Derivation of design soil coefficients (S) and response spectral shapes for Eurocode 8 using the European Strong-Motion Database

A revision is presently under way to upgrade the status of the `Designrecommendations for earthquake resistance of structures', commonlyreferred to as Eurocode 8 (or EC8). In order to improve the definition ofthe design elastic response spectra (ERS) as defined in the Eurocode 8 –Part 1 (Draft May 2001), the values of the soil amplification factors havebeen calculated for sites on sedimentary soils, both stiff (category B) andsoft (category C), with respect to rock sites (category A), such ascontemplated in EC8. The work was performed by a systematic study ofresponse spectra as a function of magnitude and site conditions, usingrecords from the European Strong-Motion Database.The results confirm the occurrence of systematic spectral amplification onsedimentary soils with respect to reference rock in a large set of Europeanstrong motion data. Such amplification has been quantified through ameasure derived from the Housner Spectrum Intensity definition.The values of the soil coefficients recommended in the current version ofEC 8 are shown to be realistic for category C, in the case of a moderateseismicity context. However the values for subsoil class B need to besignificantly increased both for the moderate and high seismicity context.     

Equivalent viscous damping for displacement-based seismic design of hysteretic damped braces for retrofitting framed buildings

A displacement-based design (DBD) procedure aiming to proportion hysteretic damped braces (HYDBs) in order to attain, for a specific level of seismic intensity, a designated performance level of a structure is proposed for the retrofitting of framed buildings. A key step for the reliability of the DBD procedure is the selection of the equivalent viscous damping in order to account for the energy dissipated by the damped braced frame. In this paper, expressions of the equivalent damping are obtained considering the energy dissipated by the HYDBs and the framed structure. To this end, dynamic analyses of an equivalent single degree of freedom system, whose response is idealized by a trilinear model, are carried out considering real accelerograms matching, on the average, Eurocode 8 (EC8) response spectrum for a medium subsoil class. Then, a three-storey reinforced concrete (r.c.) framed structure of a school building, designed in a medium-risk seismic region according to the Italian code in force in 1975, is supposed as retrofitted as if in a high-risk seismic region of the current seismic code (NTC08) by the insertion of HYDBs. Nonlinear static analyses are carried out to evaluate the vulnerability of the primary structure, characterized by the lack of interior girders along the floor slab direction, and to select optimal properties of the HYDBs. The effectiveness of the retrofitting solutions is checked referring to nonlinear dynamic analyses, considering artificially generated accelerograms whose response spectra match those adopted by NTC08 for the earthquake design levels corresponding to the serviceability and ultimate limit states.     

A stochastic based approach for a new site classification method: application to the Algerian seismic code

Building codes have widely considered the shear wave velocity to make a reliable subsoil seismic classification, based on the knowledge of the mechanical properties of material deposits down to bedrock. This approach has limitations because geophysical data are often very expensive to obtain. Recently, other alternatives have been proposed based on measurements of background noise and estimation of the H/V amplification curve. However, the use of this technique needs a regulatory framework before it can become a realistic site classification procedure. This paper proposes a new formulation for characterizing design sites in accordance with the Algerian seismic building code(RPA99/ver.2003), through transfer functions, by following a stochastic approach combined to a statistical study. For each soil type, the deterministic calculation of the average transfer function is performed over a wide sample of 1-D soil profiles, where the average shear wave(S-W) velocity, V s, in soil layers is simulated using random field theory. Average transfer functions are also used to calculate average site factors and normalized acceleration response spectra to highlight the amplification potential of each site type, since frequency content of the transfer function is significantly similar to that of the H/V amplification curve. Comparison is done with the RPA99/ver.2003 and Eurocode8(EC8) design response spectra, respectively. In the absence of geophysical data, the proposed classification approach together with micro-tremor measures can be used toward a better soil classification.     

Site Effects and Design Provisions: The Case of Euroseistest

—?Modern seismic codes usually include provisions for site effects by considering different coefficients chosen on the basis of soil properties at the surface and an estimate of the depth of bedrock. However, complex local geology may generate site amplification on soft soils significantly larger than what would be expected if we assume that the subsoil consists of plane soil layers overlaying a homogeneous half-space. This paper takes advantage of the large number of previous studies of site effects done at Euroseistest (northern Greece). Those studies have supplied a very detailed knowledge of the geometry and properties of the materials filling this shallow valley. In this paper we discuss the differences between site effects evaluated at the surface using simple 1-D computations and those evaluated using a very detailed 2-D model of the subsoil structure. The 2-D model produces an additional amplification in response spectra that cannot be accounted for without reference to the lateral heterogeneity of the valley structure. Our numerical results are extensively compared with observations, which show that the additional amplification computed from the 2-D model is real and affects by a significant factor response spectra, and thus suggests that some kind of aggravation factor due to the complexity of local geology is worthy of consideration in microzonation studies and seismic codes.     

浅谈欧洲规范Eurocode 8-结构抗震设计

简单介绍了欧洲规范(Eurocodes)的基本情况,概括给出了欧洲规范8-结构抗震设计的基本内容和特点,进而在抗震设计基本要求、建筑抗震概念设计基本原则两个方面与我国建筑抗震设计规范(GB50011-2001)进行了比较和评述,然后在场地类别、地震作用和结构抗震验算等几个方面对两规范进行了简单的对比和评述。     

土体参数对地表加速度峰值和反应谱的影响

运用一维土层地震反应的等效线性化波动方法,研究了单层均质土密度、动剪切模量比和阻尼比、土层厚度和土层剪切波速等参数变化对土层地表加速度峰值和反应谱的影响,所得到的结论对于指导地震安全性评价中的勘察试验工作、提高土层地震反应的计算精度具有一定的参考价值。     

南京河西地区设计地震动研究

本文根据南京及周围地区的地震环境,采用概率法对南京河西地区某高层公寓工程场址进行了地震危险性分析,采用等效线性化模型考虑场地土的非线性特性的影响,并用一维波动模型进行了场址土层地震反应和场地地震动效应分析,给出了５０年超越概率６３％,１０％和３％的场址基岩和地表的水平向加速度反应谱值加速度。     

Floor response spectra in RC frame structures designed according to Eurocode 8

Nonstructural components (NSCs) should be subjected to a careful and rational seismic design, in order to reduce the economic loss and to avoid threats to the life safety, as well as what concerns structural elements. The design of NSCs is based on the evaluation of the maximum inertia force, which is related to the floor spectral accelerations. The question arises as to whether Eurocode 8 is able to predict actual floor response spectral accelerations occurring in structures designed according to Eurocode 8. A parametric study is conducted on five RC frame structures in order to evaluate the floor response spectra. The structures, designed according to Eurocode 8, are subjected to a set of earthquakes, compatible with the design response spectrum. Time-history analyses are performed both on elastic and inelastic models of the considered structures. Eurocode formulation for the evaluation of the seismic demand on NSCs does not well fit the numerical results. Some comments on the target spectrum provided by AC 156 for the seismic qualification of NSC are also included.     

中美欧水平地震作用的比较

分析了中美欧抗震设计中的水平地震作用问题。首先,比较了中美欧抗震规范中建筑物重要性、强度折减系数的差异,介绍了中国规范的底部剪力法、美国规范的等效侧向荷载法以及欧洲规范的侧向荷载法。然后对一多层框架结构,分别作为办公楼和医院,计算了不同设防烈度下、不同延性等级下的水平地震作用,并进行了比较分析,获得了3种规范关于水平地震作用的一些差异。     

保定市设计地震动参数的研究

本文根据保定市及周围地区的地震地质环境，在地震危险性分析的基础上，采用等效线性一维波动方程进行土层的地震反应分析。给出50年超越概率63％、10％、2％基岩和地面的水平向峰值加速度、反应谱（场址基本烈度Ⅶ度）和地震影响系数最大值。该结果为抗震设计提供了可靠依据，具有应用价值。     

考虑地基土液化影响的高层建筑地震反应分析

为了研究地基土液化对高层建筑地震反应的影响，本文简要介绍了分时段等效线性有效应力动力分析方法，且将其中的等效线性化方法改进为逐步迭代非线性方法，并利用ANSYS程序的参数化设计语言将这一分析方法并入ANSYS程序中，最后分析了考虑液化时桩基-高层建筑体系的地震反应。认为对于单层砂土-桩基-高层建筑体系来说，砂土的液化对上部结构地震反应有较大的影响；而对于本文采用的上海土-桩基-高层建筑体系来说，砂土层的液化未对上部结构的地震反应产生明显的影响。     

Adapting earthquake actions in Eurocode 8 for performance‐based seismic design

Performance‐based seismic design (PBSD) can be considered as the coupling of expected levels of ground motion with desired levels of structural performance, with the objective of achieving greater control over earthquake‐induced losses. Eurocode 8 (EC8) already envisages two design levels of motion, for no collapse and damage limitation performance targets, anchored to recommended return periods of 475 and 95 years, respectively. For PBSD the earthquake actions need to be presented in ways that are appropriate to the estimation of inelastic displacements, since these provide an effective control on damage at different limit states. The adequacy of current earthquake actions in EC8 are reviewed from this perspective and areas requiring additional development are identified. The implications of these representations of the seismic loads, in terms of mapping and zonation, are discussed. The current practice of defining the loading levels on the basis of the pre‐selected return periods is challenged, and ideas are discussed for calibrating the loading‐performance levels for design on the basis of quantitative earthquake loss estimation. Copyright © 2005 John Wiley & Sons, Ltd.     

Influence of steel mechanical properties on EBF seismic behaviour

Among the resisting systems suitable for the design of ductile steel structures, Eurocode 8 proposes MRFs and EBFs. The formers are considered more efficient in terms of ductility, but they suffer a strong weakness in the lateral stiffness, with following cumbersome design procedures to avoid excessive lateral displacements maintaining a quite high ductile behaviour under seismic actions. Often, the design process leads to not optimized structural members, oversized with respect to the minimum seismic requirements due to lateral deformation limitations. EBFs combine high lateral stiffness, due to bracing elements, and high dissipative capacities, provided by the plastic hinges developed in links. Eurocode 8 proposes a design procedure for EBF structures in which iterative checks are required to design links with a defined level resistance dependent on all the other links’ strength. The present paper investigates the seismic behaviour of EBFs using Incremental Dynamic Analyses (IDA) to explore their mechanical response under increasing seismic action. IDAs are executed considering the influence of variability of steel mechanical properties on the behaviour of EBFs, using seven artificial accelerograms according to Eurocode 8. The aims of IDAs are the probabilistic assessment of the response of the system with respect to the variability of the material properties, the analysis of structural safety and the ability of the structures to internally redistribute plastic phenomena during the earthquake. Structural safety conditions will be defined according to a multi-level performance approach. The paper presents also some final suggestions for possible improvements and design simplifications.     

Earthquake response spectra for performance based design—A critical review

After briefly reviewing the history of the development of design codes and the contemporary performance based objectives for earthquake-resistant structures, this paper considers the linear elastic design spectra exemplified by the Eurocode 8. The amplitudes and shapes of the spectra are examined relative to what is known about strong ground motion and its empirical scaling equations. The estimation of seismic hazard is then discussed, and it is shown that hazard mapping in terms of one scaling parameter (e.g., peak ground acceleration) is neither reliable nor uniformly conservative, and is also in contradiction with the performance based design objectives. It is suggested that a reliable and robust seismic hazard mapping for use with national earthquake-resistant design guidelines (e.g. Eurocode 8) can be carried out using the Anderson–Trifunac Uniform Hazard Spectrum method.     

Engineering ground motion record selection in the ITalian ACcelerometric Archive

This paper introduces REXELite, an internet version of REXEL, a software for automatic selection of ground motion suites for nonlinear dynamic analysis of structures. REXELite was developed with the aim of integrating an advanced earthquake records’ repository, such as the ITalian ACcelerometric Archive (ITACA), with a tool to define seismic input for engineering seismic analysis according to international standards (with priority to Europe). In fact, REXELite allows to define target design spectra according either to Eurocode 8 or to the Italian building code, and to search ITACA for suitable sets of seven records (comprised of one or two horizontal ground motion components) matching such target spectra: on average, in a user-specified period range, and with the desired tolerance. The records in the set also have, individually and according to some criteria, the most similar spectral shape with respect to that of the code. Selection options include magnitude, source-to-site distance, soil conditions and, if desired, linear scaling of records to reduce further record-to-record variability of the selected suite.     

Eurocode 8-compatible synthetic time-series as input to dynamic analysis

Nonlinear dynamic analysis of existing or planned structures often requires the use of accelerograms that match a target design spectrum. Here, our main concern is to generate a set of motions with a good level of fit to the Eurocode 8 design spectra for France. Synthetic time series are generated by means of a non-stationary stochastic method. To calibrate the input parameters in the stochastic approach, we select a reference set of accelerograms for a Eurocode 8 type B site category from the PEER Ground-Motion Database, which are then adjusted to the target spectrum through wavelet addition. Then, we compute nonlinear seismic responses of a soil column, including pore pressure effects, and brittle and ductile structures to the stochastic time-series, the natural accelerograms and time-series generated using stationary stochastic approaches. The results of these calculations reveal considerable variability in response despite the similarities in terms of spectral acceleration.     

EC8-based earthquake record selection procedure evaluation: Validation study based on observed damage of an irregular R/C building

This study investigates the applicability and limitations of the Eurocode 8 earthquake ground motion selection framework for the assessment of both elastic and inelastic structural response of multi-storey, irregular R/C buildings subjected to bi-directional loading. In order to minimize modelling uncertainties inherent in the quantification of structural damage and the consideration of the supporting soil–foundation system for complex structural systems, an existing building damaged by the 2003 Lefkada earthquake was adopted as case study. This selection has an advantage in that ground excitation, soil profile and damage observations are all available, thus permitting calibration of the finite element model with the observed response, especially in terms of use of appropriate plasticity models and damage indices, plus the assessment of soil–structure interaction effects. After establishing a reliable finite element model of the structure under study, extensive parametric analyses for different EC8 compliant sets of records were conducted, permitting quantification of the discrepancy of the structural response due to record-to-record and set-to-set variability (i.e., intra-set and inter-set scatter, respectively). The results confirm that many of the observations found in the literature regarding the effect of ground motion selection on the predicted seismic performance of SDOF systems are also valid for bi-directionally excited, multi-storey, irregular buildings. Finally, the results also highlight specific limitations of the EC8 provisions that may lead to erroneous results in many practical cases.