Using orthogonal pairs of rollers on concave beds (OPRCB) as a base isolation system—part II: application to multi‐story and tall buildings

Application of orthogonal pairs of rollers on concave beds (OPRCB) isolating system to short‐ and mid‐rise buildings is presented in this paper. At first, the analytical formulation of the set of equations, governing the motion of Multi Degree of Freedom (MDOF) systems, isolated by OPRCB isolators, has been developed. Then, some multi‐story regular buildings of shear type have been considered, once on fixed bases and once installed on the OPRCB isolators. Next, some horizontal and vertical accelerograms of both far‐ and near‐fault earthquakes with low‐ to high‐frequency content, particularly those with remarkable peak ground displacement values, have been selected and normalized to three peak ground acceleration levels of 0.15 g, 0.35 g and 0.7 g, and their stronger horizontal component simultaneous with their vertical component have been used for response analysis of the considered buildings. Story drifts and absolute acceleration response histories of isolated buildings have been calculated by using a program, developed in MATLAB environment by using the fourth‐order Runge–Kutta method, considering the geometrically nonlinear behavior of isolators. Maximum relative displacement and story drifts as well as absolute acceleration responses of considered isolated buildings for various earthquakes have been compared with those of corresponding fixed‐base buildings to show the high efficiency of using OPRCB isolators in multi‐story and tall regular buildings. Copyright © 2010 John Wiley & Sons, Ltd.     

Experimental study on the sandbag isolator of buildings for subway-induced vertical vibration and secondary air-borne noise

This paper presents a new geosynthetics isolator composed of stacked sandbags for buildings’ base-isolation method to improve human comfort under subway-induced environmental vibrations. The static and dynamic mechanical properties of the isolator are tested in a laboratory first. Then field experiments are performed with a full-scale building to investigate the effectiveness of the isolator. The results indicated that the bearing capacity is mainly influenced by the soft contact interfaces between different sandbags, and is sufficient for most low-rise and mid-rise buildings. Similar to some springs in a series, the soft contact interfaces make the vertical stiffness of the isolator low enough to isolate vibrations. The friction forces between sand particles are a benefit to dissipating energy, which increases the damping ratio to 14%. After the installation of geosynthetics isolators, the vertical vibration and secondary air-borne noise inside the building are effectively suppressed, and therefore the comfort for residents is enhanced. Further, there is no negative impact caused by automobile vibrations on the isolated building because the damping ratio of the isolator is high enough to resist resonant amplitudes. These results demonstrate that the isolator is feasible for buildings near subway transportation.     

Performance of variable curvature sliding isolators in base‐isolated benchmark building

The performances of variable curvature sliding isolators like variable frequency pendulum isolator (VFPI) and variable curvature friction pendulum system (VCFPS) installed in the base‐isolated benchmark building subjected to bi‐directionally acting seven strong earthquakes have been studied. The shear type base‐isolated benchmark building is modelled as three‐dimensional linear elastic structure having three degrees‐of‐freedom at each floor level. Time domain dynamic analysis of the building has been carried out with the help of constant average acceleration Newmark‐Beta method and non‐linear isolation forces has been taken care by fourth‐order Runge‐Kutta method. The force‐displacement responses of the VFPI and VCFPS are studied under parametric variations of their key characteristics for the comparative performance evaluation. The time history variations of response characteristics and peak response evaluation criteria are also investigated for overall comparison of their performances. The performance of VFPI and VCFPS are observed both in uniform and hybrid isolation systems. The force‐displacement responses of both VFPI and VCFPS subjected to strong near‐field earthquakes show excessively large isolator displacements at higher initial radii of curvature of sliding surface. The large isolator displacements of VFPI and VCFPS can be restrained efficiently by addition of viscous fluid dampers in comparison to the increase in coefficient of friction of isolators. Copyright © 2010 John Wiley & Sons, Ltd.     

橡胶隔震支座基本力学性能试验研究

为了满足三维隔震装置对承载元件的需要,设计了一种新型的厚层橡胶隔震支座,该支座的最大特点是能够同时发生水平和竖向变形。通过水平剪切和竖向压缩试验研究了橡胶隔震支座在不同振幅、不同频率组合下的力学性能。试验研究表明,厚层橡胶隔震支座在水平方向的基本力学性能是良好、稳定的,在竖向具有大变形能力。     

隔震钢结构的实例介绍

本文通过对5栋典型隔震钢结构的比较研究,说明了隔震钢结构亦具有其它隔震结构的特点,即基本周期延长了2倍以上,具有良好的隔震效果,这5栋钢结构包括平、立面比较复杂的结构和旧建筑隔震改造。这些实例说明地下室作隔震层比较经济,而且还可以看出新建隔震建筑的高度在提高,这对隔震器的性能要求也提高了。     

Experimental study on a geosynthetics isolator for the base vibration isolation of buildings neighboring metro transportation

By replacing the polyethylene terephthalate (PET) package of stacked sandbags by polyethylene naphthalate (PEN) textile capsule from air springs, a new geosynthetics isolator is developed to improve the mechanical properties of stacked sandbags. Sand and rubber particles were mixed and used to fill the isolator as a soil skeleton. The mechanical testing results showed that the vertical stiffness and damping performance could be easily adjusted by changing the particle mixture ratio. The horizontal stiffness is approximately a quarter of the vertical value, and the horizontal damping ratio is as high as 25%. The ultimate shear strain and compressive stress are 15% and 40 MPa, respectively. Both of these values are much greater than that of stacked sandbags and are appropriate for most buildings near metro transportation. The feasibility of the isolator is verified via field experiments with a full-scale building. After installing the new isolators, the vibration comfort of occupants in the vertical and horizontal directions is significantly improved, as is the secondary air-borne noise comfort. Moreover, the resonant influences caused by vertical and horizontal low-frequency road vibrations are suppressed by the isolator's high-damping performance. Therefore, this geosynthetics isolator is feasible for the base isolation of buildings neighboring metro transportation.     

Effectiveness of sliding isolators with variable curvature in near‐fault ground motions

Recent studies have revealed that a sliding isolator with variable curvature (SIVC) can mitigate the resonance phenomenon likely to occur in seismic response of a conventional friction pendulum system (FPS) isolator due to its constant isolation frequency. The present study simulates four SIVC isolators and an FPS to find the optimum range of initial isolation period and coefficient of friction and employ them in comparing the effectiveness of SIVC in different peak ground acceleration (PGA) scales of near‐fault earthquakes. Velocity‐dependent coefficient of friction and modified viscoplasticity model have been used to simulate nonlinear friction force of the isolators. Results indicate identical performance of all SIVC isolators in PGA scales up to 0.4 g. When subjected to PGA scales from 0.4 g to 1.0 g, polynomial friction pendulum isolator (PFPI) and variable curvature friction pendulum system (VCFPS) reduce base displacement greatly, while conical friction pendulum isolator (CFPI) and variable frequency pendulum isolator (VFPI) show amplified responses. However, in mitigating structural acceleration, performance of CFPI and VFPI, unlike PFPI and VCFPS, which perform poorly, is excellent. Thus, in a strong near‐fault earthquake, PFPI and VCFPS or CFPI and VFPI can be chosen based on whether reduction of base displacement or super‐structural acceleration is the main concern of designer, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluation and refinement of closely spaced buildings’ performance under near-fault ground motions

This paper investigates the seismic behaviour of closely spaced fixed-base and isolated building structures in near-fault (NF) zones. Seismic pounding of fixed-base structures is considered at different heights, being from one or both opposite sides and at different seismic gap width. The response evaluation results of fixed-base buildings drive towards providing limited, but adequate, seismic gaps to perform seismic isolation. This aims at reducing structural responses with no seismic pounding under limited gaps, minimising the possible damage repair and diminishing the needed maintenance works due to strong NF earthquakes. To achieve that untraditionally, the paper presents a recently proposed seismic isolation system, named roll-in-cage (RNC) isolator, as a non-traditional solution to avoid direct seismic pounding of isolated buildings with their surrounding adjacent structures. It was found that the RNC isolator’s buffer mechanism is able to draw down any possible pounding of the isolated superstructure to be within the isolator solid limits. This entirely prevents direct structure-to-structure pounding but on the account of amplifying its acceleration and drift responses. However, such amplified responses might lead to only minor or moderate structural damage under sever NF earthquakes with 1.20g peak ground acceleration. Nevertheless, such damage could be avoided entirely using stiffer RNC isolators to achieve reduction of seismic response up to 69.0% under the same severe loading conditions and limited seismic gaps with no seismic pounding. Consequently, the RNC isolator could be an efficient solution for aseismic design in NF zones considering limited seismic gaps.     

Response of vibration-isolated object to ground motions with intense vertical accelerations

This paper reports the results of shaking table tests performed to assess the response of an object placed on a vibration isolator to intense ground motions whose peak accelerations are close to or over the gravity acceleration in both vertical and horizontal directions. The passive vertical and horizontal vibration isolator, developed by the authors, has rolling load-bearing elements and constant-force springs to provide piecewise-constant restoring forces. First, we performed shaking table tests, wherein only the horizontal vibration was isolated. An unexpected finding from the tests is that, when the vibration isolator was subjected to ground motions with intense vertical accelerations, not only vertical but also horizontal accelerations were amplified. This led to a large rocking response or overturning of the object placed on the vibration isolator. A likely reason for the amplification is rocking of the upper portion of the vibration isolator above the rolling load-bearing elements induced by intense vertical accelerations. Next, we performed shaking table tests, wherein both vertical and horizontal vibrations were isolated. In this case, the vertical as well as horizontal accelerations were significantly reduced and the rocking response of the vibration-isolated object was suppressed. These results suggest potential risks in conventional horizontal vibration isolators having rolling load-bearing elements when they are subjected to ground motions with intense vertical accelerations. The results also demonstrate the effectiveness of vertical vibration isolation to avoid such risks.     

Seismic response of base isolating systems with U-shaped hysteretic dampers

This paper proposes a base isolating system to reduce the seismic demands of low- or medium-rise structures and experimentally investigates its seismic response using shake-table tests. The base isolating system considered in this study consists of laminated-rubber bearings and U-shaped hysteretic (UH) dampers which are made of high toughness steel (HTS) and are machined with slotted holes to increase their deformation capacities. A base isolated 2-story specimen for shake-table tests was first designed and cyclic tests of laminated-rubber bearings and UH dampers implemented in the base isolating systems were then carried out. The component test for the laminated-rubber bearings shows typically low lateral stiffness with enough vertical stiffness to carry gravity loads. The test results for the UH dampers demonstrate that the use of HTS material and the introduction of the slotted holes details increase deformation capacities by inducing uniform stress distribution along a UH damper. Finally, shake-table tests were performed using specimens shaken with increasing ground acceleration records. The shake-table tests show that the proposed base isolating system with UH dampers limits the seismic demands of a base isolated structure by lengthening its structural period, concentrating displacement demands on the base isolating floor and adding seismic energy dissipation from the UH dampers.     

Seismic response control of base‐isolated buildings using multiple tuned mass dampers

Seismic response of a base‐isolated building equipped with single tuned mass damper (STMD), multiple tuned mass dampers (MTMDs), and distributed multiple tuned mass dampers (d‐MTMDs) under real earthquake ground motions is investigated. Numerical study is carried out using analytical models of five‐, 10‐, and 15‐storey base‐isolated buildings equipped with the STMD, MTMDs, and d‐MTMDs. The buildings are modeled as shear‐type structure with a lateral degree of freedom at each floor level, and the buildings are isolated using the laminated rubber bearing, lead‐core rubber bearing, friction pendulum system, and resilient‐friction base isolator. The coupled differential equation of motion for the buildings are derived and solved in the incremental form using Newmark's step‐by‐step method of integration. From the numerical study conducted, it is concluded that installing a tuned mass damper at each floor level of a base‐isolated building reduces the structural response in terms of top floor acceleration and bearing displacement. It is found that installing the MTMDs and d‐MTMDs are significantly beneficial in reducing top floor acceleration as compared with the STMD. Further, almost comparable reduction in the bearing displacement could be obtained by installing the STMD, MTMDs at top, and d‐MTMDs in the base‐isolated buildings. The d‐MTMDs are more beneficial as compared with the STMD and MTMDs as otherwise huge controller mass can now be divided and distributed on different floor levels.     

A study on vortex‐induced vibration of supertall buildings in acrosswind

Modern tall buildings using innovative structural systems and high‐strength materials tend to be slender and lightly damped. Hence, they are vulnerable to the dynamic action of wind. Crosswind excitation on tall buildings can result in aeroelastic problems. Vortex‐induced vibration (VIV) is the prime problem in self‐excited vibration of the flexible structures, and it should be especially observed in order to avoid the ultimate limit state in the design stage. In order to predict the vortex‐induced response of a supertall building in China with the single‐degree‐of‐freedom (SDOF) mathematical model, wind tunnel tests were carried out with an improved aeroelastic model according to the similitude. The measured top acceleration of the structure showed that VIV was quite significant at some wind speeds and should be considered in the design. Based on the experimental data, the aerodynamic parameters were determined and the characteristics of VIV were investigated in some details. The time history of acceleration at the lock‐in wind speed was then obtained using the Runge–Kutta method with the SDOF model. The numerical results are in accordance with the measurements. Copyright © 2009 John Wiley & Sons, Ltd.     

Assessment of sliding‐rubber isolator effect in progressive collapse of bridges under two scenarios

In this study, performances of 2 types of bridges, with and without seismic isolation, are addressed in 2 damage analysis scenarios, where, in the first, the side column and, in the second, the middle column are removed from the bridge piers. The performance was assessed using nonlinear dynamic analysis, and the time history and maximum structural responses were evaluated. Initially, sliding‐rubber isolators were designed according to AASHTO guide specifications, and then the bridges were modeled in OpenSees software package. Additionally, the coefficient of friction for the isolator was considered as a variable due to sudden removal of the columns and the consequent changes in the sliding velocity and axial forces. The results indicate that use of seismic isolation systems caused an increase in all maximum structural responses except that of the base shear. Considering the frictional performance of the isolators, slides in the deck are not caused by yielding of seismic isolators, and the reason for permanent displacements of the deck may be attributed to bridge instabilities in the first scenario. However, decrease in the horizontal stiffness results in increased maximum permanent displacement. In the first scenario, uplift of the deck occurred in the case of isolated bridge.     

Approximate modal analysis of multistory symmetrical buildings with restricted inelasticity

Simplified capacity curves are presented for modal decomposition of multistory inelastic cantilever bents. These structural systems are uniform over the height and plasticity is assumed to be restricted into the beams, since significant rotation ductility factors may be attained in these members without loss of strength. The approximate method of modal decomposition of multistory inelastic bents is based on the concept that the total response may be obtained from the contributions of equivalent nonlinear single‐degree‐of‐freedom (SDOF) modal systems, in combination with the technique of modal superposition. In particular, the contribution of the first mode equivalent inelastic SDOF system is examined, since its modal contribution is of higher importance. The response of such SDOF systems basically depends on their capacity curve, which may be formulated by using the dynamic properties (frequency, effective modal mass and mode shape) of the initially elastic bent, as well as the corresponding properties of the bent when the entire set of coupling beams is assumed to be yielded. The procedure is presented for plane cantilever bents, but it can be easily extended to symmetrical structural systems composed of different types of inelastic bents. Its application is illustrated by means of a 10‐story inelastic coupled‐wall bent subject to a strong earthquake motion, equal to 1·5× El Centro N–S ground excitation, and the results compare well with those obtained from a step‐by‐step nonlinear time history analysis of the discrete member model. Copyright © 2007 John Wiley & Sons, Ltd.     

Strength reduction factor for MDOF soil–structure systems

In most of the seismic design provision, the concept of strength reduction factor has been developed to account for inelastic behavior of structures under seismic excitations. Most recent studies considered soil–structure interaction (SSI) in inelastic response analysis are mainly based on idealized structural models of single degree‐of‐freedom (SDOF) systems. However, an SDOF system might not be able to well capture the SSI and structural response characteristics of real multiple degrees‐of‐freedom (MDOF) systems. In this paper, through a comprehensive parametric study of 21600 MDOF and its equivalent SDOF (E‐SDOF) systems subjected to an ensemble of 30 earthquake ground motions recorded on alluvium and soft soils, effects of SSI on strength reduction factor of MDOF systems have been intensively investigated. It is concluded that generally, SSI reduces the strength reduction factor of both MDOF and more intensively SDOF systems. However, depending on the number of stories, soil flexibility, aspect ratio and inelastic range of vibration, the strength reduction factor of MDOF systems could be significantly different from that of E‐SDOF systems. A new simplified equation, which is a function of fixed‐base fundamental period, ductility ratio, the number of stories, structure slenderness ratio and dimensionless frequency, is proposed to estimate strength reduction factors for MDOF soil–structure systems. Copyright © 2012 John Wiley & Sons, Ltd.     

建筑隔震结构地震响应计算的层间剪力校准系数法

隔震结构由于其优越的抵御地震能力而在实际建筑工程中得到了广泛的应用。针对建筑隔震结构,我国GB 50011—2010 《建筑抗震设计规范》所推荐的水平向减震系数法,通过引入水平向减震系数修正未采取隔震措施结构的反应谱法结果,间接获得采取隔震措施后结构的地震响应。该法具有较强的操作性,但其合理性一直受到工程界的关注。针对隔震结构的局部非线性行为,建立隔震结构地震响应的时域显式表达式,利用其降维列式优势实现隔震装置局部非线性自由度的高效降维迭代计算。结合随机模拟方法,统计获得非线性隔震结构在地震作用下的层间剪力。在此基础上,针对等效线性隔震结构反应谱法得到的层间剪力进行校准,获得各楼层层间剪力校准系数,然后采用该系数调整反应谱法计算得到的等效线性隔震结构各楼层的构件内力。采用所提出的层间剪力校准系数法和规范推荐的水平向减震系数法,对两个基础隔震结构进行地震作用效应分析,结果表明所提出方法能有效提高隔震结构地震响应的计算精度。     

Study on dynamic response of large and small aspect ratio isolated buildings

An experimental study of a large aspect structure, 1:4 scale five‐storey steel frame, is presented. Seismic responses of the test model to the unilateral and bilateral excitations are compared. The effects of aspect ratio on the maximum acceleration, the storey drift of the superstructure, the maximum displacement and the vertical load of the isolation system are investigated. A comparison of the large aspect ratio isolated building with the small aspect ratio isolated building showed significant difference of the effects of aspect ratio subjected to disparate earthquake ground motions. Experimental results reveal that the aspect ratio is an important factor influencing the axial load action on isolators and the tension stress of the lead‐rubber bearings. The superstructure flexibility of the large aspect ratio building‐isolation system and the effects of the axial force variation of the lead‐rubber bearings should be carefully considered for design. Copyright © 2013 John Wiley & Sons, Ltd.     

Nonlinear dynamically automated excursions for rubber-steel bearing isolation in multi-storey construction

Conventional building design includes the concept of providing more stiff structural solution to withstand the lateral seismic loading. Introduction of flexible elements at the base of a structure and at the same time ensuring enough damping is the evocative alternate option to mitigate seismic hazard. The device that is capable to meet such criteria is known as isolator. This paper covers design of base isolators for multi-storey buildings in medium risk seismicity region and the structural response evaluation. Automated nonlinear models for dynamic response investigation have been configured. Finite element method (FEM) has been incorporated to envisage the structural response behaviors. Lead rubber bearing (LRB) and high damping rubber bearing (HDRB) have been chosen for inserting isolator link element in structural base. The nonlinearities of rubber-steel bearing have been duly considered. Linear static, linear dynamic and nonlinear dynamic analyses due to site-specific earthquake accelerogram are performed for both fixed based (FB) and base isolated (BI) buildings. Both time domain and frequency domain approaches have been carried out for dynamic solution. It was found that for multi-storey buildings, base isolation diminishes muscular amount of structural responses compared to the conventional fixed base (non-isolated) structures. Allowable higher horizontal displacement induces structural flexibility. The suggested isolation technique is competent to mitigate the structural hazard even under strong seismic vulnerability in optimum manner.     

某大跨网架结构屋盖隔震整体分析

本文在分析大跨空间网架自身结构特性的基础上,提出采用柱顶隔震方法作为该类结构的减震措施。研究了橡胶支座与滑移支座两类隔震装置用于网架结构柱顶隔震的工作机理,进而以某大跨网架结构为例,对具有不同自振周期的网架结构采用不同隔震装置在不同地震波下隔震效果进行了对比。结果表明:两类隔震支座在网架柱顶隔震中均有较好的隔震效果,相对橡胶支座而言,滑移隔震支座受结构动力特性与地震波输入的影响较小。     

Nonlinear behavior of composite shear walls with vertical steel encased profiles

Research on seismic engineering of buildings using composite steel-concrete structural systems has increased in the past decade. One horizontal resisting system for buildings, placed in seismic areas, is the composite steel-concrete structural shear wall with steel encased profiles (CSRCW). The benefits of this structural system, relative to more common systems, include the performance characteristics when subjected to service or ultimate loads. The present paper summarizes the experimental results of recent research made on six experimental steel-concrete composite elements 1:3 scale, tested in laboratory under cyclic lateral loads. The experimental elements differ by the arrangement of the steel shapes embedded in the cross section of the wall and by the cross section type of the steel encased profiles. All specimens were tested under constant vertical load and cyclically increasing horizontal (lateral) loads. The tests were performed until failure. Using the recorded data during the tests, the following parameters are presented and discussed: maximum load capacity, stress and strain distribution in structural components (reinforcements, structural steel and on concrete surface), interstory drifts, cracking patterns, deformation and degradation capacity.