Effects of heating and load history on the behavior of lead-rubber bearings
Kalpakidis, Ioannis V.
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The lead-rubber bearing has been extensively used as a seismic isolation device in both seismic retrofit and new construction of buildings and bridges. Its behavior is markedly affected by heating of its lead core during cyclic motion. The phenomenon is characterized by reduction of the characteristic strength of the bearing with increasing number of cycles and has been observed in numerous experimental studies; however, it has not been analytically described so that predictions of behavior can be made for analysis and design. Moreover, a previous study demonstrated that large cumulative travel resulting from service loadings has effects on the characteristic strength of the bearing but the conditions that affect these changes are not well understood. This work presents a comprehensive investigation of the effects of lead core heating and cumulative travel on the behavior of lead-rubber bearings. First, a literature review and an experimental study of temperature effects on the mechanical properties of lead are presented. Then, a theory for the prediction of the lead core temperature when the bearing undergoes cyclic motion is developed on the basis of principles of mechanics. The theory accomplishes a reduction of the complex three-dimensional thermo-mechanical problem to an initial value problem on the history of temperature of the core and strength of the bearing that can be numerically solved. An explicit approximate solution is also presented. The accuracy of the theoretical solution is verified by comparing its results to those of experiments and complex finite element analyses of several bearings. The theoretical solution is used to establish principles of similarity and scaling that can be used in the reduced scale testing of lead-rubber bearings. Moreover, a model of behavior of lead-rubber bearings for incorporation in dynamic response history analysis programs is described. The model is capable of describing the lateral force-displacement relation of lead-rubber bearings including the lead core heating effects. Experiments conducted in cyclic and random motions are used to demonstrate the validity of the model. The model is used to conduct analyses of a representative seismically isolated structure and to demonstrate the significance of accounting for lead core heating in the prediction of the peak response. Finally, the effect of cumulative travel on the characteristic strength of lead-rubber bearings is investigated through the evaluation of existing experimental results and the conduction of a new series of tests. The results show that, depending on its amplitude, the cumulative motion may indeed affect the strength of the bearings and this should be considered in analysis and design.