An Experimental Sutdy of Elastometric Bridge Bearings With Design Recommendations

Elastomeric bridge bearings have been used in the United States since the 1950’s with overwhelming success. They are durable, economical and easily designed and employed. Recent AASHTO specifications, however, have placed a number of restrictions on their use. Of particular note is that elastomeric bearings with tapers built in to accommodate span end elevation differences were disallowed by the most current specifications even though no previous research into elastomeric bearing performance has included tests on tapered pads.

The purpose of this study sponsored by the Texas Department of Transportation was to analyze elastomeric bearing performance on the basis of elastomer hardness, shape factor, reinforcing shim orientation, degree of taper and compressive stress level with the goal of developing a simple design procedure which standardizes as many of those parameters as possible. Particular emphasis was placed on comparing the behavior of flat and tapered bearings. Experimentation included shear, compressive, and rotational stiffness tests, shear and compression fatigue loading, long-term compressive loading, and tests to determine compressive stress limits. In many cases, bearings were intentionally loaded non-uniformly to define safe limits for bearing/girder slope mismatches.

Research showed that tapered elastomeric bearings performed equally as well as flat bearings an that manufacturing tapered bearings with steel shims oriented parallel to one another offers several benefits over spacing shims radially. Additionally, bearings made from elastomers with lower hardness ratings displayed several advantages over those made from harder elastomers, particularly rotation capacity far in excess of the current AASHTO limitation. More highly reinforced bearings performed better in compression fatigue and creep tests and easily accommodated compressive stresses in excess of 6.9 MPa (1000 psi).

Also, a field and laboratory investigation was performed into bearing slip and anchorage by friction. Bearings that had slipped while in service were subjected to comparable conditions in a laboratory test apparatus to study their behavior. Research showed that secretion of antiozonant waxes to the bearings’ surfaces caused a dramatic lowering of the bearing’s friction coefficient and resulted in slipping. Several tests were performed that established friction coefficients for natural rubber against a variety of steel and concrete surfaces.

Recommended changes to the current AASHTO specification are presented.