Base isolation systems for earthquake protection of multi-storey shear structures

This paper is a study of the effectiveness of a wide range of bilinear hysteretic isolation systems in shielding multistorey 2-D shear structures from earthquake excitations. Important parameters of the isolation system are identified and their effect on structure response noted. It is shown that isolation systems can be constructed which allow the structure proper to remain purely elastic even during very strong ground motions. It is further shown that the shear responses and base displacements of structures on these isolation systems can be accurately estimated from elastic response spectra of the forcing earthquakes. The philosophy of structure isolation is discussed and an introduction given to the physical devices currently available to provide it.     

Coupled lateral-torsional behaviour of frame structures under earthquake loading

A series of parametrically defined experimental model structures has been tested under earthquake base loading using the SERC national U.K. earthquake simulator. The models have been designed with variable ratios of torsional to lateral stiffness, and with both symmetric and asymmetric mass distributions. This paper first describes the tests carried out to determine the basic dynamic model properties and the establishment of idealized analytical models which give accurate predictions of model behaviour under steady-state loading and free-vibration conditions. Secondly, a detailed discussion is made of the two highly coupled structural models having uncoupled torsional to lateral frequency ratio R_f = 1.2, commenting on the ability of the modal analysis procedures to predict accurately the maximum recorded responses. It is concluded that the theory underestimates the significance of the fundamental torsional mode of vibration in the combined structural response, and overestimates the contribution of the first lateral mode. These effects compensate each other on the side of the structure which is most severely affected by torsional response, but produce large inaccuracies on the side of the building which is commonly assumed to be affected beneficially by torsional coupling.     

日本建筑结构隔震减震研究新进展

本文介绍了日本在建筑结构隔震，减震研究方面取得的几项新进展，其中基础隔震板片结构体系是一种新型隔震结构体系，纳米结晶锌铝合金振动控制阻尼器是一种取得专利的新型减震阻尼器，无粘结钢支撑体系是一种比较新颖的减震支撑体系，跷摆振动控制设计是一个新颖的减震设计概念。     

高层隔震结构扭转分析

为研究高层隔震结构在地震作用下的扭转效应,用Etabs软件建立1个高层抗震结构和4个具有不同扭转特性的高层隔震结构的空间模型进行地震响应分析,以验证此原则上部结构质量中心与隔震层刚度中心的重合与否对结构扭转效应的影响程度,而后考察偏心高层隔震结构在偶然偏心地震作用下结构的扭转效应。结果表明:由于地震作用的减小,扭转效应要远小于原抗震结构,且隔震本身对于结构扭转效应的抑制效果要好于上述原则;扭转效应的减震率大于平动效应的减震率。布置在隔震层平面外围的铅芯橡胶隔震支座对隔震层的扭转有一定的控制作用。     

Vibration control of structures under earthquake loading by three-stage friction-grip elements

The behaviour of a building subjected to a strong-motion earthquake depends highly on its energy dissipation capacity. By introducing three-stage friction-grip elements, the energy dissipation within each storey can be ‘designed’ according to definable stages (e.g. serviceability, medium- and strong-motion earthquake). Tests on simple steel-concrete- and steel-steel-friction-grip connections have proved their very satisfactory behaviour under high dynamic loading, showing no considerable damage when well designed. Examples of horizontal stiffening elements are given to illustrate the construction of three-stage elements with either steel-steel- or steel-concrete-friction-grip joints. By computing the response of a seven storey building designed as a steel frame (‘ductile system’ philosophy), concrete core (‘stiff system’-philosophy) and three-stage truss and being subjected to the 1940 El Centre earthquake, the superior performance of three-stage elements and its economic advantage over the other two systems becomes apparent. Although this is just one illustrative example, it is nevertheless an important one, because it resembles the type of building that would respond best to the application of three-stage building brakes. Finally, the result of an online test on a three-stage element is given. This realistic earthquake simulation proved the behaviour to be adequately close to the basic philosophy and yielded important design implications for three-stage elements.     

Inelastic spectra to predict period elongation of structures under earthquake loading

Period lengthening, exhibited by structures when subjected to strong ground motions, constitutes an implicit proxy of structural inelasticity and associated damage. However, the reliable prediction of the inelastic period is tedious and a multi‐parametric task, which is related to both epistemic and aleatory uncertainty. Along these lines, the objective of this paper is to investigate and quantify the elongated fundamental period of reinforced concrete structures using inelastic response spectra defined on the basis of the period shift ratio (T_in/T_el). Nonlinear oscillators of varying yield strength (expressed by the force reduction factor, R_y), post‐yield stiffness (a_y) and hysteretic laws are examined for a large number of strong motions. Constant‐strength, inelastic spectra in terms of T_in/T_el are calculated to assess the extent of period elongation for various levels of structural inelasticity. Moreover, the influence that structural characteristics (R_y, a_y and degrading level) and strong‐motion parameters (epicentral distance, frequency content and duration) exert on period lengthening are studied. Determined by regression analyses of the data obtained, simplified equations are proposed for period lengthening as a function of R_y and T_el. These equations may be used in the framework of the earthquake record selection and scaling. Copyright © 2015 John Wiley & Sons, Ltd.     

Inelastic torsion of multistorey buildings under earthquake excitations

The inelastic earthquake response of eccentric, multistorey, frame‐type, reinforced concrete buildings is investigated using three‐ and five‐storey models, subjected to a set of 10, two‐component, semi‐artificial motions, generated to match the design spectrum. Buildings designed according to the EC8 as well as the UBC‐97 code were included in the investigation. It is found that contrary to what the simplified one‐storey, typical, shear‐beam models predict, the so‐called ‘flexible’ side frames exhibit higher ductility demands than the ‘stiff’ side frames. The substantial differences in such demands between the two sides suggest a need for reassessment of the pertinent code provisions. This investigation constitutes one of the first attempts to study the problem of inelastic torsion by means of realistic, multistorey inelastic building models. Additional studies with similar or even more refined idealizations will certainly be required to arrive at definite results and recommendations for possible code revisions. Copyright © 2005 John Wiley & Sons, Ltd.     

Mechanisms of failure for shallow foundations under earthquake loading

It is widely known that the bearing capacity of a shallow foundation is reduced when the foundation is subjected to rocking moments and horizontal loads during an earthquake event. Analytical solutions generally require an assumption to be made of the kinematic failure mechanism in the soil, when the true failure mechanism is unknown. This paper discusses a series of experiments carried out on a new 1g shaking table at Cambridge University in order to measure the displacements of a shallow foundation due to seismic loading and also the development of the failure mechanism within the soil. The failure mechanism was studied using the technique of Particle Image Velocimetry (PIV), combined with high-speed videography and photogrammetry. In this paper, the failure mechanisms observed in these experiments will be compared with the theoretical results found from upper- and lower-bound solutions and the effects of such parameters as earthquake magnitude, frequency and embedment ratio (and hence surcharge) will be discussed.     

Newmark sliding block model for pile-reinforced slopes under earthquake loading

Recent studies have demonstrated that the use of a discretely-spaced row of piles can be effective in reducing the deformations of slopes in earthquakes. In this paper, an approximate strain-dependant Newmark sliding-block procedure for pile-reinforced slopes has been developed, for use in analysis and design of the piling scheme, and the model is validated against centrifuge test data. The interaction of the pile within the slipping soil was idealised using a non-linear elasto-plastic (P–y) model, while the interaction within the underlying stable soil was modelled using an elastic response model in which (degraded) soil stiffness is selected for an appropriate amount of shear strain. This combined soil–pile interaction model was incorporated into the improved Newmark methodology for unreinforced slopes presented by Al-defae et al. [1], so that the final method additionally incorporates strain-dependent geometric hardening (slope re-grading). When combined with the strain-dependent pile resistance, the method is therefore applicable to analysis of both the mainshock and subsequent aftershocks acting on the deformed slope. It was observed that the single pile resistance is mobilised rapidly at the start of a strong earthquake and that this and the permanent slope deformation are therefore strongly influenced by pile stiffness properties, pile spacing and the depth of the slip surface. The model shows good agreement with the centrifuge test data in terms of the prediction of permanent deformation at the crest of the slope (important in design for selecting an appropriate pile layout/spacing i.e. S/B) and in terms of the maximum permanent bending moments induced in the piles (important for appropriate structural detailing of the piles), so long as the slip surface depth can be accurately predicted. A method for doing this, based on limit analysis, is also presented and validated.     

Maximum response of asymmetric structures subject to A multicomponent earthquake

A simple and general technique to obtain a six-component earthquake input motion of a rigid foundation using three-linear-components earthquake record is presented. Using the multicomponent input, the maximum structural response to an arbitrarily oriented earthquake is derived. The method is applied to determine the response spectrum for a two-d.o.f. laterally and torsionally coupled linear system. The effects of foundation geometry, structural properties and apparent wave velocity on the structural response are investigated. It is found that increasing the foundation size reduces the peak values of the input motion; this reduction is more pronounced for a square foundation than for a circular one.     

Probabilistic evaluation of liquefaction potential under earthquake loading

This paper presents a procedure to perform the risk analysis for ground failure by liquefaction. The first part of this study describes the differential equation of a smooth hysteretic model to characterize the behavior of the soil under random loading. The parameters of the proposed model to represent the experimental relationship are discussed. The second part of this study is to develop a method to calculate the probability that a specified volume of soil will liquefy at a given depth in the deposit. The liquefaction is defined as the result of cumulative damage caused by seismic loading. The fatigue life of soil can be determined on the basis of the N---S relationship and Miner's cumulative damage law. The rain-flow method is used to count the number of cycles of stress response of the soil deposit. Finally, the probability of liquefaction is obtained by integration over all the possible ground motion and the fragility curves of liquefaction potential. The sensitivity of the reliability against liquefaction to soil system parameters is also examined.     

Buckling of cylindrical liquid-storage tanks under earthquake loading

The earthquake response behaviour of a cylindrical wine storage tank similar to many that were damaged in Livermore, California during the January 1980 earthquake was studied on the University of California shaking table. Tests of the 9.5 ft diameter by 20 ft high tank, with simulated earthquake accelerations up to 0.95 g, induced buckling patterns similar to those observed after the actual earthquake. Observed peak axial compression stresses in the test tank wall were substantially higher than those assumed in typical design standards, demonstrating the need for further study of the buckling problem in tanks free to uplift during earthquake excitation.     

Base isolation by free rolling rods under basement

A new base isolation method is proposed for the protection of structures. Because of the efficiency of the isolation devices, the isolated structure may be made to remain elastic throughout major earthquakes. This device consists of two sets of mutually orthogonal free rolling rods under the basement of the structure. Since the coefficient of rolling friction of the rods is very small in practice, the structure can be isolated excellently from the support excitation. In this paper, the analytical method and the response of the isolated system for different parameters, such as the periods of the structure, the coefficient of rolling friction and the masses of rolling rods, are presented. The results indicate that the proposed method is excellent in isolating the structure from support excitations, as expected.     

Centrifuge modeling of offshore wind foundations under earthquake loading

The construction of large offshore wind turbines in seismic active regions has great demand on the design of foundations. The occurrence of soil liquefaction under seismic motion will affect the stability of the foundations and consequently the operation of the turbines. In this study, a group of earthquake centrifuge tests was performed on wind turbine models with gravity and monopile foundations, respectively, to exam their seismic response. It was found that the seismic behavior of models was quite different in the dry or saturated conditions. Each type of foundation exhibited distinct response to the earthquake loading, especially in the offshore environment. In the supplementary tests, several remediation methods were evaluated in order to mitigate the relatively large lateral displacement of pile foundation (by fixed-end pile and multi-pile foundation) and excessive settlement of gravity foundation (by densification, stone column, and cementation techniques).     

Ductility demands on buckling-restrained braced frames under earthquake loading

Accurate estimates of ductility demands on buckling-restrained braced frames (BRBFs) are crucial to performance-based design of BRBFs. An analytical study on the seismic behavior of BRBFs has been conducted at the ATLSS Center, Lehigh University to prepare for an upcoming experimental program. The analysis program DRAIN-2DX was used to model a one-bay, four-story prototype BRBF including material and geometric nonlinearities. The bucklingrestrained brace (BRB) model incorporates both isotropic and kinematic hardening. Nonlinear static pushover and timehistory analyses were performed on the prototype BRBF. Performance objectives for the BRBs were defined and uscd to evaluate thc time-history analysis results. Particular emphasis was placed on global ductility demands and ductility demands oa the BRBs. These demands were compared with anticipated ductility capacities. The analysis results, along with results from similar previous studics, are used to evaluate the BRBF design provisions that have been recommended for codification in the United States. Thc results show that BRB maximum ductility demands can be as high as 20 to 25. These demands significantly exceed those anticipated by the BRBF recommended provisions. Results from the static pushover and timehistory analyses are used to demonstrate why the ductility demands exceed those anticipated by the recommended provisions.The BRB qualification testing protocol contained in the BRBF recommended provisions is shown to be inadequate because it requires only a maximum ductility demand of at most 7.5. Modifications to the testing protocol are recommended.     

基础隔震结构随机地震响应分析的复模态法

本文对多自由度基础隔震结构的随机地震响应问题进行了系统研究，首先建立了运动方程，然后用第一振型将上部结构展开，针对所得方程为非经典阻尼、非对称质量和非对称刚度情况，用复模态法解耦，获得了以第一振型表示的结构地震响应的解析解，对单自由度体系，此解即为结构响应的精确解，从而建立了两自由度体系在任意非经典阻尼与非对称质量和刚度情况下随机地震响应解析解分析的一般方法。本文方法也可用于带TMD减震结构、无损伤“加层减震”加固结构的随机地震响应分析与优化设计。     

Local member response in offshore structures under earthquake excitations

It is pointed out that local vibrations of certain members in earthquake excited offshore structures can induce substantial stresses that may often dominate the design of these members. Such vibrations are not accounted for in dynamic analyses of structural models with masses lumped at the joints. A practical, easy to implement solution to this problem is recommended and results from an actual structure, subjected to a three-component real earthquake, are presented demonstrating the importance of local inertia effects.     

Experimental behaviour of reinforced concrete walls under earthquake loading

The experimental work and first results of a recently completed experimental research programme investigating the response of reinforced concrete (RC) walls under earthquake (EQ) loading are discussed in this paper. A brief literature review is given as a prelude to the outline of research objectives. The tests are presented in two groups according to the scale of models. For the 1:5 scale tests, a modified similitude relation for small scale reinforced concrete dynamic modelling is developed. Based on the chosen model parameters, the design of the isolated RC walls is given. The test-rig set-up and the EQ input signals suitable for testing the model on the Imperial College shake-table are also discussed. Preliminary observations regarding stiffness, strength and failure modes of the RC wall models are given. Experimental results from the shake-table are compared to tests, at the same scale, under static cyclic conditions. For the scale 1:2–5 cyclic tests a different test-rig assembly is designed. The test results are given in three pairs of flexurally similar walls followed with general observations and discussion. Finally, conclusions are drawn regarding experimental procedures and behaviour patterns of the tested models.     

A semi-empirical formula for evaluating residual strain of soils under earthquake loading

A new semi-empirical formula for evaluating the residual strain of soils under earthquake loading is presented in this paper based on the incremental method and the increment model proposed by the authors. When the incident loading is uniform, the results calculated by the new formula are nearly the same as those by the existing formula. For excitation of the random earthquake loading, the results calculated by the new formula are compared to the results obtained by dynamic triaxial tests. The dynamic triaxial tests had been performed considering different seismic waves, confining stresses,consolidation ratios, and types of cohesive soils. The comparison between the calculated and tested results indicate that the presented formula can efficiently and practically describe the time-dependent process of the soil residual strains under actual seismic loads.     

Peak earthquake response of structures under multi-component excitations

Accurate estimation of the peak seismic responses of structures is important in earthquake resistant design. The internal force distributions and the seismic responses of structures are quite complex, since ground motions are multi- directional. One key issue is the uncertainty of the incident angle between the directions of ground motion and the reference axes of the structure. Different assumed seismic incidences can result in different peak values within the scope of design spectrum analysis for a given structure and earthquake ground motion record combination. Using time history analysis to determine the maximum structural responses excited by a given earthquake record requires repetitive calculations to determine the critical incident angle. This paper presents a transformation approach for relatively accurate and rapid determination of the maximum peak responses of a linear structure subjected to three-dimensional excitations within all possible seismic incident angles. The responses can be deformations, internal forces, strains and so on. An irregular building structure model is established using SAP2000 program. Several typical earthquake records and an artifi cial white noise are applied to the structure model to illustrate the variation of the maximum structural responses for different incident angles. Numerical results show that for many structural parameters, the variation can be greater than 100%. This method can be directly applied to time history analysis of structures using existing computer software to determine the peak responses without carrying out the analyses for all possible incident angles. It can also be used to verify and/or modify aseismic designs by using response spectrum analysis.