延性需求谱在基于性能的抗震设计中的应用

基于性能的抗震设计理论涉及如何简便而合理地确定结构在指定强度地震下的弹塑性位移需求。本文给出了利用延性需求谱求解结构位移需求的一般步骤:借助模态Pushover分析将多自由度体系分解为几个非线性单自由度体系,以考虑各阶振型的影响;利用延性需求谱计算对应模态的等效单自由度体系的延性及位移需求,并以一定方式组合转化为多自由度体系位移需求。最后,通过算例分析表明:利用延性需求谱求解结构位移需求是一种具有一定精度可为工程接受的简便方法,在基于性能的抗震设计中具有较好的应用前景。     

场地类别划分与抗震设计反应谱的讨论

介绍了新的抗震设计规范(2001—07—20发布，2002—01—01实施)中场地类别划分和地震影响系数的有关内容，指出其中一些不相协调之处。最后，针对不同的问题建议相应的解决办法。     

弹塑性位移谱法的振动台模型试验验证

弹塑性位移谱法求解结构在指定强度地面运动作用下的位移需求是一种简便合理的方法。本文将弹塑性位移谱法就具体地震波计算的楼层位移需求、层间位移角需求与一比例为1／10的12层钢筋混凝土模型框架振动台试验结果作了比较。设计的12层钢筋混凝土模型框架结构在振动台上经历了7种强度等级地震波的作用,输入峰值加速度依次为：0．090g、0．258g、0．388g、0．517g、0．646g、0．775g和0．904g。求出了弹塑性位移谱法计算的楼层位移和层问位移角需求与振动台试验结果的比值,研究了二者比值的均值及方差沿楼层的分布情况。结果表明：弹塑性位移谱法的计算结果与振动台得到的位移需求值吻合较好,在基于性能的抗震设计中具有较好的应用前景。     

弹塑性地震反应谱的长周期特性研究

在基于性能抗震设计中弹塑性反应谱在计算结构地震位移反应方面越来越受到重视。利用统计分析方法研究了等强度的延性需求谱和等延性的强度折减系数谱的长周期(至5 s)区段的特性,关注的重点是等位移准则和场地条件影响。给出了若干具有工程价值的结论:一是周期介于1.5Tg(地震动特征周期)和2.5 s之间的结构可近似认为等位移准则成立且与场地条件关系不大,这样确定的强度折减系数当位移延性系数小于等于4时结果将是偏于安全的;二是结构周期大于2.5 s后以硬土场地等延性强度折减系数谱或等强度延性需求谱代替软土场地谱求解系统强度需求或延性需求,将会得到偏于安全的结果。     

地震动特性对隔震结构弹塑性位移反应谱的影响研究

根据特定震源机制、震级、断层距和场地条件选取69条地震动记录并进行分组,利用Nspectra软件计算隔震结构的弹塑性位移反应谱,分析断层距、场地条件、震级、阻尼比对弹塑性位移谱的影响,探讨隔震层的力学参数对地震能量耗散的影响。研究结果表明:相较于远场,处于近场的隔震结构最为不利,隔震层位移谱值受场地条件、地震加速度和速度大小影响较大;随着断层距的增大,位移谱值衰减较快,且在软土场地中隔震层的位移谱值衰减幅度大于硬土场地;地震震级大小对位移谱形状的影响不明显,但能够使隔震层的位移谱值产生整体缩放效应;阻尼比在小于0.4的范围内,隔震层在不同地震动特性作用下位移谱值差别较大,但在大于0.4以后,位移谱值及谱形基本趋于一致;屈服力较小(恢复力/重力小于等于1)的隔震层随自振周期增大其耗能性能更加突出。     

建筑结构基于性能的抗震评估的等ζy延性谱法

利用水平地震作用下单自由度体系的运动方程,以屈服承载力系数ζy为参量,推导出适时延性系数动力方程,从而建立了基于屈服承载力系数的延性需求谱。结合结构的非线性静力pushover分析,提出了基于性能的抗震评估方法——等ζy延性谱法。用等yζ延性谱法对一个6层钢筋混凝土框架结构进行了抗震评估,并与非线性时程分析进行了比较。结果表明,用等yζ延性谱法对结构进行抗震性能评估是可行的,并且具有较高的精度。     

基于规范弹性反应谱建立需求谱的方法

阐述了通过力的折减系数R和延性系数μ建立弹塑性反应谱的原理，并由新《建筑抗震设计规范》（GB50011-2001）的加速度反应谱建立了弹塑性需求谱，为结合新抗震规范应用能力谱方法进行结构弹塑性分析奠定了基础。同时本文比较了不同的R-μ关系对需求谱的影响。     

考虑场地类别的阻尼比修正系数研究

阻尼比修正系数B用于调整临界阻尼比不同于0．05时的反应谱值,它对减震结构、隔振结构以及弹塑性体系的简化设计与分析具有重要意义。本文按照我国现行抗震规范规定的场地分类方法,分别应用四类场地共计136条地震记录,对阻尼比为0．05、0．1、0．2、0．3、0．4、0．5的弹性单自由度结构体系的相对位移反应谱、相对速度反应谱、绝对加速度反应谱进行计算分析,从定性到定量系统研究了地震作用下场地分类、阻尼比以及结构周期对相对位移反应谱阻尼比修正系数眈、相对速度反应谱阻尼比修正系数Bv和绝对加速度反应谱阻尼比修正系数口。的影响规律,结果表明：随着阻尼比的增大,场地分类对阻尼比修正系数的影响程度逐渐增大,其中场地条件对Ba的影响更加显著,然而数据分析表明,场地条件对阻尼比修正系数最小值的影响很小。在IV场地上,阻尼对长周期结构的减震效果明显优于其它三类场地。最后通过非线性回归分析,建立了与场地类别、结构周期以及阻尼比相关的阻尼比修正系数Ba、Bd、Bv的计算公式。这部分的结果可以应用于减震结构基于．胜能的抗震设计方法中。     

对比分析中国有关场地条件对设计反应谱最大值的影响

对国内有关场地条件对设计反应谱最大值影响的研究现状作了介绍,对不同研究者所采用的研究方法和得出的结论进行了对比。通过对比发现了4点不同:1)场地类别划分标准不同;2)所用地震记录来源不同;3)地震动强度的划分依据不同;4)在计算场地系数时所参照的场地类别标准也不同。同时,发现中国规范中Ⅳ类场地的范围过于小,有必要进行调整。综合上述分析,提出用峰值加速度来表示地震动强度,按照Ⅱ类场地的峰值加速度来进行分组,以有效峰值加速度EPA作为计算标准更加合理。利用真实的地震记录,根据各个台站的地质剖面图,按照中国现行的场地类别划分标准对所得到的场地进行类别划分,通过计算不同类别场地的EPA,结合这些分析结果,建议了用于不同类别场地设计反应谱最大值的一组经验系数     

直接基于位移的结构抗震设计理论研究进展

直接基于位移的抗震设计是实现基于性能抗震设计思想的一条有效途径,其理论主要包括三方面的内容,即直接基于位移的抗震设计方法、位移需求估计方法和目标位移的确定。本文在阅读了大量文献的基础上,对国内外在这三方面的研究进行了介绍,并对今后的研究方向进行了展望。     

Consistent inelastic design spectra: Strength and displacement

A procedure for the determination of inelastic design spectra (for strength, displacement, hysteretic and input energy) for systems with a prescribed ductility factor has been developed. All the spectra are consistent (interrelated and based on the same assumptions). This is the first of two companion papers which deals with the ‘classical’ structural parameters: strength and displacement. The input data are the characteristics of the expected ground motion in terms of a smooth elastic pseudo-acceleration spectrum. Simple, approximate expressions for the strength reduction factor R are proposed. The value of R depends on the natural period of the system, the prescribed ductility factor, the hysteretic behaviour, damping and ground motion. Fairly accurate approximations to the inelastic spectra for strength and displacement can be derived from the elastic spectrum using the proposed values for R.     

Seismic inelastic design spectra

To alleviate some of the shortcomings associated with the statistically based inelastic spectral shapes, a rigorous method for the construction of inelastic design spectra is proposed. The method is based on several bounds which are derived from the differential equation of motion for a single degree of freedom system. Comparisons of the proposed spectral bounds with the actual elastoplastic response spectra and with the 84 per cent Newmark elastoplastic design spectra reveal that the proposed method yields a more economical, reliable and simple design aid. To demonstrate the quality of the proposed bounds, comparisons of the bounding spectra with their corresponding constant ductility elastoplastic response spectra for a number of records have been presented.     

土层结构对反应谱特征周期的影响

本文选取和构造了若干有工程意义的典型场地剖面,利用目前工程上广泛应用的场地地震反应分析的一维等效线性化波动方法,计算了在不同地震动输入下的不同场地剖面的地表加速度峰值和地表速度峰值。利用计算得到的地表加速度峰值和速度峰值计算了不同场地在不同地震动输入下的反应谱的特征周期。研究了不同土层结构对地表加速度反应谱特征周期的影响,获得了一些有意义的结果。     

Ductility demand spectra for multiple near- and far-fault earthquakes

This study presents ductility demand spectra for single degree of freedom (SDOF) systems under multiple near- and far-fault seismic ground motions. The main innovation has to do with the quantification of the seismic sequence effect directly into ductility demands, a phenomenon which has not been studied in the past. Due to lack of real seismic sequences records, this paper examines only artificial sequences, where they have been generated by a rational and random combination of real single events. A statistical investigation of more than 120 millions dynamic inelastic analyses is conducted to obtain expressions for the ductility demands, in terms of the period of vibration, the viscous damping, the post-yield stiffness and the force reduction factor. It is found that due to the seismic sequence effect, it is certainly insufficient to consider only the ‘design earthquake’, since this traditional hypothesis leads to underestimated ductility demands and therefore to underestimated structural damage.     

Inelastic design spectra accounting for soil conditions

This paper is concerned with the effect of soil conditions on the response of single-degree-of-freedom inelastic systems subjected to earthquake motions. The ground motions considered are 72 horizontal components of motion, most of them recorded during the 3 March, 1985 Chile earthquake (M_s = 7·8) and two main aftershocks; among these records are some of the strongest and longer duration earthquake motions ever recorded. The recording station sites were classified in one of three soil types, which can be generically referred to as rock, firm ground, and medium stiffness soil. Response results for each group were analysed statistically to obtain factors for deriving inelastic design spectra of the Newmark-Hall type, as well as alternative simplified spectral shapes suitable for code formulation. Particular attention was given to the response modification factors (R) that are commonly used in seismic codes to reduce the ordinates of the elastic spectrum to account for the energy dissipation capacity of the structure. The response modification factors, known to be function of both the natural period of vibration and the ductility factor, are found to be dependent on soil conditions, particularly in the case of medium stiffness soils. It is also shown that the indirect procedure of applying R to the elastic design spectrum is less accurate than directly using functions that represent the inelastic design spectrum.     

Capacity spectrum method based on inelastic demand spectra

By means of a graphical procedure, the capacity spectrum method compares the capacity of a structure with the demands of earthquake ground motion on it. In the present version of the method, highly damped elastic spectra have been used to determine seismic demand. A more straightforward approach for the determination of seismic demand is based on the use of the inelastic strength and displacement spectra which can be obtained directly by time-history analyses of inelastic SDOF systems, or indirectly from elastic spectra. The advantages of the two approaches (i.e. the visual representation of the capacity spectrum method and the superior physical basis of inelastic demand spectra) can be combined. In this paper, the idea of using inelastic demand spectra within the capacity spectrum method has been elaborated and is presented in an easy to use format. The approach represents the so-called N2 method formulated in the format of the capacity spectrum method. By reversing the procedure, a direct displacement-based design can be performed. The application of the modified capacity spectrum method is illustrated by means of two examples. Copyright © 1999 John Wiley & Sons, Ltd.     

Characterization of displacement demand for elastic and inelastic SDOF systems

The results of a research concerning the characterization of elastic and inelastic displacement spectral demand as a function of magnitude, source-to-site distance, and soil type are presented. The displacement spectra were computed for single degree of freedom systems subjected to a large set of strong ground motion records.In the elastic case, design displacement spectra, modeled in a simplified way with a bilinear shape in the period range 0–4 s, are then proposed for the estimation of the displacement demand to structures located on different local soil condition, at different distance from the causative fault, and for different levels of magnitude. In order to evaluate the reliability of the proposed design displacement spectra, probabilistic displacement spectra corresponding to different levels of probability of non-exceedance were also carried out.The inelastic displacement demand to elasto-plastic systems was analyzed through the ratio between inelastic and elastic spectral displacements. Simplified relationships of the inelastic displacement ratio are then proposed as a function of displacement ductility, soil condition and period of vibration. Finally, as a comparison, the inelastic displacement ratios were also estimated considering other constitutive models.     

三种土层结构反应谱特征周期的统计分析

本文人工合成了若干条能够反映不同地震动特征的基岩加速度时程,并将其作为土层地震反应分析的地震动输入。选取了若干个软弱夹层分别在底部、中部和顶部的场地剖面,利用工程上广泛应用的场地地震反应分析的一维等效线性化波动方法,计算了不同场地类别,3种土层结构在不同地震动输入下的地表加速度反应谱的特征周期。给出了不同土层结构在统计意义上的反应谱特征周期的平均值,并通过与正常剖面反应谱特征周期的比较给出了不同场地类别,3种土层结构的反应谱特征周期的影响系数。本文的研究结果对进一步考虑场地分类具有一定的意义。     

Capacity-based inelastic displacement spectra for reinforced concrete bridge columns

Capacity-based inelastic displacement spectra that comprise an inelastic displacement ratio (C_R ) spectrum and the corresponding damage index (DI ) spectrum are proposed in this study to aid seismic design and evaluation of reinforced concrete (RC) bridges. Nonlinear time history analyses of single-degree-of-freedom (SDOF) systems are conducted using a versatile smooth hysteretic model when subjected to far-field and near-fault ground motions. It is demonstrated that the Park and Ang damage index can be a good indicator for assessing the actual visible damage condition of columns regardless of its loading history, providing a better insight into the seismic performance of bridges. The computed spectra for near-fault (NF) ground motions show that as the magnitude of pulse period ranges increases from NF1 (0.5-2.5 seconds) to NF2 (2.5-5.5 seconds), the spectral ordinates of the C_R and DI spectra increase moderately. In contrast, the computed spectra do not show much difference between NF2 and NF3 (5.5-10.5 seconds) when the period of vibration T_n≤  1.5 seconds, after which the spectral ordinates of NF3 tend to increase obviously, whereas those of NF2 decrease with increasing T_n . Moreover, when relative strength ratio R  = 5.0, nearly all of the practical design scenarios could not survive NF3. On the basis of the computed spectra, C_R and DI formulae are presented as a function of T_n , R , and various design parameters for far-field and near-fault ground motions. Finally, an application of the proposed spectra to the performance-based seismic design of RC bridges is presented using DI as the performance objective.     

Consistent inelastic design spectra: Hysteretic and input energy

This is the second of two companion papers on inelastic design spectra (for strength, displacement, hysteretic and input energy) for systems with a prescribed ductility factor. All the spectra are consistent (interrelated and based on the same assumptions). This paper deals with two quantities related to cumulative damage: hysteretic and input energy. The input data for the procedure are the characteristics of the expected ground motion in terms of a smooth elastic pseudo-acceleration spectrum and the time integral of the square of the ground acceleration ∫a² dt. Simple, approximate expressions for two dimensionless parameters (the parameter γ and the hysteretic to input energy ratio E_HE_I) have been proposed. The parameter 7, which controls the reduction of the deformation capacity of structures due to low-cycle fatigue, depends on the natural period of the system, the prescribed ductility factor, the hysteretic behaviour and the ground motion characteristics. The ratio E_H/E_I is influenced by damping, the ductility factor and the hysteretic behaviour. Very good approximations to the inelastic spectra for hysteretic and input energy can be derived from the elastic spectrum using the spectra for the reduction factor R, proposed in the companion paper, and the proposed values for γ and E_H/E_I