Bridge Bearing Bearing is a mechanical device placed between superstructure and substructure to transmit vertical and horizontal load allowing some translational and rotational movement. Translational and rotational movement of bridge superstructure may be due to • Shrinkage of concrete • Elastic shortening of concrete due to prestressing • Creep of concrete • Temperature expansion and contraction • Movement due to external load Translational and rotational movement of bridge deck may be in longitudinal or transverse or other direction of bridge
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Bridge Bearing
Bearing is a mechanical device placed betweensuperstructure and substructure to transmit vertical andhorizontal load allowing some translational and rotationalmovement.
Translational and rotational movement of bridgesuperstructure may be due to• Shrinkage of concrete• Elastic shortening of concrete due to prestressing• Creep of concrete• Temperature expansion and contraction• Movement due to external load
Translational and rotational movement of bridge deck may be inlongitudinal or transverse or other direction of bridge
Types of Bridge Bearing
Bearing Fixed Bearing –Bearing, which allows rotational movement
Free Bearing (Expansion Bearing) –Bearing, which allows horizontal and rotational movement
Metalic Bearing –Bearing made up of Metal i.e. steel or cast iron
Elastomeric Bearing –Bearing made up of artificial rubber (Neoprene)
Metalic Bearing Roller Bearing
Single RollerMultiple Roller
Rocker BearingLinear RockerPoint RockerRocker Cum Roller
• Elastomeric bearing is made of synthetic rubber. Elastomer is thetrade name of Neoprene. Elastomeric bearing is designed to besufficiently soft horizontally to allow translation and sufficientlystiff vertically to prevent appreciable changes in their heightunder variable loads.
• Bearing may be reinforced or unreinforced. In reinforced bearing,mild steel plates are embedded. Unreinforced bearing may onlybe used at support of slab culverts or slab bridges.
• Elastomeric bearings are not expensive, easy to install andmaintain.
• Life of bearing is about 25 years. So there should be provision ofreplacement of the elastomeric bearings after about 25 years.
ELASTOMERIC BEARING
Elastomeric Bearing
Loads on Bearing
Vertical load• DL from superstructure• LL from superstructure• Vertical load due to braking effort • Vertical Seismic load• Vertical wind load
Horizontal load• Wind load from superstructure• Load induced by creep, shrinkage and temperature effect• Braking load• Vertical load due to seismic effect
These loads are combined according to the loadcombinations specified by IRC 6 and bearing isdesigned for critical combination of loads
Load Combination
Design of Elastomeric Bearing
Find overall length (lo), breadth (bo) and thickness (h) of elastomeric pad. Find numberof internal layers of elastomer(n), thickness of internal layers of elastomer (hi), numberof steel plates (ns), thickness of steel plates (hs), effective cover to steel plate (he) andside cover (c). Approximate sizing of bearing is done on the basis guidelines provided byIRC 83 Pt. II (Refer Table Appendix. I)
Geometrical Design
hs
hi
he
bo
h
lo
c
cSteel plate
b
l
Cross Section of Bearing
Plan of Bearing
Standard Plan Dimensions and Design Data of Elastomeric Bearing
Table Appendix I, IRC 83 Part II
Check the geometrical dimensions of bearing as follows.
• h ≥ bo / 10 and h ≤ bo / 5
• Bearing stress in concrete below bearing ≤ Allowable bearing stress in concrete
Where, Allowable bearing stress = 0.25(fck)1/2
• Shape factor (S) >6≤12
Where, S = l x b / 2 ho(l + b )
• The thickness of the internal layer of elastomer hi, the thickness of the steel platehs, and the elastomer cover at the top and bottom he should correspond to thefollowing dimensions.hi (mm) 8 10 12 16hs (mm) 3 3 4 6he (mm) 4 5 6 6
• The side cover (c) of elastomer for the steel laminates is 6 mm.
1. Check bearing for shear strain
Total shear strain in bearing ≤ 0.7
Where,Total shear strain (γd) = Strain due to creep, shrinkage andtemperature variation + shear strain due to horizontal load
2. Check bearing for rotationMaximum rotation of girder ≤ Permissible rotation
αd ≤ β n αbi,max
Where,
αd = maximum rotation, which may be taken as 400 Mmax L/(EcI) 10-3
n= number of internal elastomer layersβ = (σm/σm,max)σm = average compressive stress ; σm,max = 10N/mm2
αbi,max = (0.5 σm hi )/(bs2)M - Maximum BM at mid span L- span of girderEc – Modulus of elasticity of concrete [In short term loading ; Ec = 5000(fck)
1/2 ]I = Gross moment of inertia of main girder
bo
∆b
h
Translational Movementof Bearing
bo
αd
h
Rotational Movementof Bearing
Design
γd = ∆b /h ≤ 0.7
3. Check bearing for friction
• Total Shear Strain (γd) ≤ 0.2 + 0.1 σm
• Normal stress ‘σm ‘ ≥ 2 N/mm2 and ≤ 10 N/mm2
4. Check bearing for Shear Stress
Total shear stress due to normal and horizontal loads and rotation ≤ 5 N/mm2
τc + τr + τα ≤ 5 N/mm2
Where, Shear stress due to normal load (τc )=(1.5 σm )/SShear stress due to horizontal load ( τr ) = Total shear strainShear stress due to rotation( τα )= 0.5(b/hi)
2 αbi max
Where, σm = Normal compressive stress σm,max = 10N/mm2
