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CHAPTER 6 SUMMARY AND CONCLUSIONS

6.2 SHEAR RESPONSE

6.2.1 Influence of Elastomer Compound

Shear response is a complex characteristic for elastomeric bearings, with multiple

competing and counteracting influences from boundary and loading conditions. Apparent shear stiffness for both Type I and Type II bearings has been found experimentally to be influenced by peak strain demand and strain rate effects. Elastomer compounds are available for specialty applications offering a range of stress-strain characteristics. At one extreme, low-damping elastomers provide a response that is practically linear for a range of shear strains encompassing the anticipated structural demands. At the other extreme are high-damping elastomers, which have an initially high stiffness that transitions to a softened response, creating a source of hysteretic energy dissipation under cyclic loading. Bearing specifications used by IDOT do not explicitly require any particular damping characteristics. One of the primary means of introducing higher damping characteristics to elastomers is by the addition of carbon black. According to the bearing supplier, carbon black was added to the bearings supplied for the experiments, consistent with the supplier’s standard

manufacturing practice, to achieve a target range of shear stiffness. Consequently, although the bearings are not specifically required to provide damping or intentionally manufactured to provide high-damping characteristics, the means of achieving a desired stiffness

coincidentally resulted in tests exhibiting bilinear stress-strain response typical of high- damping bearings.

With a bilinear stiffness, the apparent linear stiffness will depend on the peak imposed shear strain. The softened stiffness branch becomes an increasingly dominant characteristic proportionately with maximum shear strain demand. For quasi-static tests, the observed stiffness at service-level strains (≤50% shear strain) was reasonably consistent with values reported by the manufacturer, but the apparent stiffness decreased as strains were

increased to levels anticipated for seismic demands. Counteracting this effect, elastomer response exhibits some limited strain rate dependency so that stiffness increases with increasing strain rate. The strain rate sensitivity was found to be relatively more significant at low levels of strain demand.

6.2.2 Type II Bearing Response

For Type II bearings, slip at the PTFE interface limits the shear strain demand imposed on the elastomer. The experimental data indicate that the shear strain demand at increased strain rates will exceed the range typically permitted for service loading, resulting in reduced apparent stiffness, but this reduction is partially offset by stiffening of the elastomer

the Type II bearing elastomer stiffness can be bounded in the range of approximately 65% to 100% of the value reported by the bearing manufacturer, with the lower values

corresponding to lower strain rates at peak displacement cycles. The upper bound is likely to be a reasonable estimate for elastomer response during a seismic event, considering that the strain rate during an earthquake will almost certainly be higher than the maximum testing capability for the experiments.

6.2.3 Type I Bearing Response

For Type I bearings, the boundary condition at the bottom of the elastomer block introduces additional aspects of complexity to the mechanical response. At the bottom surface of Type I bearings is a simple contact interface of elastomer and concrete, as opposed to the fully bonded interface of elastomer and steel provided in Type II bearings. This boundary condition leads to three separate softening effects observed during the tests. First, the slip surface for Type I bearings is at this interface of elastomer and concrete instead of at the PTFE and stainless steel interface for Type II’s. The slip force is higher between elastomer and concrete, so the Type I elastomer experiences greater shear force, greater shear strain, and a reduced apparent stiffness. Second, the shear and associated flexure in the

elastomer cause the trailing edge to curl away from the concrete surface, reducing the surface area over which the shear traction acts on the bottom of the bearing. Consequently, the bottom layer of elastomer is only partially effective in resisting shear. Finally, when displacements are sufficient to induce slip at the concrete surface, abrasion of the elastomer on the concrete will remove a portion of the leading edge of the elastomer, further reducing the area available to resist shear.

Two aspects of elastomer response contribute stiffening effects, counteracting these softening effects. First, although elastomer material response softens at moderate strains (up to about 100% to 150%), the ultimate behavior of the elastomer exhibits significant stiffening prior to material rupture. Experimental slip tended to initiate at about 125% shear strain for relatively small vertical compression stresses of 200 psi. The slip threshold

increased to about 200% for 500 psi but reached only about 250% for 800 psi. At the higher compression levels, the load-displacement response showed a slight stiffening branch. The net effect of the stiffening branch is to level off the effective apparent linear shear stiffness so that all bearing load levels converge to similar minima for an apparent linear shear stiffness prior to slip. Lastly, the Type I bearings exhibited strain rate sensitivity in the elastomer response, similar to the Type II bearings, but the effect tended to be

overshadowed by other influences at the high strain demand levels that would be expected during an earthquake.

One final aspect that affects Type I bearings is a dependency on orientation of applied loading (longitudinal versus transverse bridge direction). The elastomer itself is not inherently sensitive to the direction of loading, and the bearings are short enough that

stability effects are not significant. However, to the extent that curling of the trailing edge and loss of effective contact area through abrasion of the leading surface influence the

response, the longitudinal orientation of bearing response tends to show higher apparent sensitivity to peak strain demand and incidence of slip with lower stiffness relative to similar tests conducted with a transverse orientation.

The elastomers exhibited sporadic variability between bearings that had nominally been constructed with identical materials and methods. Results were generally consistent for the transverse tests, but they varied more significantly for the bearings used for longitudinal tests. On the basis of the test data, bounding estimates for the effective shear modulus of Type I bearings at high strains are about 60% to 100% of the bearing supplier’s documented

value for longitudinal motion and about 75% to 105% for transverse motion. These

estimates are provided assuming that there will be few slip cycles. Also, these ranges have been adjusted to include a relative increase of 5% at the lower bound and 30% at the upper bound to approximate the influence of strain rate. With multiple slip cycles, the response degrades so that the ranges would fall to about 45% to 90% for longitudinal motion and about 50% to 80% for transverse motion.

6.3 SLIDING RESPONSE

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