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

Knuckle BearingCylindrical KnuckleSpherical KnucklePin KnuckleLeaf Knuckle

Sliding Plate Bearing

Elastomeric Bearing Pad Pot

Single Roller Bearing

Multiple Roller Bearing

Linear Rocker Bearing

Point Rocker Bearing

Cylindrical Knuckle Bearing

Spherical Knuckle Bearing

Pin Knuckle Bearing

Leaf Knuckle Bearing

Elastomeric Pad Bearing

Slide Plate Bearing

Elastomeric Pot Bearing

Elastomeric Bearing

ELASTOMERIC BEARING

Elastomeric bearing is made of synthetic rubber called neoprene. It maybe used with or without reinforcement. Impregnated mild steel plates doreinforcement in the elastomeric bearing. These bearings are cheap, easy tomaintain, and are good in shear under compression.

Elastomer is the trade name of a synthetic rubber (Neoprene). Thesebearing are designed to be sufficiently soft horizontally so as to imposerelatively small horizontal forces on the supports but sufficiently stiffvertically to prevent appreciable changes in their height under variableloads.

Owing to possibility of their deterioration with age, provision should bemade in the bridge structure for replacement of the elastomeric bearingsafter about 25 years.

Design of Elastomeric Bearing

1. Geometrical Design

Find overall length , breadth and thickness (bo, ho, lo) of elastomeric pad. Find numberof steel plates (n), thickness of steel plates (hs), thickness of internal layers ofelastomer (hi), and covers(he, c)

hs

hi

he

bo

ho

lo

c

c

Steel plate

• For stability, the least plan dimension should not be less than four timesthe thickness of the bearing and The ratio of overall length to breadthshould be equal to or less than 2.

• The bearing area of the pad should be such that the compressive stressesdeveloped in concrete are within specified limits.

• The stiffness of elastomer in compression depends on the shape factor ofits individual layer. This is the ratio of loaded area to the total force freearea of the individual layer. The shape factor S should be greater than 6and less than 12.S = l x b / 2 ho(lo + bo )

• The side cover of elastomer for the steel laminates is 6 mm.

Standard Plan Dimensions and Design Data of Elastomeric Bearing

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 dimensionshi (mm) 8 10 12 16hs (mm) 3 3 4 6he (mm) 4 5 6 6

2. Check for shear strain

Shear strain due to creep, shrinkage andtemperature variation and shear strain due to

horizontal load ≤ 0.7

3. Check for rotation

αd ≤ βn αbi,max

Where,αd = actual angle of rotation, which may be taken

as 400 MmaxL/(EI) 10-3

n= number of elastomer layersβ = (σm/σm,max)σm = average compressive stressσm,max = 10N/mm2

αbi,max = (0.5 σm hi )/(bs2)

bo

bo

αd

∆b

ho

ho

Translational Movement

Rotational Movement

4. Check for friction

• Total Shear strain ≤ 0.2 + 0.1 σm

• Normal stress ‘σm ‘ ≥ 2N/mm2 and ≤ 10N/mm2

5. Check for Shear Stress

The total shear stress due to normal and horizontal loads and rotation should be less than 5 N/mm2

Where, Shear stress due to normal load τc =(1.5 σm )/S N/mm2

Shear stress due to horizontal load = Shear strain = τr N/mm2

Shear stress due to rotation = τα = 0.5(b/hi)2 αbi N/mm2

METALLIC BEARING

Metallic bearing usually made of mild steel, cast steel andstainless steel. It may be free or fixed.

Sliding or rolling surfaces of bearings are coated with Teflon,which significantly reduces the friction and consequently reduceshorizontal forces acting on pier/abutment.

In straight bridges, the following types of metallic bearings areadopted.• Sliding plate bearing• Steel roller bearing• Steel roller cum rocker bearing• Steel rocker bearing• Steel knuckle bearing

• Diameter of roller (D) ≥ 75mm• L/D ≤ 10• Lug width ≥ 10mm• No. of lug = 2 for L/D ≤ 6

= 3 for L/D > 6• P ≥ 50mm• t ≥ 20mm or ¼ of S

d ≥ 16 mm

≥ 1.1d ord + 2.5mm

0.5d

0.5d

2.5mm

Rocker Pin

Rocker cum Roller Bearing

S

P

t

t

Top Plate

Saddle Plate

Bottom Plate

Rocker Pin

Rocker

Lug for RollerRoller

L

D

• The width of plates shall not be less than 100 mm or the distance between the center-to-center distance of outermost rollers plus twice the thickness of the plate plus 10 mm.

• The top plates of sliding bearings shall project on all sides over the bottom plate by atleast 10 mm.

Design of Metallic Bearing 1. Assign approximate size of Metallic Bearing

Working loads per unit length of rollers should less or equal toFor Mild steel

Single and double rollers 8D N/mm of lengthThree or more rollers 5D N/mm of length

For Cast steelSingle and double rollers 11D N/mm of lengthThree or more rollers 7D N/mm of length

Area of plate is determined with respect to the permissible bearing stress of concrete

2. Check bearing stress in concrete and steel

Radius of rocker is determined byVertical design load per unit length ≤ 170R2σu

3/E2

3. Check radius of rocker

4. Check working load on roller

5. Check thickness of plateσb ≤ σbt

6. Check rocker pin for shearτv ≤ τva