Seismic response of light internal equipment in base-isolated structures

The seismic response of light secondary systems in a building is dependent on the response of the primary structural system to the seismic ground motion with the result that very high accelerations can be induced in such secondary systems. This response can be reduced through the use of aseismic base isolation which is a design strategy whereby the entire building can be decoupled from the damaging horizontal components of seismic ground motion by the use of some form of isolation system. The paper presents a theoretical analysis of the response of light equipment in isolated structures and a parallel experimental programme both of which show that the use of base isolation can not only attenuate the response of the primary structural system but also reduce the response of secondary systems. Thus, the design of equipment and piping in a base-isolated building is very much simpler than that for a conventionally founded structure: inelastic response and equipment-structure interaction need not be considered and multiple support response analysis is rendered unnecessary. Although an isolation system with linear elastic bearings can reduce the acceleration of the structure, it may be accompanied by large relative displacements between the structure and the ground. A system using lead-rubber hysteretic bearings, having a force-displacement relation which is approximately a bilinear loop, can reduce these displacements. A parallel experimental programme was carried out to investigate the response of light equipment in structures isolated using lead-rubber bearings. The experimental results show that these bearings can dissipate energy and limit the displacement and acceleration of the structure but are less effective in reducing the accelerations in the internal equipment. The results of both the analysis and the tests show that base isolation is a very effective method for the seismic protection of light equipment items in buildings.     

基础隔震高层建筑地震响应的理论分析

本文对某高层隔震框架－剪力墙结构的三维地震响应进行了有限元分析计算，对比了隔震结构及其相应非隔震结构的动力特性，给出了在多遇地震和罕遇地震作用下结构的最大层间剪力，层间位移等，得到一些有意义的结论。     

Seismic response of base-isolated structures with vertical-rocking coupling

A base-isolated building is liable to have a small horizontal eccentricity between the centre of mass of the superstructure and the centre of rigidity of the supporting bearings. In seismic analysis, the structure is modelled as a rigid block with tributary masses supported on massless elastomeric rubber bearings placed at a constant elevation below the centre of mass. This simplified system has three degrees of freedom: two translations and one rotation in the vertical plane. The investigation of the dynamic behaviour of a base-isolated building is carried out for both the detuned and the perfectly tuned cases. In the detuned case, the natural frequencies of the system are assumed to be well separated. In the perfectly tuned case, the uncoupled rocking frequency is assumed to be identical to the vertical translational frequency, which may result from an unusual mass distribution and/or an extreme aspect ratio of the superstructure. Perturbation methods are implemented in finding the dynamic characteristics for both cases. However, the dynamic response of the perfectly tuned case is the major concern in this investigation. The Green's functions for the displacement response of the three-degree-of-freedom system are derived for both the undamped and damped conditions. The response spectrum modal superposition method is used in estimating the maximum acceleration response. A simple method, accounting for the effect of closely spaced modes, is proposed for combining modal maxima and results in an approximate solution corresponding to a single-degree-of-freedom system. This approximate solution may be used for the preliminary design of a base-isolated structure. Numerical results for a base-isolated building subjected to the vertical component of the El Centro earthquake of 1940 were carried out for comparison with these analytical results. The proposed modal combination method showed superiority over the conventional Square Root of the Sum of the Squares method in estimating maximum responses. The results also indicated that the approximate single-degree-of-freedom system yields accurate estimations. It is shown that the effect of rocking coupling on the vertical response of base-isolated structures subjected to transient loadings, such as earthquake motions, can generally be neglected as a result of the combined effects of the time lag between the maximum translational and rotational responses and the influence of damping in the isolation system, which for elastomeric bearings can be as high as 8 to 10 per cent of critical.     

高层隔震建筑三维地震响应

对某高层框架－筒体结构进行了三维有限元时程分析，考虑了基础夹层橡胶了隔震垫的弹塑性特性和粘滞阻尼，分析了结构在地震波作用下的动力响应，为高层建筑中推广应用基础隔震技术提供了理论依据。     

软土场地基础隔震建筑减震性能研究

本文研究土与结构相互作用(SSI)对多层及中高层基础隔震建筑地震需求及隔震效率的影响规律,隔震层采用LRB铅芯橡胶与LNR普通橡胶隔震支座组合,就我国现行《建筑抗震设计规范》(GB50011-2010)中软土场地设置隔震层问题做探讨。提出土与基础隔震结构相互作用的简化计算模型,对不同场地及隔震设计目标下的多层及中高层基础隔震结构进行时程分析。研究表明:软土场地基础隔震建筑隔震层的有效隔震效率相对于硬土场地有所下降,必须通过设置具有一定规格的LRB支座来满足隔震目标。本文给出了铅芯橡胶支座极限变形需求随建筑层高及隔震目标变化的规律。     

Seismic response evaluation of base-isolated reinforced concrete buildings under bidirectional excitation

This paper reports on an investigation of the seismic response of base-isolated reinforced concrete buildings, which considers various isolation system parameters under bidirectional near-fault and far-fault motions. Three-dimensional models of 4-, 8-, and 12-story base-isolated buildings with nonlinear effects in the isolation system and the superstructure are investigated, and nonlinear response history analysis is carried out. The bounding values of isolation system properties that incorporate the aging effect of isolators are also taken into account, as is the current state of practice in the design and analysis of base-isolated buildings. The response indicators of the buildings are studied for near-fault and far-fault motions weight-scaled to represent the design earthquake (DE) level and the risk-targeted maximum considered earthquake (MCE_R) level. Results of the nonlinear response history analyses indicate no structural damage under DE-level motions for near-fault and far-fault motions and for MCE_R-level far-fault motions, whereas minor structural damage is observed under MCE_R-level near-fault motions. Results of the base-isolated buildings are compared with their fixed-base counterparts. Significant reduction of the superstructure response of the 12-story base-isolated building compared to the fixed-base condition indicates that base isolation can be effectively used in taller buildings to enhance performance. Additionally, the applicability of a rigid superstructure to predict the isolator displacement demand is also investigated. It is found that the isolator displacements can be estimated accurately using a rigid body model for the superstructure for the buildings considered.     

基础隔震结构对随机激励的响应分析

利用过滤白噪声地震动模型，对基础隔震结构进行了随机反应分析。建立了基础隔震结构的运动方程，推导了基础隔震结构位移、加速度对地面随机激励的传递函数表达式，计算了隔震结构与非隔震结构的均方根反应比值。讨论了基础隔震结构主要参数对结构反应的影响。     

Seismic response of nonstructural components attached on multistorey buildings

The maintenance of integrity and functionality of nonstructural components during earthquake excitations is of paramount importance since mechanical failure of those systems can have dramatic consequences in terms of property damage and life safety of the buildings' occupants. This paper explores the dynamic response of nonstructural elements attached on multistory buildings with well‐established floor diaphragm action. Depending on the type of support conditions, seismic response of nonstructural components may be controlled either by acceleration or displacement: Nonstructural components that are subjected to uniform support excitation are controlled primarily by the absolute spectral acceleration developing at their point of attachment on the supporting building. On the contrary, seismic response of multiply supported nonstructural components depends primarily on the relative displacements between successive support points that are imposed by the supporting building during lateral sway. These findings are illustrated from the analytical formulation and its solution through time history analysis of the governing dynamic equation of motion of the primary and secondary components of a system modeled using finite elements. The model encompasses the assembly of a multistory building along with a multiply supported gas pipeline network. It is shown that the dependence of the seismic response of nonstructural components may be linked to the deformed shape of the supporting building at the state of its maximum lateral roof displacement, thereby enabling the definition of design procedures for these systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Seismic analysis of non-linearly base-isolated soil-structure interacting reactor building by way of the hybrid frequency-time-domain procedure

The seismic analysis of a reactor building on a sliding-type base isolation is performed by way of the hybrid frequency-time-domain procedure. The frequency dependence of the foundation stiffness coefficients is duly accounted for in the analysis, although the problem is non-linear. The response results are shown to be reliable by way of comparison with the results of a time-stepping algorithm in the specialized case of constant foundation stiffness coefficients. The fact that such an analysis can be performed is an outstanding result, particularly when considering that no difficulties are encountered in the implementation and in the calculation. The flow chart used to implement the procedure is presented in the paper. The reactor building is analysed for three different sites with each site having three sets of material properties. It appears that the structural response is not strongly affected by the frequency dependence of the foundation stiffness coefficients. This also holds true as far as the response of equipment attached to the superstructure is concerned. For all practical purposes this frequency dependence may be disregarded in favour of constant spring and damping coefficients.     

Seismic response of light equipment in torsional buildings

This paper attempts to study the response of light equipment items attached to a multi-storey building that may be subjected to large torsional deformations during the earthquake excitation. To account for the effect of torsion and translation, each storey of the building is modelled as two degrees of freedom (DOFs), with one DOF for translation and the other for torsion. Perturbation techniques are employed to find closed-form expressions for the modal properties of the combined system in terms of the properties of the individual subsystems. The interaction between the equipment and building is studied for both tuned and detuned modes of vibration. Modal synthesis results obtained using the present technique based on the complete quadratic combination (CQC) method are compared with solutions obtained by other methods.     

考虑结构 - 电气设备相互作用的配电楼系统地震反应分析

对电力系统的配电楼在考虑主体结构和内部电气设备相互作用情况下,进行了线弹性阶段及弹塑性阶段的地震反应分析,并把计算结果与不考虑主体结构和电气设备相互作用的计算结果进行分析比较.结果表明,在小震作用下,结构-电气设备的相互作用对配电楼的影响不明显,配电楼的抗震设计采用传统等效荷载化方法的计算结果是偏安全的;但是在大震作用下,电气设备真实的参与了整个配电楼的动力反应,并对结构产生了相对作用,这种作用是向着不利于配电楼抗震设计的方向发展,电气设备对整个结构动力反应的贡献不能忽略,此时配电楼的抗震设计如仍采用传统等效荷载化的方法是偏于不安全的.该配电楼还存在明显的偏心扭转效应,设计时应引起重视,提高角柱的抗侧移能力.     

节能住宅地震反应分析

为研究节能房屋的抗震能力,本文基于墙体试验研究成果,建立了墙体的恢复力模型及其抗震抗剪强度公式,并对内砖外加气混凝土砌块墙体节能房屋进行了地震反应分析.本项研究为节能住宅的推广应用提供了抗震设计依据.     

Seismic response reduction analysis of large chassis base-isolated structure under long-period ground motions

Long-period structures(e.g. Isolated structures) tend to produce pseudo-resonance with low frequency components of long-period ground motions, resulting in the increase in damage. Stiffness mutation occurs due to the setback in the upper body of the large chassis structure. In the parts with stiffness mutation, the torsion effect caused by the tower is far greater than that of the chassis itself. In this study, a total of 273 ground motions are collected and then filtered into four types, including the near-field ordinary, near-field pulse, far-field ordinary, and farfield harmonic. An 8-degree(0.2 g) fortified large chassis base-isolated structure is established. Furthermore,ETABS program software is used to conduct nonlinear time history analysis on the isolation and seismic model under bi-directional earthquake ground motions. The comparison results show that the seismic isolation effect of the base-isolated structure under long-period ground motions is worse than that associated with ordinary ground motions when the seismic response reduction rate of the large base floor significantly decreases compared with that of the tower. When the inter-story displacement angle and the displacement of isolation layer of the chassis exceeds the limit of Code for Seismic Design of Buildings(GB 50011-2010), it is recommended to adopt composite seismic isolation technology or add limit devices. Under the condition of long-period ground motions, the baseisolated structure reduces the lateral-torsional coupling effect of the large chassis structure, while the torsion response of large chassis' top layer increases. Under long-period ground motions with the same acceleration peak,the response of the base-isolated structure increases much more than that of the seismic structure and the consideration of this impact is suggested to be added to the Code.     

非线性隔震立式浮顶储罐双向地震作用分析

研究浮顶振动对立式储罐地震响应的影响时,假定浮顶运动为均质刚性圆板振动,其波高沿半径方向线性变化,建立了含有波高参量的运动方程、基底剪力和倾覆力矩表达式.同时,在Park恢复力模型的基础上,给出了铅芯橡胶支座的双向耦合恢复力计算模型,并用该模型模拟了铅芯橡胶支座的力-变形非线性行为.采用时程分析方法对双向地震作用下的浮顶储罐地震响应进行分析,并与未考虑浮顶作用以及单向地震作用的储罐地震响应进行对比.数值结果表明,考虑浮顶作用具有降低地震响应的效果;特定工况下,地震响应峰值可能出现在非主震方向.     

隔震建筑结构的强震观测与初步分析

福建省防灾减灾隔震大楼自2005年8月架设强震观测结构台阵以来,已多次记录到大楼地震动力反应,这些宝贵的结构反应资料对于评价隔震效果、分析结构反应特性具有十分重要的价值。本文从中选取了震级在3.7~7.6、震中距在189~3 595km的24次地震的台阵资料,对大楼的振动特性和隔震效果进行了初步分析,得到的结论是:(1)大楼场地土层的特征周期在0.63~0.68 s;(2)中心大楼在小地震作用下表现的振动卓越频率分别为4.7Hz(垂直向)、1.31Hz(南北向)、1.40Hz(东西向),大楼水平向第一阶振型的频率分别为1.41Hz(南北向)、1.59Hz(东西向),即自振周期为0.70 s(南北向)、0.63 s(东西向);(3)大楼在小地震时隔震效果不明显,其南北向隔震效果比东西向好;随着地下室记录峰值的增大,顶层放大倍数有所降低,大楼卓越频率也有所降低;(4)初步分析表明大楼在小震时仍有扭转效应,其扭转周期为0.5 s,有待进一步分析。     

强震观测建筑结构的地震反应分析

选择取得强震观测资料的建筑结构为研究对象，采用有限元法建立结构的动力反应分析模型，利用SAP2000程序分析了结构的地震反应，并同实际记录到的结构地震反应进行了对比分析，讨论了结构模型参数估计和一些不确定因素对模型精度的影响。     

Seismic response of tall building considering soil-pile-structure interaction

The seismic behavior of tall buildings can be greatly affected by non-linear soil-pile interaction during strong earthquakes. In this study a 20-storey building is examined as a typical structure supported on a pile foundation for different conditions: (1) rigid base, i.e. no deformation in the foundation: (2) linear soil-pile system; and (3) nonlinear soil-pile system. The effects of pile foundation displacements on the behavior of tall building are investigated, and compared with the behavior of buildings supported on shallow foundation. With a model of non-reflective boundary between the near field and far field, Novak’s method of soil-pile interaction is improved. The computation method for vibration of pile foundations and DYNAN computer program are introduced comprehensively. A series of dynamic experiments have been done on full-scale piles, including single pile and group, linear vibration and nonlinear vibration, to verify the validity of boundary zone model.     

Seismic response analysis of an irregular base isolated building

This paper assesses the reliability of code-compliant linear and nonlinear dynamic analyses for irregular buildings with base isolation system (BIS). Comprehensive analyses are carried out for a case study comprising a large reinforced concrete multi-storey framed hospital with 327 high-damping rubber bearings. Spectral and time history (linear and nonlinear) analyses were performed on the three-dimensional (3D) finite element model (FEM) of the structure; simplified analyses were also conducted on single-degree-of-freedom (SDOF) systems. It is found that, at damageability limit state, the values of maximum interstorey drifts (d/h) computed with spectral analyses on the three-dimensional FEM range between 1/6 and 1/10 of the code limit (d/h = 0.33%); thus more stringent code limits should be required for buildings with BISs. The maximum floor acceleration is reduced by about 70% with respect to the ground acceleration (free field site); the acceleration profile is uniform along the height of the multi-storey frame. Threshold values of floor accelerations to assess the seismic performance of equipments in buildings with BIS are lacking. At ultimate limit state (ULS), spectral analyses provide values of actions and deformations that are less conservative than those derived through time history analyses. To perform reliable dynamic analyses of base isolated buildings it is crucial to select natural earthquake ground motions compliant with the fundamental period of vibration of the structural system. Nevertheless, it is not straightforward to select adequate natural strong motions in the catalogues available world-wide; buildings incorporating BISs possess periods of vibration which are generally higher than 2.0 s. As a result, distant and high-magnitude earthquakes are effective for base isolated buildings; nevertheless, such earthquakes are scarce in the seismic databases. The outcomes of the present study also demonstrate that simplified linear analyses tend to provide estimates of the response quantities, displacements of base isolators and base shear of the superstructure, which can be reliably employed at preliminary design stage. Spectral analysis results of the 3D model tend to match those of the SDOF systems, even for irregular superstructure, provided that modal mass participating ratios are greater than 85–90%. The results of spectral analyses on both SDOF and three-dimensional FEM envelope the outcomes of linear time histories.     

Structural performance of a base-isolated reinforced concrete building subjected to seismic pounding

The effects of seismic pounding on the structural performance of a base-isolated reinforced concrete (RC) building are investigated, with a view to evaluate the influence of adjacent structures and separation between structures on the pounding response. In particular, seismic pounding of a typical four-story base-isolated RC building with retaining walls at the base and with a four-story fixed-base RC building is studied. Three-dimensional finite element analyses are carried out considering material and geometric nonlinearities. The structural performance of the base-isolated building is evaluated considering various earthquake excitations. It is found that the performance of the base-isolated building is substantially influenced by the pounding. The investigated base-isolated building shows good resistance against shear failure and the predominant mode of failure due to pounding is flexural. Copyright © 2012 John Wiley & Sons, Ltd.     

Seismic response of a RC frame building designed according to old and modern practices

In the paper the seismic response of different variants of the three-story reinforced concrete frame structure SPEAR is compared. The basic structure is representative of building practice before the adoption of seismic codes. This structure has been compared with four modified variants, which were designed partly or completely in accordance with the Eurocode family of standards. For seismic assessment the practice-oriented nonlinear N2 method was used. The results demonstrate the low seismic resistance of buildings designed for gravity loads only. On the other hand, the advantages of new standards are clearly apparent. By taking into account the requirements of Eurocode 8 it is possible to ensure adequate strength, stiffness and ductility. By means of capacity design it is possible to ensure a global plastic mechanism. All these characteristics contribute to the high seismic resistance of structures designed according to Eurocode 8 and to their satisfactory behaviour during earthquakes.