Curved Surface Sliders Research Articles

Seismic mitigation analysis of base-isolated structure with sliding magnetic bearings considering local-site conditions

Base-isolation systems applied in building structures and infrastructures often suffer from damage even failure during strong earthquakes. The adaptability of isolators to local sites is a main concern for the robust design of base-isolation systems. In this study, seismic mitigation analysis of base-isolated structure with newly-developed sliding hydro-magnetic bearing (HMB) and sliding implant-magnetic bearing (IMB) considering local-site conditions is carried out. Extension of modeling of sliding magnetic bearings is first conducted based on the data of shaking-table tests of a reduced-scale base-isolated structure. To assess the instantaneous frequency for quantifying sliding magnetic bearings’ resisting force, the Hilbert–Huang transform (HHT) is applied. The influences of local-site conditions upon mitigation performance of the sliding magnetic bearings are then addressed, including far-field and near-field, ground motions with and without velocity pulses, and hard-soil and soft-soil. For comparison purposes, the seismic mitigation analysis of the base-isolated structure attached with lead rubber bearing (LRB) and curved surface slider (CSS) is carried out as well. Numerical results show that the sliding magnetic bearings outperform the state-of-the-art seismic isolators in both seismic mitigation and deformation constraint, and the IMB has excellent applicability for engineering since its perfect adaptability to local sites. However, the LRB cannot achieve the desired seismic mitigation under near-field pulse-like ground motions.

Seismic Fragility Reduction for Base Isolated RC Frame Buildings by Curved Surface Sliding Bearings with Over-Stroke Displacement Capacity

ABSTRACT Recently, experimental studies on failure conditions of buildings equipped with curved surface sliding isolators have shown that when no displacement restraining elements are employed and the concave plates feature a flat rim, the inner slider can run on the edge of the sliding surfaces producing lateral displacement larger than the nominal isolator capacity. The over-stroke displacement capacity reduces the probability of seismic collapse of code-conforming base-isolated buildings for earthquake stronger than the design one. In order to quantify the benefit of the over-stroke displacement capacity of double concave curved surface slider (DCCSS) bearings on the seismic fragility of base isolated buildings, four case studies of six-storey reinforced concrete framed structures, consisting of new constructions and retrofit of existing structures located in high and medium hazard seismic sites, have been investigated in this paper. In all cases, two configurations of the isolation system have been considered, with end-stop displacement or with over-stroke displacement capacity. The seismic performance of the buildings has been investigated by multi-stripe nonlinear time-history analysis. The results of the nonlinear dynamic analysis at the collapse limit state have been compared with nonlinear static analysis in terms of maximum displacement and corresponding base shear. Fragility curves highlight a higher safety margin against collapse for seismic intensities beyond the design limit state of the isolation system with over-stroke capacity.

Friction coefficient of sliding isolators in icing conditions

This paper investigates the variation in static friction coefficient of a double curved surface slider (DCSS) due to low temperature exposure. In the experimental campaign, a full-scale DCSS was subjected to displacement controlled cyclic motions after exposure to different temperatures (20, 0 and −20 °C) for different levels of exposure times (3, 12 and 24 hrs). Tests were repeated for several normal stresses (40, 60 and 80 MPa) and maximum loading velocities (250, 500 and 750 mm/s). Force-displacement curves obtained from the tests were used to calculate static friction coefficients. It is revealed that exposure of DCSS to low temperature leads to formation of an ice layer on the sliding surface. The corresponding sliding is no more a function of properties representative of dry-dry friction, but the ice layer behaves as a lubricant. Thus, static friction coefficient decreases in case of an ice layer on the sliding surface.

Shaking table tests of a full-scale base-isolated flat-bottom steel silo equipped with curved surface slider bearings

This paper presents a series of shaking table tests on a full-scale flat-bottom steel silo filled with soft wheat, under isolated-base conditions. The tested specimen is a 3.64 m-diameter 5.50 m-height corrugated-wall cylindrical silo, representing the smallest manufactured silo available in the catalogue of an Italian commercial silo provider. Curved Surface Slider isolators were introduced between the shaking table and the reinforced-concrete plate, on which the silo was mounted, to realize the isolated configuration where the main results have been investigated and compared with other results for the same silo specimen under fixed-base conditions. A detailed description of the silo components, the filling bulk material properties as well as the test setup is provided, including the full testing protocol. Multiple sensors were used to monitor the dynamic response of the filled silo system, including accelerometers, pressure cells, LVDT displacement transducers. Numerous unidirectional dynamic tests were conducted consisting of random signals, sinusoidal inputs, pulse-like inputs, and both artificial and real earthquake records. The results were processed to evaluate the performance of the isolators and their effectiveness on the silo response in terms of measured accelerations and dynamic overpressures. The efficiency of the isolation on the reduction of both acceleration amplifications over the silo wall height as well as the captured dynamic overpressures were detected in the range 30%–80% depending on the input type and magnitude.

Remarks on the experimental behaviour of curved surface sliders of the new Polcevera Bridge in Genoa

This work presents the results of the procedure of certification and acceptance for the sliding behavior of non-standard devices curved surface sliders, used for the seismic isolation of the New Polcevera Bridge. Indeed, the deck is seismically isolated from the piers. The supporting scheme withstands the seismic actions, wind actions and thermal actions, very relevant in a 1067 m length bridge without middle expansion joints. The paper aim is to describe the approach followed during the construction supervision process for the functional certification and acceptance tests of the bearing devices coherently with Italian and European codes. The paper highlights the main issues encountered during the application of the different specifications. A description of the different functional tests required during the construction supervision process is presented as well as an overview of the different acceptance criteria of the tests results. The intent is to prepare a series of useful recommendations for the definition of the seismic acceptance tests for bearing devices for highway bridges in Italian and European contest. Finally, all the results regarding the static and dynamic frictional coefficients, have been used to derive regressions laws, by also comparing them with the outcomes of previous literature studies.

Seismic vulnerability assessment of steel storage tanks protected through sliding isolators

AbstractAn adequate seismic protection of steel storage tanks of industrial, chemical and petrochemical plants is essential since potential consequences of failures, besides the economic losses, can be catastrophic for the environment and for people living nearby. Among the most common damages suffered by steel tanks in recent earthquakes, there are the elephant‐foot‐buckling of the perimeter wall and the tanks’ uplift or overturning. Due to the uncertainties related to the estimation of the mechanical properties of filled tanks and related seismic loads (i.e. dynamic overpressures generated by the content), conventional design approaches may lead to unsafe or, on the contrary, oversized solutions. Among effective alternatives, base isolation through Curved Surface Slider isolators (CSSs) offers some valuable advantages like high load bearing and displacement capacity with compact dimensions, independence of the oscillation period on the supported mass, and minimisation of torsional effects. Despite being neglected by most common commercial FEM codes, as a drawback, recent studies pointed out that the initial peak of friction force at CSSs’ motion breakaway can lead to significantly increased lateral deformations and accelerations of the superstructure. Within this framework in this study, a refined numerical model is developed in OpenSees FEM code, and an exhaustive parametric study is carried out to investigate the effect of the breakaway on isolated tanks providing some useful insights for the selection of the most suitable isolators.

Experimental Investigation of Wear Effects on the Friction Coefficient of a Curved Surface Slider: Comparison of Small- and Full-Scale Tests

ABSTRACT The current practice to determine friction properties of sliding isolation systems is to perform displacement-controlled dynamic tests where the effects of several loading conditions with different amplitudes, loading velocities, and vertical forces are of concern. The number of cycles in these tests is generally three for most of the loading conditions assumed to be representative of seismic conditions. On the other hand, the isolator may undergo small-amplitude but large-cycles of service motions due to thermal deformations and accidental loads. In that case, the methodology requires to perform cyclic tests on small-scale samples of the slider material (not the isolator) considering very large accumulated sliding distances such as 1000 m for use in buildings, and 10,000 m for use in bridges. However, two unsettled issues are regarded: (i) the suitability of small-scale slider tests to estimate the characteristics of full-scale sliding isolation systems and (ii) the change in seismic performance of a sliding isolation system after it is subjected to a large amount of accumulated sliding distance. In the present study, both a full-scale double concave friction pendulum (DCFP) and small-scale sample composed of the slider of the DCFP are subjected to a total accumulated sliding distance of 1000 m. The corresponding variations in friction properties are comparatively presented, demonstrating that small-scale test results provide reasonable estimates for the friction properties of the tested full-scale DCFP. Moreover, the seismic response of the investigated DCFP remains almost the same regardless of the considered sliding distance.

Seismic performance of a clutched inerter for structures with curved surface sliders

The performance of the clutched inerter in limiting the seismic response of structures isolated by curved surface slider (CSS) is investigated. The chosen structure is a flexible multi-floor building with an idealized linear shear-type behaviour. The equations governing the motion of an isolated structure with a clutched inerter under earthquake excitation have been derived. Initially, using the equivalent linearization technique, the isolated building's stationary floor absolute acceleration, isolator displacement, and device forces are evaluated. The stochastic earthquake excitation is considered to be a filtered white-noise Kanai-Tajimi model. The performance of the clutched inerter in mitigating the response of the isolated structure is studied by varying system parameters. The important parameters included are the inertance ratio of the clutched inerter, the period and friction coefficient of the CSS, and the flexibility of the superstructure. Further, there exists an optimum inertance of the clutched inerter for which the structure's highest floor absolute acceleration reaches its minimum level. The influence of important system parameters on the optimum inertance of the clutched inerter is also investigated. Finally, the performance of the clutched inerter as a supplemental device for the base-isolated structure is investigated when subjected to recorded earthquake motions consisting of near-fault as well as far-field types. The trends of results of isolated structures with clutched inerter under the recorded earthquake motions were in good agreement with that under stochastic excitation. It is demonstrated that the supplemental clutched inerter is effective in reducing the displacement response of base-isolated structures.

Experimental Validation of Fast Design Rules for Curved Surface Slider Devices through the Hybrid Simulation Technique

In this work fast design procedures for a get-started definition of the seismic isolation system for buildings have been validated through the analysis of the outcomes of experimental hybrid tests. More specifically, earthquake simulations have been performed, by adopting both numerical and physical substructures, represented by an equivalent Multi Degree of Freedom for the building and a full-scale Double Curved Surface Slider isolator for the whole isolation system. A suite of seven natural seismic events has been selected, by ensuring the proper spectrum compatibility with respect to the hazard of the considered case study structure, in terms of acceleration response spectrum. Special attention has been focused on global results of the isolation layer, in terms of displacement and total force responses, together with the evaluation of the effective protection of the superstructure against eventual plastic deformation demands. The response parameters returned by both the single-events and as average values among the suite of selected events have been compared to the performance initially estimated through fast design rules. Furthermore, Non-Linear Time History Analyses have been performed, by adopting the same analytical model of the building used within the hybrid tests algorithm, for sake of comparison between numerical and experimental simulations.

Mechanical model of the over-stroke displacement behaviour for double concave surface slider anti-seismic devices

For double concave curved surface slider (DCCSS) isolators with a flat rim and lacking restrainers, such as those most commonly used in Europe, the rigid slider can exceed the geometrical capability of the housing plate during earthquakes stronger than those produced in simulations. During this over-stroke displacement, DCCSSs preserve the ability to support superstructure gravity loads and the capacity to dissipate energy. There are currently no applicable hysteresis rules or available algebraic solutions that can be used to predict over-stroke behaviour for response-history analysis. This study presents an algebraic solution to extend basic theories for estimating the actual limit displacement of DCCSS devices with over-stroke capacity. DCCSS behaviour in the over-stroke sliding regime was modelled with a focus on geometrical compatibility and kinematics. The proposed analytical formulation was calibrated on the basis of experimental controlled-displacement tests performed on single DCCSS devices. A case study of a six-storey reinforced concrete frame isolated building was modelled using a combination of non-linear elements that are currently available in several structural analysis software packages and able to correctly model over-stroke displacement behaviour for non-linear time history analyses. The DCCSS model was augmented with a friction model capable of accounting for torsional effects, axial load, and velocity variabilities. Comparison with non-linear dynamic analysis outcomes shows that the forces and displacements in the over-stroke sliding regime are predictable and therefore useful for the designer.

On optimal triple friction pendulum base-isolation design for steel moment-frame buildings employing value-based seismic design methodology

This paper uses the value-based seismic design methodology to optimize the mechanical and geometrical properties of triple friction pendulum bearings (TFPBs). An optimization procedure is proposed within this methodology. Initial construction cost and lifetime seismic losses are picked as the value components. Extreme failure conditions of the TFPB, less considered in the current literature, such as the uplift and exit failure modes of the curved surface sliders at the end edges, have been carefully considered in the seismic behavior of the structure. Besides, the minimum code provisions and practical limitations (e.g., the critical overturning diameter of sliders) for friction pendulum isolated buildings are regarded as design constraints. Moreover, a new cost function for TFPBs is developed from real projects of isolated buildings. The FEMA P-58 procedure is exploited to evaluate the lifetime seismic repercussions, comprising the reparation cost, reparation time, injuries, and fatalities. The endurance time method is incorporated in the FEMA P-58 procedure to estimate the structural seismic responses. A set of three buildings are chosen for evaluation, representing low-to mid-rise isolated moment-resisting frames (MRFs) with TFPBs. Each building is optimally designed with two approaches: the value-based design (VBD) via maximizing the total building value and the code-based design (CBD) via minimizing the construction cost. It is found that the total value of each isolated MRF building optimized using the CBD approach is moderately acceptable. Nevertheless, it can be notably increased using the VBD approach since it sufficiently diminishes the probability of the uplift and exit failure modes at the end edges of TFPBs.

Experimental Hybrid Simulation of Severe Aftershocks Chains on Buildings Equipped with Curved Surface Slider Devices

In this research work the outcomes of a hybrid experimental campaign are analyzed, in order to evaluate the influence of aftershock events on the frictional response of sliding-based isolation devices for buildings. To achieve this, a hybrid testing framework was accordingly defined, by considering a numerical substructure, in terms of a simplified analytical model of a case study structure, and a physical substructure, as a full-scale Curved Surface Slider device, tested within the Bearing Tester System of the EUCENTRE Foundation Laboratory in Pavia (Italy). The tested isolator was equipped with a special sliding material, made up of a Poly-Tetra-Fluoro-Ethylene-based compound (PTFE), filled with carbon fibers and with a solid lubrication. The hybrid tests were performed, in terms of earthquake simulations, and the response of the base-isolated structural system was computed, by applying single-events, rather than aftershock chains. Results lead to a better understanding of the behavior of sliding-based seismic isolation systems, characterized by medium-to-high tribological properties, in terms of peak and residual displacements for both the single-event and the mean responses. Specifically, this work provides hybrid experimental evidence of the influence of an initial displacement offset on the overall behavior of the considered structural system.

Definition and Validation of Fast Design Procedures for Seismic Isolation Systems

The research on traditional and innovative seismic isolation techniques has grown significantly in recent years, thanks to both experimental and numerical campaigns. As a consequence, practitioners have also started to apply such techniques in real applications, and nowadays, seismic isolation is widespread in regions characterized by a high level of seismic hazard. The present work aims at providing practitioners with very simple procedures for the first design of the isolation devices of a building, according to the most common typologies of isolators: Rubber Bearings, Lead Rubber Bearings and Curved Surface Sliders. Such Fast Design Procedures are based on simplified approaches, and the mechanical properties of the implemented devices can be obtained by assuming a performance point of the overall structural system, namely effective period and equivalent viscous damping. Furthermore, some important parameters are defined, according to the outcomes of a statistical analysis of the test database of the EUCENTRE Foundation in Italy. Finally, results of a validation study have been provided by analyzing a case-study structure through a Multi Degree of Freedom oscillator and a full 3D Finite Element model.

Mitigation of the Seismic Impact on Storage Gas Tanks by Using Isolation System

The paper presents the studies carried out on a big storage gas tank for the seismic vulnerability assessment and the retrofit through isolation system. This Gasometer, constructed in the early eighties, consists in a steel cylinder of 70 m height and 44 m diameter. The tank bearing structure is composed by a classical steel frame closed with curved steel plates. Inside the tank, a piston-fender and rubber sealing system allow the variation of the container volume and control the gas pressure. The latter represents half of the seismic mass and its position can vary along the tank height. The seismic vulnerability of the steel structure of the Gasometer on its as-it-is state is assessed through the execution of Incremental Dynamic Analyses, placing particular attention on the definition of possible limit states. Subsequently, the seismic retrofit through base isolation system, adopting Double Curved Surface Sliders, is proposed and numerically assessed. The results and comparison between the as-it-is state and the retrofitted state supplies important information about the effectiveness of base isolation for the risk mitigation of gas tanks.

Estimation of the instantaneous friction coefficients of sliding isolators subjected to bi-directional orbits through a nonlinear state observer

One of the most challenging task for earthquake engineers is the accurate prediction of the dynamic response of structures implementing sliding seismic isolators, like the curved surface sliders. Indeed, the force–displacement behaviour of these devices strictly depends on the coefficiens of friction developed at the two sliding surfaces whose values vary instantaneously depending on variable compression load, sliding velocity, and contact temperature developed during the seismic motion. However, only the overall (or effective) friction coefficient of the isolator can be estimated, as weighted average of the values at the two sliding surfaces, through tipycal displacement controlled prototype experimental tests. This information is not suitable for the calibration of predictive isolator models (e.g. FEM analyses) or to characterize the tribological behaviour of potentially different friction pads at the two sliding surfaces. In this paper, an estimation approach, based on a Constrained Unscented Kalman Filter (CUKF) integrated with a Random Walk Model technique (RWM), that is capable to identify the two distinct friction coefficients, and their time-variations during displacement-controlled test is presented. The proposed tool allows the identification of the distinct frictional properties without any a-priori knowledge about the design properties at the two sliding surfaces that can widely differ in terms of adopted sliding materials, presence of lubrificant, and radius of curvature. The developed tool is firstly validated through the comparison with the results of FEM analyses and then applied on experimental tests carried out on a full-scale isolator prototype demonstrating its suitability for the assessment of the actual friction coefficients.

Multibody Kinematics of the Double Concave Curved Surface Sliders: From Supposed Compliant Sliding to Suspected Stick-Slip

AbstractThe double-concave curved-surface slider isolators are tribological systems composed of the assemblage of three rigid bodies that include two concave spherical plates and a straight cylinde...

Shake table testing and numerical modelling of a steel pallet racking structure with a seismic isolation system

This study aims to evaluate the performance of a steel storage pallet racking system equipped with a novel seismic isolation system, which is effective along the cross-aisle direction alone. The isolation system consists of two IsolGOODS® isolators, which are curved surface slider bearings as framed into the European Standards for anti-seismic devices. The efficiency of the proposed device in reducing the seismic effects on pallet racks is investigated. Firstly, uniaxial real-time shake table tests are performed, showing a great reduction of floor accelerations. With the help of a herein validated numerical model, advanced knowledge is obtained. Six accelerograms, two normal-fault records, two pulse-like, and two spectrum-compatible series are selected for drawing generally valid statements. As proposed in this work, the outcomes confirm that the isolation system provides substantial reduction in terms of floor accelerations. In contrast, no up-lift of the base-connections is recorded: the uprights are always engaged in compression. On the whole, the most severe condition comes from the pulse-like record (Emilia 2012), in which displacement demand must be carefully considered while defining the isolators to be put in place.

Preliminary study on the threshold stiffness and residual displacement of the multangular-pyramid concave friction system (MPCFS) for seismic isolation

In this research, the threshold stiffness and residual displacement of the MPCFS are both investigated. The MPCFS has a higher threshold (breakaway) stiffness and no residual displacement after earthquakes or ambient vibrations, which makes it different from the conventional Curved Surface Slider (CSS). These two features can enable the MPCFS to be more stable when experiencing micro-to-small shakings, and always restore to its central point after earthquakes. With the aim of testifying the two features, a series of analytical simulations are conducted on a four-storey building model equipped with MPCFS. The analytical results are compared with that obtained with CSS. The simulation results validate the aforementioned virtues of MPCFS over the CSS. This indicates that MPCFS has great potential in the engineering practice of seismic isolation.

Adaptive Curved Surface Slider for Improved Seismic Structural Performance

AbstractThe design of conventional curved surface sliders (CSSs) uses appropriate selections of effective radius and friction coefficient. These two design parameters are commonly optimized for the...

Onset of Motion of Curved Surface Sliders Used in Seismic-Isolation Systems

Curved surface sliders are generally preferred for the seismic isolation of buildings because of their technical features, such as the fact that the value of the vibration period is theoretically independent of the mass. Given the experimental evidence that indicates that the isolated system did not work under low seismic actions, in this paper, the influence of the static friction on the global behavior of some structures is studied in detail. By adopting a suitable model of friction, the onset of motion is analyzed with reference to simple isolation systems in which the influence of the friction values and its variability among the isolation devices are analyzed. The results pointed out that the behavior of curved surface sliders is not independent of the vertical load and highlights the importance of a suitable choice of the design friction, which must be different for devices with different vertical loads.