bearings
General
Bearings are usually located between superstructure and substructure; they serve the purpose of
transmitting the loads from the superstructure to substructure while making allowance for
translational and/or rotational movement. Bearings come in variety of types and characteristics,
each has its own size and can restrain specific movement depending on the design requirements
plus the loads and movements magnitude, but they serve the same function.
The process of choosing and designing the bearing must include the anticipated movement and
applied loads. Generally speaking, Bearings can be fixed in both longitudinal and transverse
directions, fixed in one direction and expanding in the other, or expanding in both directions.
bearing types
there are many types of bearings each used for a specific application, the five most common
bearing types are mentioned below:
Cylindrical Bearing
Spherical Bearing
Rocker Bearing
Pot Bearing
Elastomeric Bearing
Selection of bearing
After defining design requirements for the bearings of the bridge, the selection of the most
suitable bearing type could be done using the table 14.6.2-1 provided by the AASHTO:
In which:
S = Suitable, U = Unsuitable, L = Suitable for limited applications
R = May be suitable, but need extra considerations or components such as sliders or guideways,
Long. = Longitudinal axis, Trans. = Transverse axis, Vert. = Vertical axis.
Elastomeric bearing is considered to be a more common type for the small to moderate size
bridges. While the High-load multi-rotational (HLMR) bearing is well suited for the large
bridges in which the elastomeric bearing won’t satisfy the design requirements. The HLMR
bearing include several types, the most common one is the pot bearing.
Pot bearing is a good choice for the design of this bridge as it can withstand large vertical loads,
while providing a good range to facilitate rotation through deformation of elastomeric pad
located in the pot. The fixed pot bearing resists the Horizontal loads through the contact of the
pot by the piston. Guide bars are provided to aid in the movement in on direction of the guided
pot bearing. Bearings that are not guided can move in any direction and cannot handle horizontal
loads in any direction.
The following figure shows the primary components of the pot bearing:
Bearing design
The AASHTO provides design requirements for the elastomeric disc, sealing rings, pot, and
piston in article 14.7.4 for the pot bearing design.
Pot bearing can be designed using the following basic steps, with accordance to the AASHTO
article 14.7.4:
Obtain required design input.
Select preliminary bearing properties.
Design the elastomeric disc.
Design the sealing rings.
Design the pot.
Design the piston.
Check the concrete or grout support.
Required design input includes maximum and minimum vertical loads for service limit state,
maximum vertical loads, and Maximum design rotation as well as total horizontal load for
strength limit state.
A sample of calculation for a selected bearing in the bridge is shown in the manual calculation
chapter.
Results
The following table summarizes the bearing types and their geometry:
Bearing Location type Pot Sliding Sliding Bearing Net
diameter plate length plate width height weight
(mm) (mm) (mm) (mm) (kg)
Start abutment Guided 580 640 580 125 191
Start abutment Multi- 410 460 410 89 85
directional
Before Guided 410 460 410 104 100
expansion joint
Before Multi- 370 420 370 84 67
expansion joint directional
after expansion Guided 480 530 480 113 146
joint
after expansion Multi- 370 420 370 84 67
joint directional
end abutment Guided 370 420 370 99 77
end abutment Multi- 370 420 370 84 67
directional
