Curved Surface Sliders Research Articles

The Multangular-Pyramid Concave Friction System (MPCFS) for seismic isolation: A preliminary numerical study

The Multangular Pyramid Concave Friction System (MPCFS) is introduced in this study. Different from the conventional Curved Surface Slider (CSS), the concavity of the MPCFS is designed as a multangular (mostly triangular or rectangular) pyramid, and the slider is removed from the isolator. While maintaining the virtues of the CFS, the MPCFS can provide greater vertical sustaining capacity with a more stable friction force. When combined with a cross uplift restraint, the MPCFS offers numerous virtues such as increased uplift stability, a greater initial yielding force, and resonance-dodge ability during near-fault earthquakes (variable frequency). Frequency-sweeping and acceleration time-history numerical investigations are conducted on a four-storey building model using the MPCFS. The revised State-Space Method (SSM) is expanded from two-degrees-of-freedom to multi-degrees-of-freedom as to accommodate more complex analysis such as dynamic analysis of high-rise, multi-storey structure installed with MPCFS. The simulation result verifies the merits of the MPCFS, indicating that the MPCFS, viewed as a beneficial complement to the existing well-established isolation techniques, may be a promising tool for the seismic isolation of high-rise, and super high-rise buildings in near-fault earthquake prone zones.

Base Isolation of Buildings with Curved Surface Sliders: Basic Design Criteria and Critical Issues

Curved surface sliders are being used more and more in the seismic isolation of buildings. They are preferred not only because of their lower cost with respect to elastomeric isolators but also of their technical characteristics, such as the fact that the value of the period of vibration is independent of the mass and the automatic coincidence between the gravity mass center of the superstructure and the stiffness center of the isolation system. In this paper, these features are analysed with reference to simple structures, pointing out the possibility of rotations of the superstructure and the loss of contact in some devices. Finally, the importance of the static friction is also emphasized showing the experimental seismic response of an isolation system under a low‐energy earthquake. For all these reasons, the use of nonlinear analysis, revised and detailed in this paper, is advisable for the isolation system made of curved surface sliders.

Shaking table test of a four-tower high-rise connected with an isolated sky corridor

A 4-tower high-rise building connected with an isolated sky corridor on the top is designed in the seismic region of China. The 300-meter long sky-corridor bridges the four 230-m-high towers at the top floor. The seismic design of the building is challenging due to the structural complexity. Passive control strategy is employed to reduce earthquake responses and member forces of the towers and the sky corridor. Connections between the towers and the sky corridor are designed as flexible links. Curved surface sliders (CSSs) and viscous dampers (VDs) are installed at the connection joints. The characteristics of the seismic isolation system should remain unchanged during the service life, and the CSSs shall be protected strictly from humidity and dust. To study the seismic performance of the 4-tower connected structure, a 1/25 scale model is tested by shaking table tests subject to minor, moderate, and major earthquake. According to the Chinese code, peak ground accelerations subject to the 3 levels are specified as 0.025, 0.07, and 0.175 g. Eight earthquake records with different frequency spectrum properties were selected to test the model structure. Detailed dynamic similitude design of towers, CSSs, and VDs are described. The maximum acceleration and deformation responses of the towers and the sky corridor are measured, as well as the seismic performance of the CSSs and VDs, the dynamic characteristics, and the cracking pattern of the building. Results show that no serious damage occurs on the 4-tower connected structure. The protective system that consists of the CSSs and VDs reduces the seismic responses of the sky corridor. The sky corridor keep in elastic state under the high-intensity earthquake.

Experimental dynamic response of spherical friction-based isolation devices

ABSTRACTThis article presents a summary of the experimental dynamic response of curved surface sliders as resulting from more than 1000 dynamic tests on about 60 different types of devices performed at the laboratory of the EUCENTRE foundation. Two different kinds of low-friction sliding materials have been employed in the devices, derived from polyamide (PA), or polyethylene (PE) materials.The rate of change of the dynamic friction coefficient as a function of sliding velocity and contact pressure is assessed and the transition phase between dynamic and static friction, with the consequent possible evidence of static/stick-slip phenomena, has been carefully considered.

Investigation of the Consequences of Mounting Laying Defects for Curved Surface Slider Devices under General Seismic Input

ABSTRACTExperimental studies performed on friction-based isolators have shown widely sparse frictional response, as direct effect of intrinsic properties of the devices and the vertical load variation. Moreover, in practical applications mounting laying defects consisting of uneven inclinations of the sliding surfaces with respect to the horizontal plane have been recognized as a potential cause of deviation from the response assumed in the design phase. In this paper mono and biaxial motions of a base-isolated RC building have been investigated; the aim of the work is to identify the effects of both laying defects and friction variation on the seismic response.

11.63: Seismic protection of gas tanks

ABSTRACTRecent earthquakes demonstrated the susceptibility of industrial plants to such events. In extreme situations, as the earthquake in Fukoshima (Japan), the impact of the seismic action on these infrastructures leads to catastrophic results with consequences for the environment, neighbour population and economy. The impact of an earthquake on an industrial plant may result in a local failure, leading to an interruption on the operation, or a total collapse. In any of these cases, it is imperative the improvement of such facilities with high importance class. It is to this end that the European project PROINDUSTRY is ongoing. The objective is the development of innovative seismic protection systems for industrial buildings or infrastructures made of steel. In the present paper the numerical study of a gas tank is presented. For this type of infrastructures, amongst the different devices under development, the base isolation system was selected as it is seen as the most suitable. The proposed device is the Curved Surface Slider (CSS). In order to evaluate the performance of such device, numerical studies, namely Incremental Dynamic Analysis, were conducted. The results of these analyses lead to a calibration of the main properties of this device, static friction and vibration period, and allowed to verify the effective protection of the gas tank upon an earthquake event.

Novel procedure for reliability-based cost optimization of seismically isolated structures for the protection of critical equipment: A case study using single curved surface sliders

The optimized reliability-based design of seismic isolated structures requires a consideration of the failure probability of the system, in addition to the optimization objectives. The stochastic nature of a given system (e.g., the probable input ground motion) must therefore be properly included in the analysis model. To address this challenge, a novel procedure is developed and examined on a 3-story isolated concrete building model, to minimize the total construction cost of the system with regards to protecting sensitive equipment located within the structure. In this procedure, the structure performance and reliability were first evaluated using time-consuming Monte Carlo simulations. We then employed artificial neural networks as a response surface to facilitate the prediction of the failure probability for the supposed structure. To simulate seismic excitations, we generated artificial ground motion combining random high-frequency and long-period components. Also, a newly developed sensitivity analysis method was used to identify the critical uncertain parameters of the system. Finally, by using a simulated annealing algorithm, we determined the optimal design variables of the structure and isolation system for a range of desired probabilities of failure. The optimal results indicate that, for different target failure probability ranges, some design variables are more significant than others.

“Total displacement of curved surface sliders under nonseismic and seismic actions: A parametric study”

Summary The re-centring capability is recognized as a fundamental function of any effective isolation system, not only because it is associated to small or negligible deformation at the end of the earthquake but rather because it prevents displacement build up that may limit the capability of the structure to withstand aftershocks and future earthquakes. The current Eurocode recommends to estimate the maximum total displacement of the isolated system as the superposition of the nonseismic offset displacement resulting from permanent actions, long-term deformations and thermal movements of the structure, and of the amplified seismic displacement induced by the design earthquake. For systems endowed with low re-centring capability, the estimation shall also account for the possible accrual of displacements during the lifetime of the structure. However, the aforementioned criteria have never been evaluated for curved surface sliders, which are characterized by an inherent nonlinear behaviour. The study aims at giving more insight into the matter by conducting a parametric study based on one-directional nonlinear response time history analyses and considering a variety of seismic scenarios. The first part of the study investigates the effect of a nonseismic displacement on the earthquake-induced displacement and formulates a criterion to evaluate the capability of curved surface sliders to provide a seismic response independent of the offset displacement. The response of the isolation system to natural sequences of earthquakes, where the offset displacement is the residual displacement from the previous shake, is addressed in the second part of the paper. The provisions of the Eurocode are eventually checked against the observed data.

Radial basis function neural network algorithm for semi-active control of base-isolated structures

Curved surface slider (CSS) is considered as an effective isolation device for structures subjected to earthquake ground motions. Due to constant frequency, CSS may encounter a resonance problem when subjected to near-fault earthquake ground motions. To overcome this problem, we propose CSS combined with a control device in this study. The control device consists of variable orifice fluid damper, and its damping coefficient is controlled by a radial basis function-based neural network algorithm. Numerical simulations are performed to evaluate the effectiveness of the proposed technique for only one-directional horizontal seismic excitations without any evaluation concerning the durability of CSSs. The results of the investigation demonstrate that the proposed technique is effective to reduce both the base shear and the sliding displacement of the isolated structure. In addition, the response predicted by the proposed technique is almost similar to the response of isolated structure with passive damper at optimum damping ratio.

Study of wire rope devices for improving the re-centering capability of base isolated buildings

Wire rope devices have found numerous applications in the shock and vibration isolation of industrial machinery and equipment. The aim of the work is to present a preliminary investigation of their usefulness as a mean to increase the re-centering capability of the isolation system of a civil structure. In the first part of the study, analytical formulations have been developed to predict the restoring stiffness of wire rope devices depending on their material and geometrical properties, and the direction of loading. In the second part, a preliminary assessment of wire rope springs has been performed based on unidirectional shake table tests on a scaled steel structure isolated with curved surface slider and either or not provided with wire rope springs. The changes in the seismic response of the isolation system are analyzed and some suggestions for improving the use of wire rope devices are drawn. Copyright © 2016 John Wiley & Sons, Ltd.

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

Summary 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. Copyright © 2016 John Wiley & Sons, Ltd.

Mutual seismic assessment and isolation of different art objects

New developments of a research programme dedicated to the seismic performance assessment and base isolation protection of art objects are presented in this paper. The investigation field has been extended from massive marble statues, considered at first step of this study, to slender free-standing columns, and bronze statues with highly vulnerable geometrical portions. A semicircular marble column and an equestrian bronze sculpture located in the main hall of the Restoration Laboratories of the Opificio delle Pietre Dure Institute in Florence are examined as representative case studies of the two classes, respectively. Rocking and sliding effects are simulated in the time-history assessment analyses of both artefacts, carried out by introducing all simultaneous components of seismic action as input. The results show an overturning-related near-collapse response of the column, and a plasticization-induced collapse of the statue, at the maximum considered earthquake level. In view of the different material and structural characteristics of the two artworks and several other art objects and equipments situated in the same hall, a mutual advanced seismic protection strategy is adopted, consisting in the base isolation of the floor by means of double curved surface sliders. This retrofit measure guarantees completely undamaged response conditions of the column and the statue, without requiring direct interventions on the two artworks. At the same time, the base-isolated floor constitutes a highly protective support for the hi-tech equipments housed in the Laboratories, as well as for any other artefact to be placed on it in the future.

Introduction of the convex friction system (CFS) for seismic isolation

This work introduces an innovative seismic isolation system named the convex friction system (CFS). This newly introduced isolation system has a sliding concavity with a circular cone-type surface, and exhibits some distinct features compared to conventional isolation techniques, such as increased uplift stability, improved self-centring capacity, and resonance dodge when subjected to near-fault earthquakes. A series of comprehensive analytical and numerical investigations are performed to verify these features of the CFS. First, the force–displacement relation of the CFS is established to describe the underlying philosophy of the system. The analytical model is then incorporated into numerical simulations to evaluate the seismic isolation performance of the CFS. Various ground accelerations, such as near-fault shakings, are included in these numerical calculations. Furthermore, the numerical results are rigorously investigated to illustrate the feasibility of the CFS. Finally, the limitations of the CFS study are discussed, and conclusions are drawn. The analytical and numerical results show that the CFS performs well in seismic isolation applications. The structural response can be reduced by approximately 30% with the CFS when compared to that with the Curved Surface Slider (CSS) with a spherical-surface concavity for some near-fault earthquakes, which verifies the aforementioned advantages of the CFS. Copyright © 2016 John Wiley & Sons, Ltd.

Seismic assessment and retrofit of two heritage-listed R/C elevated water storage tanks

A seismic assessment and advanced retrofit study on two heritage-listed reinforced concrete (R/C) elevated water storage tanks is presented in this paper. The two structures were built between the late 1920s and the early 1930s as water suppliers for a coal power plant in Santa Maria Novella Station in Florence, and are still in service. The first, taller tank has a R/C frame supporting structure and is currently used as water supplier for trains and platform services. The second, shorter tank, with a shaft-shell supporting structure, is used as water tower for the Station. The dynamic behaviour of the fluid is simulated by means of a classical convective and impulsive mass model, for which a discrete three-dimensional schematization is originally implemented in the finite element analysis. The time–history assessment enquiry highlights numerical collapse of the frame structure in the taller tank, and unsafe tensile stress states in a large portion of the shaft structure of the shorter one, under seismic action scaled at the maximum considered earthquake level. Based on these results, two retrofit hypotheses are proposed, and namely a dissipative bracing system incorporating pressurized fluid viscous spring-dampers, for the taller tank, and a base isolation system including double curved surface sliders, for the shorter one. The mechanical parameters, design criteria and technical implementation details of the two rehabilitation strategies are illustrated. The verification time–history analyses in protected conditions show that a substantial enhancement of the seismic response capacities of both structures is attained as compared to their original configurations, with little architectural intrusion, quick installation works and competitive costs.

Seismic isolation technique of marble sculptures at the Accademia Gallery in Florence: numerical calibration and simulation modelling

Isolation technique based on small-size double concave curved surface sliders was studied and applied to Michelangelo’s sculptures placed at the “Galleria dell’Accademia” in Florence, Italy. Starting from preliminary results carried out by some of the authors on such kind of devices, the effect of an enhanced calibration of friction parameters was presented and a number of numerical simulations of both experimental tests and real sculptures under different dynamic conditions were carried out. In particular the experimental tests results were analysed and compared with those obtained for the statues subjected to sinusoidal inputs. Then the sculptures in isolated conditions were subjected to a set of generated accelerograms, compatible with the design spectrum characterizing the Florence site and the main results of the analyses are presented and compared with the case of non-isolated systems.

Seismic risk mitigation technique for art objects: experimental evaluation and numerical modelling of double concave curved surface sliders

The opportunity and the advantages of applying base isolation technologies developed for civil structures to small objects are investigated and some considerations related to the scale effects are discussed. A series of experimental tests carried out at SRMD facility in San Diego are presented with the aim to investigate the dynamic response of a system designed to simulate a sculpture isolated by means of Double Concave Curved Surface Sliders. Such devices are specifically re-designed to fit the peculiar situation of light weight objects. In particular the samples used in the experimentation are designed with geometric and inertia characteristics fit for representing the Michelangelo’s sculptures exposed at the “Galleria dell’Accademia” in Florence, Italy. Finally, the main aspects concerning the numerical simulation of the seismic response of the proposed isolated system are discussed, also to investigate the efficiency of the existing numerical models, which are developed for traditional devices, when they are applied on such small and particular objects. The results of the experimental/numerical campaign show a general efficiency of the isolation system in terms of limiting the transferred action. Further study is needed to understand more clearly some local unexpected phenomena emerged from the experimental tests.