Shake table testing of un‐reinforced brick masonry building test model isolated by U‐FREI

This paper presents a detailed study on feasibility of un‐bonded fiber reinforced elastomeric isolator (U‐FREI) as an alternative to steel reinforced elastomeric isolator (SREI) for seismic isolation of un‐reinforced masonry buildings. Un‐reinforced masonry buildings are inherently vulnerable under seismic excitation, and U‐FREIs are used for seismic isolation of such buildings in the present study. Shake table testing of a base isolated two storey un‐reinforced masonry building model subjected to four prescribed input excitations is carried out to ascertain its effectiveness in controlling seismic response. To compare the performance of U‐FREI, same building is placed directly on the shake table without isolator, and fixed base (FB) condition is simulated by restraining the base of the building with the shake table. Dynamic response characteristic of base isolated (BI) masonry building subjected to different intensities of input earthquakes is compared with the response of the same building without base isolation system. Acceleration response amplification and peak response values of test model with and without base isolation system are compared for different intensities of table acceleration. Distribution of shear forces and moment along the height of the structure and response time histories indicates significant reduction of dynamic responses of the structure with U‐FREI system. This study clearly demonstrates the improved seismic performance of un‐reinforced masonry building model supported on U‐FREIs under the action of considered ground motions. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural and nonstructural performance of a seismically isolated building using stable unbonded fiber‐reinforced elastomeric isolators

Stable unbonded fiber‐reinforced elastomeric isolators (SU‐FREIs) exhibit a characteristic horizontal softening and stiffening response, similar to other adaptive devices such as the triple friction pendulum and sliding systems with variable curvature. The transition between the softening and stiffening occurs at a displacement corresponding to a unique deformation known as full rollover. In this paper, the full rollover displacement of SU‐FREIs is altered by using modified support geometry (MSG), a geometric modification of the upper and lower supports applied to tailor the hysteresis loops of the isolator. Experimental results are used to calibrate a numerical model of a base‐isolated structure. The model demonstrates that the stiffening regime provides minimal restraint against displacements during events that meet or exceed the maximum considered earthquake. A parametric study revealed that the level of stiffening required to restrain displacements during large events is significant. This increase in stiffness is reflected in an increase in the response of the structure and light nonstructural components. Full rollover and MSG is considered advantageous to maintain horizontal stability and provide control over the stiffening of SU‐FREIs. Copyright © 2015 John Wiley & Sons, Ltd.     

Sensitivity analysis of carbon fiber-reinforced elastomeric isolators based on experimental tests and finite element simulations

Conventional steel-based rubber bearings are being replaced by fiber reinforced elastomeric isolators (FREI) due to their high weight and manufacturing cost. Compared to existing rubber bearings, FREIs have superior performance and as a result can control the seismic response of structures more efficiently. This study aims to simulate the performance of rectangular carbon FREIs (C-FREIs) produced through a simple and cost-effective manufacturing process. Additionally, the effect of different factors including the number and the thickness of rubber layers, as well as the thickness of carbon fiber reinforced sheets are investigated on the performance of C-FREIs through sensitivity analyses based on the results obtained from finite element simulations. The results show that by increasing the number and thickness of rubber layers, the efficiency of C-FREIs degrades in terms of vertical strength and damping capacity, however, the performance improves in terms of lateral flexibility. Another important observation is that the increasing thickness of fiber-reinforced layers can increase the vertical rigidity of the base isolator. The vertical stiffness has the most sensitivity to the thickness of elastomeric layers and the thickness of CFR sheets while, when the number of rubber layers increases, the effective lateral stiffness is mostly affected.     

方形多铅芯橡胶支座力学性能研究

本文通过对方形多铅芯橡胶支座竖向压缩性能试验,水平剪切性能试验以及其等效刚度、屈服强度、屈服后刚度、等效阻尼比等水平特征参数与水平剪切应变和竖向压应力的关系,特别是对其在不同方向上压缩剪切变形状态下的性能试验,分析了这种隔震支座各种水平特征参数在不同方向上变化的相关规律。得出在这种类型橡胶隔震支座在双向水平荷载同时作用下,竖向性能和水平性能较为稳定,是较为理想的桥梁结构的减隔震装置。     

Shake table study on an ordinary low‐rise building seismically isolated with SU‐FREIs (stable unbonded‐fiber reinforced elastomeric isolators)

This paper reports on the investigation of novel fiber reinforced elastomeric isolator (FREI) bearings, which do not have thick end plates, and are used in an unbonded application. Owing to the stable lateral load‐displacement response exhibited by the unbonded FREI bearings, the proposed bearings are referred to as stable unbonded (SU)‐FREIs. A shake table test program was conducted on a two‐story test‐structure having well‐defined elastic response characteristics. Compared with the results for the corresponding fixed base (FB) structure, the peak response values, distribution of lateral response throughout the height of the structure, and response time histories of the tested base isolated (BI) structure indicate that significantly improved response can be achieved. This study clearly indicates that SU‐FREI bearings can provide an effective seismic isolation system. Copyright © 2009 John Wiley & Sons, Ltd.     

Theory and experimental study for sliding isolators with variable curvature

Because a conventional isolation system with constant isolation frequency is usually a long‐period dynamic system, its seismic response is likely to be amplified in earthquakes with strong long‐period wave components, such as near‐fault ground motions. Seismic isolators with variable mechanical properties may provide a promising solution to alleviate this problem. To this end, in this work sliding isolators with variable curvature (SIVC) were studied experimentally. An SIVC isolator is similar to a friction pendulum system (FPS) isolator, except that its sliding surface has variable curvature rather being spherical. As a result, the SIVC's isolation stiffness that is proportional to the curvature becomes a function of the isolator displacement. By appropriately designing the geometry of the sliding surface, the SIVC is able to possess favorable hysteretic behavior. In order to prove the applicability of the SIVC concept, several prototype SIVC isolators, whose sliding surfaces are defined by a sixth‐order polynomial function, were fabricated and tested in this study. A cyclic element test on the prototype SIVC isolators and a shaking table test on an SIVC isolated steel frame were all conducted. The results of both tests have verified that the prototype SIVC isolators do indeed have the hysteretic property of variable stiffness as prescribed by the derived formulas in this study. Moreover, it is also demonstrated that the proposed SIVC is able to effectively reduce the isolator drift in a near‐fault earthquake with strong long‐period components, as compared with that of an FPS system with the same friction coefficient. Copyright © 2011 John Wiley & Sons, Ltd.     

Lateral load performance and seismic demand of unbonded scrap tire rubber pad base isolators

The scrap tire rubber pad(STRP) made by natural or synthetic rubber and high strength reinforcing cords exhibits substantial vertical stiffness and horizontal flexibility, and these properties can be regarded as suitable for seismic isolators for structures. The use of environmentally burdensome scrap tires as STRP isolators might be convenient as an efficient and low-cost solution for the implementation of aseismic design philosophy for low-to-medium rise buildings, especially in developing countries. Finite element analyses of unbonded square and strip-shaped STRP isolators subjected to a combination of axial and lateral loads are conducted to investigate its lateral deformation performance under seismic loading. The rubber of the isolator is modelled with Mooney-Rivlin hyperelastic and Prony viscoelastic materials, including the Mullins material damage effect. The influence of the length-to-width ratio and bearing height on the isolator performance is assessed in terms of the force-displacement relationship, horizontal stiffness, damping, and isolation periods. It is shown that the dependence of stiffness on the length-to-width ratio is significant in the longitudinal direction and minor in the transverse direction. The STRP isolators following the proposed design criteria are shown to satisfy the performance requirement at different levels of seismic demand specified by the ASCE/SEI 7-2010 seismic provisions.     

Performance of seismic‐isolated bridges with and without elastic‐gap devices in near‐fault zones

In this paper, the efficiency of providing elastic‐gap devices (EGDs) to improve the performance of seismic‐isolated bridges (SIBs) in near‐fault (NF) zones is investigated. The device is primarily made of an assembly of circular rubber bearings and steel plates to provide additional elastic stiffness to the SIB upon closure of a gap. The EDG is intended to function at two performance levels under service and maximum considered design level (MCDL) NF earthquakes to reduce isolator displacements while keeping the substructure forces at reasonable levels. A parametric study, involving more than 500 nonlinear time history analyses of realistic and simplified structural models of typical SIBs, is conducted using simulated and actual NF ground motions to investigate the applicability of the proposed solution. It is found that providing EGD is beneficial for reducing the isolator displacements to manageable ranges for SIBs subjected to MCDL NF ground motions regardless of the distance from the fault and characteristics of the isolator. It is also found that providing EGD resulted in an improved performance of the isolators in terms of the reduction of heat generated by the isolators. Further analyses conducted using a realistic structural model of an existing bridge and five NF earthquakes confirmed that EGD may be used to reduce the displacement of the isolators while keeping the substructure base shear forces at reasonable ranges for SIBs located in NF zones. Copyright © 2008 John Wiley & Sons, Ltd.     

Study on effects of damping in laminated rubber bearings on seismic responses for a ⅛ scale isolated test structure

The effects of damping in various laminated rubber bearings (LRB) on the seismic response of a ?‐scale isolated test structure are investigated by shaking table tests and seismic response analyses. A series of shaking table tests of the structure were performed for a fixed base design and for a base isolation design. Two different types of LRB were used: natural rubber bearings (NRB) and lead rubber bearings (LLRB). Three different designs for the LLRB were tested; each design had a different diameter of lead plug, and thus, different damping values. Artificial time histories of peak ground acceleration 0.4g were used in both the tests and the analyses. In both shaking table tests and analyses, as expected, the acceleration responses of the seismically isolated test structure were considerably reduced. However, the shear displacement at the isolators was increased. To reduce the shear displacement in the isolators, the diameter of the lead plug in the LLRB had to be enlarged to increase isolator damping by more than 24%. This caused the isolator stiffness to increase, and resulted in amplifying the floor acceleration response spectra of the isolated test structure in the higher frequency ranges with a monotonic reduction of isolator shear displacement. Copyright © 2002 John Wiley & Sons, Ltd.     

橡胶座非线性弹性拉伸特性的理论和试验研究

本文系统地研究分析了橡胶隔震支座(简称橡胶座)拉伸相关的基础理论,提出了原点拉伸纵弹性模量、拉伸刚度因子、原点拉伸刚度、偏拉原点刚度等基本概念,还提出了计算橡胶座非线性段拉伸刚度的双刚度和原点拉伸刚度应力应变模型等理论。基于原点拉伸刚度和偏拉原点刚度,提出了预测橡胶座的拉伸变形量计算理论。针对新提出的基本概念和计算理论采用低弹性天然橡胶和铅芯橡胶座原型试件进行了试验研究工作,结果表明本文建立的计算理论能够较理想地定量分析橡胶座拉伸相关的刚度和变形特性。     

Experimental and analytical studies of structures seismically isolated with an uplift‐restraining friction pendulum system

This paper presents findings from a comprehensive analytical and experimental study on the uplift‐restraining XY‐FP sliding isolation system. To investigate the effectiveness of the XY‐FP isolator and provide a rational basis for evaluating the efficacy of the developed mathematical model, an extensive experimental program was conducted on the earthquake simulator at the University at Buffalo. The experimental program involved a slender, five‐storey, scale‐model frame seismically isolated with four XY‐FP isolators subjected to simulations of historical horizontal and vertical ground motions. The experimental response demonstrates the validity of the concept and provides evidence for the effectiveness of the XY‐FP isolator in preventing uplift. A comprehensive analytical model capable of emulating the mechanical behaviour of the XY‐FP isolator is developed and implemented in program 3D‐BASIS‐ME. The newly enhanced program is used to predict the dynamic response of the seismically‐isolated model structure. Comparison of analytical predictions with experimental results attests to the efficacy of the analytical model for simulating the response of the XY‐FP isolator. With its appealing conceptual simplicity and its proven effectiveness, the new uplift‐restraining isolator has the potential to facilitate the application of seismic isolation even under the most extreme of conditions, including but not limited to near‐fault strong ground motions and uplift‐prone structural systems. Copyright © 2005 John Wiley & Sons, Ltd.     

桥梁隔震支座压缩剪切变形状态下竖向变形量研究

本文通过对不同参数的橡胶支座在压缩剪切变形状态下的竖向变形量进行了试验,分析了在压缩剪切变形状态下竖向变形量的特点。得出对RB支座,由剪切和弯曲产生的竖向变形随剪切应变增加而增大较快;对LRB支座,在达到一定剪切应变时,竖向变形量的增加缓慢。     

Estimation of seismic demands on isolators in asymmetric buildings using non‐linear analysis

A procedure based on rigorous non‐linear analysis is presented that estimates the peak deformation among all isolators in an asymmetric building due to strong ground motion. The governing equations are reduced to a form such that the median normalized deformation due to an ensemble of ground motions with given corner period T_d depends primarily on four global parameters of the isolation system: the isolation period T_b, the normalized strength η, the torsional‐to‐lateral frequency ratio Ω_θ, and the normalized stiffness eccentricity e_b/r. The median ratio of the deformations of the asymmetric and corresponding symmetric systems is shown to depend only weakly on T_b, η, and Ω_θ, but increases with e_b/r. The equation developed to estimate the largest ratio among all isolators depends only on the stiffness eccentricity and the distance from the center of mass to the outlying isolator. This equation, multiplied by an earlier equation for the deformation of the corresponding symmetric system, provides a design equation to estimate the deformations of asymmetric systems. This design equation conservatively estimates the peak deformation among all isolators, but is generally within 10% of the ‘exact’ value. Relative to the non‐linear procedure presented, the peak isolator deformation is shown to be significantly underestimated by the U.S. building code procedures. Copyright © 2003 John Wiley & Sons, Ltd.     

Analysis of a kinematic self‐centring seismic isolator

This investigation deals with the analytical formulation and experimental validation of a prestressed reinforced concrete seismic isolator with kinematic constraints at both ends. The kinematic isolator was proposed initially as a low‐cost solution for seismic protection of low‐income people housing usually placed at the periphery of big cities where regular to bad soil conditions are common. So, the isolator is also a pile foundation with a central prestressed cable and two rolling steel surfaces at the top and bottom ends. By varying the shapes of the end rolling surfaces, different force–deformation constitutive relationships for the isolator may be obtained. Energy dissipation is introduced by yielding of passive reinforcement at the rolling interphase. Apart from stating the large‐deformation formulation of the element, several relevant aspects of the behaviour of these devices are studied herein, such as the increase in the tension of the central prestressed cable, responsible for the self‐centring action of the isolator, the floor uplift that results from the geometry of the isolator, and the vertical stability of the system. Experimental and theoretical results obtained for a group of 9 testing specimens show an excellent agreement in the force–deformation constitutive relationship. Although not the intent of this article, the device proposed may be extended directly as a coupling beam element for shear wall systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of air temperature on the cyclic behavior of elastomeric seismic isolators

The mechanical properties of elastomers can change significantly due to air temperature variations. In particular, prolonged exposure to subzero temperatures can result in rubber crystallization, with a considerable increase in the shear stiffness of the material. As a result, the seismic response of structures with elastomeric isolators can be strongly influenced by air temperature. Current seismic codes, indeed, require an upper and lower bound analysis, using suitable modification factors, to account for the changes in the cyclic behavior of elastomeric isolators due to air temperature variations. In this study, the sensitivity of the cyclic behavior of elastomeric isolators to air temperature variations is investigated based on the experimental results of an extensive test program on six different elastomeric compounds for seismic isolators, characterized by a shear modulus ranging from 0.5 to 1.2 MPa at 100% shear strain and 20°C. The cyclic tests have been performed on small-size specimens, subjected to shear strain amplitudes and frequency of loading typical for elastomeric seismic isolators, at seven different air temperatures, ranging from 40 to −20°C. The effects of rubber crystallization due to prolonged exposure to low-temperatures have been also investigated. A finite element model for the evaluation of the temperature contour map inside a full-size elastomeric isolator exposed to low air temperatures has been also developed. In the paper, the experimental outcomes are compared with the modification factors provided by the current seismic codes to account for the temperature effects on the mechanical properties of elastomeric isolators.     

昆明长水国际机场航站楼隔震橡胶支座更换技术

本文介绍了昆明长水国际机场航站楼1m直径隔震橡胶支座的更换技术。针对机场航站楼隔震承台轴力不同,隔震橡胶支座自重大,人工无法移动,且隔震层上下承台之间距离小等实际情况,定制了特殊的柴油机械传动叉车和液压千斤顶。更换支座的上承台顶升与周边上承台的顶升采用异步顶升方法。本文介绍了异步顶升方法千斤顶的布置和上承台顶升位移限值的确定方法。通过现场测量上法兰板螺栓孔在下法兰板投影的位置,按照投影位置加工上法兰板螺栓孔的方法,解决了由于隔震支座发生水平剪切变形使上下法兰板孔竖向不对齐的问题。通过两个措施大大减小了上承台顶升高度:（1）提前施加压力,减小了新支座的高度;（2）把旧隔震支座上下法兰板连接起来,阻止了支座竖向压缩变形的恢复。该技术成功应用于昆明长水国际机场42个侧鼓变形隔震橡胶支座的更换。该技术简单、经济、可行,可为其他工程隔震橡胶支座更换提供借鉴,为相关规范标准的制定提供参考。     

Effect of stiffness variability on the response of isolated structures

Results are presented of an investigation, the objective of which was to determine the relationship between the stiffness variability of the bearings of an isolation system and the response variability of the structure. The system is modeled as a rigid, rectangular structure that is free to translate and rotate. The isolation system consists of N isolation bearings arranged in a rectangular pattern, each with a stiffness k_i that is an independent, normally distributed, random variable. Response spectrum analysis is used to obtain the analytical solution for the structure response. Approximate closed‐form expressions are obtained for the variance of the centreline displacement, rotation, corner displacement and base shear, that are in terms of the variability of the isolator stiffness, aspect ratio of the structure, and the number and layout of isolation bearings. Results show that the standard deviation of the centreline displacement and base shear decrease with increasing number of isolation bearings, and are independent of the aspect ratio and layout of isolators, and in all cases are less than 1/4 the standard deviation of the isolator stiffness. The standard deviation of the corner displacement is a function of all of the system parameters, and is bounded below by the standard deviation of the centreline displacement and above by the standard deviation of a bar aligned perpendicular to the direction of ground motion with m isolation bearings distributed along the length. The approximate expressions are shown to be in good agreement with the results of Monte Carlo simulations. The results should be of use to designers of isolated structures and manufacturers of isolation systems, in assessing the significance of stiffness variability on the response of the isolated structure. Copyright © 2000 John Wiley & Sons, Ltd.     

Vertical vibration isolator having piecewise‐constant restoring force

We present a vertical vibration isolator having a piecewise‐constant restoring force, which belongs to a class of passive and nonlinear vibration isolators. In vertical vibration isolation, direct use of low‐stiffness elements leads to unacceptably large deformations due to self‐weight. To overcome the difficulty, we apply a combination of constant‐force springs, each of which sustains a constant load regardless of its stretch. By arranging the constant‐force springs, so that the isolator has a piecewise‐constant restoring force, we alleviate the problem of the excessive deformation caused by self‐weight, provide stability at the static equilibrium state along with the self‐centering capability, and realize a large stroke while keeping the mechanism simple and compact. Further, we attempt to limit the response acceleration within a tolerance regardless of the frequency spectrum and the magnitude of earthquake ground motions. We demonstrate the effectiveness of the present isolator through shaking table tests and numerical simulations. Copyright © 2009 John Wiley & Sons, Ltd.     

低硬度大直径橡胶隔震支座竖向荷载作用下钢板应力研究

采用大型通用有限元程序,对低硬度大直径橡胶隔震支座在竖向荷载作用下的基本性能进行了精细有限元分析。分析了在竖向荷载作用下支座上下封板、内部钢板的各种应力分布,以及支座顶部是否施加竖向同位移约束、支座孔径比、橡胶材料G值、内部单层橡胶厚度与内部单层钢板厚度之比对支座内钢板受力的影响。结果表明,内部钢板最大Mises应力、最大径向水平正应力约为支座竖向压力的4倍、2.5倍;支座顶部是否施加竖向同位移约束对支座用钢板内力影响较大,特别是支座上封板;橡胶G值的变化对内部钢板受力基本没影响;支座孔径比对靠近孔边缘部分内部钢板应力不均匀性分布影响较大;内部单层橡胶厚度与内部单层钢板厚度的比值对内部钢板的Mises应力、径向正应力、环向正应力影响较大,但对其竖向正应力基本没影响。