Because rubber undergoes no change in its volume when loaded (Poisson’s ratio = 0.5), there is virtually no compression under load.

If a freely loaded plate, bearing on such a pad, is tilted, the rubber flows within the confined space with almost no resistance to the rotation. Since the rubber reacts like a viscous fluid, pressure is uniformly distributed within the closed system. Figure 1 illustrates the complete system.

Practical bearings using this principle can be designed for any combination of loads (including uplift), angle of rotation, or horizontal load found in today’s bridges. Only pot bearings have all the following performance advantages:

• Negligible Vertical Deflection: In designing a structure, the engineer only needs to consider a maximum 0.06” compression of pot bearings. By contrast, unconfined, unreinforced elastomer disc or pad bearings will compress (i) under dead load, (ii) under live load, and (iii) with time. All will have potentially damaging effects on expansion joints, rail connections, and water mains or utility pipes crossing the bridge.
• Uniform Loading: A line of pot bearings along a pier will accept loading uniformly since the very small compression (0.06” max.) is typical for all pot bearings. By contrast, unconfined elastomer discs have a compression that varies with size and hardness. A normal hardness tolerance of 4 durometer points (Shore) equates to a change in Young’s Modulus of approx 30%! Therefore, a line of these bearings along a pier may compress differently from one another by as much as 30%, giving rise to significant secondary stresses within the structure.
• Minimal Load Eccentricity: Tilting of the piston caused by end rotation of the bridge beam results in a slight deformation. The resistance to this deformation is so small that the load eccentricity is no more than 3% of the disc diameter. Thus, pot bearings have the least resistance of any bridge bearing in the critical range of 0.01 to 0.022 radians.
• Low Stress on Bearing Seats: Due to the basically hydraulic design principles, the characteristic load distribution curve under a pot bearing is flatter than other bearing types, which means lower peak pressures on the supports.
• High Horizontal Load Capacity: The naturally large bearing area of the piston against the pot wall inherently withstands high side forces. Although a figure of 10% of vertical capacity is normal, 50% or more can be provided by simply increasing this bearing area. Compare this with bearings having central pin-and-sleeve restraints. These restraints become so large when designed for high horizontal forces that higher load-range bearings must be used.