Bridge Bearing Bearing is a mechanical device placed between superstru tructure and substruc ructure to tra transmit mit vertic tical and hori horizzont ontal load load allo allowi wing ng some some tran transl sla ation tional al and and rota otationa ionall movement. Translational and rotational movement of bridge superstructure may be due to • Shrinkage of concrete prestressing • Elastic shortening of concrete due to prestressing • Creep of concrete expansi on and contraction • Temperature expansion • Movement due to external load Translational and rotational movement of bridge deck may be in longitudinal 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 Roller Multiple Roller Rocker Bearing Linear Rocker Point Rocker Rocker Cum Roller Knuckle Bearing Cylindrical Knuckle Spherical Knuckle Pin Knuckle Leaf Knuckle Sliding Plate Bearing
Single Roller Bearing
Multiple Roller Bearing
Linear Rocker Bearing Cylindrical Knuckle Bearing
Point Rocker Bearing Spherical Knuckle Bearing
Slide Plate Bearing
Elastomeric Bearing
Pin Knuckle Bearing
Pad Pot
Elastomeric Pad Bearing
Leaf Knuckle Bearing
Elastomeric Pot Bearing
ELASTOMERIC BEARING •
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Elastomeric bearing is made of synthetic rubber. Elastomer is the trade name of Neoprene. Elastomeric bearing is designed to be sufficiently soft horizontally to allow translation and sufficiently stiff vertically to prevent appreciable changes in their height under variable loads. Bearing may be reinforced or unreinforced. In reinforced bearing, mild steel plates are embedded. Unreinforced bearing may only be used at support of slab culverts or slab bridges. Elastomeric bearings are not expensive, easy to install and maintain. Life of bearing is about 25 years. So there should be provision of replacement of the elastomeric bearings after about 25 years.
Elastomeric Bearing
Loads on Bearing Vertical load •
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DL from superstructure LL from superstructure Vertical load due to braking effort Vertical Seismic load Vertical wind load
Horizontal load •
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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 load combinations specified by IRC 6 and bearing is designed for critical combination of loads
Load Combination
Design of Elastomeric Bearing
Geometrical Design Find overall length (lo), breadth (bo) and thickness (h) of elastomeric pad. Find number of internal layers of elastomer(n), thickness of internal layers of elastomer (h i), number of steel plates (ns), thickness of steel plates (h s), effective cover to steel plate (h e) and side cover (c). Approximate sizing of bearing is done on the basis guidelines provided by IRC 83 Pt. II (Refer Table Appendix. I) bo
hs hi
h
Cross Section of Bearing
he
Steel plate c c l
lo
b
Plan of Bearing
Table Appendix I, IRC 83 Part II
Standard Plan Dimensions and Design Data of Elastomeric Bearing
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The thickness of the internal layer of elastomer hi, the thickness of the steel plate hs, and the elastomer cover at the top and bottom h e should correspond to the following dimensions. hi (mm) 8 10 12 16 hs (mm) 3 3 4 6 he (mm) 4 5 6 6 The side cover (c) of elastomer for the steel laminates is 6 mm.
Check the geometrical dimensions of bearing as follows. •
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h ≥ bo / 10 and h ≤ bo / 5 Bearing stress in concrete below bearing ≤ Allowable bearing stress in concrete Where, Allowable bearing stress = 0.25(f ck )1/2 Shape factor (S) >6 ≤ 12 Where, S = l x b / 2 ho( l + b )
Design bo
1. Check bearing for shear strain Total shear strain in bearing ≤ 0.7 γd = ∆b /h ≤ 0.7
∆b
h
Where, Total shear strain (γd ) = Strain due to creep, shrinkage and temperature variation + shear strain due to horizontal load
Translational Movement of Bearing
2. Check bearing for rotation Maximum rotation of girder ≤ Permissible rotation αd ≤ β n αbi,max Where, αd = maximum rotation, which may be taken as 400 Mmax L/(E cI) 10-3 n= number of internal elastomer layers ) β = (σ m / σm,max σ m = average compressive stress ; σ m,max = 10N/mm2 αbi,max = (0.5 σ m hi )/(bs2 ) M - Maximum BM at mid span L- span of girder E c – Modulus of elasticity of concrete [In short term loading ; E c = 5000(f ck )1/2 ] I = Gross moment of inertia of main girder
αd
h bo Rotational Movement of Bearing
3. Check bearing for friction •
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Total Shear Strain (γ d) ≤ 0.2 + 0.1 σm Normal stress ‘σm ‘ ≥ 2 N/mm2 and ≤ 10 N/mm2
Where, σ m = Normal compressive stress σ m,max = 10N/mm2
4. Check bearing for Shear Stress Total shear stress due to normal and horizontal loads and rotation τ + τ + τ α ≤ 5 N/mm2 c
r
Where, Shear stress due to normal load ( τc )=(1.5 σ m )/S Shear stress due to horizontal load ( τ r ) = Total shear strain 2 Shear stress due to rotation( τ α )= 0.5(b/h ) i αbi max
≤
5 N/mm2
