Circular Elastometric Bearings

Reinforced and unreinforced elastometric bearing pads have been used for bridges in NJ for about five years. The shape of these bearings is square or rectangular and their orientation is generally in the direction of thermal movements. Although the direction of thermal movement for straight bridges can be reasonably predicted, the direction of displacements of skewed and curved bridges may not be well defined. For rectangular bearings, if the direction of movement is not oriented along one of the principal axes of the bearing, distortion of the bearing may occur. The problem gets worse if the fatigue loading is significant which could cause delamination at the elastomer-steel shim interface. For very wide bridges, circular bearings have a better performance than square or rectangular bearings because transverse as well as longitudinal movement need to be considered and the direction of movement is not along the centerline of the beam. Moreover, rectangular bearings often need to be notched to provide edge clearance for certain capital geometries, which increases the cost of the bearing and adds more corners to its shape. Also in some instances, these bearings may not be properly oriented in the field as required by the contract drawings. Circular elastomeric bearings are not direction dependent and they exhibit the same behavior in all directions. The circular shape, moreover, does not have edge corners, which eliminates stress concentrations and the possibility of distortions. These bearings are less likely to be notched compared to the rectangular bearings and their orientation in the field is simple. Their advantages on skewed, curved, and wide bridges make them an attractive alternative to rectangular bearings, however, their behavior needs to be studied and evaluated. A nationwide survey has been conducted to evaluate state DOT's experience with circular elastomeric bearings and a finite element investigation was conducted to compare various bearing geometries. Survey results showed circular elastomeric bearings are being used in several states and more states are willing to consider them. Results from the analytical investigation showed that circular bearings have higher translational and rotational stiffness compared to square and rectangular bearings. The analysis also showed non-linear distribution of normal stresses across the bearing and the existence of tensile stresses at the interface between the elastomer top layer and the steel shim under compressive and shear loads.