Mechanical behaviour of rubber blocks

This study investigates the behaviour of rubber blocks bonded between two plates under combined compression and shear loading, using experimental and numerical analyses, and also approximate analytical theories. First, experimental data from a series of compression and shear tests of rubber blocks with different aspect ratios are presented. Next, numerical simulations are carried out with three-dimensional finite element (FE) models, allowing insight to be gained into the stress and strain fields within the blocks. Existing analytical theories for blocks under compression and combined compressive and shear loading are then reviewed, and their accuracy is evaluated against test and numerical results. The study shows that those theories accounting for the effect of the axial shortening of the blocks provide a better description of the combined compression and shear behaviour, compared to theories, developed for laminated structural bearings with many thin rubber layers, that ignore this effect. An improved theory is also proposed, which better describes the effects of the bulging of the compressed blocks on their shear and flexural parameters and provides a better fit to experimental and numerical results

Mechanical behaviour of rubber bearings with low shape factor

This study investigates the mechanical behaviour of elastomeric bearings with a low shape factor (LSF). Such bearings can offer an effective solution for three-dimensional seismic isolation of structures, that is, isolation in vertical as well as horizontal directions. They could also be employed for developing low-cost isolation systems for developing countries due to their reduced weight and manufacturing cost. The first part of the study describes tests carried out at Tun Abdul Razak Research Centre (TARRC) on low-damping rubber double-shear test pieces and LSF bearings. The material tests are used to inform the development of a finite element (FE) model of the bearings, which is validated against the bearing test results. It is shown that the proposed FE model can be used to describe accurately the global non-linear horizontal force-displacement behaviour of the compressed bearings, while providing an insight into the local distribution of stresses and strains. It can also be used to investigate the bearing response under boundary conditions that differ from the one considered in the tests. The second part of the study illustrates the numerical simulations of shaking table tests carried out at the University of Naples Federico II on a structural prototype mounted on the low-damping LSF bearings. Useful insights are provided into the effect of the vertical bearing flexibility on the response and the attainment of critical conditions of zero tangent horizontal stiffness under horizontal displacements

Rolling-Ball Rubber-Layer isolation system: state of the art, performance and design procedure

A rolling-ball rubber-layer (RBRL) isolation system was developed at TARRC to enable isolation of lightweight structures. The system is very versatile, a great range of equivalent natural frequencies and coefficients of damping being achievable through the independent choice of rubber spring and rubber-layer rolling track. It is suitable for isolating light structures, and much more effective at low excitations than an equivalent sliding system would be. In this paper the state of the art and the dynamic behaviour of RBRL isolation system will be restated. Subsequently, a simple and efficient procedure will be described for the design of the system: this is aimed to get the principal value of the system parameters to meet the chosen values of isolation period and damping ratio. In particular, it will be emphasized that a certain value of rolling resistance of the device could result from different combinations of the device parameters, thus leaving the final specification to be made on the basis also of preferences regarding small-deflection behaviour and cost. Finally, a future application of the isolation system for the seismic protection of a statue is presented and discussed

Dynamic behaviour and seismic response of structures isolated with low shape factor bearings

This study investigates the mechanical behaviour of laminated elastomeric bearings with a low shape factor (LSF) and the dynamic response of structures mounted on them. Axial loads have a significant influence on the mechanical behavior of the LSF bearings. Most of existing theories and mechanical models for laminated bearings cannot be employed for LSF bearings because they disregard the important effects of axial shortening and bulging of the rubber layers on the horizontal bearing stiffness. In this study, a simplified model originally developed for slender rubber blocks is employed for describing the mechanical behavior of LSF bearings, and validated against the experimental results on low-damping LSF bearings manufactured and tested at Tun Abdul Razak Research Center (TARRC). The proposed model is then used to simulate the seismic response of a structural prototype mounted on the low-damping LSF bearings and tested at University of Naples Federico II on a shaking table under horizontal seismic input. Further analyses are carried out to evaluate how the bearing shape factor affects the dynamic and seismic response of the prototype. The study provides some useful insight into the complex mechanical behavior of LSF bearings and of structures mounted on them

Rolling-Ball Rubber-Layer Isolation System – small deflection and vibrational behaviour

The small-deflections behaviour of the Rolling-Ball Rubber-Layer (RBRL) seismic isolation system is investigated through numerical analyses, performed on the test results of a previous shaking-table experimentation, and by monoaxial sinusoidal tests specifically carried out at TARRC. The efficacy of the RBRL system in seismic mitigation is shown even for seismic events that induce only small deflections across the isolators (i.e. < 5mm). This good performance is due to the indentations developed by the balls in the rubber track under static load, caused by relaxation phenomena of the rubber. This peculiarity makes the system much more effective at low excitations than an equivalent sliding isolation system. Other useful conclusions were obtained from tests about the influence of: load on each ball, type of rubber and dwell time under static load. Finally, the results show a good retention of performance of the system when retested 15 years after its manufacture. The system, suitable for isolating light structures, is relatively economical and is easy to tailor for the specific case, in terms of geometry and performance, a great range of equivalent natural frequencies and coefficients of damping being achievable

Effectiveness of the RBRL isolation system: evidences from seismic tests and numerical simulation

The Rolling-Ball Rubber-Layer (RBRL) system was developed to enable seismic isolation of low-mass structures, such as works of art or special equipment, and is very versatile, a great range of equivalent natural frequencies and coefficients of damping being achievable through the independent choice of the system parameters. The paper presents new results from a previous campaign of shaking-table tests (PSTRBIS ECOEST 2 Project, 1999), related to a superstructure model consisting of two concrete slabs separated by four M16 studs 500mm long, which give a first mode fixed-base response at about 2.5 Hz. In particular, attention is given not only to the global behaviour of the system, which includes the steady-state rolling, but also to its small-deflections behaviour, influenced by the creation of pits in the rubber layer due to its viscoelastic properties. These experimental results are compared to those obtained from numerical simulations, conducted in OpenSees, using a FE fixed-base model previously calibrated using other shaking-table tests performed at fixed-base. These comparisons, isolated (test) versus fixed (model) case, are presented in terms of peak values of acceleration and inter-storey drift, time-history accelerations and displacements and by means of response spectra ratios for both the slabs, and show the effectiveness of the system not only at large displacement but also for small deflections if compared with an equivalent sliding isolation system. Attention is here restricted to uniaxial behaviour. Finally, some considerations are made regarding a possible characteristic frequency of roll-out of the balls from their initial pits

Isolation of light structures with Rolling-Ball Rubber-Layer system - characteristics and performance

The characteristics of the Rolling-Ball Rubber-Layer (RBRL) seismic isolation system are presented through results for force versus displacement, covering a range of amplitudes and varying sinusoidally with time, and through results for the acceleration and drift of the upper slab of an isolated model SDOF superstructure subjected to seismic excitations. It is shown how these characteristics may be described approximately by equivalent linear viscoelastic parameters K/ and K//, or alternatively K* and δ, these being functions of frequency and amplitude. This may be thought of as a frequency-domain approach. Alternatively, they may be described approximately using a non-linear time domain model, and two alternative ones are assessed here. The first has been presented previously, and a new one is presented for the first time. An objective way of comparing the accuracy of such time domain models is to compare the equivalent linear viscoelastic parameters extracted from their predictions for sinusoidal excitations, and this reveals that the new model agrees considerably better with the directly measured behaviour of the actual system. The system is very versatile, a great range of equivalent natural frequencies and coefficients of damping being achievable through the independent choice of rubber spring and rubber rolling track layer. It is suitable for isolating light structures, and much more effective at low excitations than an equivalent sliding system would be