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33 Chapter 3 Interpretation, Appraisal, and Applications 33 3.1 Proposed Design Rules 36 3.2 Design Examples 45 3.3 QC/QA Procedures 47 Chapter 4 Summary, Conclusions, and Recommendations 47 4.1 Summary 47 4.2 Conclusions 47 4.2.1 Conclusions on Behavior Measured in Tests 48 4.2.2 Conclusions on Analytical and Numerical Modeling 48 4.2.3 Conclusions on Development of Design Procedures 49 4.3 Recommendations 49 4.3.1 Recommendations for Implementation 49 4.3.2 Recommendations for Further Research 50 Bibliography 52 Notation 55 Appendixes
S U M M A R Y The response of elastomeric bridge bearings to imposed rotations was studied using testing and analysis. The program concentrated on steel-reinforced elastomeric bearings. The demand on typical bearings was evaluated by analyzing a range of bridges and eval- uating the amplitudes of the axial loads and rotations on the bearings that are likely to occur in practice. During this process, it was found that cyclic axial forces cause larger strains in the elastomer than do the cyclic rotations. However, resource limits in the program pre- cluded cyclic axial testing, and the discovery was anyway made quite late in the program. Thus, in the development of design procedures, the effects of cyclic axial load were taken into account by analysis alone. The capacity of the bearings to accommodate the loads and rotations without exces- sive damage was evaluated by a program of testing and analysis of the bearings them- selves. The test program included material tests, static and cyclic tests on full bearings, and diagnostic tests on full bearings to evaluate their instantaneous state of damage. In all, 78 bearings were tested. The bearings were purchased from the four largest manufac- turers in the country. Static and low-cycle repeated load tests were conducted under axial load with or with- out a fixed rotation. Stresses up to 12,000 psi were applied. Bearings with high shape fac- tors (9 and 12) were able to carry the loads with no damage whatsoever, while bearings with lower shape factors suffered various levels of damage. The cyclic rotation tests were con- ducted in a specially constructed test machine that is capable of applying simultaneously constant axial load, constant shear displacement and cyclic rotation. Peak capacities are 800 kips axial load and +/â 8% rotation. Cyclic loading was found to cause progressive damage, analogous to fatigue in metals. However, the damage was in the form of progres- sive debonding of the elastomer from the internal steel shims, so failure was never sudden. At the end of every cyclic test, the bearing could still easily carry the axial load, even though other properties had degraded. This finding creates the need for judgment in establishing a level of damage that constitutes failure. Failure of a component is usually expressed in terms of critical stress or strain, and the same approach was used with the bearings studied here. However, measuring local strains in rubber is difficult, so analysis was necessary to relate the internal stress and strain fields to the external loadings that caused them. Classical, closed-form analytical techniques for laminated incompressible materials were used for the purpose, and nonlinear Finite Ele- ment Analysis (NLFEA) was used to verify that the closed-form methods were sufficiently accurate to warrant their use in design procedures. Empirical fatigue models were also gen- erated to predict the progress of damage under cyclic loading. The results of the tests and analyses were combined to develop design procedures suitable for adoption in the AASHTO LRFD Bridge Design Specifications. Major proposed changes 1 Rotation Limits for Elastomeric Bearings
2from the present specifications include the removal of the absolute limit on compressive stress, so that the design of high shape factor bearings for high stresses will be possible, the removal of the previous âno-upliftâ provisions, which were causing difficulties for design- ers, and a change in the testing requirements so that the second-level, more rigorous testing is required for large bearings rather than any bearing deigned by the more comprehensive âMethod B.â