汶川地震河流阶地建筑物的震害机制研究

Study on the seismic damage mechanism of buildings on the river terrace in Wenchuan earthquake

  • 摘要: 基于2008年汶川MS8.0地震现场的震害调查资料,揭示了河流阶地地形对建筑物震害分布的影响规律。阶地前缘冲洪积层较厚区域的建筑物遭受了严重的破坏,而阶地后缘坡积层较薄区域则相对受损较轻,总体上呈现出前缘震害显著大于后缘的特征。为研究该震害特征的机制,利用FLAC3D有限差分软件构建三维阶地模型,模拟分析其在脉冲荷载作用下的地震动响应。计算结果表明:各级阶地前缘的地震动水平均要显著高于后缘,同时随上覆土层增厚,加速度峰值、相对持时值、反应谱平台值以及放大系数均呈现上升趋势。而随阶地级数的减小,峰值、持时和反应谱特征周期却呈下降趋势,且级数的增加还会显著增强地震动低频成分的放大效应。因此阶地级数与上覆土层厚度是影响地震动响应的关键因素,高阶地和厚覆盖土层区域对地震动的放大效应更强,从而导致建筑物破坏严重。

     

    Abstract:
    The seismic ground motion topographic effect, as an important research content in the field of seismic engineering, the study of the mechanism of complex terrain on the ground motion characteristics can provide basis for engineering seismic defense.A large number of post-earthquake site investigations have shown that the complexity of local terrain has a significant impact on the distribution of seismic damage, especially the irregular terrain can change the intensity and spectral characteristics of ground motion. As a type of local irregular topography widely found in nature, the unique geometric shape of river terraces can cause complex scattering and diffraction of seismic waves, resulting in differences of ground motion in its local areas, and then affecting the seismic damage degree of surrounding buildings.
    Based on the on-site seismic damage investigation data of the river terraces in Wenchuan MS8.0 earthquake in 2008, it was found that buildings in the area with thicker alluvial at the front edge of the terrace suffered serious damage, while those in the area with thinner slope deposit at the rear edge of the terrace suffered relatively less damage. In general, the seismic damage at the front edge was significantly greater than that at the rear edge. At the same time, in order to study the mechanism of this seismic damage feature, the river terraces were selected as the research object, and the FLAC3D finite difference software was used to establish three-dimensional river terrace analysis models with different thicknesses of overburden soil layers, simulating and calculating the ground motion response under impulse loading, further revealing the influence law and internal mechanism of river terrace topography on ground motion characteristics and the distribution of seismic damage to buildings.
    For the same level terrace, the peak values of the horizontal and vertical acceleration and the 90% energy duration all show an upward trend with the increase of the overlying soil thickness, reaching the maximum value at the front edge of the terrace and the transition point with the steep slope. Meanwhile, the ground motion level at the front edge of each terrace is significantly higher than that at the rear edge. As the terrace grade decreases, the peak values of the horizontal and vertical acceleration and the 90% energy duration at the corresponding area also gradually decrease. Similarly, the trend of the Fourier spectrum amplitude and ratio at different monitoring points on the same level terrace is basically consistent, but the amplitude and ratio increase gradually as the monitoring point approaches the edge of the terrace facing the steep slope and the front edge of the area with increased soil thickness.As the terrace grade increases (the terrain height rises), the natural frequency of the structure increases accordingly, thereby significantly enhancing the amplification effect of low-frequency ground motion. The characteristic period value, platform value, and platform amplification coefficient in the standard response spectrum of seismic acceleration for different monitoring points are all affected by the terrace grade and overlying soil thickness. The Tg value decreases gradually as the terrace grade decreases; the platform value increases as the overlying soil thickness at the front edge of the terrace increases; however, the platform amplification coefficient β value decreases as the terrace grade increases, and the amplification coefficient at the front edge of the terrace is significantly larger than that at the rear edge.
    River terraces have a significant impact on the propagation of ground motion and the degree of seismic damage to buildings. The change of the thickness of the overlying soil layer leads to different distributions of building damage by affecting the amplification of ground motion, while the number of terrace grades exacerbates or mitigates the seismic damage by affecting the spectral characteristics and overall level of ground motion.Therefore, the number of terrace grades and overlying soil thickness are the key factors affecting ground motion response, and the amplification effect of ground motion is stronger in high terrace and thick covering soil layer, which leads to serious damage of buildings.

     

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