Seismic performance evaluation of a jointed arch dam

Seismic performance and safety of a jointed arch dam, as an arch-shaped mass concrete structure, are investigated through the nonlinear incremental dynamic analysis. In this way, 12 proper ground motions are selected, each of them is scaled to 12 successively increasing intensity levels and applied to the dam. Three and seven contraction joints are inserted within the dam body, and stage construction is taken into account. Several main assumptions including dam–reservoir–foundation dynamic interaction, absorbing boundary conditions at the far-ends of the reservoir and foundation, and material and joint nonlinearities are considered. The failure modes of the dam are determined according to the distribution of damaged zones through the dam body. The results of the static analysis under dam self-weight and hydrostatic load show that the inclusion of the contraction joints and the stage construction do not considerably alter the tensile principal stresses; however, they increase the compressive stresses. In seismic analysis, increasing the number of the contraction joints generally reduces the joints opening and sliding displacements. The contraction joints can significantly decrease the damage imposed to the dam body. Finally, it is concluded that inserting the contraction joints can increase the seismic safety of the arch dam.     

Probabilistic seismic response and uncertainty analysis of continuous bridges under near-fault ground motions

Performance-based seismic design can generate predictable structure damage result with given seismic hazard. However, there are multiple sources of uncertainties in the seismic design process that can affect desired performance predictability. This paper mainly focuses on the effects of near-fault pulse-like ground motions and the uncertainties in bridge modeling on the seismic demands of regular continuous highway bridges. By modeling a regular continuous bridge with OpenSees software, a series of nonlinear dynamic time-history analysis of the bridge at three different site conditions under near-fault pulse-like ground motions are carried out. The relationships between different Intensity Measure (IM) parameters and the Engineering Demand Parameter (EDP) are discussed. After selecting the peak ground acceleration as the most correlated IM parameter and the drift ratio of the bridge column as the EDP parameter, a probabilistic seismic demand model is developed for near-fault earthquake ground motions for 3 different site conditions. On this basis, the uncertainty analysis is conducted with the key sources of uncertainty during the finite element modeling. All the results are quantified by the “swing” base on the specific distribution range of each uncertainty parameter both in near-fault and far-fault cases. All the ground motions are selected from PEER database, while the bridge case study is a typical regular highway bridge designed in accordance with the Chinese Guidelines for Seismic Design of Highway Bridges. The results show that PGA is a proper IM parameter for setting up a linear probabilistic seismic demand model; damping ratio, pier diameter and concrete strength are the main uncertainty parameters during bridge modeling, which should be considered both in near-fault and far-fault ground motion cases.     

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.     

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.     

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。     

近断层地震动空间分布特征对斜拉桥地震响应影响

为研究近断层地震动空间分布特征对斜拉桥地震响应的影响规律,以台湾集集地震近断层地震动为研究对象,根据台站与断层的空间相对位置,将 近场波分为破裂前方区域(Forward District,FD)、破裂区域(Middle District,MD)和破裂后方区域(Backward District,BD)三类波,对其进行频谱分析 表明,MD区域和FD区域地震动低频成分显著而BD区域高频成分显著。以某主跨为406m的斜拉桥为依托工程,对三个区域地震动作用下地震响应规律进行对比 研究,计算结果表明三个区域地震响应规律和量值均有显著差异,不同区域结构动力响应与地震动特征参数相关程度迥异。MD区域塔顶纵向位移较BD区域增 加3.03倍,塔底弯矩和剪力增加1.0倍;FD区域塔顶纵向位移较BD区域增加1.06倍,塔底弯矩和剪力较BD区域却分别下降了35.5%和32.2%。基于上述结果, 建议对于位于破裂区域的结构进行抗震设计时,须同时提高强度与变形需求,采用PGV及PGA作为结构动力响应参数的评估指标,还应选择合理地震动方向; 对于破裂前方区域,结构动力响应受输入能和PGA影响显著,须重点控制结构的变形;对于破裂后方区域,应着重提高结构的强度需求,首选断层距及PGV作 为地震动输入的控制参数。     

基于本征模态函数重构的近断层地震动识别与响应谱验证

为了充分考虑近断层地震动对中长周期结构的不利影响,提出了一种近断层速度脉冲型地震记录的量化识别方法。该方法基于经验模态分解(EMD)数据自驱动特征,充分考虑了本征模态函数(IMF)对原始记录脉冲尺度和形状的自适应匹配能力,以及单阶IMF固有的平滑特征,借助Tanimoto相似度的去中心化表达,给出了一种基于IMF信号自适应重构的地震记录去噪算法,用于识别主脉冲的幅值和周期。为了对主脉冲包含的能量进行度量,给出了基于指数函数的脉冲能量指示器,并验证了其有效性。通过与既有方法在周期、波形、反应谱等各方面的对比表明,本征模态函数与原始地震记录信号间的相似度可以作为筛选本征模态的重要依据。动力响应反应谱分析表明,抽取的脉冲对主脉冲中长周期的动力特性保留更为完整。总体上,由于经验模态分解方法的数据自驱动效应,所提方法对波形显著非对称的脉冲和多脉冲记录识别更为有效。     

近断层脉冲型地震作用下防屈曲支撑-钢筋混凝土框架结构的抗震性能

具有向前方向性效应和滑冲效应的近断层脉冲型地震动对建筑结构的破坏已受到工程界的广泛关注。为了解设置防屈曲支撑（BRB）的混凝土框架在近断层脉冲型地震动激励下的抗震性能,采用基于能量平衡的塑性设计方法完成了3个V形BRB支撑的RC框架结构的抗震设计。分别选取具有向前方向性效应和滑冲效应的脉冲型以及非脉冲型三组共36条近断层地震动,对结构进行罕遇地震作用下的非线性动力分析,研究了结构的最大层间位移角、最大顶点加速度、最大顶点位移和BRB的轴向性能;分析和评估了结构在3条典型地震动激励下的地震响应。结果表明：近断层脉冲型地震动比非脉冲型地震动对结构会产生更大的地震响应,且响应显著集中于速度脉冲时刻;BRB能充分发挥其耗能特性,提高RC框架结构体系的抗震性能。     

Behaviour of liquid storage tanks with VCFPS under near-fault ground motions

Earthquake response of slender and broad liquid storage steel tanks isolated with variable curvature friction pendulum systems (VCFPSs) is investigated under near-fault motions. The tanks isolated with VCFPS are idealised with three-degrees-of-freedom associated with convective, impulsive and rigid masses. The frictional forces mobilised at the interface of the VCFPS are assumed to be velocity independent. The governing equations of motion of isolated tank are derived and solved in the incremental form using Newmark's method. For comparative study, the seismic response of liquid storage tanks with the VCFPSs is compared with that of same liquid storage tanks isolated using the friction pendulum systems (FPSs). The seismic response of isolated liquid storage tanks is also compared with that of the non-isolated tanks. Further, a parametric study is carried out to critically examine the behaviour of liquid storage tanks isolated with the VCFPSs. The important parameters considered are the friction coefficient of VCFPS, the fundamental period at the centre of the sliding surface of VCFPS and the tank aspect ratio. It is observed that under near-fault ground motions, the VCFPS is quite effective in controlling the seismic response, viz. the base shear, the sloshing displacement and the impulsive displacement, of liquid storage tanks.     

近断层地震动作用下基础隔震结构的一体化动力优化设计

首先讨论了近断层地震动的脉冲运动特征和特性参数,并以台湾集集地震实际脉冲型近震记录作为地震动输入,以上部结构最大层间位移和构件体积最小化为目标,应用含潜在约束策略的序列二次规划算法,对安装铅芯橡胶隔震支座的钢筋混凝土框架隔震结构进行一体化优化设计,同步确定隔震器参数和上部结构构件截面几何尺寸。然后输入ElCentro(1940)、Hachinohe(1968)非脉冲型近断层地震动记录进行隔震结构一体化优化设计。计算结果表明,本文隔震结构优化设计得到的结果是合理的,与其他作者通过参数分析确定的隔震器最优参数具有可比性;对考虑脉冲型近断层地震动作用的隔震结构进行参数优化设计后,该隔震结构能同时满足脉冲型和普通非脉冲型近震作用的结构设计需求。反之,对非脉冲型近断层地震动作用的隔震结构进行参数优化设计后,该隔震结构不能满足脉冲型近震作用的设计需求。     

近场地震作用下型钢-混凝土组合结构桥易损性分析

考虑近场速度脉冲型地震动特征,对型钢-混凝土组合结构桥梁的地震易损性进行分析。以一座工字型钢-混凝土组合梁桥为例,从PEER数据库挑选60条近场地震动记录作为输入,对桥梁结构整体性能和局部性能两个层次的易损性进行分析,给出了桥梁结构的整体和局部地震易损性曲线。通过与远场地震作用下的桥梁结构地震易损性分析结果的对比研究,发现：近场速度脉冲型地震引起的桥梁结构整体和局部地震需求均显著大于远场地震|在近场地震作用下,桥墩是型钢-混凝土组合桥梁的最易损构件,混凝土横梁与栓钉的易损性水平低于桥墩。     

近断层向前方向性地震动作用下RC框架结构易损性研究#br#

向前方向性效应是近断层地震动的重要特征之一,使得在垂直于断层方向上表现出明显的速度脉冲分量,引起结构的严重破坏。为考察该类特殊地震动作用下RC框架结构的损伤程度,基于传统的增量动力分析方法,建立考虑材料不确定性和输入地震动不确定性的结构-地震动样本,对一个12层框架结构进行地震易损性研究。利用易损性分析结果计算出结构的破坏状态概率,结合群体结构震害评估中的震害指数经验值,得到多遇地震、设防地震和罕遇地震的易损性指数。研究结果表明：近断层向前方向性地震动作用下,依据我国抗震规范设计的RC框架结构能够满足小震不坏、中震可修、大震不倒的抗震设防目标。尤其在大震作用下,结构仍处于中等破坏水平,表明结构在发生倒塌之前具备充分的富裕度。     

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。     

Earthquake performance assessment of concrete gravity dams subjected to spatially varying seismic ground motions

This paper investigates the earthquake performance of concrete gravity dams under spatially variable seismic excitations. A nonlinear finite element model is developed and validated using shake table experimental results. The model is then subjected to spatially varying earthquake ground motions incorporating the wave passage effect, with values for apparent propagation velocities consistent with the source-site geometry and the shear wave velocity in the foundation rock. The evaluation reveals that different response patterns occur when spatially non-uniform and uniform seismic ground motions are applied as input excitations to the model, because spatially non-uniform excitations induce the quasi-static response, whereas uniform excitations do not, and, in addition, the dynamic response caused by different input motions varies. Notably, spatially non-uniform excitations produce larger opening at the heel of the dam and severer slipping at its toe; this latter observation can have a significant effect on the global equilibrium and stability of the dam during an earthquake.     

近断层/远场地震动作用下隧道结构易损性研究

以川藏铁路线控制性工程——折多山隧道为研究对象,建立隧道动力时程分析模型.结合场地地震动设计反应谱,选取近断层脉冲型地震动及远场地震动记录,用于增量动力分析隧道工程结构的抗震性能水平.初步探讨适用于隧道结构的地震动强度指标IM,分析不同特征部位隧道结构易损性,对比分析近断层脉冲型地震动及远场地震动作用下隧道结构的地震易...     

不同震源机制的近断层脉冲型地震动频谱特性及强度指标研究

将不同场地类别和震源机制上的118条近断层地震记录进行分类,基于基础隔震结构的双自由度体系简化模型,利用状态空间法及四阶龙格-库塔法对结构的弹塑性反应谱进行求解,研究了场地类别和震源机制对反应谱频谱特性的影响。通过分析近断层地震记录的3个强度指标与基础隔震结构最大弹塑性响应的相关性,探讨了基础隔震结构地震反应分析时强度指标的选取问题。结果表明,三种场地类别条件中,速度反应谱规律不明显,但逆断层地震动的平均加速度反应谱和逆斜断层地震动的平均位移反应谱均最大。场地类别和震源机制不同,地震动强度指标与结构响应的相关程度不同。因此在进行近断层脉冲型地震动作用下基础隔震结构地震反应分析时,建议考虑结构所在场地及地震动的震源机制后采用不同地震动强度指标来选择和调整地震动输入。     

Determination of symmetric arch dam vibration periods using empirical formulae

Some simple formulae which allow us to estimate the first six vibration periods of a symmetric arch dam (designed by following the guidelines of the US Bureau of Reclamation) are presented. Polynomial type expressions are indicated for both empty and full reservoir situations. Dam-reservoir interaction is taken into account by means of a modified three-dimensional Westergaard technique. Likewise, a tentative formulation, which accounts for the foundation flexibility effects is given.     

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.     

近断层地震作用下斜拉桥黏滞阻尼器参数简化设计方法

黏滞阻尼器是漂浮体系斜拉桥常用的减震耗能装置,但传统的阻尼器参数设计需要通过反复的非线性有限元时程计算来确定,这种方法不够简便有效。本文根据近断层地震的脉冲特点以及斜拉桥动力特性,通过建立斜拉桥的双质点模型的运动微分方程,推导出近断层脉冲作用下的动力反应计算公式;并基于等效线性化方法,得到非线性黏滞阻尼器参数与等效阻尼比的关系,从而利用主梁目标位移反向确定所需设置的黏滞阻尼器参数;最后通过一座斜拉桥实例验证本文方法的正确性,并提出了黏滞阻尼器参数设计简化方法的流程。     

近场区竖向地震动对钢筋混凝土框架结构抗震性能的影响

对竖向地震动进行统计分析,采用与水平地震动相比较的方法来间接反映竖向地震的特性。对断层距、震级、场地条件、震源机制进行分类,考察诸因素对加速度峰值比的影响。分析断层距对加速度反应谱比的影响,在近场区及短周期段,竖向地震动伪加速度反应谱值大于水平方向的谱值,远大于规范中所采用的0.65的系数。采用OpenSees程序对一地震模拟振动台的框架试验结果进行分析,验证了分析方法的可行性。8层钢筋混凝土框架结构的非线性动力分析表明,在竖向和水平地震动共同作用下,近场区钢筋混凝土框架柱的承载能力可能由拉-弯和拉-剪受力状态控制。