Bridge Bearing-function

BRIDGE BEARING-COMPONENTS, FUNCTIONS, AND WORKING PRINCIPLE

         Agencies responsible for bridge movement:

Bridge bearing, is a small size component on which the superstructure of a bridge is placed, takes a strong role  on the satisfactory performance of the  bridge structure as a whole. Bridges are always subjected to constant invisible movement and undergoes deformations due to various factors like  

• Expansion and contraction of the bridge structures due to temperature changes.
• Shrinkage and creep of concrete.
• Settlement of support.
• Vibration due to Road/Rail traffic , braking forces.
• Elastic Deformation e.g for prestressed girder
•  Lateral forces such as wind load
• Seismic Vibration.
• Longitudinal forces-Tractive/Braking/skidding/accelerating
• Soil pressure on the Bridge abutments
• Vehicular collisions
•  Deflections, rotations due to dead load, live load and other forces

     

                           

     Effects when girder movement is restrained fully or partially:

Magnitude of these movements and induced force thereof  is largely depended upon the span of the bridge (Distance between consecutive Supports),support conditions, type of material and other factors like variation of temperature, nature of traffic load etc. Huge forces may be generated in the bridge structures if these movements are restrained. For allowing free movement of bridge superstructure , suitably designed ‘bridge bearing’ is most essential to give required freedom of movement so as stresses beyond certain limit,  tending to develop in the sub structure and superstructure are released quickly. It can potentially cause significant problems, distresses/cracks in substructures, superstructures if bearings do not function properly.

This may be due to frozen bearing, improper design, improper installation and subsequent maintenance in uncared manner. Bearings are commonly located in the area prone to accumulation of dust, debris which may further aggravate in the populated town/cities. People in some places, treats bridges/piers like an open dustbin. This can result corrosion, frozen bearing and worsening in its functioning. Bearings are to be designed so that their components can be easy accessible, inspected, cleaned, will have provision of lifting of superstructure for subsequent maintenance and replacement as required. Lifting of the superstructure for maintenance /replacement of bearings shall be foreseen in the design and detailing of substructure and superstructure. Therefore, bridge bearings may cause vigorous problems on the service life of the bridge structure especially when it does not get proper attention. 

Major part of these  movements in bridges are accommodated by bearings. At the same time it is important that bridge structure including substructure has to be capable of resisting extra forces and moment that generated when the bridge  movement  is  restrained fully (lateral movement) or partially in any direction. 

Principal function of bridge bearings:

• To transmit the vertical loads received from superstructures on the Bed Block so that stresses induced in the bed block comes within a permissible limit and evenly distributed on the substructure as far as possible.

• Bearing allows rotation & displacement of superstructure as required by the designer with very low resistance during the life of the structure. Thus only a minimum force may be  transferred to superstructure, substructure & foundation.

• To permit rotary motion due to deflection of the girder. Bearing if not designed/installed properly will cause unequal load distribution and stress concentration on the substructure.

• To accommodate and transfer dynamic forces and vibrations without causing wear or destruction to the substructure.

• To restrict the movement of the superstructure in the desired direction by providing suitable guiding system.

• To Connect the bridge superstructure to the substructure with a aim to reduce shear force on the top of the piers, viaducts or abutments.

• To act as seismic protectors that arrest and dissipate energy during earthquake and other seismic activities. During seismic occurrence force is transferred from   bridge foundations towards the superstructure causing rigorous vibration and movements of the bridge. 

       Factors in the selection of bearing:

• Bearing capabilities i.e vertical load, longitudinal forces, maximum longitudinal & rotational movement.
• Life of Bearing shall be the compatible with life of bridge. Usually life of the bridge bearing is less  than the life span of the bridge itself. However, with appropriate choice of bearing, its arrangement, articulation, subsequent monitoring, maintenance etc,  replacement turn can be minimized to once in the lifetime of the bridge.
• Life cycle maintenance effort on Bearings should be minimum. Maintenance of some types of bearing require huge involvement like temporary suspension of traffic, temporary support structure to lift the heavy girder, heavy equipment which in turn leads to additional direct cost and indirect cost in the form of  traffic suspension. To minimize maintenance cost it is important to make an appropriate choice of bearing in the planning and design stage. In corrosion prone region bearing of steel material may be avoided.
• Cost of Bearing in the selection process, is a big factor which includes initial cost, subsequent maintenance &   replacement cost. Replacement, if unavoidable at all, shall be simple and easy.   
•  Overall height of the bearing assembly shall be limited so as  it is not unstable.
•  Material should have dampening ability to take care of seismic force.   Elastomeric bearing is more effective in resisting earthquake effects.
• Choice of environment friendly material for bearing.
• Seismic region of bridge location to consider e.g. Roller bearing is  sensitive to seismic vibration.

    Translational/rotational movement of Superstructure:

    There are possible 06 degrees of freedom in any bridge support

• Translation in 03 directions i.e along the span (say X-Axis), across the span (say Y-axis)  & in vertical direction (say Z-Axis)   
•  Rotation about X-Axis, Y-axis & Z-Axis as denoted above. 

     A bridge bearing may allow movement in all directions or in none.

1. Translation is facilitated by any one or combination of these mode of  action of bearings- by sliding action, by rolling action and by shearing strain( as in elastomeric bearing)
2. Rotation  is facilitated by any one or combination of the these mode of  action of bearings- by rocking/hinge action, by differential compression as in elastomeric pads, by deformation of the shape of elastomer as in Pot bearing, by sliding as in spherical bearing.  

      Types of bearings widely adopted:
  Some of the bearings widely adopted in steel girder/composite        steel/concrete bridges  in Indian Railway are mentioned below:
       1.   Sliding bearing
       2. Rocker & Roller bearing at the free end 
       3. Rocker Bearing at the fixed end
       4. Elastomeric  bearing (Plain, Laminated)
       5. POT/PTFE bearing
       6. Spherical bearing

        Materials commonly used for bearings are Mild Steel, Stainless steel,      

           Bronze, Elastomer and PTFE. All the above bearings will be discussed in    

        detail in consecutive articles with necessary sketches.

   Note: Readers' suggestion for betterment , modification,  is always    welcome and appreciated.