26. Bridge Bearings

8/12/2019 26. Bridge Bearings

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BEARINGS FOR BRIDGES

Dr. S. Venkateswara RaoDepartment of Civil Engg.,

National Institute of Technology, Warangal

NEO CONSTRUCTION MATERIALS


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Bridge Bearings


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Function of Bearings

Bridge bearings are used to transfer forces from thesuperstructure to the substructure, allowing the followingtypes of movements of the superstructure:

Translational movements; andRotational movements

Until the middle of this century, the bearings used

consisted of following types:PinRollerRocker

Metal sliding bearings


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Pin Bearing

Steel Bearing

Rotational Movement is allowed

Lateral and Translational Movements are Restricted


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A pin bearing is a type of fixed bearings that accommodatesrotations through the use of a steel pin.

Translational movements are not allowed.

The pin at the top is composed of upper and lower semicircularly recessed surfaces with a solid circular pin placed

between.

Usually, there are caps at both ends of the pin to keep the pinfrom sliding off the seats and to resist uplift loads ifrequired.

The upper plate is connected to the sole plate by either boltingor welding. The lower curved plate sits on the masonry plate.


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Roller Bearings

Single Roller Bearing

Multiple Roller Bearing

A general drawback to this type of bearing is its tendency tocollect dust and debris.


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Roller type Bearing

Roller type Bearing with gear arrangement

Longitudinal movements are allowed

Lateral Movements and Rotations are Restricted


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Rocker type Bearing


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A rocker bearing is a type of expansion bearing that comes ina great variety.

It typically consists of a pin at the top that facilitatesrotations, and a curved surface at the bottom thataccommodates the translational movements.

Rocker and pin bearings are primarily used in steel bridges.


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Sliding Bearing


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Sliding Bearings

A sliding bearing utilizes one plane metal plate slidingagainst another to accommodate translations.

The sliding bearing surface produces a frictional force that isapplied to the superstructure, substructure, and the bearingitself.

To reduce this friction force, PTFE(poly tetra fluoroethylene)is often used as a sliding lubricating material.

PTFE is sometimes referred to as Teflon, named after awidely used brand of PTFE


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Sliding Bearing

Sliding Bearings be used alone or more often used as acomponent in other types of bearings.

Pure sliding bearings can only be used when the rotationscaused by the deflection at the supports are negligible.

They are therefore limited to a span length of 15 m or less by

ASHTTO [10.29.1.1]


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Knuckle Pinned Bearing


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It is special form of Roller Bearing in which the Knuckle pinis provided for easy rocking.

A knuckle pin is inserted between the top and bottom casting.The top casting is attached to the Bridge superstructure, whilethe bottom casting rests on a series of rollers.

Knuckle pin bearing can accommodate large movements andcan accommodate sliding as well as rotational movement.


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Pot Bearing


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Plain Elastomeric Bearing


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Laminated Elastomeric Bearing

Elastomeric material interspersed with steel plates


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Laminated Elastomeric Bearing


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Other types of Bearings


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Consist of thin rubber sheets bonded onto thin steel plates andcombined with an energy dissipation mechanism.The rubber sheets are vulcanized and bonded to the thin steel

plates under pressure and heat.it is designed in such a way that bearing is very stiff and strongin vertical direction, but flexible in horizontal direction.Thick mounting steel plates are bonded to the bottom and topsurfaces allowing the isolator to be firmly connected to thefoundation below and the superstructure above.

3 . 0

E l a s

t o m e r i c

B e a r

i n g s

Fig: Basic structure of rubberbearing

ELASTOMERIC BEARINGS


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A laminated elastomeric bearing is an elastomeric rubber blockreinforced with steel plates vulcanised when built.

This bearing is the connection between a structure and itssupport, and should make the following possible throughelastic deformation:

transmission of normal forces; horizontal movements; rotation of the structure in any direction; transmission of horizontal forces, within defined limits.

It may also be provided with a sliding plane for withstandinglarge movements of the structure and also having a one or twohorizontal movement locking systems.


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Elastomeric Bearings are designed to accommodate threetypes of movement.1. Vertical2. Shear3. Rotation


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Types of Elastomeric Bearings

Standard bearings

Bearings are individually moulded and the reinforcing plates arecompletely buried in the elastomer with at least 4mm lateral coverand generally 2.5mm thick (type B) outside layers with theexception of bearings provided with thick external bands (typeC).

These bearings may have a rectangular, square or circular section.They may also have holes for running specific materials through(cross bar anchor), for fitting damping material or for reducingvertical stiffness.


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Bearings with anchor plates


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Bearings with sliding surfaceLaminated elastomeric bearings can be fitted with a slidingsurface comprising a PTFE (poly tetra fluoroethylene) sheetdipped in the elastomer of the bearing or notched in the topthick outer plate and a sliding plate provided with an austeniticsteel sheet for decks requiring significant movement.

Therefore, the built up bearing only provides very littlemovement resistance (limited to PTFE/ austenitic steel friction).


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THANK YOU

Documents

26. Bridge Bearings