Standard Specifications and Code of Practice for Road Bridges, Section IX — Bearings, Part I: Metallic Bearings
2014 Edition

According to IRC 83 Part I (Section 4), metallic bridge bearings should be constructed from structural steel grades compliant with relevant IS standards, often carbon or alloy steels. Sliding surfaces utilize composite materials such as PTFE-based compounds and metal meshes like CuSn6 bronze sintered into PTFE for low friction. Steel bearing surfaces require hard chromium plating, which must pass the Ferroxyl integrity test to ensure absence of defects. PTFE overlays must meet specified thickness and adhesion criteria as detailed in the standard. Surface finishes must be inspected to confirm smoothness and absence of imperfections, ensuring durability and corrosion resistance.

Verification of bending and shear stresses involves ensuring restraining rings comply with allowable bending stress limits, preferably being monolithic with the base component. If welding is necessary, continuous welds on all sides with tongue-and-groove or Allen-key bolt locking are required. The welds must be checked for combined stresses including shear, bending, and tension. Guide bars are assessed by confirming their equivalent stress does not exceed the ultimate tensile strength divided by the product of width and a safety factor (usually 1.25). Welding of guide bars must be performed by certified manufacturers adhering to recognized standards.

IRC 83 sets a minimum backing plate thickness of 12 mm and a required side clearance of at least 20 mm on the radius. Recess shoulders must be sharp and square with a root radius not exceeding 1 mm. Sliding surface fit tolerances depend on diameter: 0.5 mm max gap for 75-500 mm, 1.0 mm for 501-1000 mm, and 1.5 mm for 1001-1500 mm diameters. Surface flatness deviation must be within the greater of 0.0003 times the diameter or 0.2 mm. Protrusion tolerances are ±0.2 mm for diameters up to 750 mm and ±0.3 mm for larger sizes. Thickness tolerance for sliding surface sheets ranges between +0.3/-0.0 mm and +0.5/-0.0 mm depending on diameter.

The standard mandates protective coatings conforming to ISO 12944 for all exposed steel parts such as backing plates, intermediate plates, and welded zones. Durability classifications depend on location: interior environments require high durability coatings (>15 years, category C4), whereas coastal or industrial regions necessitate very high durability systems (categories C5-I or C5-M). Steel embedded in concrete must receive a zinc-rich primer with a minimum dry film thickness of 50 microns. Stainless steel sheet attachments welded continuously require only cleaning of the backing plate, but those fixed by screws or rivets demand full corrosion protection. Regular inspections and prompt maintenance are essential.

Design considerations include evaluating the resultant horizontal forces caused by sliding friction and ensuring the bearing's curved sliding surface can sustain these without instability or separation. Stability is verified using a formula incorporating projected area, radius of curvature, angles related to load direction, and maximum allowable contact stress. Frictional moments due to internal forces and eccentricities must be accounted for. The coefficient of friction varies with loading conditions, and external or internal guides may be employed to resist horizontal forces effectively while allowing smooth rotation. Prescribed thickness and protrusion dimensions for sliding surfaces and guides are specified to maintain structural integrity.