Bearings and expansion joints
A bridge deck is never still. It expands and contracts with temperature, shortens permanently through concrete creep and shrinkage, rotates at its supports as it deflects under traffic, and moves sideways in wind and earthquakes. Bearings and expansion joints let these movements happen in a controlled way while transferring loads safely to the substructure.
Common bearing types each suit a range of loads and movements. Elastomeric bearings, laminated layers of rubber and steel plates, carry vertical load by compression and allow movement and rotation by shear and tilt of the rubber; they are simple and cheap for small and medium spans. Pot bearings hold an elastomer disc confined in a steel pot to carry high loads with rotation, with a sliding PTFE-stainless steel surface for free bearings. Spherical bearings allow large rotations. Restraints and guides fix the deck in chosen directions.
Expansion joints bridge the gap in the road surface at the end of each continuous unit. Small movements are handled by asphaltic plug or strip seal joints; larger ones by modular joints with several seals, or finger joints. They must be watertight, because water and salts leaking through joints are a major cause of deterioration of bearings, pier caps and girder ends.
Design starts with the movement range from temperature, creep, shrinkage and prestress, plus construction tolerances. IRC 83 covers bearings in India. Both bearings and joints need regular inspection and are planned to be replaceable, with jacking points provided.
Provide jacking points and access from the start; every bearing will be replaced at least once.
Thermal movement: Δ = α L ΔTElastomeric bearing shear strain = horizontal movement / total elastomer thickness
Sizing the joint for temperature only and forgetting creep and shrinkage of a prestressed deck.
Problem. A 40 m simply supported concrete deck (α = 12 × 10⁻⁶ per °C) sees a temperature range of 35 °C. With a fixed point at mid-length, find the movement at each end and the shear strain in an elastomeric bearing with 50 mm total rubber thickness.
- Total movement Δ = 12 × 10⁻⁶ × 40 000 × 35 = 16.8 mm.
- About mid-length, each end moves half: 8.4 mm.
- Shear strain = 8.4 / 50 = 0.168.
- Add creep and shrinkage movement and check the strain limit and the joint gap for the total.
Answer. 16.8 mm total; 8.4 mm at each end; shear strain about 0.17 from temperature alone
Quick quiz
1How does an elastomeric bearing allow movement?
By shear deformation of its rubber layers.
2Why must expansion joints be watertight?
Leakage carries water and salts onto bearings and girder ends, causing deterioration.
3Which IRC code covers bearings?
IRC 83.
- IRC:83 (Parts I to IV) (India): Road bridge bearings: metallic, elastomeric, pot and spherical types by part.
- IRC:6 (India): Standard specifications for road bridges: loads and load combinations.
- EN 1337 (series) (Europe): Structural bearings.
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- D. Johnson Victor, Essentials of Bridge Engineering, Oxford and IBH, latest edition.
- N. Krishna Raju, Design of Bridges, Oxford and IBH, latest edition.
- S. Ponnuswamy, Bridge Engineering, McGraw Hill Education (India), latest edition.
Free and official sources
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