Friction of Polymers Sliding on Smooth Surfaces

Friction plots of polymers sliding on smooth metal surfaces are generally characterized by two regions of distinct dependency on the normal load, with low sensitivity at low stress levels followed by a sharp change in the rate of decrease of friction with increasing pressure at levels above the plastic flow limit of the polymer. A simplified model is proposed to describe this behavior which accounts for the effect of the normal load on the growth of the real contact area and the shear stress at the interface between the polymer and the mating surface. The model has a wide generality when expressed in terms of dimensionless variables, allowing to rationalize the friction behavior of different polymers within a single framework

High performance materials for pendulum sliding bearings

At Politecnico di Milano a novel thermoplastic material for pendulum bearings operating in seismic isolation of buildings and structures has been recently developed. The material properties were characterized in small scale tests, and the dependence of the coefficient of friction on operating conditions like pressure, temperature and velocity was determined at speeds and durations typical of seismic events. The effect of extended durations of excitation on the stability of friction was assessed. Two prototypes of pendulum bearing operating with the new material were manufactured and tested. The results confirmed the good performance of real scale isolators in terms of low horizontal stiffness, high damping and good stability of the dynamic propertie

Effect of Pressure on the Shear Properties of Elastomeric Compounds

High Damping Rubber (HDR) is used in the manufacturing of elastomeric bearing for seismic isolation of building and structures. In service rubber bearings are subjected to a permanent vertical load which may change at the occurrence of the earthquake, and potential shear deformation induced by the seismic ground motion. This study investigates the response of five commercial elastomeric compounds used in the manufacture of HDRB with shear modulus between 0.4MPa and 1.4 MPa, and equivalent viscous damping between 0.06 and 0.16 under combined compression and shear, with compressive stresses ranging from 0 up to 18 MPa. Soft rubbers seem to be more sensitive to the compression load than hard rubbers as far as the damping factor is concerned, whereas the influence of the compression load on the shear modulus seems to be peculiar of every mixture and not apparently related on the hardness, which can explain the inconsistencies found in literature relevant to the studies on full scale rubber bearings

Total displacement of Curved Surface Sliders during an earthquake: effect of coexisting non-seismic actions

The restoring capability is a fundamental function of any effective isolation system, required to prevent accrual of displacements that may limit the capability to withstand aftershocks and future earthquakes. For systems endowed with good restoring capability the displacement capacity is determined according to Eurocode 8 by adding to the design seismic displacement the non-seismic offset induced from permanent and thermal actions and long-term deformations of the structure. However it is noted that the design provisions of EC8 were developed for bilinear hysteretic systems and their validity for nonlinear isolation systems with high damping capacity has not yet been evaluated. The study investigates the re-centering capability of Curved Surface Sliders (CSSs) and the influence of the non-seismic offset on the displacements induced from the design earthquake. Nonlinear Time-History Response analyses are conducted, considering five non-seismic offset displacements combined with a wide range of device characteristics covering the current design practice. Twenty-four natural ground motion time histories are selected from a database and classified in terms of the predominant period of the quake and the level of impulsivity. Based on the results of the analyses, an improved formulation of the Eurocode’s re-centering criterion for Curved Surface Sliders is eventually proposed

Experimental Assessment of High Damping Rubber Under Combined Compression and Shear

High damping rubber (HDR) is used in the manufacturing of elastomeric bearings for seismic isolation of building and structures. In practical situations, rubber bearings are subjected to a permanent vertical load which may change at the occurrence of the earthquake, and concurrent shear deformation, due to either service movements of the structure or earthquake-induced ground motion. The study presents an experimental procedure for the assessment of HDR specimens under combined compression and shear, reproducing the same typical load regimes which rubber isolators experience in service. Five commercial HDRs were tested according to the procedure. The results point to the importance of considering the influence of the compression stress for a correct understanding of the behavior of HDRs under cyclic shea

Experimental investigation of the re-centring capability of curved surface sliders

The re-centring capability is recognized as a fundamental function of the isolation system, because it is intended to prevent substantial permanent deformation at the end of the earthquake that may affect the serviceability of the structure and eventually limit the capability of the isolators to withstand aftershocks and future earthquakes. In this study, the re-centring behaviour of isolation systems composed of sliding bearings with curved surfaces is investigated in shake-table tests carried out on a one-storey steel frame with rectangular plan, scaled at one third-length scale and isolated with four bearings. The coefficient of friction of the bearings is varied by changing the material or lubrication condition of the pads, providing different equivalent damping ratios to the isolation system. The response of the base isolated structure to selected natural ground motion waveforms is assessed in terms of the residual displacement after a single event and the accrual of displacements during a sequence of quakes, and considerations on the influence of the coefficient of friction on the re-centring behaviour, as well as on the effect of an initial displacement offset are drawn. The re-centring provision of the current European design code is eventually checked against the experimental data

Effects of permanent offset on the response of the Curved Surface Sliders

The large worldwide diffusion of isolation systems with Curved Surface Sliders (CSS), also known as the Friction Pendulum System®, requires detailed knowledge of their behavior and improved modelling capability under seismic conditions. Restoring capability after the earthquake is one of the fundamental functions required to seismic isolation systems. It is noted the dependency of residual displacements on ground motions characteristics and isolator mechanical properties, namely the radius of curvature and the coefficient of friction. Current standards on antiseismic devices (EN15129, AASHTO) and design codes (Eurocode 8) provide criteria to ensure good restoring capability. An analytical model has also been formulated based on numerical analysis to predict the residual displacement of the isolation system. In mainshock-aftershock sequences it is possible that at the occurrence of the aftershock the isolation system present an offset from its original configuration as a result of the main shock. The main concern is whether or not an increase in the maximum or residual displacement consequent to the displacement accrual starting from the offset position may lead, under certain conditions, to exceed the displacement capacity of CSS device and the structural integrity or compromise the serviceability of the structure. In this study, some hundreds of nonlinear time-history analyses of SDOF systems were conducted within an extensive parametric study aimed to investigate the effects of a non-centered initial position on the CSS response, in terms of maximum and residual displacements. Five different initial offsets were considered coupled with a wide range of devices and earthquakes, characterized by different values of the isolator design parameters and characteristics of the ground motion. Twenty different CCS isolators were considered varying five radii of curvature (from 2 200 mm to 5 000 mm) and four friction laws, covering the current design practice. Twenty-four natural ground motion time histories were selected from a database and classified in terms of the predominant period of the quake and the level of impulsivity (to this scope, a “Pulse Index” was formulated based on the rate of transmission of the kinetic energy). The effects of an initial offset are discussed herein in terms of maximum and residual displacements of the isolators. The displacement responses obtained in presence of the offset are eventually compared with the relevant response provided by the isolation system that moves from its centred configuration

Numerical modelling of heating effects in curved surface sliding isolators

The Curved Surface Slider, also known as the Friction Pendulum System, has become in the last years a very popular antiseismic hardware for base isolation of buildings and structures. A potential issue for sliding isolation systems is the degradation of the coefficient of friction caused by the temperature growth within the bearing due to the dissipation of the seismic energy as frictional heat. Both experimental and numerical investigations have pointed to the importance of the issue, and models accounting for the temperature dependence of the coefficient of friction at the material level have been recently proposed, which can be used in finite element analyses of the whole isolator. In this study, a three-dimensional finite element model of a Curved Surface Slider unit developed by the Authors is used to investigate in detail the influence of the path of motion on the temperature growth. In the first part, the finite element formulation and its validation are presented. The generation of frictional heat is reproduced in the model by locating a heat source on the surface of the sliding pad, with intensity of the heat flux depending on the coefficient of friction, the axial pressure, and the velocity. The coefficient of friction at the material level is routinely adjusted by the software at each calculation step on the current levels of pressure, velocity and temperature. In the second part, either unidirectional and multidirectional displacement-controlled orbits are challenged in finite element thermal-mechanical analyses in order to investigate the temperature growth inside the bearing and the relevant changes in the mechanical response of the device. The result point to the unsuitability of the unidirectional trajectories performed in the tests prescribed in current standards to reproduce the temperature rises that may possibly occur within the Curved Surface Slider unit under more general multidirectional orbits typical of real earthquakes

Effect of friction on the re-rentring capability of sliding bearings with curved surfaces

A complete understanding of the mechanical response of sliding bearings with curved surfaces, like e.g. the Friction Pendulum SystemTM, has not yet been achieved. Among the arguments under debate is their ability to present small residual displacements at the end of the ground motion, and the possible accrual of displacement in presence of an initial offset displacement. Current re-centring criteria provided in the standards were indeed formulated for low-damping bilinear hysteretic systems, and their suitability for curved sliding bearings, which present an inherent nonlinear behavior due to the effect of friction, has been never deepened. In this research, the re-centring capability of curved surface sliding bearings is investigated in shake table tests performed on a structural mock-up consisting of a single story steel frame, scaled at one-third length and isolated with four bearings. Three bearing surfaces lining materials were considered in the study, with design coefficient of friction of 2%, 5% and 10% respectively. A detailed characterization of the sliding behavior of the lining materials was performed on a custom biaxial bench and the variation of the coefficient of friction with velocity was assessed over a range from 1 to 200 mm/s. The base isolated mock-up was tested under a set of seven natural ground motions in two series of tests: in the first series, the bearings were undeformed at the beginning of each ground motion time history, whereas in the second series an offset displacement was imposed before the run of the ground motion. The results of the tests were analyzed in terms of maximum and residual displacement of the isolation system. Additionally, the effect of the static coefficient of friction of the lining material on the response of the isolation system, the level of ground acceleration that promotes the breakaway, and the acceleration induced in the superstructure were assessed. It is eventually concluded that the re-centring criteria provided in the codes may be not conservative in presence of high friction forces developed by the isolators; an offset displacement may affect the maximum displacement during the earthquake for system with low re-centring capability; the amplification of ground acceleration through the isolated structure may be even substantially affected from the static coefficient of friction developed before the breakaway

Response of Curved Surface Sliders under bidirectional displacement orbits

The dynamic performance of Curved Surface Sliders (CSS) used in base isolation is affected from the generation of heat at the sliding surface occurring at high forces and speeds, and the consequent temperature rise that promotes the degradation of the coefficient of friction. The study presents a numerical investigation of the heating effect in a typical Curved Surface Slider. A 3D finite element thermal-mechanical model of the CSS is formulated, and numerical analyses are performed considering either unidirectional and bidirectional displacement-controlled orbits to investigate the temperature growth inside the bearing and the changes in the mechanical response of the device. A recursive algorithm is used to adjust at each iteration step the coefficient of friction based on the calculated temperature. The results demonstrate the fundamental importance of accounting for the temperature growth for a correct determination of the response of the Curved Surface Slider, and point to the inability of unidirectional trajectories performed in the tests regulated in the current standards to reproduce the temperature rises that may possibly occur under bidirectional orbits during real earthquakes