Structural Bearings for Bridges: Elastomeric, Pot, and Spherical Types

Every bridge span, whatever its type, has to rest on something at each end, and that resting point cannot simply be a rigid connection to the pier or abutment below. A span changes length with temperature, deflects and rotates under live load, and on longer bridges shortens over time from concrete creep and shrinkage; a bearing's job is to carry the vertical reaction and whatever horizontal load is assigned to that support while accommodating those movements without locking up and dragging unplanned forces into the substructure.

Elastomeric Bearings

The simplest and most common bearing on short- and medium-span bridges is a laminated elastomeric pad − layers of rubber bonded to thin steel shim plates, stacked and vulcanized into a single pad. The steel shims do not carry load directly; they restrain the rubber layers from bulging outward under compression, which lets the pad support far higher vertical load per unit area than an unreinforced rubber block could. Horizontal movement and end rotation are both accommodated by shear deformation and compression of the rubber itself rather than by any moving mechanical part, which is why elastomeric bearings need essentially no maintenance and have no wearing surfaces to inspect − their limitation is capacity, since very heavy reactions or very large rotations eventually call for a pad too large or too tall to be practical.

Pot Bearings

Where vertical loads exceed what an elastomeric pad can handle in a reasonable footprint, a pot bearing confines an unreinforced elastomeric disc inside a rigid steel cylinder (the "pot"), with a matching steel piston bearing directly on top of the confined rubber. Because the rubber is fully enclosed rather than free to bulge sideways, it behaves almost like a confined fluid and can transmit very high vertical loads through a comparatively compact bearing. Rotation is accommodated by the piston rocking slightly within tight tolerances against the confined disc, but pot bearings provide little to no horizontal movement capacity on their own, so bridges use pot bearings in combination with a separate sliding element (a PTFE-on-stainless-steel sliding surface) wherever thermal expansion also has to be released at that support.

Spherical Bearings

A spherical bearing replaces the flat piston-on-rubber arrangement with a curved, machined concave-and-convex steel or PTFE-lined surface, which allows much larger rotations than a pot bearing's near-flat contact can tolerate without concentrating stress at one edge. This makes spherical bearings the common choice on long-span and segmental bridges, discussed in our overview of segmental bridge construction methods, where deep girders and long spans generate rotations at the support well beyond what a pot or elastomeric bearing could absorb without excessive local stress. Spherical bearings are also frequently specified with a sliding PTFE interface built into the same unit, combining rotation capacity and horizontal movement capacity in a single, compact assembly rather than stacking separate components.

Movement Design and Bearing Selection

Choosing a bearing type is really a movement-and-load budgeting exercise done alongside the deck's expansion joints, covered in our piece on expansion joints: total thermal movement, live-load rotation, and any long-term shortening from creep and shrinkage in a concrete superstructure all have to be added up and checked against what the chosen bearing type can actually deliver at that support, with one end of the span typically fixed (or nearly so) and the other left free to translate so that the whole span isn't fighting itself as it moves. Federal transportation guidance on bearing selection, inspection, and replacement is publicly documented (fhwa.dot.gov) and reflects decades of field performance data on exactly which bearing types hold up under which combinations of load and movement.