Effect of two horizontal components of earthquake on nonlinear response of torsionally coupled base isolated structures

Torsion in base‐isolated structures using bilinear rubber isolators effected by two horizontal components of earthquake simultaneously is investigated under various principal parameters including number of storeys; ratio of uncoupled torsional frequency to lateral frequency in the superstructure; ratio of uncoupled torsional frequency to lateral frequency in the isolation system; mass eccentricity in the superstructure and isolation system; and, in addition, the direction of eccentricity. Structure was supposed a shear structure and it was modeled by using concentrated mass and springs. Isolators are modelled by using nonlinear springs considering the interaction of isolator behaviour in two directions. The effect of variation in considered parameters on the behaviour of the superstructure and isolation system is investigated under seven ground motions. It is demonstrated that the parameters affect asymmetric structure behaviour with respect to symmetric one. The results of our investigations help us to understand the asymmetric structure behaviour under bidirectional earthquake by comparing it with a symmetric one. These results demonstrate that asymmetry in the superstructure or isolation system could have a significant effect on the torsional behaviour of isolated structures. In addition, the results show that the use of rigid superstructures and calculation of dynamic torsion by multiplying eccentricity by the dynamic base shear are unacceptable assumptions. Torsional effect on the isolated structural behaviour is amplified by considering two horizontal components of earthquake and comparing them with just one horizontal component. Copyright © 2010 John Wiley & Sons, Ltd.     

组合基础隔震房屋模型振动台试验研究

基础隔震能有效地降低上部结构的地震作用,但不同的隔震体系,隔震效果不同.本文根据叠层橡胶支座和滑板摩擦隔震支座的特点,提出组合基础隔震系统,以中国和日本国的在建隔震房屋为工程背景,进行了组合基础隔震房屋模型和基础固定房屋型模拟地震动振动台试验,并对两种试验结果进行了能量分析,分析结果表明组合基础隔震系统中叠层橡胶支座能自动复位,滑板摩擦隔震支座具有良好的耗能能力,隔震效果明显,组合基础隔震系统是一种简单、经济、有效具有广泛应用价值的基础隔震形式.本文获得的大量振动台试验结果可以为隔震设计和加固中使用这种隔震系统及进一步的理论分析提供重要依据.     

Cost viability of a base isolation system for the seismic protection of a steel high-rack structure

In this study the effects and costs of implementing a base isolation system for the mitigation of the seismic risk of an existing externally-braced steel frame rack structure are analysed by means of nonlinear static (pushover) analysis. Various plan asymmetric variants, with different realistic distributions of the payload mass and occupancy levels, have been investigated under two seismic intensities. The results obtained are presented as floor plan projection envelopes of the top displacements and as plastic hinge damage patterns of the superstructure. In the presented cost evaluation, the cost of the implementation of the proposed base isolation system is compared with the estimated costs of structural repairs to the damaged structural members of the superstructure, as well as with estimated expenses of the downtime period and content damage. The results have shown that base isolation is, in general, not economically feasible for lower ground motion intensities, whereas it could be of great benefit in the case of moderate and high intensities. A simple rough cost estimation study, based on the obtained plastic hinge patterns, showed that the inclusion of the downtime period costs and content damage costs might be important parameters, which — if taken into account — could make such an isolation system viable also for lower ground motion intensities. The other benefits brought by seismic isolation, such as savings on the building design costs, reductions in the threat to employees’ lives, and others, were, however, not included in the presented study. The comparison is done only for two deterministic scenarios of seismic attack, e.g. for design ground motion intensity (a _g =0.175 g) and for increased intensity with a _g =0.25 g indicating the Maximum Considered Earthquake level.     

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.     

Seismic friction base isolation performance using demolished waste in masonry housing

The conversion of large amount of demolished waste into alternate source of building material will contribute not only as a solution to growing waste problem of waste disposal, but also it will conserve the natural resources of other building material and thereby reduce the cost of construction. The present work makes an effort to safe and economic use of recycled mortar as a supplementary material. Conventional and recycled brick prisms were casted with varying percentage of demolished waste added (0%, 10%, 20%, 30%) replacing cement and tested under compression testing machine. As the replacement is increasing, the strength is decreasing. A 1:4 scale single storey brick model was fabricated on shake table for dynamic testing using pure friction isolation system (friction material for coarse dry sand, μ = 0.36). Pure friction isolation technique can be adopted economically in developing countries where low-rise building prevails due to their low cost. The superstructure was separated from the foundation at plinth level, so as to permit sliding of superstructure during severe earthquake. The observed values of acceleration and displacement responses compare fairly with the analytical values of the analytical model except in displacement beyond 2.4 kN. It also concluded that 20% replacement of cement by demolished waste could be safely adopted without endangering the safety of the masonry structures under seismic load.To have an idea that how much energy is dissipated through this isolation, the same model with fixed base was tested and results were compared with the isolated free sliding model and it has been observed that more than 70% energy is dissipated through this pure friction isolation technique. In case of base isolation, no visible cracks were observed up to a table force of 4.25 kN (1300 rpm), whereas for fixed base failure started at 800 rpm and complete bond failure was observed at 1300 rpm.     

Limitations of height‐to‐width ratio for base‐isolated buildings under earthquake

The limitation of height‐to‐width ratio (HWR) for a base‐isolated building with elastomeric rubber bearings is of considerable concern to structural design engineers. Guidelines and codes on this type of building have to deal with this issue. Nevertheless, until now, no systematical and quantitative studies have been done on this problem for base‐isolated buildings. For this reason, the main objective of this paper is to focus on investigations on the limit of the HWR for the isolated building with rubber bearings under different conditions subjected to earthquake excitations. The simplified formulation is derived to explore the rules of seismic responses for the structural system and some influential factors, such as the site soil conditions, seismic ground motion intensity, period of the isolated system, period of the superstructure and layout of isolators, are studied and discussed. According to the numerical results, it has been found that the effects of site soil conditions on the HWR limit values are important: the softer the site is, the smaller the HWR limit value is under different seismic intensities. The predominant period of an isolated building also plays a considerable role in the HWR limit value, namely, the isolated building with a longer period may have a relatively large HWR value; and the stiffness of the superstructure affects the HWR limit value little. Furthermore, an effective method to improve the HWR limit value is proposed. Copyright © 2006 John Wiley & Sons, Ltd.     

Analytical and Numerical Study of a Smart Sliding Base Isolation System for Seismic Protection of Buildings

The seismic response of a single–story steel building frame with a smart base isolation system is evaluated. The isolation system consists of sliding bearings combined with an adaptive fluid damper. The damping capacity of the fluid damper can be modulated in real time based on feedback from the earthquake ground motion and superstructure response. The adaptive capabilities of the fluid damper enable the isolation system displacement to be controlled while simultaneously limiting the interstory drift response of the superstructure. This paper concentrates on the development of analytical models of the smart isolation system and control algorithms for operation of the system. In general, the results from numerical simulations demonstrate that, for disparate earthquake ground motions, the smart isolation system is capable of simultaneously limiting both the response of the isolation system and the superstructure.     

几种隔震支座性能的对比试验研究

近年来,基础隔震的研究与应用取得了飞速的进展.对基础隔震中常见几种隔震支座(踏步支座、滑移支座、橡胶支座以及未隔震的固定支座)的性能进行对比评价,采用相同的框架结构模型作为上部结构,在不同支座的情况下输入不同强度的E1 Centro波和Northridge波,并记录分析上部结构的相应反应,以分析不同支座在同一地震下以及...     

Earthquake analysis of isotropic asymmetric multistory buildings

Isotropic multistory buildings are the ones characterized by the property: all load‐resisting planar frames have proportional lateral stiffness matrices. In the present paper it is proved that the modal analysis of an N‐story isotropic asymmetric, torsionally coupled, building (a problem of order 3N) can be separated into two independent sub‐problems: (a) a sub‐problem that corresponds to a single‐story asymmetric, torsionally coupled, building (a problem of order 3); and (b) a sub‐problem that corresponds to an N‐story, torsionally uncoupled, planar frame (a problem of order N). It is also demonstrated that the orientation of peak modal seismic forces of the building is independent of the orientation of seismic excitation, which affects only their size. The separation provides a better insight into the structural behavior of asymmetric multistory buildings under earthquake ground motion. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of vertical component of earthquake on the response of pure-friction base-isolated asymmetric buildings

This study has been conducted to evaluate the effects of the vertical component of earthquake on the response of pure-friction base-isolated asymmetric buildings. The pure-friction base-isolated asymmetric building is idealized as a three-dimensional single-storey building resting on sliding supports. The sliding surface is modeled by interface rigid contact elements. The response of this idealized system subjected to three-component (including vertical component) and two-component (excluding vertical component) of earthquake excitations is investigated. As an example, the Northridge 1994 earthquake record is used on the structural model. The performance of asymmetric pure-friction base-isolated system is investigated with the help of peak response ratio of sliding to fixed base systems. It is observed that the vertical component of the strong earthquake excitation, significantly affects the response of pure-friction base-isolated torsionally coupled system.     

橡胶垫隔震支座结构高宽比限值研究

本文对叠层橡胶垫隔震结构的高宽比限值进行了研究,导出了实用的简化计算公式,分析了地震动峰值、场地条件、隔震层刚度、上部结构刚度(反映了上部结构类型)、橡胶垫的布置等因素对隔震结构高宽比限值的影响及其变化规律。给出了不同条件下具有统计意义的隔震结构高宽比限值的定量范围,并提供了提高隔震结构高宽比的有效方法,得出了一些具有实际意义的结论,为工程计算与设计提供了方法和依据。本文结果可供编制有关规范和规程参考。     

房屋建筑中间层隔震研究

房屋建筑隔震的新体系 中间层隔震不同于基底隔震。本文通过SAP2 0 0 0程序对某工程二层框架平台上多塔楼与平台间隔震与不隔震的对比分析 ,探讨了中间层隔震的隔震效果     

Using orthogonal pairs of rollers on concave beds (OPRCB) as a base isolation system—part I: analytical,experimental and numerical studies of OPRCB isolators

A somehow new isolating system is introduced for short‐ to mid‐rise buildings. It does not need high technology for manufacturing and is not costly, contrary to other existing systems like lead‐rubber bearing or friction pendulum bearing systems. Each isolator of the proposed system consists of two Orthogonal Pairs of Rollers on Concave Beds (OPRCB). Rolling rods installed in two orthogonal directions make possible the movement of the superstructure in all horizontal directions. The concave beds, in addition to giving the system both restoring and re‐centring capabilities, make the force–displacement behaviour of the isolators to be of hardening type. The results of the studies on the specifications of the proposed isolating system and its application to buildings can be presented in two parts. Part I relates to the analytical formulations and the results of experimental and numerical studies of the system's mechanical feature, including its dynamical properties, and part II focuses on the effectiveness of the proposed isolation system in seismic response reduction of low‐ to mid‐rise buildings. In part I of the work, presented in this paper, at first general features of the OPRCB isolator are explained and the analytical formulation, governing its dynamic motion, is derived and discussed in detail. Then, the results of experimental and numerical investigations, including the lateral load displacement relationship of the OPRCB isolators under various vertical loads, obtained by both Finite Element Analyses (FEA) and laboratory tests are presented (FEA results have been verified by the laboratory tests). Finally, responses of some Single Degree of Freedom (SDOF) systems, isolated by OPRCB devices, subjected to simultaneous effect of horizontal and vertical ground motions, are presented and compared with responses of their fixed‐base counterparts. Based on the numerical calculations, it is observed that the oscillation period of the isolated SDOF system is independent of its mass, the initial amplitude of its free vibration response and the value of rolling resistance coefficient. With regard to seismic response reduction it is seen that the amount of absolute accelerations in the SDOF systems, isolated by OPRCB devices, can be reduced drastically in comparison with the fixed‐base systems. Results also show that if the rollers and cylindrical beds are made of high‐strength steel materials, the system can be used effectively under the vertical loads of about the axial forces of ground floor columns in ordinary buildings up to 14 storeys. Copyright © 2010 John Wiley & Sons, Ltd.     

基于弹塑性限位装置的基础隔震建筑地震碰撞行为

在近场速度脉冲型地震作用下,基础隔震建筑可能产生过大的支座变形,为了防止结构和隔震沟边缘产生刚性碰撞并对上部结构造成损伤,建议在隔震层高度用弹塑性限位装置来防止结构产生过大位移并实现一定程度的消能减震。提出了一组动力学方程,将建筑结构模型和Bouc-Wen单元串联,可以考虑隔震间隙的非线性动力响应。考虑了速度脉冲周期对结构响应的放大作用,根据弹塑性动力碰撞分析方程,以3组不同高度的钢框架结构基准模型为算例,计算了限位装置的弹性刚度和屈服力等因素对不同结构的弹塑性响应。结果表明：恰当选取用于碰撞限位的弹塑性限位装置的碰撞刚度与屈服力,既能有效限制隔震层的水平位移,上部结构也不会产生过大的反向动力响应,能够充分保护结构;在较易激发类共振效应的速度脉冲型地震动作用下,上部结构最大响应同弹塑性限位装置的弹性刚度与屈服力呈正相关性。     

基础隔震大跨异形钢连廊连体结构地震易损性分析

针对地震中大跨连廊与主体结构连接处容易破坏的特征,以某大跨异形钢连廊连体结构为研究对象,开展基础隔震与非隔震连体结构的地震易损性对比分析.首先,对原连体结构进行基础隔震设计,并采用有限元软件SAP2000分别建立隔震与非隔震连体结构的数值模型.然后,定义隔震连体结构中框架-剪力墙结构、连廊橡胶支座端、连廊预埋锚固端和基...     

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.     

What is the relationship between indoor air quality parameters and airborne microorganisms in hospital environments? A systematic review and meta-analysis

Airborne microorganisms in hospitals have been associated with several hospital-acquired infections (HAIs), and various measures of indoor air quality (IAQ) parameters such as temperature, relative humidity, carbon dioxide (CO₂), particle mass concentration, and particle size have been linked to pathogen survival or mitigation of pathogen spread. To investigate whether there are quantitative relationships between the concentration of airborne microorganisms and the IAQ in the hospital environment. Web of Science, Scopus and PubMed databases were searched for studies reporting airborne microbial levels and any IAQ parameter(s) in hospital environments, from database inception to October 2020. Pooled effect estimates were determined via random-effects models. Seventeen of 654 studies were eligible for the meta-analysis. The concentration of airborne microbial measured as aerobic colony count (ACC) was significantly correlated with temperature (r = 0.25 [95% CI = 0.06–0.42], p = 0.01), CO₂ concentration (r = 0.53 [95% CI = 0.40–0.64], p ˂ 0.001), particle mass concentration (≤5 µg/m³; r = 0.40 [95% CI = 0.04–0.66], p = 0.03), and particle size (≤5 and ˃5 µm), (r = 0.51 [95% CI = 0.12–0.77], p = 0.01 and r = 0.55 [95% CI = 0.20–0.78], p = 0.003), respectively, while not being significantly correlated with relative humidity or particulate matter of size >5 µm. Conversely, airborne total fungi (TF) were not significantly correlated with temperature, relative humidity, or CO₂ level. However, there was a significant weak correlation between ACC and TF (r = 0.31 [95% CI = 0.07–0.52], p = 0.013). Although significant correlations exist between ACC and IAQ parameters, the relationship is not definitive; the IAQ parameters may affect the microorganisms but are not responsible for the presence of airborne microorganisms. Environmental parameters could be related to the generating source, survival, dispersion, and deposition rate of microorganisms. Future studies should record IAQ parameters and factors such as healthcare worker presence and the activities carried out such as cleaning, sanitizing, and disinfection protocols. Foot traffic would influence both the generation of microorganisms and their deposition rate onto surfaces in the hospital environment. These data would inform models to improve the understanding of the likely concentration of airborne microorganisms and provide an alternative approach for real-time monitoring of the healthcare environment.     

Assessment of the stability of isolated vibratory building services systems and the use of inertia blocks

Inertia blocks are commonly used by engineers to stabilize vibration-isolated building services equipment. However, no comprehensive guidelines for the selection of inertia blocks or indices for assessing the effect of inertia blocks on the stability of the vibratory system are available. In this paper, the level of “power transmissibility” is used to assess the performance of vibration isolators, and the level of the “vibration velocity transmissibility” of the supporting floor structure, the “mounted vibration velocity”, and the “mounted rotational velocity level” of the vibratory machine are proposed to assess the stability of the vibratory system with various inertia blocks. The results primarily indicate that the use of an inertia block does not affect the performance of vibration isolation. Instead, it decreases the vibration velocity and rotational velocity of the isolated vibratory machine; so it can increase the stability of the vibratory system regardless of whether the machine has slightly or highly uneven mass distribution. The results reveal that, for the machine with highly uneven mass distribution, the degree of an larger inertia block is required to enhance the stability of the isolated vibratory system. Therefore, in addition to proposing indices for assessing the stability of the vibratory system, this paper also provides an insight into the selection of the inertia block based on the mass distribution of an isolated vibratory machine.     

自动复位基础隔震体系的试验研究

本文建议了一种多自由度自动复位基础隔震体系,并建立了其计算模型和运动方程。对两质点以滚球隔震模型进行了振动台试验。结果表明,与基础固定钉比,隔震体系对上部结构的地震反应具有明显隔震效果。     

Seismic behavior of piled raft with ground improvement supporting a base-isolated building on soft ground in Tokyo

The static and seismic behavior of a piled raft foundation, supporting a 12-story base-isolated building in Tokyo, is investigated by monitoring the soil–foundation–structure system. Since the building is located on loose silty sand, underlain by soft cohesive soil, a piled raft with grid-form deep cement mixing walls was employed to cope with the liquefiable sand as well as to improve the bearing capacity of the raft foundation. To confirm the validity of the foundation design, field measurements were carried out on the ground settlements, the pile loads, the contact pressure and the pore-water pressure beneath the raft from the beginning of the construction to 43 months after the end of the construction.On March 11, 2011, 30 months after the end of the construction, the 2011 off the Pacific coast of Tohoku Earthquake struck the building site. The seismic response of the ground and the foundation–structure system was successfully recorded during the earthquake, and a peak horizontal ground acceleration of 1.75 m/s² was observed at the site of the building. Based on static and dynamic measurement results, it was found that there was little change in the foundation settlement and the load sharing between the raft and the piles before and after the earthquake. It was also found that the horizontal accelerations of the superstructure were reduced to approximately 30% of those of the ground near the ground surface by the input losses due to the kinematic soil–foundation interaction in addition to the base isolation system.Consequently, the piled raft with grid-form deep cement mixing walls was found to be quite stable in the soft ground during and after the earthquake.