摘要
大跨度桥梁的地震响应分析一直是桥梁工程行业内关注的焦点问题,传统桥梁结构时程分析都是基于地面运动在整个基础范围内一致的假定,但是对于大跨度桥梁结构这种假定并不符合实际情况。因为地震时从震源释放出来的能量以地震波的形式传至地表,而地表各点接收到的地震波是经由不同的路径、不同的地形地质条件而到达的,因而反映到地表的震动必然存在差异。是否考虑这种效应的影响,大跨度桥梁结构的地震反应分析结果将存在很大的差异。大量的震害也表明,地表地形对地面运动有很强的放大和缩小作用,直接影响到震害的分布,因而也对桥梁震害有很大的影响。
本文结合西部山区不断涌现的高墩梁式桥为研究对象,以探讨考虑地形影响的桥梁地震反应和碰撞反应规律为研究目标,通过数值计算结果分析,着重开展了以下几个方面的研究工作,并初步得到了一些有意义的结果:
1、高墩梁式桥在西部的山区不断涌现,由于跨越地形复杂,河谷、深沟众多,导致桥墩各支承点的地震动输入有很大的不同。本文考虑地震动在复杂地形条件下的各种效应,分析了均匀粘弹性半空间上局部相邻对称山脊平台加河谷地形对地震波的散射作用,分析了各个控制观测点在单脉冲输入下的反应、传递函数以及在实际近断层大速度脉冲地震动输入下的时程反应。结果表明:山脊平台各点放大效应十分明显,山脊平台和山坡的地形突变处出现最大放大倍数,山坡点和山脚点呈缩小趋势,峡谷各点放大现象显著,尤其是峡谷和山脚平台地形突变点处,这种放大作用更加突出。这对跨深大河谷的桥梁工程的抗震设防不利。
2、在考虑地形影响的多点输入模式下对桥梁结构进行多点多维地震反应分析,并与一致激励模式下的桥梁结构地震反应分析作结果对比。结果表明:桥梁结构地震反应时程曲线不仅在形状上(时程包线形状)有一定变化,同时地震反应峰值也有所变化;两侧桥墩处位移都有一定的放大,而且从反应时程曲线上看与中间桥墩位移差动明显,并因此引起了较复杂的内力响应;考虑地形影响使桥梁的墩顶位移、速度、加速度与墩底的剪力、弯矩等各相关量的衰减都更慢。
3、对桥梁支座和伸缩缝进行了合理的模拟后,在考虑地形影响的多点输入模式下对桥梁结构进行碰撞反应分析。结果表明:由于考虑地形影响后两侧梁的位移差动增大,考虑地形影响的多点激励与一致激励下相比桥梁碰撞次数增多,桥梁碰撞延续时间增长;地形影响使碰撞单元所在桥墩的地震反应都有不同程度的放大效应,地形影响使桥梁的墩顶位移、墩底的剪力和弯矩的衰减都有很明显的延迟,使桥梁的各相关量衰减得更慢;尽管考虑桥梁梁板碰撞并考虑地形影响与考虑碰撞而不考虑地形影响相比,在一定程度上减小了中间桥墩的内力和位移反应,但却增大了左右墩处支座的反应,使其发生剪切失效的可能性大大增加。
The analysis of seismic responses on long-span bridges remains focused. Obviously, it’s unreasonable to say the anti-seismic design of bridge, based on the assumption that every part of whole basement moves uniformly. The energy from the earthquake source is transmitted in the form of seismic waves, and the ground motions at different observation points are caused by waves traveling through different paths, different topographic and geological conditions, so the ground motions at supporting points of bridges are not uniform. If we do not consider this fact there will be a large analysis error. Many investigations of earthquake damages manifest that surface topography has significant amplification or reduction effects on ground motions. Therefore, it may influence the distribution of ground motions of engineering sites and bridge damages directly.
The objective of this thesis is to study the law of seismic responses and collision responses of long-span bridges for multi-supported excitations and topographic effects. Through numerical experiments and result analysis, the following problems are analyzed. And some applicable results are obtained.
1、High-pier bridges in western China have emerged constantly. Due to the complicated topographic effects, the bridges across valley and deep ditch, the ground motions at supporting points of bridge is not uniform. In order to consider the various effects of local topography of engineering sites on ground motions, we analyze the scattering effects of seismic waves of local symmetry mountain ridge and adjacent canyon on visco-elastic half-space. Also we analyze the response properties of time history and transfer function at the various reference points under a single pulse input and real near-fault acceleration time history input. The results show that, ground motions are amplified clearly at the mountain ridge, especially largest amplification appears at the mutation point of the ridge and the slope of mountain. The ground motions of the canyon enlarge clearly, especially at the mutation point of the canyon and platform of the foot of the mountain.
2、We study the law of seismic responses of long-span bridges under multi-supported excitations and topographic effects, and compare seismic responses of long-span bridges with those of uniform excitations. The results show that the seismic response curves of the bridge are not only different in shape (on the waveform) but in peak ground acceleration, the displacements on the roof of the bridge piers have some amplified, and the largest amplification appears on the left and right sides. The shape of displacement time histories of the piers are different so that more complicated responses of internal forces are caused. The displacement, velocity and acceleration on the roof of the bridge piers and the shear force, moment on bottom of the bridge piers decay more slowly due to the topographic effects.
3、After reasonable simulation for the bridge bearing and extension joints, we study the law of collision responses of long-span bridges under multi-supported excitations and topographic effects. The results show that by the reason of the topographic effects, the displacement of the roof of the bridge piers has some amplified, and the shape of displacement time histories of the piers is different, and also the collision frequency and duration of the bridge increase. The displacement, velocity and acceleration of the roof of the bridge piers and the shear force, moment of the bottom of the bridge piers decay more slowly due to the topographic effects. Although the internal forces and displacements of the middle pier are reduced by considering effects of topography of engineering site and the collision of bridge units, response values of other quantities increase obviously, so that the possibility of shear failure of the bridge bearing increases greatly.
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