Dynamic soil–structure interaction effects on the seismic response of asymmetric buildings

An approach is formulated for the linear analysis of three-dimensional dynamic soil–structure interaction of asymmetric buildings in the time domain, in order to evaluate the seismic response behaviour of torsionally coupled buildings. The asymmetric building is idealized as a single-storey three-dimensional system resting on different soil conditions. The soil beneath the superstructure is modeled as linear elastic solid elements. The contact surface between foundation mat and solid elements of soil is discretised by linear plane interface elements with zero thickness. An interface element is further developed to function between the rigid foundation and soil. As an example, the response of soil–structure interaction of torsionally coupled system under two simultaneous lateral components of El Centro 1940 earthquake records has been evaluated and the effects of base flexibility on the response behaviour of the system are verified.     

Earthquake‐induced interaction between adjacent reinforced concrete structures with non‐equal heights

The influence of the structural pounding on the ductility requirements and the seismic behaviour of reinforced concrete structures designed to EC2 and EC8 with non‐equal heights is investigated. Special purpose elements of distributed plasticity are employed for the study of the columns. Two distinct types of the problem are identified: Type A, where collisions may occur only between storey masses; and Type B, where the slabs of the first structure hit the columns of the other (72 Type A and 36 Type B pounding cases are examined). Type A cases yielded critical ductility requirements for the columns in the pounding area mainly for the cases where the structures were in contact from the beginning of the excitation. In both pounding types the ductility requirements of the columns of the taller building are substantially increased for the floors above the highest contact storey level probably due to a whiplash behaviour. The most important issue in the pounding type B is the local response of the column of the tall structure that suffers the hit of the upper floor slab of the adjacent shorter structure. In all the examined cases this column was in a critical condition due to shear action and in the cases where the structures were in contact from the beginning of the excitation, this column was also critical due to high ductility demands. It can be summarized that in situations of potential pounding, neglecting its possible effects leads to non‐conservative building design or evaluation that may become critical in some cases. Copyright © 2004 John Wiley & Sons, Ltd.     

Inelastic response and design criteria of plan-wise asymmetric systems

The study of the torsional response of buildings in the inelastic range of behaviour is of great interest since the ability of structures to resist strong earthquakes mainly relies on their ductility and capacity for energy dissipation. Furthermore, an examination of the performance of structures during past earthquakes demonstrates that plan-asymmetric buildings suffered greater damage due to torsional response. The paper deals with this subject by analysing a model which idealizes a one-storey building with resisting elements oriented along two perpendicular directions. In addition to the parameters of the elastic behaviour, the inelastic system response depends on full yield capacity and plan-wise strength distribution. The influence of the criterion adopted for the design of resisting elements on local ductility demand and damage has been evaluated by parametric analysis. In particular, a comparison has been carried out between systems with equal design levels for all elements and systems with design levels dependent on the element location. For a given elastic behaviour and total capacity, the strength distributions in plan have been defined which minimize ductility demand and structural damage. Finally, based on these findings, responses from models designed according to several seismic codes have been compared.     

使用偏心支撑减小不规则高层建筑的扭转振动效应

文章对比分析了一个不规则高层钢筋混凝土框-剪结构和在原结构基础上增设偏心支撑后的新结构的抗震性能,比较了它们的自振特性以及层间位移等地震作用效应。针对不规则高层建筑在地震作用下不可避免的扭转振动问题,提出在结构中适当地增设偏心支撑来减小结构的扭转振动效应及其它结构地震反应这种简单可行、经济适用的方法。     

On the dynamic response of regular structures exhibiting tilt

The mechanical characteristics of earthquake‐resistant structures and the ground motion properties that have an important influence on the seismic performance of structures with asymmetric load–deformation behaviour, are identified from the results obtained from a parametric study of the dynamic response of single‐degree‐of‐freedom systems. The dynamic response of tilted single and multi‐degree‐of‐freedom systems, designed according to Mexican requirements for the design of asymmetric structures is studied. After a general understanding of this phenomenon is offered, shortcomings in current design requirements for tilted structures are discussed. Research needs to establish a rational design approach of this type of structures are identified. Copyright © 2000 John Wiley & Sons, Ltd.     

SMA阻尼器偏心结构平-扭震动控制试验研究

为了验证提出的新型筒式自复位形状记忆合金阻尼器(telescopic recentering shape memory al-loy damper,TRSMAD)对结构平动-扭转耦联振动反应的抑制作用,进行了偏心结构消能减震体系的振动台试验。设计了一个1/4缩尺的三层两跨单向偏心的钢框架模型,将提出的新型SMA阻尼器安装在结构底层的一侧,通过振动台分别对无控条件下和装有阻尼器的有控条件下的结构反应进行了研究。试验结果表明:(1)在各地震波作用下,TRSMAD对结构的平动反应有很好控制效果,而对结构各层扭转角位移的控制效果稍低;(2)不同地震波下的控制效果有所不同:对结构的平动位移而言,天津波的减震率最高,El Centro波次之,最后为Taft波;对结构扭转角的控制,平均而言,除了天津波作用下第二层为特例外,对El Centro波的减震效果最好,其次为Taft波,最后为天津波;(3)同一地震波下,阻尼器对结构模型一层的位移控制效果较其他层为优。     

Performance of masonry enclosure walls: lessons learned from recent earthquakes

This paper discusses the issue of performance requirements and construction criteria for masonry enclosure and infill walls.Vertical building enclosures in European countries are very often constituted by non-load-bearing masonry walls, using horizontally perforated clay bricks.These walls are generally supported and confined by a reinforced concrete frame structure of columns and beams/slabs.Since these walls are commonly considered to be nonstructural elements and their influence on the structural response is ignored,their consideration in the design of structures as well as their connection to the adjacent structural elements is frequently negligent or insufficiently detailed.As a consequence,nonstructural elements,as for wall enclosures,are relatively sensitive to drift and acceleration demands when buildings are subjected to seismic actions. Many international standards and technical documents stress the need for design acceptability criteria for nonstructural elements,however they do not specifically indicate how to prevent collapse and severe cracking,and how to enhance the overall stability in the case of moderate to high seismic loading.Furthermore,a review of appropriate measures to improve enclosure wall performance and both in-plane and out-of-plane integrity under seismic actions is addressed.     

双向偏心结构扭转耦联地震反应的序列最优控制

本文分析了不对称建筑结构平移-扭转耦联振动的动力特性及地震作用下的响应;根据地震动输入结构的过程,推导出一种更为一般的最优控制算法,所获得的控制力表达式同时包括地震响应和地震激励。通过对一非规则四层框架结构的扭转耦联地震反应控制分析表明,该算法不仅能有效地控制结构的平移地震反应,而且更有效地抑制结构的扭转耦联地震反应。     

Implications of design philosophies for seismic response of steel moment frames

During a severe earthquake, steel moment resisting frames are expected to experience significant inelastic deformation in their members and joints. This behaviour is dependent upon several design parameters such as member sizes, frame's overstrength, member deformation capacities and the detailing of components. In this study, the influence of such aspects on the inelastic response of frames is investigated. Inelastic static and dynamic analyses were performed on four frames of different designs for a typical six-storey building. The frames were designed and detailed in accordance with current North American code requirements. The computed response of each frame was compared with the behaviour expected by the codes. Recommendations for a design procedure are suggested for improving the structural performance of low-rise steel frames subjected to strong earthquake excitation.     

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.     

Evaluation of code torsional provisions by a time history approach

Presented in this paper is a detailed parametric study of the coupled lateral and torsional response of single-storey building models subjected to earthquake base loadings. The aim is to assess the influence of torsional coupling on the elastic responses of buildings subjected to transient ground motion records, and to make comparisons with current code provisions which make allowance for coupling effects by means of empirical design procedures. The study of building responses to selected earthquake excitations shows that the qualitative effects of the controlling parameters on the maximum translational and torsional responses of the coupled system are similar to those observed in analyses using idealized response spectra to represent the input ground motion. It is also demonstrated that for particular ranges of the key parameters defining the structural system, typical of the properties of many actual buildings, torsional coupling induces significant amplification of earthquake forces. This amplification is shown to be inadequately accounted for in the current design provisions of major building codes. Recommendations for improving existing design practice for asymmetric structures are outlined.     

A simplified model for analysis and design of asymmetric-plan buildings

A new simplified model for analysis and design of multistorey buildings is developed. The model is based on a single super-element per building storey capable of representing the elastic and inelastic properties of the storey. This is done by matching the stiffness matrices and ultimate yield surface of the storey with that of the element; this surface relates storey shear and storey torque. For practical convenience, these surfaces are parametrized in terms of seven important physical parameters controlling the seismic response of asymmetric structures. Several numerical studies showed that the accuracy of the super-element model is satisfactory for most design purposes; the errors in peak responses are expected to be less than 20 per cent for most practical structures. Among the important advantages of this simplified model is that the time required in formulating, analysing and interpreting the structural model and its response is at least an order of magnitude smaller than for any conventional 3-D inelastic model. This enables the engineer to try different structural configurations and, thus, produce designs that have the desired seismic behaviour and are cost-effective. Furthermore, it has been shown through a multistorey building example that the super-element model is a powerful tool for conceptual design of a building. In spite of its simplicity, the model uses an accurate representation of the storey-shear and torque surfaces, which enables it to capture the fundamental features controlling the inelastic behaviour of the building.     

多维地震作用下偏心结构的磁流变阻尼器半主动控制

理论研究与震害经验表明,地震时结构会产生不可忽略的平动与扭转耦合的空间振动。本文提出了基于线性最优控制理论的部分状态反馈次优控制策略和基于遗传算法的控制策略,以Marlab和Simulink为平台,采用磁流变阻尼器对双向水平地震作用下的偏心结构平．扭耦联反应进行半主动控制。对-6层双向偏心框架结构进行控制效果仿真分析的结果证明,本文提出的2种控制策略是有效的。     

UNCOUPLED FREQUENCY RATIO IN ASYMMETRIC BUILDINGS

Structural eccentricity and Uncoupled Frequency Ratio (UFR) are two system parameters that strongly influence the elastic response of eccentric structures. The opinions differ regarding their influence on the inelastic response. Different investigators arrived at contradictory conclusions on the influence of uncoupled frequency ratio. The reasons for these contradictions are the use of different definitions and different models in the inelastic studies. In this paper, three possible definitions of the uncoupled frequency ratio are identified, and their influence is studied on the inelastic response. An ensemble of 10 real earthquakes of 20 s duration is used. The response is measured in terms of the mean plus one standard deviation of the ductility ratio. The value of the UFR obtained by each of the three possible definitions is not always admissible. It is concluded that the inelastic response depends upon the eccentric system, definition employed for the UFR, time period of the eccentric systems and the basis of strength distribution among the various lateral elements.     

Effective strength eccentricity concept for inelastic analysis of asymmetric structures

A large number of parameters affect the inelastic response of an eccentric system. The centre of resistance no longer remains constant due to continuous loading and unloading of various elements of the system in and out of the inelastic region. The objective of this study is to develop the concept of strength eccentricity for asymmetrical structures excited well into the inelastic region in the event of a severe earthquake. A single mass monosymmetric three-element system is selected. The torsional flexibility parameter is varied so as to obtain nine eccentric configurations covering the entire spectrum of such systems. These models were subjected to S00E component of the El Centro earthquake of 1940 which is considered to be a very severe earthquake. The concept of effective strength eccentricity is introduced which is defined as strength eccentricity at the point of intersection of ductility ratio curves of REE and FEE. It is proposed to provide additional strength in the elements of a torsionally unbalanced system independent of the design eccentricity. Empirical equations are proposed to account for design eccentricity and additional strength as well as distribution of the design strength. The proposed formulation is compared with the torsional provisions of UBC 1991, NBCC 1990 and NZC 1992. It is concluded that additional ductility demand on the REE and FEE designed based on the proposed formulation is almost nil as compared to that given by the three codes. The NZC and UBC require maximum ductility on the REE. The increase in overall strength of the system is least for the proposed formulation as compared to that given by the three codes. NBCC consistently requires the maximum total strength ratio.     

Parametric earthquake response of torsionally coupled buildings with foundation interaction

A study is made of the effect of soil-structure interaction on the coupled lateral and torsional responses of asymmetric buildings subjected to a series of historical free-field earthquake base motions. It sh shown that for particular classes of actual buildings the equivalent rigid-base responses are significantly increased for structures founded on medium-stiff soils, and hence the assumption of the major building codes that a conservative estimate of response is obtained by considering the structure to be fixed rigidly at its base is shown to be inconsistent with the presented dynamic results. It is shown that foundation interaction produces greatest amplification of torsional coupling effects for structures subjected to a particular class of European strong-motion earthquake records, identified by similarities in their spectral shape, for which the vibrational energy of the ground motion is distributed approximately uniformly over the range of frequencies which are of interest for real structures. It is recommended that provision be made in the torsional design procedures of building codes for the increase in the coupled torsional response due to soil-structure interaction as indicated in this study. Such provision should be based on the results of comprehensive parametric studies employing a wide selection of earthquake records and accounting for expected variations in localized soil conditions.     

建筑结构监测与抗震韧性评估

建筑结构响应是有效反映结构动力特性的最直接参数,开展结构动力响应实时监测可为结构抗震韧性评估提供准确的地震动输入。本文基于非结构构件损失构建结构抗震韧性评估方法,研究确定位移敏感型和加速度敏感型非结构构件的易损性模型。选择某六层钢筋混凝土框架结构进行实时监测系统建设,基于监测数据开展结构抗震韧性评估。通过构建建筑信息模型（BIM）,并在有限元分析软件OpenSees中建立结构弹塑性分析模型,利用实时监测数据实现结构模型更新,直至监测数据与模型分析结果一致。由于实时监测数据峰值较低,结构不会发生塑性变形,因此选择10条双向非脉冲地震动模拟实时监测地震记录。根据层间位移角和楼面加速度分布,开展结构功能损失评估,得到该建筑结构的抗震韧性得分。分析表明,该结构抗震性能较好,在遭受地震破坏后,会发生非结构构件脱落,需要采取有效措施进一步提升建筑抗震韧性水平。     

Influence of lateral-load-resisting system on the earthquake response of structures—a system identification study

Analysis and comparison of the dynamic responses of three well instrumented (with accelerographs) high-rise buildings shaken during the 1984 Morgan Hill earthquake are presented. The buildings examined in the present work are (i) the Town Park Towers Apartment building, a 10-storey, concrete shear wall building; (ii) the Great Western Savings and Loan building, a 10-storey building with concrete frames and shear walls; and (iii) the Santa Clara County Office building, a 13-storey, moment-resistant steel frame building. The structures are located within 2 km of each other and, as may be confirmed by visual inspection of the recorded seismograms, experienced similar ground motions. One-dimensional and three-dimensional linear structural models are fitted to the observations using the modal minimization method' for structural identification, in order to determine optimal estimates of the parameters of the dominant modes of the buildings. The time-varying character of these parameters over the duration of the response is also investigated. Comparison of the recorded earthquake response of the structures reveals that the type of lateral-load-resisting system has an important effect on the dynamic behaviour of the structures because it controls the spacing of the characteristic modes on the frequency axis. The Santa Clara County Office building has closely spaced natural frequencies and exhibits strong torsional response and modal coupling. Its dynamic behaviour is contrasted with that of the Great Western Savings and Loan building which has well separated natural frequencies and exhibits small torsional response and no modal coupling. Strong modal coupling causes a beating-type phenomenon and makes earthquake response of structures different from that envisioned by codes.     

Improved earthquake resistant design of eccentric steel buildings

Recently, a design modification has been proposed for eccentric, torsionally stiff, braced steel buildings, designed according to the current Eurocodes 3 and 8, that improves noticeably their inelastic response under the action of design level earthquakes. The improvement consists in a more uniform distribution of ductility demands throughout the building. In the present paper, a similar, though differently derived, modification is applied to torsionally flexible eccentric buildings and their response is again evaluated under pairs of design earthquake motions. A substantial improvement of their inelastic response is also observed, similar to what had been obtained for torsionally stiff buildings. The new approach is also tested with torsionally stiff buildings and leads to similarly satisfactory results. Thus it may be recommended for general application.     

Damping‐adjusted combination rule for the response spectrum analysis of base‐isolated buildings

The classical response spectrum method continues to be the most popular approach for designing base‐isolated buildings, therefore avoiding computationally expensive nonlinear time‐history analyses. In this framework, a new method for the seismic analysis and design of building structures with base isolation system (BIS) is formulated and numerically validated, which enables one to overcome the main shortcomings of existing techniques based on the response spectrum method. The main advantages are the following: first, reduced computational effort with respect to an exact complex‐valued modal analysis, which is obtained through a transformation of coordinates in two stages, both involving real‐valued eigenproblems; second, effective representation of the damping, which is pursued by consistently defining different viscous damping ratios for the modes of vibration of the coupled BIS‐superstructure dynamic system; and third, ease of use, because a convenient reinterpretation of the combination coefficients leads to a novel damping‐adjusted combination rule, in which just a single response spectrum is required for the reference value of the viscous damping ratio. The proposed approach is specifically intended for design situations where (i) the dynamic behaviour of seismic isolators can be linearised and (ii) effects of nonproportional damping, as measured by modal coupling indexes, are negligible in the BIS‐superstructure assembly. Copyright © 2012 John Wiley & Sons, Ltd.