Bearing and Railing

1. Fundamentals of Bridge Design (Infolink College-Hawassa Campus) Page 1
7. BEARINGS AND RAILINGS
BEARINGS
Bearings are structural devices positioned between the bridge superstructure and the substructure.
Their principal functions are as follows:
1. To transmit loads from the superstructure to the substructure, and
2. To accommodate relative movements between the superstructure and the substructure.
Movements should be accommodated properly and these result from
a) longitudinal movement due to temperature changes-reversible
b) rotation due to deflection of girders- live load deflection is reversible
c) vertical movement due to settlement of support- irreversible
d) movement due to shrinkage, creep or prestressing-irreversible
Bearings are categorized in to two as
a) Fixed bearings – are fixed against translation, but free to rotate
b) Expansion bearings-are free to translate and rotate
Selection of the type depends on the type of superstructure, type of substructure supports and the
length of the span.
- A simply supported span requires fixed bearings at one and expansion joints at the other.
- A continuous girder require fixed bearing at one and expansion bearing on all other supports
- A two span girder will have fixed bearing at the centre support and expansion bearings at the
two abutments
Bearings are important components of bridges and call for care in design, skill in execution, regular
attention in maintenance. Faulty design or malfunctioning bearings due to clogging by debris have
often resulted in collapse of bridges due to the high stresses induced due to restrained motion.
On certain major bridges bearing cost is as high as 10-15% of the total cost. It is worth designing
carefully and inspecting regularly for maintenance.
Types of Bearings
For slab bridges no special bearings are provided because the translational movement is very small.
Edges of supports are beveled to allow for rotation.
The following are the different types of bearings for girder bridges
- Sliding plate bearing- is the simplest type of bearing. This utilizes one plate sliding on
another. This type is used for bridges up to 30m span. Flat plates were common but due to
freezing and high frictional forces the surface of the upper plate is slightly curved as shown
in Figure below.

2. Fundamentals of Bridge Design (Infolink College-Hawassa Campus) Page 2
Figure: Sliding plate bearing
- Roller bearing- in this type rotation is accommodated by the pin and translation by the use
of a roller or a group of rollers. See Figure below. This is used for spans of moderate length.
Figure: Roller bearing
- Rocker bearing- this is used for longer spans and is used to accommodate large live load
deflections. See Figure below.
Figure: Rocker bearing

3. Fundamentals of Bridge Design (Infolink College-Hawassa Campus) Page 3
In seismic areas suitable guides should be provided to prevent the roller and rocker
components from being displaced off abutments and piers during earthquakes.
- Elastomeric bearing- is made of a polymeric substance called elastomer (either natural or
synthetic rubber).
This can be reinforced with steel plates, which are usually the case, or may be unreinforced.
This is very strong in compression and relatively week in shear, thus allowing horizontal
displacement. This has shown good weathering performance and so incurs low maintenance
cost. This type is favored for bearings. This is used for medium to large spans. See Figure
below.
Figure: Elastomeric bearing
- Lead Rubber bearing-is an elastomeric bearing equipped with lead core and is used in
highly seismic areas. See Figure below. Under normal loading conditions, the lead core
resists lateral loads. Under seismically induced dynamic loads, however, the lead core yields
and assists the elastomeric material in resisting loads while at the same time accommodating
lateral flexibility.
RAILINGS
Railings are provided along edges of bridges for protection of traffic and pedestrian. There are three
types of railings.
1. Traffic railings 3. Pedestrian railings
2. Combination railings

4. Fundamentals of Bridge Design (Infolink College-Hawassa Campus) Page 4
The following guideline indicates the application of various types of rails.
- Traffic railings is used when a bridge is for the execution use of highway traffic
- A combination railing in conjunction with raised curb and side walk is used on low-speed
highways.
- On high speed highways the pedestrian path should have both an out board pedestrian railing
and an inboard combination railing
Aesthetics and freedom of view from passing vehicles should also be taken in to consideration.
Traffic Railing
The primary purpose of traffic railings is to contain and redirect errant vehicles. Specifically they
serve the purposes of
- primarily containing the average vehicle
- protecting the occupants of a vehicle in collision with the railing
- protecting other vehicles near the collision
- protecting people and property on roadway and nearby areas underneath
A traffic railing should provide a smooth and continuous face on the traffic side.
Steel and concrete posts should set back from the face to the rail.
Pedestrian Railing
Pedestrian railing can be made from concrete, metal and timber.
A pedestrian walkway should be separated from roadway by a barrier curb, traffic railing or
combination railing.
Combination Railing
It consists of traffic and pedestrian railings.