【作者】 周国良；

【导师】 李小军；

【作者基本信息】 中国地震局工程力学研究所 ， 防灾减灾工程及防护工程， 2010， 博士

【摘要】 受不规则地形影响,地震波在河谷地形中的传播与平坦场地相比有很大不同。因此,河谷地形地震动分布在小尺度空间上并非一致,呈现出明显的差异性,有必要评估这种差异对多支撑大跨桥梁地震反应的影响。本文回顾了局部场地及地形因素对地震波传播的影响及大跨桥梁地震反应考虑局部地形影响的有关研究,由此引出了研究河谷地形因素对地震波传播影响的重要性及其对跨河工程抗震分析与设计的重要意义,介绍了本文的研究目的和主要任务。基于5.12汶川地震山梁及河谷地形效应的震害调查和自贡西山公园地形效应观测台阵强震动记录分析,介绍了具有工程意义的小尺度空间内,局部场地及地形因素对地震动空间分布影响的间接证据和直接证据。归纳总结了5.12汶川地震山梁和河谷的地震地形效应,分析了西山公园地形效应观测台阵强震动记录的基本特征。以二维河谷地形为例,分析了均匀粘弹性半空间上河谷地形对地震波的散射。分别将具有不同频谱特征的3条强震动记录视为SH波和SV波形式入射,考虑不同入射角度,分析了河谷地形地震动分布特征。结果表明:从河谷两侧角点到河谷底部,PGA和PGD基本呈现逐渐减小的趋势,但受局部地形因素的影响,这种变化并非简单的单调性变化;入射方向对河谷场地各点地震动分布有重要影响:与入射波垂直或近似垂直的一侧PGA明显大于河谷另一侧。斜入射情形下,河谷各场点间的相对位移差动(任选参照点)明显大于垂直入射情况。入射波的频谱特征对河谷各场点的反应有显著影响,高频分量丰富的地震波放大比较明显,这种高频放大在一定程度上受河谷局部地形特征影响。系统地回顾了多点激励时程分析的基本方法,指出了传统大质量法、位移输入法、大刚度法采用Rayleigh阻尼时的误差来源、适用条件和计算精度,基于严格理论推导分别提出了修正方法。结果表明:采用Rayleigh阻尼假定时,大质量法和大刚度法不适用于多点激励分析,位移输入不适用于一致激励分析,否则会造成显著的误差。大质量法和位移输入法误差与Rayleigh阻尼的质量相关系数有关,大刚度法与Rayleigh阻尼的质量相关系数和刚度相关系数都有关系。本文提出了基于地震动加速度修正的改进大质量法和基于地震动位移修正的改进大刚度法,与原方法相比该改进方法大大地提高计算精度,总体误差控制在2%以下,可以满足工程结构抗震分析的需要。以两座不同结构形式的大跨桥梁为例,考虑河谷地形影响,分析了多点激励下,大跨桥梁地震反应特点,并与一致激励情况进行了比较。结果表明:地形效应对大跨桥梁地震反应具有重要影响。受入射波频谱特征、入射角度及自身结构形式的影响,结构地震反应表现复杂。对于桥型布置对称,桥墩高差很小的连续刚构桥而言,河谷地形效应总体上没有使桥梁关键部位遭受不利反应;而桥墩高差明显的刚构桥,当考虑河谷地形SV波斜入射时,关键部位遭受不利反应的可能性大大增大。考虑SH波入射下河谷地形的影响时,两重桥型地震反应表现复杂,与一致激励相比,难以发现规律性。总体而言,SV波入射时,地形效应对结构地震反应的影响比SH波入射时更显著,尤其是SV波斜入射时,结构地震反应非常复杂,关键构件内力有可能大大增加,不可忽视。以一连续刚构桥为例,分析了不同的视波速下,桥梁的地震反应特点并与地形效应作了比较。结果表明,较低的视波速使得行波效应对连续刚构桥呈现出不利的影响,尤其是对主梁和桥墩顶部内力而言。但行波效应相比地形效应,仍不能完全控制结构的最不利响应。地形效应的影响依然应当受到重视。为研究多点激励作用下,大跨桥梁拟静力反应和动力反应的特征和规律,提出并验证了一种基于DM和改进LMM的大跨结构多点激励分析拟静力反应项和动力反应项的分离方法。以一座大跨连续刚构桥多点激励分析为例,研究了其拟静力反应项、动力反应项的基本特点,揭示了大跨结构遭受多激励时的复杂反应过程及其特征。结果表明:结构的拟静力反应与各支撑输入的地震动位移差动密切相关,对于连续刚构桥,位移差动越大,拟静力内力越大。斜入射情形下,结构的拟静力反应远大于垂直入射情况;而动力反应除了在主梁轴力偏大外,其它内力总体上小于垂直入射情况。结构的拟静反应内力在垂直入射情况除了主梁轴力外,总体上远小于动力反应内力,其峰值约为动力反应内力峰值的10%-20%;而在斜入射情况下,拟静反应内力相比动力反应是一个不可忽视的量,甚至还大于动力反应内力。拟静反应对总反应的贡献,在斜入射时明显变大。结构的动力反应主要地震动的频谱特征有关,对于周期较长的刚构桥,长周期分量丰富的地震动使得结构的动力反应明显增强。多点激励作用下,桥梁地震反应的复杂性与拟静力项和动力项的相互作用过程有关。结构的总反应大小取决于拟静力反应和动力反应的相互过程。对于连续刚构桥而言,考虑河谷地形影响时,SV波斜入射情形下两种反应相互叠加的情况可能大于垂直入射情况。因此,SV波斜入射情况是结构动力分析考虑地形效应多点激励的一种值得关注的情况。更多还原

【Abstract】 Induced by the topographic irregularity, the seismic wave propagation, reflections and refractions become quite complex. The propagation of seismic waves on canyon site distinctly differs from that on flat surface. The distribution characteristics of ground motions are influenced by small-scale topographic features, which cause non-uniform ground motions. So, the extended structures, such as long-span bridges, are influenced significantly by the non-uniform ground motions considering the canyon topography effects. And it is necessary to evaluate canyon topography effects on long-span multi-support bridges during earthquake.The survey of the state of the art in topographic effects on seismic wave propagation and seismic response of multi-support bridges has been given to demonstrate the significance of this thesis. The task of this dissertation is introduced.Based on the seismic damage investigations and strong ground motion data from Xishan-Park Array in Zigong city, the indirect and direct evidences are presented to demonstrate that the local topographic effects on seismic wave propagations. The typical mountain topography effects and canyon topography effects during Wenchuan earthquake are summarized.The two-dimensional site finite element model is formed, and the seismic analysis of a canyon has been carried out on visco-elastic half-space respectively under SV waves and SH waves using the assumptions of vertical incidence and oblique incidence. It indicates that the distribution of ground motions is affected by small-scale canyon topographic features and the incidence angle of the waves. In the case of vertical incident SV/SH waves, the peak ground accelerations increase greatly at the upper corners of canyon; whereas the peak ground accelerations decrease at the bottom corners of canyon. In the case of oblique incident waves, the shaking of the slope perpendicular to the incidence direction is more severe, however on the opposite side, the peak ground acceleration values decrease. The relative displacements of ground motions mainly depend on the incident direction of waves. Generally, the relative displacement increases obviously under oblique incident wave, compared with those under vertical incident wave. And the high frequency content of incident waves has been amplified obviously.The analytical methodologies for multiple-supported seismic excitation are reviewed, which include the Relative Motion Method (RMM), the Large Mass Method, the Displacement Method and Large Stiffness Method (LSM). With the assumption of Rayleigh damping, the error origins and applicability of the LMM, the DM and LSM are discussed. It is found that The LMM and LSM can cause distinct errors when the Rayleigh damping is adopted .And the improved LMM and improved LSM are presented and validated respectively. It has been proved that the improved LMM and improved LSM can be applicable to non-uniform seismic excitation analysis with the assumption of Rayleigh damping. And the improved methods can yield results that are identical to theoretical method with a high precision.Based on the ground accelerations obtained from seismic response of a canyon, the seismic responses of two bridges are performed considering the canyon topography effects using the improved LMM presented in this dissertation. The response characteristics of bridge are summarized. It indicates the canyon topography influences on the response of bridges greatly and complexly. In terms of the incidence direction,frequency content of incident waves and structural styles,the characteristics of seismic responses have become complex. The responses of structure members considering the canyon topography effects may be enlarged or decreased compared with the results of uniform seismic excitation case. The canyon topography effects under SV incident waves on structure are much more obvious than that under SH incident waves. In general, in the case of oblique incident SV waves, the rigid frame bridges suffer most critical responses.Based on the analysis of wave passage effects on rigid frame bridge with assumption of different apparent velocity, the response characteristics were compared with those in the case of canyon topographic effects considered.It indicates that the low apparent velocity brings critical responses to the bridge; however the topographic effects are still not neglected.Finally, an acquisition technique for quasi-static response components and dynamic response components in multi-support seismic excitation analysis is presented and validated based on the improved LMM and DM. The quasi-static response components and dynamic response components of a long-span rigid frame bridge subjected to non-uniform seismic excitations considering the canyon topography effects under incident SV waves are successfully obtained, and the characteristics of quasi-static response components and dynamic response components are compared. It indicates that the quasi-static response components mainly rely on the relative displacements between bridge supports and the dynamic response components depend on the frequency content of incident seismic ground motions. And in the case of vertical incident waves on the canyon, the quasi-static force components is much smaller than dynamic force components, however it become quite bigger and can not be ignored in the case of oblique incident waves.It also indicates that in the case of oblique incident SV waves the quasi static components and dynamic components resonate with a higher probability.The total response of structures subjected to non-uniform seismic excitations consists of quasi-static components and dynamic components, which contribute to the complexity of structural responses. The canyon topography effects under oblique incident SV waves should be considered seriously in seismic response analysis of multi-support bridges.更多还原