Bearing pads are designed as per AASHTO LRFD Bridge Design Specifications-2007. The design rule is based on the assumption that rubber is a viscoelastic material, the deflection of which, under compressive loads, is influenced by its shape. Reinforcing plates in the bearings are chemically bonded to the elastomer to prevent any relative movement. Design of the bearing pads results in achieving the sizes, which would be stable under the specified loads, translation, and rotations of the given axis.
Loads and Deflections on a Bearing Pad
Laminated Elastomeric Bearing Pads (Steel Reinforced)
Laminated elastomeric bearing pads can accommodate movement in any direction and rotational movement of any axis by elastic deformation. They are sually used where bridges are susceptible to high loading, high translation and rotations. Alternate layers of steel and rubber are vulcanised together to attain the desired load bearing capacity. Laminated bearings can be either rectangular or circular as per the requirement at the abutments.
Cotton Duck Pad
Bearing pads can also be reinforced with cotton duck fabric, which is commonly known as cotton duck pads (CDP). CDP are known to be quite stiff and to have large compressive load capacity. Because of this great stiffness, the rotational capacity and translational movements of CDP has been severely limited. The below sizes are nominated as per the AASHTO standards and calculations:
Plain Elastomeric Bearing Pads
Plain elastomeric bearing pads are solid blocks of rubber without any steel reinforcement plates. They are not generally used in bridges. They are predominantly used where there is low pressureor loads as well as static conditions.
Plain elastomeric bearing pads are simple, low-cost rubber separation strips, capable of carrying compressive loads and providing limited translational movement and rotational capacity. They have a varied application in prefabricated structures.
Guided Elastomeric Bearing Pads
This type of special purpose bearing pad consists of top and bottom load plates with a guide to restrict the movement to the required direction. The top and bottom load plates are hot-vulcanised to the rubber. Anchor rods are also provided in the load plates for anchoring to the bridges as per BS Standards. The rubber pads could either be plain or reinforced as per the load conditions prevailing.
Sliding Bearing Pads (Frictionless Bearing Pads)
Sliding type bearing pads are provided where there is a requirement of higher translation with respect to available area in the abutment, especially in large malls or airports where spans are very wide and higher relative translations.
PTFE sheets with mirror-finished stainless steel plates are used for obtaining the desired result of higher translations. PTFE is either chemically bonded or fixed using adhesives to the top portion of the bearing pads. Bearing pads are either plain or reinforced with respect to the loads defined.
Pot bearings are special types of bearing pads for taking larger loads, translation and rotation in confined areas. Pot bearings are often regarded as suitable for use when bridge bearing rotations are too large.
Rotation may occur on any axis and is accommodated by deformation of the elastomeric pad. Pot bearings are fixed against all translations unless they are used with a PTFE sliding surface. The pad is deformable but almost incompressible and the pot bearing has large reserve capacity against vertical loads.
Pot bearings are divided into three types:
Standards and Specifications
The basic elements of a pot bearing:
- Shallow cylinder or pot
- Elastomeric pad
- Set of sealing rings
- PTFE Sliding surface
- Stainless steel mating surface
- Sliding plate
- AASHTO LRFD Bridge Design Specifications
- AASHTO M251 (Standard Specification for Bridge Bearing Materials)
When compared to normal elastomeric bearing pads, it has the following advantages:
- Takes larger loads where space is a constraint as compared to that of elastomeric bearing pads
- It can sustain larger cycles of rotation and movement
- Maintenance is economical, as the replacement is effected only for the rubber disc
- Long term serviceability
- Effect of atmospheric corrosion is minimal
ORF Spherical Bearings are designed to transmit vertical forces in the bridge sub-structure and to facilitate rotations by means of convex top plate and concave lower part. The flat sliding surface between the top of the calotte and a sliding plate accommodates longitudinal and transverse movements. Guide bars and a restraining ring are used to transmit horizontal forces. Depending on whether the bearing is fixed, guided or free sliding, it accommodates vertical loads, horizontal forces, rotations and longitudinal or transverse movements.
- Qualified and experienced specialized staff
- Sophisticated and reliable individual components
- High-quality certified materials: min.
- Strip with bronze backing, long-lasting silicone grease, etc.
- High quality standard (certified to ISO9001)
ORF Rubber Factory LLC is specialized in the design and manufacturing of various types of bearings for bridges and all types of infrastructure projects. In addition to the standard Elastomeric Bearings, Pot Bearings and Spherical Bearings, ORF Rubber Factory has now perfected a solution for the bearing application where extensive live load effects come into play, as such in rail projects.
Disc Bearings have been extensively used on railroad bridges as well as highway bridges. The Disc Bearing with its Polyurethane Disc element gives a cushioning effect, durability and is designed to handle the excessive live loads, often present in railroads. The Polyurethane elements in the Disc Bearings posses good fatigue resistance properties, giving the Disc Bearings advantage over the Pot Bearings when subjected to excessive cyclic loading. ORF Disc Bearings meet the extensive and stringent requirements of railroad industry guidelines.
Major Parts of Disc Bearings
The Polyurethane Disc in the Bearing accomodates very high vertical design loads, at the same time maintaining its ability to provide rotation in both longitudinal and transverse directions. The size and geometry of the disc is designed as per the specific requirements of the project.
Shear Force Resisting Pin
A high strength shear pin transfers horizontal loads from the lower plate to the top bearing plate and protects shear loads from the Polyurethane Disc. While calculating stress on the Polyurethane Disc, the cross section area of the shear pin is reduced from the total area of the Disc.
PTFE and The Polished Stainless Steel Interface
The PTFE, along with the Polished Stainless Steel Interface, provides a low friction surface, facilitating the horizontal movement requirements of the bearings.
Typical section showing the details of a Free Sliding and Guided Sliding Disc Bearing.
Bearing Fixing Accessories
Bearing Grout (Flo-Grout BP) – Bearing Grout is a non-shrink cementitious material used as a bedding material for bearings. It possess high strength and cyclic load bearing capacity, without failure.
Bearing Adhesive (Quickmast 341C) – Two component, solvent-free, high strength adhesive and patching compound. Formulated to give an easily applied paste and suitable for sealant gun application, Quickmast 341C is thixotropic in nature and is suitable for vertical and overhead bonding and structural repairs. Quickmast 341C adheres well to most building materials without need for a primer and will bond and cure under damp conditions, as well as exhibiting very good abrasion and chemical resistance along with negligible shrinkage.
- Aluminium: Aluminium alloy grade AA1100.H14 sheet with powder coating
- Powder Coating Type: Durable polyester TGIC powder coating
- Sizes: 3mm thick with various widths of 200mm to 600mm depending on the gap
- Jacking Plate: This SS plate is useful during replacement of the bearings in order to locate the jack
- SS Material: SS316 L-2B Finish
- Size: Dia 500mmx 3mm to 5mm thick as per the specification requirement
**The above products are of M/S. DCP International, Jordan Factory. Powder Coated Aluminium Bearing Cover Plate.