmit vertical and horizontal loads and allow for rotations.


2.Movable bearings: A movable bearing in addition, allows for horizontal movements of the structure


Classification of bearings

1.Functional Requirements: There are possible 6 degrees of freedom at any support. These are translation in three directions and rotation about these three axes. A bearing may permit movement in any of these 6 degrees of freedom or in none.

2.Material Used: A number of different materials have been used for making bearings such as steels of various types, elastomer, PTFE etc. In many forms of bearings, there is a combination of two materials.


Steel Bearings-------

MoRT&H Specifications


Steel Bearings


MATERIALS:


1. The cast steel used for bearings shall conform to Grade 280-520 N of IS : 1030.

2.For the purpose of checking the soundness of cast steel components,castings shall be ultrasonically examined following procedures as per IS:7666 with acceptance standard as per IS:9565.

3.The castings may also be checked by any other accepted method of non destructive testing as specified in IS:1030.

4.Quality level of castings shall be level 3 as per IS: 9565.

5.Grease for bearings shall conform to the requirements of IS: 503 (Grade 4).


Construction Operations:


1.All parts of the bearings shall fit accurately together.

2.Knuckle pins, rolling surfaces of the rollers and bearing surface of the bearing plates shall be machined and all bolt holes shall be drilled.

3.However in the case of bearings which are to be grouted or bedded on a suitable yielding material or on any surface which is to be in permanent contact with the grout or the yielding material may be left unmatched.

4.In pre stressed concrete construction where launching of girders is employed, in order to avoid slipping or jumping of rollers due to vibration or jolts, adequate measures may be taken to ensure that the roller assembly is not disturbed.

5.During concreting of girders, the bearings shall be held in position securely by providing temporary connection between the top and bottom plates in case of fixed bearings and between top plate, base plate and saddle plate in case of roller cum rocker bearings.

6.In pre stressed pre cast girders, where recesses are left on the underside of the girders to receive the anchor bolts, grout holes extending to the beam sides or to the deck level shall be provided. The cement sand grout shall have a mix of 1:1.


Placing:

1.On supporting structures, pockets shall be provided to receive bolts. The pockets shall be filled with mortar of mix 1:1 and the concrete bearing area also shall be finished level by a thin and stiff mortar pad of mix 1:1 just before placing of bearing assemblies or bottom plate on the concrete seat.’[

2.In case of pre cast girders a recess of 6 mm shall be provided on the underside with a level finish for housing the bearing plate. A thin and stiff mortar pad of mix 1:1 with thickness not exceeding 3 mm shall be provided over the top plate before lowering the pre cast beam in position in order to ensure full and even pressure on the plate surface.


Testing:

1.A suitable number of complete bearings shall be tested to 1.25 times the design load.Recovery should be 100 percent.

2.In case the lot size of similar bearings exceeds 12 sets, one extra set of bearings for each 24 sets of bearings shall be manufactured and tests performed on it.


Elastomeric Bearings


In the conventional steel bearings, there are problems pertaining to corrosion of metal and freezing of moving parts. Elastomer as material for making bridge bearing has been found to satisfy these requirements.It offers the following advantages:

1.Requires minimum maintenance compared to all other bearings.

2.Installation is easy.

3.Permits movement of the structure in all directions.

4.Occupies small space.

5.Serves as a shock absorber due to anti vibration properties of elastomer.

6.As an aid to better management of longitudinal forces.


Elastomer is a form of rubber, a class of polymeric substance obtained after vulcanization and it posses rubber like properties i.e its ability to regain shape almost completely even after large deformations.


>>Elastomeric bearings shall be composed of multiple laminate of elastomeric material separated by steel reinforcing.

>>The overall size of the bearing, the number of laminate and thickness of laminate are designed in accordance to the requirement of Load to be Transferred.

>>Bearings having steel plates as reinforcement shall be cast/compression molded/vulcanized as a single unit in a mould under pressure and heat.


Difference between a plain rubber block and laminated elastomeric bearings



Plain rubber block under vertical load compressed down where as a laminated bearing bulge out with each laminations absorbing most of the vertical load.


How a plain rubber block and laminated bearing reacts against horizontal load.


POT BEARINGS


>>Elastomeric bearing because of its inherent flexibility, will always permit deformations and rotations in all the directions.

>>The ordinary elastomeric bearing therefore can not be used as a fixed bearing.

>>On the other hand, the translation allowed by the elastomeric bearing is restricted by its thickness, being of the order of 0.5 to 0.6 of the total thickness of the elastomer.

>>There is limit to the vertical load which the elastomeric pad can safely withstand.

>>Larger vertical loads would require large plan size of the elastomeric pads or would result in greater amount of compression and bulging of the rubber.

>>On account of these constraints, the recommended movement capacity of elastomeric pads is limited to 50-60 mm and the vertical load range to 200 tons.



The need to take advantage of the beneficial properties of elastomer in fixed bearings as well as for design of large expansion bearings have led to the development of a special category of bridge bearings known as POT Bearings.



>>The design principle of the POT Type bearing is that of a fixed bearing which does not permit longitudinal or transverse movements but permits rotational movement of the bridge superstructure about any horizontal axis.

>>The rotational movement is permitted by the shear deformation of an elastomeric pad.

>>Restraint against translational movement is provided by completely encasing the elastomer pad in a steel casing (POT). Therefore these bearings are called Pot bearings.

>> The elastomer inside the pot behaves like an incompressible fluid maintaining constant volume under all round pressure and therefore no deflection takes place under the vertical loads.

>>The addition of a sliding component on top of the pot converts the fixed bearing into a sliding bearing.

>>The sliding movement may be permitted in all the directions or by provision of appropriate stopper plates it can be regulated in one direction only.




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