Bridge Bearings -Types of Bearings for Bridge Structures and Details
Bridge bearings are structural equipment or devices installed between bridge substructure and superstructure to transfer the applied load including earthquake loads; wind loads; traffic loads; and superstructure self-weight.
Bridge bearings also makes rooms for relative movements between superstructure and substructure, for instance, rotation movements and translational movements in horizontal and transverse direction.
Bearing used in the construction of bridge structure is divided into two major categories namely expansion bearings and fixed bearings. The former permits both translational and rotational movements whereas the latter allow rotational and limited translational movements.
There are several types of bridge bearings which have been employed in bridge construction which are discussed below.
Types of Bridge Bearings and their Details
Different types of bearings for bridges include:
- Sliding bearings
- Rocker and pin bearings
- Roller bearings
- Elastomeric bearings
- Curved bearings
- Pot bearings
- Disk bearings
Sliding Bearings for Bridges
Sliding bearing consist of two metal plates, commonly stainless-steel plates, that slide relative to each other and hence makes room for translational movement and lubricating material between them as illustrated in Figure-1.
Fig.1: Sliding Bridge Bearing
A friction force is generated in sliding bearing and it is imposed on substructure, superstructure and sliding bearing itself. So, it may be required to provide lubricant such as polytetrafluoroethylene (PTFE) to decline generated friction.
It is specified by ASSHTO that sliding bearing cannot be used unless the bridge span is smaller than 15m. This is because sliding bearing cannot be purely used if the bridge experiences rotation movement.
However, the span restriction for sliding bearing utilization can be disregarded when it is used in combination with other bearing types.
Rocker and Pin Bearings for Bridge Structures
Rocker is an expansion bearing composed of curved surface at the bottom, which accommodate translational movement and a pin at the top makes room for rotation movement as illustrated Figure-2 and Figure-3 in detail.
Fig.2: Rocker Bridge Bearing
Fig.3: Rocker Bridge Bearing
Pin bearing is a fixed bearing that make room for rotation movement through the application of steel pin. It has similar structure and component like rocker bearing apart from the bottom of pin bearing which is flat and fixed to the concrete pier, as can be observed in Figure-4.
Fig.4: Pin Bearing for Bridges
Both rocker and pin bearings are mainly employed in steel bridge structure. Rocker and pin bearing should be considered when the bridge movement is adequately known and described, since such bearings can make rooms for both translational and rotational movements in one direction only.
These bearings are likely to suffer deterioration and corrosion, so it is necessary to conduct regular inspection and maintenance.
Roller Bearings for Bridges
Roller bearing can be used in the construction of reinforced concrete and steel bridge structure. There are two main configurations including single roller bearing which is composed of one roller placed between two plates and multiple roller bearing that consist of several rollers installed between two plates.
The former as shown in Figure-5 can accommodate both rotation and translation movement in longitudinal direction and it is cheap to manufacture but its vertical load capacity is limited.
In contrary, the latter as shown in Figure-6 can make room for translation movement only and rotation movement can be accommodated if rollers are combined with pin bearing. Multiple roller bearings are expensive and support considerably large vertical loads.
Regular inspection and rehabilitation should be conducted since roller bearing are susceptible to corrosion and damages.
Fig.5: Single Roller Bridge Bearing
Fig.6: Multiple Roller Bridge Bearing
Elastomeric Bearings for Bridge Structures
It consists of elastomer manufactured from synthetic or natural rubber and can take both translation and rotation movements through elastomer deformation. The ability of elastomer to carry large vertical loads is because of reinforcement provision that prevents lateral bulging of elastomer.
There are number of elastomeric bearing pads classified based on types of reinforcements used. For example, steel reinforced, plain, fiberglass reinforced and cotton duck reinforced elastomeric bearing pads.
Strength and response of each type is different, steel reinforced elastomeric bearing is the strongest one and plain elastomeric pad is the weakest.
Elastomeric bearing is neither expensive nor requires considerable maintenance, that is why it the most desired bearing type. Figure-7 show details of elastomeric bearing and its application in bridge structure.
Fig.7: Elastomeric Bridge Bearing
Curved Bearings for Bridges
It consists of two curved plate that match each other as shown in Figure 8. If curved bearing is cylindrical, then it only accommodates rotation movements. However, both rotation and translational movements can be dealt with if curved bearing is spherical.
Due to the fact that both gravity loads and curved geometry generate lateral resistance against and consequently lateral movement would be limited, that is why polytetrafluoroethylene (PTFE) slider is adhered to the bearings in order to make rooms for lateral movements. Details of curve bearings can be seen in Figure-8.
Fig.8: Spherical Bearing for Bridges
Pot Bearings for Bridge Structures
As shown in Figure-9, pot bearing consists of elastomeric disk confined in a pot, steel piston that is properly tailored into the pot wall and flat sealing rings which keeps elastomeric inside the pot.
Pot bearing can support considerable vertical loads and it is commonly transferred through steel piston to the elastomeric disc which is almost incompressible. As far as lateral load is concerned, it is transferred as the steel pistol moves toward pot wall.
Translational movement is limited in pure pot bearing that is why PTFE are introduced to the sliding surface to make rooms for translation movement.
Fig.9: Pot Bridge Bearing
Disk Bearings for Bridges
Different components of disk bearing are shown in Figure-10 in detail. Rotation movement is accommodated through the deformation of elastomer whereas the translation movement is considered through the application of PTFE slider.
Fig.10: Disk Bearing
The utilized elastomer should be adequately hard to support vertical loads without suffering large deformations and sufficiently flexible to allow rotational movement.
Both vertical loads and lateral loads are supported by elastomeric disk and metal ring in the center of the bearing respectively.