Elastomeric shape factor

The most important design parameter for reinforced elastomeric bearings is the shape factor. The shape factor is defined as the plan area of the bearing divided by the area of the perimeter free to bulge (plan perimeter multiplied by elastomeric layer thickness). Figure 1.16 illustrates the shape factor concept for a typical steel reinforced elastomeric bearing and for a fabric pad bearing. 

Limiting instantaneous live load deflections is important to ensure that deck expansion joints are not damaged. Steel reinforced elastomeric bearings exhibit nonlinear compressive load-deflection behavior. Compressive stiffness of an elastomeric layer substantially increases with increasing shape factor. The total compressive deformation of an elastomeric bearing is equal to the sum of the compressive deformation of all its constituent elastomeric layers. 

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