The design and development of elastomeric bridge bearings has been almost entirely empirical, and many conflicts exist regarding current specifications and practices. Design information on shape-related elastomeric bearing behavior is meager. Only limited usable materials- properties information is available; current materials specifications are quality- rather than performance- related. Little effort has been made to validate empirical predictions with actual bearing field perfommance. It is therefore difficult to judge the potential of new materials without testing. An effort was made to define and separate those parameters that significantly affect the behavior and performance of elastomeric bridge bearings, toward the end of improving current designs and establishing design criteria. The research included a survey of the literature and available test data, an evaluation (testing) program, and the analysis of data generated. The evaluation program investigated (1) shear modulus and stress relaxation, (2) compressive behavior, (3) shear and compression of commercial bearings, and (4) cyclic shear under constant compression. The major work was accomplished with 50- to 70-durometer neoprene bonded to steel and neoprene bonded to dacron; some limited testing was done with natural rubber and ethylene propylene dimonomer. The shear of neoprene was investigated down to -40 f. Compression tests included a range of shape factors from 1 to 15 and a range in number of laminates from 1 to 18. It was found, as expected, that the higher the durometer hardness, the greater the shear modulus; there was some disagreement with published curves. Lower temperatures produced a marked nonlinear increase in the shear modulus of neoprene. Stress relaxation in shear showed a general increasing trend with increased shear modulus and, therefore, also with decreased temperature. A large increase in shear modulus (probably due to crystallization) with time at low temperatures was noted for neoprene; the greatest increase occurred at about -20f. Size and shape did not affect shear modulus to any significant degree, provided bending of tall bearings was properly accounted for. A relationship between shape factor and compressive strain for bonded elastomeric bearings was developed for several values of hardness; there was significant deviation of these results from both empirical (shape factor approach) and theoretical predictions. The theoretical approach appeared valid for low shape factors; the empirical approach, for intermediate shape factors; and neither, for factors. In view of these and other findings, several modifications of the current aasho "standard specifications for highway bridges" (10th ed., 1969) are suggested so that these specifications will more closely agree with desired performance and material property requirement.
Abstract