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Current models describing the behavior for the triple friction pendulum (TFP) bearing are based on the assumption that the resultant force of the contact pressure acts at the center of each sliding surface. Accordingly, these models only rely on equilibrium in the horizontal direction to arrive at the equations describing its behavior. This is sufficient for most practical applications where certain constraints on the friction coefficient values apply as a direct consequence of equilibrium. This paper presents a revised model of behavior of the TFP bearing in which no assumptions are made on the location of the resultant forces at each sliding surface and no constraints on the values of the coefficient of friction are required, provided that all sliding surfaces are in full contact. To accomplish this, the number of degrees of freedom describing the behavior of the bearing is increased to include the location of the resultant force at each sliding surface and equations of moment equilibrium are introduced to relate these degrees of freedom to forces. Moreover, the inertia effects of each of the moving parts of the bearing are accounted for in the derivation of the equations describing its behavior. The model explicitly calculates the motion of each of the components of friction pendulum bearings so that any dependence of the coefficient of friction on the sliding velocity and temperature can be explicitly accounted for and calculations of heat flux and temperature increase at each sliding surface can be made. This paper presents (a) the development of the revised TFP bearing model, (b) the analytic solution for the force-displacement relations of two configurations of the TFP bearing, (c) a model that incorporates inertia effects of the TFP bearing components and other effects that are useful in advanced response history analysis, and (d) examples of implementation of the features of the presented model. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents seismic behaviour of isolated curved bridges in the earthquake prone regions. For the seismic isolation of bridges, double concave friction pendulum bearings are placed between the deck and the piers, and the abutments as isolation devices. A curved bridge is selected to exhibit the application for seismic isolation. The mentioned bridge is a three-span featuring cast-in-place concrete box girder superstructure supported on reinforced concrete columns found on drilled shafts and on integral abutments founded on steel pipe piles. Additionally, the bridge is located on site underlain by a deep deposit of cohesionless material. The drilled shaft-soil system is modelled by equivalent soil springs method and is included in the finite element model. The soil modelled as a series of springs is connected to the drilled shaft at even intervals. The multi mode method of analysis is typically implemented in a computer program capable of performing response spectrum analysis. The response spectrum specified for the analysis is the 5%-damped spectrum modified for the effects of the higher damping. Each isolator is represented by its effective horizontal stiffness with a linear link element. As seen from the results of the outlined analysis, usage of the isolation devices offers some advantages for the internal forces on the deck for the considered curved bridge as per the non-isolated curved bridge. The response spectrum analysis is substantially required to make a decision of the displacement capacity of the double concave friction pendulum bearings used in the study. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Horizontal seismic isolation is a viable approach to mitigate risk to structures, systems and components (SSCs) in nuclear power plants (NPPs) under extreme ground shaking. This paper presents a study on an NPP seismically isolated using single concave Friction Pendulum™ (FP) bearings subjected to ground motions representing seismic hazard at two US sites: Diablo Canyon and Vogtle. Two models of the NPP, five models to describe friction at the sliding surface of the FP bearings, and four levels of ground shaking are considered for response-history analysis, which provide insight into the influence of 1) the required level of detail of an NPP model, 2) the vertical component of ground motion on response of isolated NPPs, and 3) the pressure-, temperature- and/or velocity-dependencies of the coefficient of friction, on the response of an isolated NPP. The isolation-system displacement of an NPP can be estimated using a macro model subjected to only the two orthogonal horizontal components of ground motion. The variation of the coefficient of friction with temperature at the sliding surface during earthquake shaking should be accounted for in the calculation of isolation-system displacements, particularly when the shaking intensity is high; pressure and velocity dependencies are not important. In-structure floor spectra should be computed using a detailed three-dimensional model of an isolated NPP subjected to all three components of ground motion. [ABSTRACT FROM AUTHOR]

Seismic isolation using low damping rubber and lead–rubber bearings is a viable strategy for mitigating the effects of extreme earthquake shaking on safety-related nuclear structures. The mechanical properties of these bearings are not expected to change substantially in design basis shaking. However, under shaking more intense than design basis, the properties of the lead cores in lead–rubber bearings may degrade due to heating associated with energy dissipation, some bearings in an isolation system may experience net tension, and the compression and tension stiffness may be affected by the lateral displacement of the isolation system. The effects of intra-earthquake changes in mechanical properties on the response of base-isolated nuclear power plants (NPPs) are investigated using an advanced numerical model of a lead–rubber bearing that has been verified and validated, and implemented in OpenSees. A macro-model is used for response-history analysis of base-isolated NPPs. Ground motions are selected and scaled to be consistent with response spectra for design basis and beyond design basis earthquake shaking at the site of the Diablo Canyon Nuclear Generating Station. Ten isolation systems of two periods and five characteristic strengths are analyzed. The responses obtained using simplified and advanced isolator models are compared. Strength degradation due to heating of lead cores and changes in buckling load most significantly affect the response of the base-isolated NPP. [ABSTRACT FROM AUTHOR]