Bearings in bridges

Types of bearings in Bridges, their parts, recommended bearing type for various lengths and their maintenance are presented in this slide.

1. 2016233028 Elamathy M BEARINGS IN BRIDGES

2. Bearings are important elements in large infrastructures such as bridges, heavy buildings and high rise buildings, ensuring load transfer, movement, rotation, vertical or horizontal restraints as required by the design at critical locations in a structure Bearings

3. Bridge Bearings Bridge Bearings are structural devices positioned between the bridge superstructure and the substructure. Their principal functions are as follows: 1. To transmit loads from the superstructure to the substructure 2. To accommodate relative movements between the superstructure and the substructure.

4. Forces on Bridge Bearings Self-weight Traffic loads Wind loads Earthquake loads

5. Movements on Bridge Bearings Translations ( both transverse and longitudinal directions ) caused by Creep, shrinkage and temperature effects Rotations caused by Traffic loading, Construction tolerances and uneven settlement of the foundation

6. Development of Bearings Until the end of the 18th century all structures of any appreciable size were built of stone, brick or mixed masonry. These structures, generally massive, are little affected by environmental changes and any slight movements which may occur are compensated either by deformation of the constituent materials or by small displacement of the supports.

7. Development of Bearings In the 19 th century, with introduction of cast iron and steel larger structural spans accompanied by lightening of the structure which becomes-more slender and flexible and loses some of the thermal inertia. They must then be fitted at their support points with simple devices, called bearings, which can withstand movement and, more particularly, the expansion or contraction due to temperature changes.

8. Rail Bridge that connects Piermont, NY (Hudson River) to Dunkirk. 

9. Bearings used at that time were metal plates sliding one on the other or, of rollers or, of a combination of both. They were gradually improved by incorporation of swivel arrangements to provide rotational movements

10. North Pivot Bearing of Sydney Harbor Bridge

11. In 20 th century, RCC structures developed but, initially they were as massive as masonry with little significant support movements. But at the end of Second World War the necessity of rebuilding rapidly the structures favored the rapid development of reinforced concrete and, even more so, of Pre-stressed concrete. Structures which were more and more slender, needed bearing devices to allow for movements and rotations

12. Types of Bearings Allow rotations but restrict translational movements Allow both rotational and translational movements

13. SLIDING BEARINGS A sliding bearing utilizes one plane metal plate sliding against another to accommodate translations. The sliding bearing surface produces a frictional force that is applied to the superstructure, the substructure, and the bearing itself. To reduce this friction force, PTFE (Polytetrafluorethylene) is often used as a sliding lubricating material.

14. Sliding bearings can be used alone or more often used as a component in other types of bearings. Pure sliding bearings can only be used when the rotations caused by the deflection at the supports are negligible. They are therefore limited to a span length of 15 m or less by ASHTTO. A guiding system may be added to a sliding bearing to control the direction of the movement.

15. ROCKER AND PIN BEARINGS

16. A rocker bearing is a type of expansion bearing that comes in a great variety. It typically consists of a pin at the top that facilitates rotations, and a curved surface at the bottom that accommodates the translational movements Rocker and pin bearings are primarily used in steel bridges.

17. The moment and lateral forces induced from the movement of these bearings are very small and negligible. However, metal bearings are susceptible to corrosion and deterioration. A corroded joint may induce much larger forces. Regular inspection and maintenance are, therefore, required.

18. ROLLER BEARINGS Roller bearings are composed of one or more rollers between two parallel steel plates. Single roller bearings can facilitate both rotations and translations in the longitudinal direction, while a group of rollers would only accommodate longitudinal translations.

19. Single roller bearings are relatively cheap to manufacture, but they only have a very limited vertical load capacity. Multiple roller bearings, on the other hand, may be able to support very large loads, but they are much more expensive. Like rocker and pin bearings, roller bearings are also susceptible to corrosion and deterioration. Regular inspection and maintenance are essential.

20. Single Roller Bearing Multiple Roller Bearing

21. ELASTOMERIC BEARINGS Bearings are formed of horizontal layers of synthetic or natural rubber in thin layers bound between steel plates. These bearings are capable of supporting high vertical loads with very small deformations. These bearings are flexible under lateral loads. Steel plates prevent the rubber layers from bulging. Lead cores are provided to increase damping capacity as plain elastomeric bearings does not provide significant damping. They are usually soft in horizontal direction and hard in vertical direction.

22. It consist of a laminated elastomeric bearing equipped with a lead cylinder at the center of the bearing. The function of the rubber-steel laminated portion of the bearing is to carry the weight of the structure and provide post-yield elasticity. The lead core is designed to deform plastically, thereby providing damping energy dissipation. Lead rubber bearings are used in seismically active areas because of their performance under earthquake loads.

23. CURVED BEARINGS A curved bearing consists of two matching curved plates with one sliding against the other to accommodate rotations. The curved surface can be either cylindrical which allows the rotation about only one axis or spherical which allows the bearing to rotate about any axis. Lateral movements are restrained in a pure curved bearing and a limited lateral resistance may be developed through a combination of the curved geometry and the gravity loads .

24. To accommodate lateral movements, a PTFE slider must be attached to the bearings. The lower convex steel plate that has a stainless steel mating surface is recessed in the masonry plate. The upper concave plate with a matching PTFE sliding surface sits on top of the lower convex plate for rotations. Between the sole plate and the upper concave plate there is a flat PTFE sliding surface that will accommodate lateral movements.

25. POT BEARINGS A pot bearing comprises a plain elastomeric disk that is confined in a shallow steel ring, or pot. Vertical loads are transmitted through a steel piston that fits closely to the steel ring (pot wall). Flat sealing rings are used to contain the elastomer inside the pot. The elastomer behaves like a viscous fluid within the pot as the bearing rotates.

26. Because the elastomeric pad is confined, much larger load can be carried this way than through conventional elastomeric pads. Translational movements are restrained in a pure pot bearing, and the lateral loads are transmitted through the steel piston moving against the pot wall. To accommodate translational movement, a PTFE sliding surface must be used. Keeper plates are often used to keep the superstructure moving in one direction.

27. DISK BEARINGS A disk bearing utilizes a hard elastomeric (polyether urethane) disk to support the vertical loads and a metal key in the center of the bearing to resist horizontal loads. The rotational movements are accommodated through the deformation of the elastomer. To accommodate translational movements, however, a PTFE slider is required. In this kind of bearings, the polyether urethane disk must be hard enough to resist large vertical load without excessive deformation and yet flexible enough to accommodate rotations easily.

28. SELECTION OF BEARINGS Loads acting on bearings are calculated and suitable types are decided For the various qualified bearing alternatives, preliminary designs are performed to determine the approximate geometry and material properties in accordance with design specifications. It is likely that one or more alternatives will be eliminated now because of an undesirable attribute such as excessive height, oversize footprint, resistance at low temperature, sensitivity to installation tolerances, etc. At the end of this step, one or more bearing types may still be feasible and they will be included in the bid package as the final choices of the bearing types.

29. Recommended type of bearing for various span lengths and support flexibility conditions Where a simple span deck is supported over rigid supports and the span is less than 7.5m, no special bearing devices are necessary. Only tar paper or a felt layer is adequate. For spans betweens 7.5m to about 15m, mild steel plate bearings, sliding type over free supports and rocking type over the fixed support, may be used. For spans in excess of 15m, metallic rocker type bearing is provided over the fixed support and a roller cum rocker type bearing is provided over the free supports. Alternatively suitably designed laminated neoprene bearings may be provided.

30. MAINTENANCE RECOMMENDATIONS Power-wash steel sliding plates and steel rocker bearings (especially at abutments) annually when washing decks. Make sure to concentrate efforts between rocker surfaces and base plates to remove dirt that could accumulate and impede movement. Touch up steel with high-quality epoxy paint as needed. Neoprene bearings require little maintenance except occasional power washing to keep dirt and debris from building up around bearings

31. REFERENCES 1) Raina VK (1994) , ‘ Concrete Bridge Practice – Analysis, Design and Economics’, Second edition, Tata McGraw-Hill Publishing Company Limited, New Delhi 2) Standard Specifications and Code of Practice for Road Bridges, Section-1 General Features of Design (Eighth Revision), IRC-5:2015, Indian Road Congress, New Delhi 3) Bridge Engineering Handbook, CRC Press, New York Washington DC

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