Earthquake responses of RC moment frames subjected to near‐fault ground motions

There are three objectives in this paper. The first objective is to compare the dynamic behaviour of a reinforced concrete building structure subjected to near‐fault and far‐field ground motions. A twelve‐storey and a five‐storey reinforced concrete building with moment resisting frames were selected in this study. The Chi‐Chi earthquake was selected as a first set in this study to test near‐fault earthquake characteristics. Further, another earthquake record of an event at the same site was selected to test the far‐field earthquake characteristics for comparison. Through nonlinear time history analyses, the results show that the near‐fault earthquake results in much more damage than the far‐field earthquake. The second objective of this paper is to compare the predictions for ductility demand by the nonlinear time history analyses with those obtained by the pushover analysis procedure. The third objective is to explore the parameters that will more significantly affect the the building structure's dynamic response characteristics of base shear reduction and displacement amplification. Copyright © 2001 John Wiley & Sons, Ltd.     

Modeling of lightly reinforced concrete walls subjected to near‐fault and far‐field earthquake ground motions

Reinforced concrete bearing walls with low vertical reinforcement ratios of less than 0·2% are referred to as lightly reinforced walls. Recently, Eurocode 8 and the French code PS 92 adopted a special design concept for lightly reinforced concrete walls based on the multifuse principle favouring rupture occurrence at several storeys. This design leads to lower reinforcement ratios with their optimized distribution allowing wide cracks to take place with large energy dissipation potential. In addition, the vertical displacement of the mass results in energy transformation from kinematic to potential. The objective of the investigation is to analytically predict the response of such lightly reinforced walls when subjected to near‐fault and far‐field ground motion records up to failure to establish the load‐carrying capacity and ductility of the walls. A wall was modeled using six‐node two‐dimensional panel elements. The panel elements have lumped flexural/axial plasticity at their top and bottom fibre sections. The response of the wall was evaluated in terms of pushover, spectral, displacement‐based, and time history analyses. The model and the response data were verified against available measurements from a test program conducted using a shake table. The comparison indicated that the model closely represented the behaviour observed in the test. Copyright © 2007 John Wiley & Sons, Ltd.     

Plastic hinge length of reinforced concrete columns subjected to both far‐fault and near‐fault ground motions having forward directivity

In a strong earthquake, a standard reinforced concrete (RC) column may develop plastic deformations in regions often termed as plastic hinge regions. A plastic hinge is basically an energy dampening device that dampens energy through the plastic rotation of a rigid column connection, which triggers redistribution of bending moments. The formation of a plastic hinge in an RC column in regions that experience inelastic actions depends on the characteristics of the earthquakes as well as the column details. Recordings from recent earthquakes have provided evidence that ground motions in the near field of a rupturing fault can contain a large energy or ‘directivity’ pulse. A directivity pulse occurs when the propagation of the fault proceeds at nearly the same rate as the shear wave velocity. This pulse is seen in the forward direction of the rupture and can cause considerable damage during an earthquake, especially to structures with natural periods that are close to those of the pulse. In the present paper, 1316 inelastic time‐history analyses have been performed to predict the nonlinear behaviour of RC columns under both far‐fault and near‐fault ground motions. The effects of axial load, height over depth ratio and amount of longitudinal reinforcement, as well as different characteristics of earthquakes, were evaluated analytically by finite element methods and the results were compared with corresponding experimental data. Based on the results, simple expressions were proposed that can be used to estimate plastic hinge length of RC columns subjected to both far‐fault and near‐fault earthquakes that contain a forward‐directivity effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance‐based seismic rehabilitation of existing steel eccentric braced buildings in near fault ground motions

The devastating effects observed in the recent earthquakes, in terms of loss of lives as well as immediate and long‐term economic losses, have prompted the need to provide documents concerning the assessment and improvement of the structural performance of existing buildings at the time of an earthquake. In this regard, performance engineering is defined as performance‐based seismic design and rehabilitation. There are many reasons for rehabilitation of existing buildings. Changing the building's usage is one of the most common reasons. In the present study, the residential steel buildings were subject to performance‐based rehabilitation, converting to educational use. Several steel frames with dual lateral‐resistant systems (MRF–EBF) and different numbers of stories were initially designed as residential buildings. The frames were rehabilitated according to the current seismic rehabilitation codes and regulations. Cover plates were used to strengthen structural elements. Variations in structural responses were evaluated before and after retrofitting by the use of nonlinear analysis. Moreover, the performance of rehabilitated structures was evaluated, considering the gross features observed in near‐field records. Copyright © 2013 John Wiley & Sons, Ltd.     

Variability of seismic demands according to different sets of earthquake ground motions

It is a challenging task to predict seismic demands for earthquake resistant design, particularly for tall buildings. In current seismic design provisions, seismic demands are expressed as a design base shear of which the key components are linear elastic design response spectra, force reduction factor (‘R factor’), and building weight. For tall buildings, response spectrum analysis or response history analysis is recommended in current design provisions. In recent years, new methods for predicting seismic demands have been developed, such as the capacity spectrum method (CSM) and displacement coefficient method. This study investigates the effect of different earthquake ground motion (EQGM) sets on seismic demands. Key components of the base shear and performance points in the CSM are considered as the seismic demands to be tested. For this purpose three EQGM sets are collected independently at rock sites. This study found that seismic demands can vary significantly according to different EQGM sets even though those sets were obtained at sites with similar soil conditions. This study also attempts to provide a criterion for reducing the variability in seismic demands according to different EQGM sets. Copyright © 2007 John Wiley & Sons, Ltd.     

Collapse analysis of building structures under excitation of near‐fault ground motion with consideration of large deformation and displacement

The dynamic analysis of structural stability with consideration of material and geometrical non‐linearity is necessary for near fault‐earthquake that is rich in long‐period components and often induces the non‐linear large displacement and deformation response of a building structure. A macro‐element bilinear geometric stiffness model and simplified analytical model are proposed and developed to analyze the P‐Δ effects of structural dynamic response using a numerical approach. A structural stable threshold diagram is then proposed to evaluate the geometric stability of a building structure with large deformation under the excitation of a near‐fault earthquake. The analysis results reveal: (1) the simplified geometric stiffness analytical model is useful for analyzing structural dynamic P‐Δ effects and acquire very good accurate results even though the structural geometric stiffness varies between elastic and plastic zone; (2) stable threshold diagrams, based on dynamic analysis and statistical analysis procedures, are conducted by application of this proposed model to easily evaluate structural geometric stability with larger deformation imposed by a near‐fault earthquake. This method can supplement the insufficient capability for the static pushover analysis procedure to estimate the seismic proof demands for building without dynamic P‐Δ effects analysis; (3) the analysis results of stable threshold diagrams indicate that when stability coefficient θ of a building is greater than 1 or base shear factor (V/W) of the building is less than 0·2, static P‐Δ effects become noticeable. Copyright © 2007 John Wiley & Sons, Ltd.     

Estimation of seismic demands of steel frames subjected to near‐fault earthquakes having forward directivity and comparing with pushover analysis results

High statistics of damages in modern structures (buildings structured based on new codes) exposed to near‐fault earthquake illustrates the necessity of more studies on this kind of earthquake effects on the structures. A specification of near‐fault earthquakes is the directivity effects. Existing records of near‐fault quakes containing directivity effects including records of Iran and abroad were modified and used for linear time history analysis of three steel moment frames (5, 8 and 12 story frames), and the results were compared with nonlinear time history analysis and pushover analysis of far‐fault quakes in this paper. The results showed that these records (near fault) motivate high modes of the structure, and especially for the 12‐story structure, high response was detected, but none of these results made the frames collapse. By comparing nonlinear dynamic analysis (time history) with nonlinear static analysis (pushover), it was concluded that various lateral load patterns in pushover cannot cover the time history result needs. Load distribution pattern based on the first vibration mode covers these demands in the lower floors, but in higher floors, this method is not applicable. Copyright © 2012 John Wiley & Sons, Ltd.     

Effectiveness of modified pushover analysis procedure for the estimation of seismic demands of buildings subjected to near‐fault earthquakes having forward directivity

Near‐fault ground motions with long‐period pulses have been identified as being critical in the design of structures. These motions, which have caused severe damage in recent disastrous earthquakes, are characterized by a short‐duration impulsive motion that transmits large amounts of energy into the structures at the beginning of the earthquake. In nearly all of the past near‐fault earthquakes, significant higher mode contributions have been evident, resulting in the migration of dynamic demands (i.e., drifts) from the lower to the upper stories. Due to this, the static nonlinear pushover analysis (PA) (which utilizes a load pattern proportional to the shape of the fundamental mode of vibration) may not produce accurate results when used in the analysis of structures subjected to near‐fault ground motions. The objective of this paper was to improve the accuracy of the pushover method in these situations by introducing a new load pattern into the common pushover procedure. Several PAs are performed for six existing reinforced concrete buildings that possess a variety of natural periods. Then, a comparison is made between the PA results (with four new load patterns) and those of FEMA‐356 with reference to nonlinear dynamic time‐history analyses. The comparison shows that, generally, the proposed pushover method yields better results than all FEMA‐356 PA procedures for all investigated response quantities, and is a closer match to the nonlinear time‐history responses. In general, the method is able to reproduce the essential response features providing a reasonable measure of the likely contribution of higher modes in all phases of the response. Copyright © 2009 John Wiley & Sons, Ltd.     

Fragility of skewed bridges under orthogonal seismic ground motions

The paper evaluates seismic fragility characteristics of skewed bridges under simultaneous action of orthogonal ground motion components. The effect of skew angle on bridge seismic fragility characteristics is investigated through nonlinear time-history analyses of Painter Street Overpass, a 38.5° skewed bridge located in Rio Dell, CA, and six representative bridges with skew angles varying between 0° and 50°. Ground motion incident angle is varied from 0° to 180° to investigate the effect of the direction of ground motion incidence on bridge seismic performance. Bridge seismic response is used to generate fragility curves and contours plots that quantify the sensitivity of bridge fragility characteristics on skew angle and incident angle. For any value of incident angle, bridge seismic vulnerability increases with an increase in skew angle; however, no such general trend is found to describe the effect of incident angle on bridge fragility characteristics. Results show that the variation of maximum rotation of bridge columns for an earthquake does not follow any particular trend with the change in skew angle and incident angle. Analysis-based fragility curves are further compared with empirical fragility curves generated using real-life seismic damage data of skewed bridges and a reasonable agreement is observed between these two.     

基于抗震规范和地震动区划图的风险导向地震动决策参数分析

基于GB 50011—2010《建筑抗震设计规范》和GB 18306—2015《中国地震动参数区划图》中给出的地震动强度,对普通建筑结构的风险导向地震动决策参数进行分析,探讨目标倒塌风险、易损性对数标准差,以及大震、中震、巨震条件倒塌概率的取值。基于抗震规范和地震动区划图中的地震动强度进行风险导向地震动决策,验证了决策参数分析结果的合理性,给出了我国单体结构和群体结构的风险导向地震动决策参数建议值,并对风险导向地震动决策参数进行了相关分析。结果表明:对于单体结构,易损性对数标准差取0.3时,目标倒塌风险及大震、中震、巨震的条件倒塌概率分别为1.0%、10.0%、0.04%、54.7%,易损性对数标准差取0.4时,该值分别为1.0%、11.0%、0.2%、44.6%;对于群体结构,目标倒塌风险取1.0%时,易损性对数标准差及大震、中震、巨震的条件倒塌概率分别为0.6、10.4%、1.1%、30.2%,目标倒塌风险取0.4%时,该值分别为0.6、3.6%、0.2%、13.2%。通过分析可以发现:易损性对数标准差不大于0.6时,随着易损性对数标准差的增大,中震的条件倒塌概率逐渐增大,巨震的条...     

Efficiency of active systems in controlling base‐isolated buildings subjected to near‐fault earthquakes

It has been pointed out that base‐isolated structures may be vulnerable during near‐fault earthquakes and special considerations are required in the design of isolated structures in near‐fault areas. This paper investigates the efficiency of active control systems in reducing the responses of base‐isolated structures with various isolation parameters. The design of hybrid control systems using base isolation and active systems are optimized in order to accomplish different design purposes. Also for some cases, equivalent passive control systems are introduced which result in comparable responses with respect to hybrid control systems. Copyright © 2009 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。     

Robustness of base‐isolated high‐rise buildings under code‐specified ground motions

The robustness of base‐isolated high‐rise buildings is investigated under code‐specified ground motions. Friction‐type bearings are often used in base‐isolated high‐rise buildings to make the natural period of those buildings much longer. While additional damping can be incorporated into every story in passive controlled structures with inter‐story type passive members, that can be incorporated into the base‐isolation story only in the base‐isolated building. This fact leads to the property that, as the number of stories of the building becomes larger, the damping ratio reduces. This characteristic may cause some issues in the evaluation of robustness of base‐isolated high‐rise buildings. The purpose of this paper is to reveal the robustness of base‐isolated high‐rise buildings. A kind of inverse problem for the target drift in the base‐isolation story is formulated in order to determine the required quantity of additional viscous damping. It is demonstrated numerically that, as the base‐isolated building becomes taller, the damping ratio becomes smaller and the ratio of the friction‐type bearings in the total damping becomes larger. This may lead to the conclusion that base‐isolated high‐rise buildings have smaller robustness than base‐isolated low‐rise buildings. Copyright © 2007 John Wiley & Sons, Ltd.     

远场长周期地震动反应谱拐点特征周期研究

研究远场长周期地震动反应谱两个拐点周期的统计特征以及主要因素的影响规律。考虑震中距、震级和场地类型不同,筛选长周期成分丰富的破坏性浅源强震数字化记录,通过反应谱分析,基于Newmark-Hall模型计算加速度反应谱第一下降段特征点周期和第二下降段特征点周期,分析特征周期的变化规律。结果表明：第一下降段特征点周期和第二下降段特征点周期随震中距和震级的增大而增大,场地类型对第一下降段特征点周期有显著影响,而对第二下降段特征点周期的影响不显著;震中距和震级与第一和第二下降段特征点周期的秩相关性处于中等偏弱水平,表明该两个特征点周期随震中距和震级而变化,采用单一的震中距和震级估算设计反应谱的特征点周期,并非可靠;第一和第二下降段特征点周期的线性相关性为中等偏弱水平,现有规范以第一下降段特征点周期估算第二下降段特征点周期的线性模型并不适用于长周期地震动反应谱。考虑震中距、震级和场地类型的综合影响,给出了长周期地震动反应谱第一和第二下降段特征点周期的计算模型;依据四类场地上的统计平均值,给出第一和第二下降段特征点周期建议取值,可供规范修订时参考。     

Effects of a controlling geological discontinuity on the seismic stability of an underground cavern subjected to near-fault ground motions

The mechanism for the influence of near-fault ground motion on seismic issues for underground rock caverns has seldom been addressed, especially for caverns controlled by large geological discontinuities. In this paper, a nonlinear joint model was used to simulate the effects of unfavorable geological discontinuities under seismic excitation. The influence of near-fault ground motion on unfavorable geological discontinuities was analyzed using a large sample of ground-motion records collected from the NGA-West2 database. A damage potential index (DPI) for unfavorable geological discontinuities was proposed and discussed. The #1 surge chamber of the Baihetan Hydropower Plant, which is dominated by interlayer shear weakness zone (ISWZ) C2, was used as a study case to investigate the differences between pulse-type near-fault ground motion, non-pulse-type near-fault ground motion, and far-field ground motion. The results of the study indicate that (1) significant velocity and displacement as well as a stronger long-period response spectrum are key characteristics of pulse-type near-fault ground motions, whereas non-pulse-type near-fault ground motions display characteristics similar to those of far-field ground motions; (2) the velocity pulse is responsible for the destructive capabilities of near-fault ground motions; (3) the peak ground velocity (PGV) was shown to be the most suitable DPI of several ground-motion parameters for large geological discontinuities under seismic excitation (applicable to both near-fault and far-field ground motions); and (4) PGV was verified to be the most effective DPI for ISWZ C2 at the Baihetan #1 surge chamber. The cavern became fragile when subjected to near-fault ground motions, so special seismic reinforcement measures are recommended. These findings may provide a reference for the seismic design of underground caverns.     

Resonant behaviour of base‐isolated high‐rise buildings under long‐period ground motions

The resonant behaviour of base‐isolated high‐rise buildings under long‐period ground motions is investigated. The long‐period ground motions are known to be induced by surface waves. While the acceleration amplitude of such long‐period ground motion is small, the velocity amplitude is fairly large. It is expected that high‐rise buildings and base‐isolated buildings with long fundamental natural periods are greatly influenced by these long‐period ground motions. Especially base‐isolated high‐rise buildings with friction‐type bearings may have remarkable mechanical characteristics unfavourable for these long‐period ground motions. The purpose of this paper is to reveal that the long‐period ground motions recorded in Japan have the intensity to make base‐isolated high‐rise buildings in resonance with long‐period components and that careful treatment is inevitable in the structural design of these base‐isolated high‐rise buildings. It is pointed out that the friction‐type bearings are effective in general in avoiding the resonance with ground motions with a narrow‐range frequency characteristic, but are dangerous for ground motions with a wide‐range frequency characteristic in the long period range. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

基于改进迟滞阻尼模型的混合结构抗震分析SRSS法

混合结构的阻尼矩阵不再满足经典阻尼条件,无法直接采用模态叠加法。迟滞阻尼模型具有每周期耗散能量与外激励频率无关的优点,且时域计算结果稳定收敛,但不满足能量守恒原则。利用结构每周期耗散能量与阻尼做功相等,对迟滞阻尼模型的阻尼系数进行修正,得到了改进迟滞阻尼模型,并提出了对应的混合结构模态叠加法。在此基础上,分析改进迟滞阻尼模型的反应谱特点,计算最不利地震作用效应组合,提出了基于改进迟滞阻尼理论的混合结构抗震分析SRSS法。算例分析表明,改进迟滞阻尼模型计算的地震作用效应比粘性阻尼模型计算的地震作用效应更大,其增大幅度约为15%～20%。