Automated classification of near‐fault acceleration pulses using wavelet packets

This study proposes a new algorithm for automatically classifying two types of velocity‐pulses that are integral of a distinct acceleration pulse (acc‐pulse) or a succession of high‐frequency one‐sided acceleration spikes (non‐acc‐pulse). For achieving this, wavelet packet transform is used to filter the high‐frequency content and to extract the coherent velocity‐pulse. Then, the pulse period is unequivocally derived through the peak point method. Following the determination of the pulse‐starting (t_s) and pulse‐ending (t_e) time instants in the velocity time‐history, a local acceleration time‐history truncated by t_s and t_e is obtained. The maximum relative energy of the pulse between two adjacent zero crossings is then employed as indicator for distinguishing the two types of velocity‐pulses. The criteria for identifying acc‐pulses and non‐acc‐pulses are calibrated using a training data set of manually classified ground motions from the Next Generation Attenuation West 2 project. Finally, significance of such a classification between velocity‐pulses of different characteristics is assessed through the comparison of elastic acceleration response spectra of the two categories of pulse‐like records.     

地震位移反应谱特性的研究

研究了地震地面运动的加速度谱、位移反应谱与富里叶谱之间的对应关系 ,结果表明 ,加速度谱可较好地表现地震动高频部分的特性 ,位移谱可较好地表现地震动低频部分的特性。提出了位移谱特征周期Tgd的概念 ,并以弹性位移谱上Tgd对应的点为基准点对位移谱进行标准化 ,较好地消减了不同地震动对位移谱曲线形状的影响。最后 ,根据多条地震动的计算结果 ,总结了此标准化方法下的平均位移谱的特征 ,为基于位移抗震设计理论的研究提供了基础     

近场脉冲型地震作用下黏滞阻尼器对双柱#br# 摇摆桥墩减震作用研究

为研究黏滞阻尼器对双柱摇摆桥墩在近场脉冲型地震作用下的减震作用,基于摇摆刚体假定和拉格朗日方程,对采用黏滞阻尼器的双柱摇摆桥墩进行实例分析、参数分析和抗倒塌能力分析。研究结果表明：黏滞阻尼器可减小双柱摇摆桥墩的地震位移反应,且作为速度型阻尼器,其在近场脉冲型地震作用下的减震效果比远场地震更为显著;在近场脉冲型地震作用下,随着黏滞阻尼器的无量纲阻尼常数(λ)的增大和阻尼指数(n_v)的减小,桥墩位移反应随之变小,当阻尼器失效位移大于120mm时桥墩峰值位移反应无显著变化;黏滞阻尼器的减震作用可有效提高双柱摇摆桥墩在近场脉冲型地震作用下的抗倒塌能力,λ越大和n_v越小,桥墩的抗倒塌能力越强。     

近断层脉冲地震动对基础隔震结构放大效应的量化分析

为量化分析近断层脉冲地震动对基础隔震结构的地震放大效应,建立多层和高层两个基础隔震结构模型,根据隔震结构基本周期,采用基于能量的脉冲量化识别方法,得到40条脉冲地震动,并剔除其中的主脉冲,产生40条非脉冲地震动。将这80条地震动分别输入两个隔震结构,量化分析脉冲地震动对其层间位移角、楼层剪力、楼层加速度和隔震层位移的放大效应。结果表明：脉冲地震动作用下基础隔震结构的反应明显大于非脉冲地震动,脉冲地震动对隔震层位移放大系数最大,而楼层加速度放大系数对脉冲地震动比较敏感,呈现出波浪形的变化特征。两个隔震结构所得到的隔震层位移的放大系数平均值约为2.0,层间位移角、楼层剪力、楼层加速度的放大系数平均值约为1.5。     

Reliability assessment of structure subjected to horizontal-vertical random earthquake excitations

Statistical analysis of the response spectra due to both horizontal and vertical ground excitations is conducted. A method is presented for generating a uniform hazard response spectrum making use of the seismic hazard curve expressing annual probability of exceedance as a function of peak ground acceleration (PGA), a set of normalized mean response spectra, and a coefficient-of-variation function for the random normalized spectral values in terms of period T. The response spectra of the system are calculated by considering both horizontal and vertical excitations simultaneously. Comparison on the uniform hazard response spectrum for case of single input (horizontal excitation only) and for case of multiple inputs (horizontal and vertical excitations) are made. It is important to note that the effect of load ratio (Mg /P_cr) as well as the PGA ratio between horizontal and vertical ground accelerations are two important parameters in the analysis of structural systems subjected to both horizontal and vertical ground motions.     

Effects of near-fault ground motions and equivalent pulses on multi-story structures

The focus of this paper is the structural response of multi-story structures to near-fault ground motions, and whether structural response is dominated by the ground motion pulses present in forward-directivity ground motions. Also considered is whether simplified pulses are capable of representing the effects of these pulses on structural response. Incremental Dynamic Analysis was employed to assess the effects of forward-directivity pulses on the response of near-fault multi-story structures. Three different generic multi-story shear buildings were subjected to fifty four near-fault ground motions including ordinary and forward-directivity records. The Maximum Story Displacement Ductility Demand was selected as the Engineering Demand Parameter. Results showed that pulse-like forward-directivity ground motions impose a larger ductility demand to the structure compared to ordinary ground motions. Moreover, the response of the structures to forward-directivity motions shows higher scatter than the response to ordinary ground motions when correlated with simple intensity measures such as PGA or spectral acceleration at the first mode period. The only intensity measure that appears to be valid for both ordinary and forward-directivity ground motions is the peak ground velocity. The structural response to the forward directivity ground motions was reproduced using an equivalent pulse model based on the modified Gabor Wavelet pulse. It is shown that when the ratio of pulse period to the fundamental structural period falls in a range of 0.5-2.5, the equivalent pulse model appropriately represents the structural response to forward-directivity ground motions. The simplified pulse parameters can be predicted using existing relationships and can be incorporated into probabilistic seismic hazard analysis to develop a seismic reliability analysis. Finally, the effects of damping ratio and P-Δ were investigated for forward-directivity ground motions. The effect of variations in the damping ratio on the ductility demand was insignificant while P-Δ-effects on the ductility demand are significant.     

Seismic fragility assessment of effectiveness of viscous dampers in R/C buildings under scenario earthquakes

Fragility curves are used to represent the probabilities that the structural damages, under various level of seismic excitation, exceed specified damage states by means of earthquake intensity–damage relations. In this study, the fragility curves have been developed for comparative seismic evaluation of several retrofitting measures by incorporation of fluid viscous (VS) dampers applied to a representative high-rise reinforced concrete (R/C) office building located in Istanbul. In the retrofitting strategies considered, similar type of VS dampers was used and designed to provide the structure with three different effective damping ratios of 10%, 15%, and 20%. In the fragility analysis, a set of 240 artificially generated earthquake ground motions compatible with the design spectrum selected to represent the variability in ground motion was employed to study nonlinear dynamic responses of the structures before and after retrofit. Four damage states: slight, moderate, major, and collapse were defined to express the condition of damage. The fragility curves in this study are represented by lognormal distribution functions with two parameters and developed as a function of peak ground acceleration (PGA), spectral acceleration (S_a), spectral displacement (S_d). Comparison of the fragility curves indicated that the VS dampers were very effective in attenuating seismic structural response under various earthquake ground motions. It was also found that a two-fold reduction in the probability of exceeding damage states might be achieved by introducing passive VS damper systems.     

Appropriate ground motion intensity measures for estimating the earthquake demand of floor acceleration-sensitive elements in super high-rise buildings

To characterise the functionality resilience of super high-rise buildings subjected to earthquakes, an investigation is conducted on the selection of appropriate ground motion intensity measures (IMs) for developing an efficient and sufficient correlation with peak floor acceleration (PFA) and floor acceleration response spectrum (FARS) for floor acceleration-sensitive building contents and equipment. The efficiency and sufficiency of 53 collected IMs are assessed with respect to PFA and FARS at selected floors of two example-frame core-tube super high-rise buildings. Two sets of ground motions, i.e. far-field and pulse-type near-field motions, are chosen to reflect the record-to-record variability in the investigation. The investigation indicates that with respect to PFA, the collected IMs can be divided into three different groups based on their efficiency. The ground-acceleration-related IMs, i.e. PGA (peak ground acceleration), EDA (Effective design acceleration) and A₉₅ (threshold of acceleration when 95% of Arias intensity is achieved), are shown to be efficient in either set of ground motions. Regarding FARS, the appropriate IMs are notably different from those in the case of PFA, from the perspective of efficiency as well as sufficiency. None of the collected IMs can maintain its sufficiency with respect to the magnitude and source-to-site distance in any case.     

地震动速度脉冲对不同高宽比基础隔震结构抗震性能的影响

为研究近断层地震动的速度脉冲对基础隔震结构地震反应的影响,建立了不同高宽比的传统抗震及基础隔震框架结构有限元模型,选取了6条具有向前方向性效应和滑冲效应速度脉冲的实际近断层强震记录作为结构基础输入地震动,对8个模型进行了非线性动力时程分析,对比分析了传统抗震及基础隔震框架结构模型的层间位移角、支座位移和基底剪力等反应。结果表明：在近断层速度脉冲型地震动作用下,随着结构高宽比的增加,基础隔震结构的层间位移角和基底最大剪力逐渐增加,而隔震支座位移有先增大后减小的趋势,且随着地震动峰值速度与峰值加速度比值的增大,隔震支座位移也逐步增大;基础隔震对高宽比小于3的结构具有较好的减震效果,且高宽比越小其减震效果越好,但是当结构高宽比为4时,基础隔震效果较差;近断层地震动的速度脉冲对基础隔震结构底部楼层的不利影响会导致结构出现倒塌破坏。     

近断层地震作用下混合阻尼隔震结构的响应分析

本文以实际的地震波El-Centro波为底波,在此基础上叠加三角型脉冲运动模拟近断层地震地面的激励;以人工合成的近断层地震波作为输入,研究其对混合阻尼隔震结构的动力反应特性,绘制了上部结构最大加速度和基底最大滑移量的时程图,并讨论了混合阻尼隔震结构的摩擦系数、粘滞阻尼比及滞回阻尼等参数对隔震结构产生的重要影响.研究表明:在混合阻尼隔震结构中,摩擦系数存在一个较优值,粘滞阻尼比对脉冲能量较大的B、C类波的控制有较显著的效果,但对原始波及A类波控制效果不明显,滞回阻尼对隔震结构Smax和amax均有一定影响,通过分析给出了该参数的合理取值.     

近断层速度脉冲地震动对边坡滑移的影响分析

 采用基于小波变换的多向地震动速度脉冲特性鉴定方法,从NGA(next generation attenuation)数据库及汶川地震动记录中选取196条速度脉冲地震动;基于Newmark非耦合滑动模型,考虑土体动力非线性,通过对比近断层脉冲及非脉冲地震动引起的不同参数边坡的滑移特征,分析近断层地震动速度脉冲特性及边坡本身参数对滑动位移值的影响;同时对近断层速度脉冲地震动参数与边坡滑移量的相关性做了对比分析。结果表明：(1) 近断层速度脉冲地震动引起的边坡滑动位移值远大于近断层非脉冲地震动,对长周期边坡影响尤为明显;(2) 相比非脉冲地震动,脉冲地震动引起的边坡滑动时间较短,且主要发生在地震动起始阶段的较短时间内(速度脉冲段),滑移的坡体具有更大的滑移速度,携带更高的能量,从而对周边环境产生更强的破坏作用;(3) 近断层速度脉冲地震动引起的边坡滑移特征与其速度脉冲特性(持时短、周期长、速度峰值大)密切相关,且峰值地震动速度PGV与边坡滑移量具有高度相关性。最后,通过比较原始速度脉冲地震动及相应的等效脉冲作用下不同参数边坡的滑动位移值,对提取出的等效脉冲预测原始地震动引起的滑动位移的有效性进行分析,并基于此建立脉冲参数–滑动位移曲面。     

超高层结构地震位移及剪力响应峰值时滞效应研究

近年结构震害表明,超高层结构在远场地震动作用下出现了楼顶长时间大幅值晃动现象且时滞效应明显,其中大幅值晃动造成非结构构件破坏严重,现行设计方法对地震作用下超高层结构大变形的控制还有不足。因此,文中简要分析了结构响应峰值机理,对长周期结构进行了近场及远场地震波作用下的时程分析,研究了长周期结构响应峰值与地震波中和结构基本周期对应频率的脉冲族之间的相关性,讨论了结构弹塑性对超高层结构响应峰值时滞特征的影响。结果表明：超高层结构与地震波中对应频率的脉冲族产生类共振是引起结构大幅值晃动的主要原因,仅基于加速度反应谱拟合精度选出的地震波不能较好预测超高层结构的大变形;类共振出现在地震波衰减段时会引起时滞效应;结构弹塑性引起的刚度退化会影响结构响应峰值及其出现时刻。     

基于速度脉冲地震动的边坡地震位移统一预测模型

采用基于小波变换的多向地震动速度脉冲特性鉴定方法,从NGA数据库及汶川地震动记录中提取出了196条速度脉冲地震动数据;基于Newmark非耦合滑动模型,考虑土体非线性特征。通过计算速度脉冲地震动作用下不同强度(k_y)及初始自振周期(T_s)边坡模型的地震位移,对比分析了边坡地震位移与速度脉冲地震动参数之间的相关性,并建立了适用于近断层速度脉冲地震动的边坡地震位移统一预测模型。结果表明：既有边坡地震位移预测模型低估了近断层速度脉冲地震动引起的位移值,而用于预测近断层非脉冲地震动引起的边坡地震位移离散较小;近断层速度脉冲地震动引起的边坡地震位移与其速度脉冲特性密切相关,地震位移与速度脉冲地震动参数(峰值速度PGV)相关性最好;采用1.5倍边坡自振周期对应的加速度反应谱(S_a(1.5T_s))和 PGV分别代表速度脉冲地震动的频谱成分及速度脉冲特征,能够综合反映速度脉冲地震动对边坡地震位移的影响;建立了基于边坡参数(k_y, T_s)和脉冲地震动参数(PGV, S_a(1.5T_s))的边坡地震位移预测模型,为考虑近断层地震动速度脉冲特性影响的边坡地震位移概率灾害分析提供了基础。     

Statistical evaluation of the damage potential of earthquake ground motions

This study focuses on the damage potential of earthquake ground motions based on the inelastic dynamic response of equivalent single degree of freedom structures. Their yield resistances are selected in accordance with seismic design codes. An index accounting for the accumulation of damage due to inelastic excursions is used to represent structural damage. A set of 94 ground motions are employed for this analysis, which are all scaled to the same peak ground acceleration of 0.4 g. Earthquake ground motions are classified with respect to both the ratio of peak velocity to peak acceleration (V/A ratio) and their effective excitation duration. The effect of these parameters on damage potential is investigated by using sensitivity analysis and probabilistic techniques. It is concluded that both V/A ratio and effective duration significantly influence the damage potential of earthquake ground motions, although they are not represented appropriately by the spectral definitions of earthquake excitations in seismic design codes.     

带限位装置的摩擦隔震结构动力特性及地震反应分析

提出了一种新型基础隔震模型,即带限位装置的摩擦隔震体系 (S- LF);基于此隔震模型,利用 Poincare映射法研究地面谐运动下 S- LF结构的运动特征;并利用高精度的精细时程积分法,通过对地震作用下 S- LF动力响应的计算,绘制了上部结构最大加速度反应谱和基底最大滑移量反应谱,并研究了各种结构参数对隔震的影响;通过与恢复力摩擦隔震系统 (R- FBI)和纯摩擦力滑移隔震系统 (P- F)的比较表明, S- LF的隔震性能优于 P- F和 R- FBI;最后通过对计算实例的分析发现,地震作用下 S- LF结构的层间最大剪力和最大绝对加速度反应分布较一般传统结构有很大区别。     

考虑地震动摇摆分量作用的输电塔线体系响应

为探究输电塔线体系在地震动水平-摇摆耦合作用下的响应,进行了振动台试验和理论分析。地震动摇摆分量通过小波分析的方法从原始地震记录中取得,此方法以单摆式地震仪在水平和竖向的响应差别为基础,通过摇摆倾斜位移的傅里叶谱和竖向速度相似作验证。采用实际输电塔线体系的简化缩尺模型,进行单塔模型和三塔两线模型在顺线向作用下的振动台试验,考察在地震动水平、摇摆及水平-摇摆耦合作用下的结构响应。推导了地震动水平、摇摆加速度和摇摆位移耦合作用下输电塔线体系的动力方程,在方程的激励项中以等效侧向力的形式考虑摇摆转角位移产生的附加P-Δ效应。研究结果表明：理论分析和试验结果吻合较好,证明了理论分析的正确性;地震动摇摆分量不可忽略,其对输电塔线体系的地震响应影响较大,其中由摇摆转角位移产生的附加P-Δ效应会增大输电塔线体系的动力响应,并造成塔体产生一定程度的非对称位移响应;本次试验中,在多维地震作用下,输电线会减弱输电塔主体结构的水平位移和加速度响应,但相对于水平地震动作用,考虑摇摆分量后,输电线对输电塔主体结构响应的减弱效果将会被削弱。     

竖向地震效应下近断层边坡永久位移预测模型

利用改进的能量法从美国NGA数据库的4 669条地震记录中量化识别出305条近断层速度脉冲地震动。脉冲具有方向性效应,所有地震动均转换到最强脉冲方向。基于Newmark刚塑性理论模型,考虑竖向地震效应,输入拟合后的地震动,计算了边坡永久位移值。根据大量计算结果统计发现,竖向地震效应对近断层内的边坡永久位移有显著影响,既有边坡地震位移预测模型的预测值离散性较大。通过相关性分析,发现竖向效应作用下,边坡永久位移值与近断层地震动的峰值速度相关性显著。采用统计学回归分析方法,建立了基于峰值速度(PGV)、Arias强度(I_a)、临界加速度(a_c)的预测模型,该模型简单实用,可应用于近断层区域的边坡滑坡地质灾害风险评估。     

A new proxy for ground motion selection in seismic collapse assessment of tall buildings

Tall buildings are long‐period structures that are sensitive to the long‐period content of ground motions. Selection of appropriate ground motions is an important step in seismic collapse assessment of tall buildings using nonlinear dynamic analyses. Epsilon (ε_Sa) and eta (η) are two spectral shape indicators, which have been recently proposed for ground motion selection in the technical literature. In this study, a new parameter gamma (γ) is proposed, which has considerable correlation with the collapse capacity of long‐period structures having a fundamental period greater than 1 s. This parameter is a linear combination of ε_Sa and the displacement spectrum intensity epsilon (ε_DSI). The parameter γ is obtained and optimized by applying the particle swarm optimization algorithm. Since γ has significant correlation with the collapse capacity of long‐period structures, it can be used as an efficient proxy for ground motion selection in seismic collapse assessment of tall buildings. The results of this study show that ground motion selection considering the new proxy γ causes reduction in the dispersion of structural response and also decrease in the mean annual frequency of collapse, when compared with ground motion selection based on ε_Sa and η. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of deflection amplification factor in steel moment‐resisting frames

Steel moment‐resisting frames (SMRFs) are the most common type of structural systems used in steel structures. The first step of structural design for SMRFs starts with the selection of the structural sections on the basis of story drift limitation. ASCE 7 (2010) requires that the inelastic story drifts be obtained by multiplying the deflections determined by elastic analysis under design earthquake forces with a deflection amplification factor (C_d). For special moment‐resisting frames, C_d is given as 5.5 in ASCE 7 (2010). Lower C_d values will increase the overall inelastic response of the structure. On the other hand, the inelastic response of the structure is expected to be less severe when designed for higher C_d values. The performance objective is that the structure should sustain the inelastic deformation demand imposed due to design earthquake ground motions. This study aims at investigating the inelastic seismic response that low‐rise, medium‐rise and high‐rise SMRFs can experience under design earthquake ground motions and maximum considered earthquake (MCE) level ground motions and evaluating the deflection amplification factors (C_d) for SMRFs in a rational way. For this purpose, nonlinear dynamic time history and pushover analyses will be carried out on SMRFs with 4, 9 and 20 stories. The results indicate that the current practice for computing the inelastic story drifts for SMRFs is rational and the frames designed complying with the current code requirements can sustain the inelastic deformations imposed during design earthquake ground motions when seismically designed and detailed. Copyright © 2013 John Wiley & Sons, Ltd.     

近断层地震动向前方向性效应和滑冲效应对高层钢结构地震反应的影响

为考察近断层地震动向前方向性效应和滑冲效应引起的两种速度脉冲运动对高层钢框架结构地震反应的影响,选择具有向前方向性效应、滑冲效应和无速度脉冲的近断层地震动作为输入,利用SAP2000软件对一座20层平面钢框架进行非线性时程分析。计算结果表明,含滑冲效应和向前方向性效应的脉冲地震动主要激发结构基本振型反应,而无速度脉冲的地震动能够激起结构的高阶振型反应,而且,脉冲型地震动的结构破坏作用远强于无速度脉冲地震动。最后,引入了单自由度体系的能量耗散系数,从能量耗散和高阶振型影响的角度对钢结构动力反应计算结果和损伤破坏状态给出了合理解释。