Bridge Bearing-Pure Sliding Bridge Bearing

Fig-1

STEEL SLIDING BRIDGE BEARING:

Sliding plate bearings are the simplest form of bearings consisting of two metal plates, one sliding over another plate on a smooth interface. Plan view of a typical flat  sliding bearing is shown in Fig-1.  The sliding bearing surface produces considerable amount of frictional force that is transmitted to the superstructure, the substructure and also to the bearing itself. This type of bearing can permit translation in long and in  transverse direction if not guided by some arrangement. No rotation is permitted by this bearing. Pure sliding bearing may be used when rotation due to deflection and induced force is negligible for small span generally up to 12.2m. For long span bridges rotation of  girder may be increased due to higher loading and span,depending on  the stiffness of the girder. For longer span, flat sliding bearing is not suitable as due to larger rotation in addition to translation force , more  stress is developed towards the inner side of  bed block/ substructure, also subjected to eccentric and unequal stress distribution. Such stress concentration may cause failure or crack in concrete bed block, substructure. Effect of stress concentration can be minimized by a beveling cut at  the inside edge of the slide/flat bearing plate taking care of the better rotation. Many old bridges of Indian Railway having span 12.2m or more up to 24m was having flat bearing with large size. With adoption of  increased loading standard it was essential to reduce the effect of force/moment/shear force on the substructure which could be done by developing/redesigning existing flat bearing in place of costly rebuilding or heavy repairing/strengthening of substructure. The development by beveled cutting  the edges of flat bearing is known as centralized bearing (  Fig-1A elevation). Many Such flat bearing plate in the past was widely adopted in the steel plate girder and were converted into centralized bearing in Indian Railways during the year 1985 to 2000.  Materials generally used for pure sliding bearing  are

a)   Mild Steel over mild steel

b)   Mild Steel over phosphor bronze

c)   Stainless steel over PTFE

Sliding-Mild Steel over Mild Steel: This bearing permits translation only, however by beveling  the bottom edge as in fig-2, vertical eccentricity can be reduced with better stress distribution compare to plain sliding . This has  inherent higher frictional coefficient (0.2 to 0.3) which aggravates due to sticking of dusts/debris during operation.  Thus free movement of the girder is affected to a higher extent compare to other two above, thereby steel over steel bearings transmit more forces on the substructure, may develop shear crack on the substructure near bed block location particularly in longer span where larger force is generated. To overcome the situation this type of bearing needs periodical lubrication in both ends at an interval not more than three years so as superstructure can have expansion and contraction with least resistance and thus longitudinal forces transmitting  to the substructure can be minimized considerably. For cleaning and lubrication, lifting of girder is involved Steel slide bearing may be used for lighter vertical load with movement up to 25mm. Components of the sliding bearings  as marked in the FIG-1 & 1A are listed as under:

a) stopper plate/Locking strip/Location strip: is provided to regulate the movement in the direction of predominant traffic with a tolerance of 10 to 20 mm depending upon designed gap required for the movement of superstructure. 

b) Guide strip:  is provided to arrest lateral movement of the girder within a very small tolerance of 1.5 to 2mm  to keep the girder in its installation alignment.

c) Stopper plates and guide strip are connected with Bedplates by welding or with countersunk rivets :

d) Bed plate is placed on  Bed Block made of high performance concrete and firmly fixed by Anchor Bolts at its proper location.

e) Bearing Plates( upper part) are connected with the girder bottom flange  with counter sunk rivets/welding or similar.

f) Lubricating materials used are graphite, grease, oils etc. For proper functioning of the bearing periodic lubrication is required.

       

FIG-1A

                                                      

Sliding-Mild Steel plate over phosphor bronze lower plate:

This is also a sliding type like centralized articulated bearing permitting translation only. This bearing may also be beveled at the edge to avoid stress concentration.   Since the coefficient of frictional force (0.15) in sliding of steel over phosphor bronze is considerably less, it is advantageous to provide these bearings, in place of steel over steel bearings,  with considerable reduction in avoidable longitudinal forces on the substructure. Further greasing/lubrication is not required in phosphor bronze bearing as they are corrosion resistant and may retain the smooth surface. Phosphor bronze bearing is widely used in Composite girder bridge of span of 18.3m or more.

At the Fixed end,  Bearing plate assembly may rest on  Steel bed plate(masonry plate).

At the free end,  Bearing plate assembly attached to the girder end slides over phosphor bronze plate fixed with bed plate. A Sketch of Phosphor bronze bearing  is shown in Fig-2.
                                                              

         

Sliding-Stainless steel over PTFE:

Fig-3

A sketch of typical plain sliding PTFE bearing is shown in Fig-3.This is also a sliding bearing further developed over old conventional sliding bearing permitting translation only with least induced force. Other sliding bearing surface as discussed above  produces larger frictional force that is applied to the superstructure, substructure, and the bearing itself. For further reduction of frictional force, lubricated PTFE (poly tetra fluoro ethylene) is often used as a sliding surface. PTFE and lubricated Stainless steel are common choices of sliding surfaces for bearings because of very low coefficient of friction at the interface.

Advantages of PTFE sliding bearing:

1. PTFE is a fluro carbon polymer which possesses good chemical resistance and can function in a wide range of temperature.
2. PTFE has the lowest coefficients of static and dynamic friction of any solid (less than 0.08). A beneficial property of PTFE is that the coefficient of friction between the mating surface and the confined PTFE pad will be at a minimum when the stress on the PTFE layer is maximum.
3. It is   highly insoluble in most solvents or chemicals i.e weather resistant.
4. PTFE’s density is also very high, at 2200 kg/m3
5. There is no stick-slip action
6. PTFE bearings are far less bulky than alternative assemblies
7. Vibrations are damped to some extent
8. In a certain range of span, PTFE structural bearings are superior to conventional expansion plates, rollers and rocker  type.
9. They are used in combination with a wide range of other bearing systems. PTFE sliding surfaces are used to accommodate large translations, when combined with spherical or cylindrical bearings.

PTFE structural bearing  may be unfilled or filled with glass fibre or other reinforcing material. PTFE has the draw back of high thermal expansion and its poor compressive strength. Therefore it is not suitable for large span/Heavy weight superstructure. However this can be further improved by filling with glass fibre and bronze.  Further its bond property with steel is not strong and to take care of this, PTFE is preferably placed in a confinement so that a part of its thickness is located inside the recess of  the metallic  plate and remaining part projected above the recess as in Fig-3. In this Bearing, the upper plate is attached with stainless steel plate which slides over confined PTFE. This stainless steel plate should be larger than PTFE surface without exposing the PTFE any way during movement, thus contamination to dirt and rubbish can  be avoided or minimized. These bearings also require periodic cleaning of the bearing surface. Lubricating the mating surface by silicon grease will further reduce the coefficient of friction. PTFE requires replacement after a period of time as it wears under service conditions and contamination of the sliding interface increase the wear rate. 

Movement of steel sliding bearing:

Translation: Permits translation in long and also capable in transverse direction if not restricted. No translation is permitted in vertical direction.
Rotation: No rotation is permitted about any axis by this bearing.


Merits of steel sliding bearings:
1. Steel bearings can be easily fabricated by steel manufacturers and can be       easily installed
2. Steel bearings are relatively cheap

Demerits of steel sliding bearings:
1. Failure of anchoring components and connections (rivets, bolts, welds) that prevents uplift and shear.
2. Periodical lubrication is required to avoid wear of sliding parts and induced forces. 
3. Partial uplift and excessive wear of steel
4. Tilting of masonry plate under high load.
5. All functioning parts are made of steel hence the problem of durability,
rust and corrosion prone,  leading to frozen bearing.
6. Not suitable where rotation is essentially a matter of consideration.