1

Bearings for Bridges

Dr. Shahzad Rahman
NWFP University of Engg & Technology,

Peshawar

2

Bridge Bearings

3

Bridge Bearings

Function Of Bearings



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



Translational movements; and



Rotational movements

4

Bridge Bearings

Until the middle of this century, the bearings

used consisted of following types:



Pin



Roller



Rocker



Metal sliding bearings

5

Pin Bearing



A pin bearing is a type of fixed bearings that

accommodates rotations through the use of a steel



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 pin from sliding off the seats and to resist uplift

loads if required.



The upper plate is connected to the sole plate by

either bolting or welding. The lower curved plate sits

on the masonry plate.

6

Pin Bearing

Steel Pin

• Rotational Movement is allowed

• Lateral and Translational Movements are
Restricted

7

Roller Type Bearings

Multiple Roller Bearing

Single Roller Bearing

• AASHTO requires that expansion rollers be equipped with “substantial

side bars” and be guided by gearing or other means to prevent lateral
movement, skewing, and creeping (AASHTO 10.29.3).

•

A general drawback to this type of bearing is its tendency to

collect dust
and debris.

8

Roller Type Bearings

Roller Type Bearing with Gear Arrangement

• Longitudinal movements are allowed

• Lateral Movements and Rotations are
Restricted

9

Rocker Type Bearing

• A rocker bearing is a type of expansion bearing that comes in

a great variety.

• It typically consists of a pin at the top that facilitates

rotations, and a curved surface at the bottom that
accommodates the translational movements

• Rocker and pin bearings are primarily used in steel bridges.

10

Sliding Bearings

• A sliding bearing utilizes one plane

metal plate sliding against another to
accommodate translations.

• The sliding bearing surface produces a

frictional force that is applied to the
superstructure, substructure, and the
bearing itself.

• To reduce this friction force, PTFE

(polytetrafluoroethylene) is often used
as a sliding lubricating material. PTFE
is sometimes referred to as Teflon,
named after a widely used brand of
PTFE

11

Sliding Bearings

• Sliding Bearings be used alone or more often used as a component in

other types of bearings

• Pure sliding bearings can only be used when the rotations caused 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]

12

Knuckle Pinned Bearing

• It is special form of Roller Bearing in which the Knuckle pin is provided for

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

• Knuckle pin bearing can accommodate large movements and can

accommodate sliding as well as rotational movement

13

Pot Bearings

14

Pot bearing

15

Pot Bearings



A POT BEARING consists of a shallow steel

cylinder, or pot, on a vertical axis with a

neoprene disk which is slightly thinner than the

cylinder and fitted tightly inside.



A steel piston fits inside the cylinder and bears

on the neoprene.



Flat brass rings are used to seal the rubber

between the piston and the pot.



The rubber behaves like a viscous fluid flowing

as rotation may occur.



Since the bearing will not resist bending

moments, it must be provided with an even

bridge seat.

16

Plain Elastomeric Bearings

17

Laminated Elastomeric Bearings

Elastomeric material interspersed with steel plates

18

Laminated Elastomeric Bearings

• consist of a laminated elastomeric

bearing equipped with a lead cylinder
at thecenter of the bearing.

• The function of the rubber-steel

laminated portion of the bearing is to
carry the weight of the structure and
provide post-yield elasticity.

• The lead core is designed to deform

plastically, thereby providing damping
energy dissipation.

• Lead rubber bearings are used in

seismically active areas because of
their performance under earthquake
loads.

19

Other Types of Bearings

20

Selection of Bearing Type



AASHTO LRFD provides guidelines for selection of

suitable bearings for bridges as per requirements in

Table 14.6.2-1

21

Selection of Bearing Type

Long

Trans

Long

Trans

Vert

Long

Trans

Vert

S

L

R

U

L

L

S

S

L

L

L

L

2

6

0

0

18

S

S

S

S

L

L

L

L

4

4

0

0

20

U

U

U

U

U

L

L

S

1

2

0

5

7

S

S

S

S

L

L

L

S

5

3

0

0

21

S

S

U

U

S

R

R

S

4

0

2

2

14

R

R

S

S

S

R

R

S

4

0

4

0

16

R

R

U

S

U

R

R

S

2

0

4

2

10

R

R

S

S

L

S

R

S

4

1

3

0

17

R

R

S

S

U

R

R

S

3

0

4

1

13

R

R

S

S

L

S

S

S

5

1

2

0

19

S

U

U

S

U

U

R

S

3

0

1

4

10

U

U

U

S

U

S

R

S

3

0

1

4

10

S

U

U

S

U

U

R

S

3

0

1

4

10

S

U

U

U

U

U

U

S

2

0

0

6

6

Score

Rank

Type of Bearing

S

=

suitable,

U

=

unsuitable,

L

=

suitable for limited applications,

R

=

may be suitable, but requires special considerations or additional

elements such as slider or guideways.

Bearing Suitability:

Disk bearing

Pot bearing

Rocker bearing

AASHTO Table 14.6.2-1

Axis indicated

Single roller bearing

Multiple roller bearing

Rotation about bridge

Resistance to Loads

Curved sliding spherical
bearing
Curved sliding cylindrical
bearing

Double cylindrical bearing

Knuckle pinned bearing

Fiberglass reinforced pad

Cotton duck reinforced pad
Steel-reinforced elastomeric
bearing
Plane sliding bearing

Movement

Plain elastomeric pad

22

Elastomeric Bearing Design -Example

23

Elastomeric Bearing Design -Example

24

Elastomeric Bearing Design -Example

Loading Data

25

Elastomeric Bearing Design -Example

Corrected for skew

mgr = r

skew

x mg

Where,
r

skew

= Correction Factor for Skew

mg = Uncorrected Distribution Factor neglecting skew

26

Elastomeric Bearing Design -Example

826

.

0



MI

V

mg

Uncorrected Distribution Factor =
For Shear, Interior Beams

762

.

0



SE

V

mg

Uncorrected Distribution Factor =
For Shear, Exterior Beams

746

.

0



MI

M

mg

Uncorrected Distribution Factor =
For Moment, Interior Beams

762

.

0



SE

M

mg

Uncorrected Distribution Factor =
For Moment, Exterior Beams

27

Elastomeric Bearing Design -Example

Correction Factor for Skew

For skewed bridges the Distribution Factor for Shear may be modified by
Multiplying it with a Modification Factor given as: [A4.6.2.2.3 c-1]



tan

2

.

0

0

.

1

3

.

0

3

















Kg

Lts

r

skew

o

30





0

.

1

3















Kg

Lts

 

115

.

1

)

577

.

0

(

0

.

1

2

.

0

0

.

1

3

.

0







skew

r

28

Elastomeric Bearing Design -Example

Correction Factor for Moment

For skewed bridges the Distribution Factor for Moment may be modified by
Multiplying it with a Modification Factor given as: [A4.6.2.2.2 e]

5

.

0

25

.

0

3

1

5

.

1

1

25

.

0

)

(tan

0

.

1































L

S

Lts

Kg

c

c

r

skew



o

30





mm

S

mm

L

Lts

Kg

2440

,

670

,

10

,

0

.

1

3



















948

.

0

)

30

(tan

0

.

1

5

.

1

1







c

r

skew

12

.

0

10670

2400

1

25

.

0

5

.

0

25

.

0

1

















x

x

c

29

Elastomeric Bearing Design -Example

Modified Distribution Factors for Shear and Moment

30

Elastomeric Bearing Design -Example

Bearing Load Calculation

31

Elastomeric Bearing Design -Example

Bearing Load Calculation

32

Elastomeric Bearing Design -Example

33

Elastomeric Bearing Design -Example

Maximum Longitudinal Movement at the Abutment

34

Elastomeric Bearing Design -Example

35

Elastomeric Bearing Design -Example

Preliminary Thickness of Bearing

36

Elastomeric Bearing Design -Example

Preliminary Thickness of Bearing

37

Elastomeric Bearing Design -Example

Preliminary Thickness of Bearing

38

Elastomeric Bearing Design -Example

Check Stresses in Trial Bearing Size

39

Elastomeric Bearing Design -Example

40

Elastomeric Bearing Design -Example

0.033

41

Elastomeric Bearing Design -Example

42

Elastomeric Bearing Design -Example

43

Elastomeric Bearing Design -Example

44

Elastomeric Bearing Design -Example

45

Elastomeric Bearing Design -Example

46

Elastomeric Bearing Design -Example

47

Elastomeric Bearing Design -Example

48

Elastomeric Bearing Design -Example

49

Elastomeric Bearing Design -Example

50

Elastomeric Bearing Design -Example

51

Elastomeric Bearing Design -Example

52

Elastomeric Bearing Design -Example

53

Elastomeric Bearing Design -Example

15mm

7mm

0.6mm

L = 240 mm

62.4mm

W

=

3

50

m

m

FINAL DESIGN