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This paper investigates the validity of theories for the prediction of the temperature increase at the sliding interfaces of sliding isolators. The prevailing theory considers the isolator to be indefinite in depth and the heat flux generated at the sliding interface(s) to be infinite in extent. In reality, the depth is finite and small, and bounded by an insulating boundary, whereas the heat flux is bound by the circular area of contact. New theories are presented in which the restrictions of the past theory are relaxed. The resulting solutions have been implemented in program OpenSees for single and for triple friction pendulum isolators but are very complex and time‐consuming to be used in response history analysis with many isolators. Analysis of the response of sliding isolated structures with these models demonstrates that consideration of the finite depth of the isolators and of the limited extent of the heat flux does not have any important effects on the calculated isolator displacement demands in either short‐duration or long‐duration ground motions. It is shown that the finite depth has some effect on the peak temperature at the sliding interface and, more importantly, on the duration of the high temperature in long‐duration ground motions. This is of some importance in the use of these theories for the development of testing specifications for isolators with unusually thin end plates. [ABSTRACT FROM AUTHOR]

This paper augments the series spring model for Triple Friction Pendulum (TFP) isolators to predict the relative displacement and velocity at each of its four sliding interfaces. This enables the prediction of the rise in temperature at each sliding interface that results from frictional heating and the introduction of temperature‐dependent friction coefficient. Moreover, the model directly accounts for the velocity‐dependence of the friction coefficient as it can predict the actual velocities at each sliding interface, whereas the original series model only approximately accounted for the velocity effects. It also accounts for the effects of instantaneous bearing pressure on the friction coefficient. The model has been implemented in program OpenSees and has been verified by comparison to results obtained using a much more advanced and complex model that is unsuitable for computations with a large number of isolators. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of inherent damping modeling on the computed seismic response and collapse performance of selected seismically isolated buildings. The analyzed seismically isolated buildings were designed by the procedures of the ASCE/SEI 7–16 standard. The structure is a six‐story perimeter frame building designed with special moment resisting frames or with special concentrically braced frames (SCBF) for a location in California. Three different seismic isolation systems are considered: (i) triple friction pendulum (TFP) bearings without moat walls, (ii) TFP bearings with moat walls (double concave [DC] friction pendulum bearings with moat walls have effectively the same ultimate behavior), and (iii) DC friction pendulum bearings without moat wall. The superstructure inherent damping schemes considered are (i) zero damping, (ii) modal damping, (iii) Zareian‐Medina damping, (iv) added virtual viscous dampers with and without force‐caps, and (v) added virtual viscous dampers with the same damping constant value. The response parameters computed are peak floor accelerations, peak story drift ratios, peak residual story drift ratios, peak isolator horizontal displacement, and floor acceleration spectra. Also, the probability of collapse in the maximum considered earthquake (MCER) is computed. It is shown that the modeling approach for the inherent damping has minor effects on the computed responses and the collapse probability of the studied seismically isolated buildings, except for the peak floor acceleration and the floor response spectra for periods below one second. It is suggested that a convenient way to model inherent damping is to use virtual viscous dampers with all having the same damping constant. [ABSTRACT FROM AUTHOR]

Lead-rubber bearings are seismic isolators that have been used extensively to protect buildings, bridges, and mission-critical infrastructure from the damaging effects of earthquake shaking. In bridges, lead-rubber bearings are subjected to continuous movement due to service (traffic) and temperature effects. This paper presents data on the effects of cumulative travel on the mechanical properties of lead-rubber bearings based on an experimental investigation of both medium- and small-sized bearings. A theoretical interpretation of the test results is presented. DOI: 10.1061/(ASCE)ST.1943-541X.0000141 CE Database subject headings: Lead; Rubber; Service loads; Thermal factors; Strain hardening; Isolation; Seismic effects; Earthquakes. Author keywords: Lead-rubber bearing; Cumulative travel; Service load, Thermal load; Strain hardening; Characteristic strength; Recrystallization.

The energy dissipated per cycle and the characteristic strength of lead-rubber bearings reduce with increasing number of cycles as a result of heating of the lead core. A theory which is capable of predicting the histories of temperature rise in the core, of the energy dissipated and of the characteristic strength of lead-rubber bearings has been presented in a companion paper. This paper presents a comparison of predictions of this theory to experimental results obtained in the testing of six lead-rubber bearings. The analytical predictions are found to be sufficiently accurate. Moreover, finite-element analyses results are presented that provide insight into the distribution of temperature in the lead core and end and shim plates of the bearings. These distributions confirm the assumptions made in the development of the theoretical solution. DOI: 10.1061/(ASCE)ST. 1943-541X.0000071 CE Database subject headings: Lead; Rubber; Tests; Heat transfer; Temperature effects; Stress; Energy dissipation; Verification.

The energy dissipated per cycle and the characteristic strength of lead-rubber bearings reduce with increasing number of cycles as a result of heating of the lead core. The reduction is substantial depending on the geometric characteristics of the bearing and the speed of motion. This paper presents a theory that is capable of predicting the dependency on time of the characteristic strength and energy dissipated of lead-rubber bearings. The theory is based on predicting the temperature of the lead core and relating the temperature to the strength. A much simpler theory is also presented based on neglect of heat conduction through the steel shim and end plates of the bearing. The theory is useful in predicting the cyclic behavior of lead-rubber bearings for simplified analysis, in extrapolating experimental data from one scale to another and in developing models for dynamic response history analysis of seismically isolated structures that account for the time-dependent mechanical properties of the bearings. A companion paper presents results of eight tests of six lead-rubber bearings that provide verification of the theoretical solution. DOI: 10.1061/(ASCE)ST.1943-541X.0000072 CE Database subject headings: Lead; Rubber; Heating: Temperature effects; Stress; Energy dissipation; Theories; Seismic effects.

An experimental study investigating the influence of lateral displacement on the vertical stiffness of elastomeric and lead-rubber seismic isolation bearings is summarized. Two identically constructed low-damping rubber and lead-rubber seismic isolation bearings were subjected to a series of tests with varying levels of combined lateral displacement and axial (compressive) loading to study this relationship. The results of these tests showed the vertical stiffness decreases with increasing lateral displacement for each bearing tested. Additionally, the vertical stiffness data are used to evaluate four formulations for the estimation of the vertical stiffness as a function of the lateral displacement. From this comparison, two formulations, one based on the Koh-Kelly two-spring model and the other on a piecewise linear relationship, showed good agreement with the experimental data over the wide range of lateral displacements considered in this study. DOI: 10.1061/(ASCE)0733-9445(2007) 133:9(1227) CE Database subject headings: Elastomer; Rubber; Stiffness; Lateral displacement; Experimentation.

An analytical study of buildings without and with seismic damping systems was conducted to investigate the dynamic response of secondary systems. The buildings were represented by steel frames designed by the procedures described in the 2000 NEHRP Recommended Provisions. Nonlinear response analyses were performed for the undamped frames and the damped frames equipped with different energy dissipation systems and for far-field, near-field, and soft-soil earthquake histories. The study shows that significant benefits may be provided by viscous damping systems in terms of reduced floor spectral accelerations and floor absolute velocities. CE Database subject headings: Damping; Dynamic response; Nonlinear response; Nonstructural elements.

Isolation bearings experience tensile forces or uplift under certain conditions during seismic excitation. In general, this phenomenon is undesirable. This paper discusses an uplift prevention mechanism that uses prestress to develop sufficient compressive force on the isolator. A simplified theory of prestressed isolators is presented for three types of isolators, namely flat sliders, spherically shaped sliding bearings friction pendulum system (FPS), and elastomeric bearings. The behavior of prestressed isolators has been experimentally studied and presented. It has been observed that, consistent with the theory, the prestress has significant effects on the behavior of the prestressed isolators. The primary effect of prestress to increase the axial load on the isolators and prevent the uplift phenomenon was clearly observed in all three types of bearings tested in the study. The secondary effect of prestress to introduce additional lateral stiffness was also observed. While the additional axial load affects the behavior of elastomeric bearings, the effects were observed to be minor for the tested elastomeric bearings. In all cases of tested bearings, the experimental response could be predicted with sufficient accuracy by analytical means. DOI: 10.1061/(ASCE)0733-9445(2005)131:6(857) CE Database subject headings: Isolation; Axial loads; Excitation; Tensile loads; Uplift resistance.

Seismic isolation bearings consisting of lubricated sliding bearings and steel dampers exhibit nearly elastoplastic behavior. Accordingly, they can transfer force to bridge substructures which is independent of the level and frequency content of seismic excitation. While this is a desirable property, it is achieved at the expense of potential for development of significant permanent displacements. This paper reports on a study of one such system that consists of E-shaped steel dampers and which found a number of applications in bridge seismic isolation. The paper demonstrates that indeed significant permanent displacement develop, particularly in earthquakes with shock-loading characteristics. Moreover, the paper demonstrates the significance of restoring force in isolation systems and concludes that the current American Association of State Highway and Transportation Officials (AASHTO) design guidelines for these systems are appropriate.

Seismic isolation in the United States is gaining wide acceptance as an attractive and alternate means to upgrade historic structures seismically with minimal disturbance to architecturally significant features. One important historic-landmark structure that underwent such an upgrade is the Ninth Circuit U.S. Court of Appeals building located in San Francisco, which suffered damage during the 1989 Loma Prieta earthquake and was declared an unsafe building for further occupancy. The present paper summarizes the comprehensive approach adopted for implementation of the seismic-isolation technique for the seismic retrofit of this monumental structure. This historically significant structure is the first federal building to be retrofitted using seismic isolation. The paper discusses in detail the procedure adopted for evaluating the existing building strength, establishing the design approach, and evaluating and selecting the optimum isolator placement location and isolation system best suited for the building's structural performance and architectural usage.

Summary: This paper presents an investigation of the effects of the duration of earthquake ground motion on the peak displacement and total dissipated cumulative energy in seismic isolation systems designed by the procedures of ASCE/SEI 7‐16. The seismic isolation systems considered in this study are elastomeric with lead and natural rubber bearings and sliding in the form of single and triple friction pendulum bearings. Long‐ and short‐duration ground motions were selected so that the 5%‐damped spectral acceleration values of each motion were closely matched over periods of 0.05 to 6 s. The results of the analysis demonstrate that the duration of ground motions have insignificant effects on the isolation system peak displacement—particularly in the lower bound conditions and sliding isolation bearings—but have important effects on the total cumulative energy for all conditions. This has implications in the testing of isolators, which are presented in an evaluation of the isolator test procedures of the ASCE/SEI 7‐16 standard. The evaluation reveals that, if the cumulative energy dissipated by the isolation system is used as the controlling criterion, the specified tests are generally appropriate for locations characterized by short‐duration ground motions, but are unconservative for locations where long‐duration motions are anticipated. Additional results and discussion demonstrate that the cumulative energy may not be an appropriate criterion for designing isolator tests as it may lead to unrealistic demands on the tested isolators. [ABSTRACT FROM AUTHOR]