【作者】 周芬；

【导师】 尚守平； 张友亮；

【作者基本信息】 湖南大学 ， 建筑与土木工程， 2005， 硕士

【摘要】 对于现行的结构抗震设计,第一阶段的工作仍然是进行结构的弹性分析。结构的自振频率计算是弹性分析的重要内容,自振频率的大小直接影响到结构在抗震设计中的地震作用。 通过脉动试验和牵引激振试验测定了土—箱型基础—框架结构的自振频率。两种试验方法的测试结果非常吻合,这也为下一步的理论计算提供了准确的实测结果。通过在地基两个方向上安装速度传感器,利用牵引试验研究了上部结构振动对地基的影响。在土-结构动力相互作用体系中,当周围土体和基础混凝土粘结较好时,地面运动的速度分量会以上部结构振动方向为主。位于结构纵向的测点,其纵向速度的频率组成以低频为主;位于结构横向的测点,其速度的频率组成以高频为主。 本文采用有限元软件Patran、Nastran对大比例框架模型进行了自振频率分析、牵引激振分析。自振频率分析中考虑了边界条件、单元形式、梁板偏移、配筋率、上下部刚度、网格划分疏密、地基刚度分布等因素的影响。计算结果表明,当不考虑梁板偏移、结构配筋时,对基于刚性地基假定的梁板模型,采用传统设计方法计算得到的自振频率往往比实测值低。在小震情况下按照现行规范进行抗震设计偏于不安全。在这种情况下计算结构自振频率不能考虑上下部共同作用。本文针对不同的场地类别给出了结构一阶自振频率的调整系数。本文还给出了梁、柱的配筋率与结构一阶频率的二次曲线关系式。此外,文中指出考虑土—结构相互作用时,有限元建模中地基大小的取值范围并非定值,而与地基的刚度分布有关。当地基刚度增大时,所取范围可以减小。结构低阶频率受基础周围地基刚度影响较大,而结构的高阶频率则受深层地基刚度影响较大。 牵引激振有限元计算结果表明,在上部结构振幅不大且土体和基础混凝土粘结较好的情况下,上下部结构可以将振动平稳地传递到地基中。刚性地基模型的加速度幅值要大于考虑地基模型。结构上部的加速度幅值大于下部的加速度幅值。基础附近地基上测点的加速度幅值与基础接近。地基中测点平行于牵引方向的加速度幅值大约是垂直牵引方向的加速度幅值的10倍。更多还原

【Abstract】 For current structure seismic design, the first-phase work is still elastic analysis. Calculation of structural natural-frequency is the important content of elastic analysis, and its value directly influences the seismic action in seismic design.The natural frequency of soil-box foundation-frame structure is obtained by puisateon test and traction excitation test. The results of two test methods are coincident with each other, which afford accurate test results for theoretical calculation next step. Velocity sensors fixed on soil in two directions, the effects of upper-structure on soil are studied through traction test. In soil-structure dynamic interaction system when ambient soil bonds to foundation concrete well, the velocity component of ground movements will give priority to the direction of upper-structure vibration. The frequency composition of velocity for longitudinal test points of structure is mainly low frequency and for transverse test points of structure is mainly high frequency.FEM programs Patran and Nastran are adopted to perform natural frequency analysis and traction excitation analysis for the large-scale frame model. In natural frequency analysis the effect of some factors are taken into consideration, such as boundary conditions, element type, beam-plate offset, ratio of reinforcement, stiffness of upper-lower structure, meshing density and distribution of soil stiffness. Theoretical results shows that without consideration of beam-plate offset and structural reinforcement, the natural frequency calculated by traditional design method for beam-plate model based on rigid-soil assumption is always lower than test results. In case of small seisms, the seismic design carried out according to current code is not safe. Under this condition the interaction of upper-lower structure can not be considered in the calculation of natural frequency. The adjusting factors of the first-order frequency for different kinds of grounds are given. The conic expression between reinforcement ratio of beam and column and the first-order frequency of structure is also advanced. It is pointed out that when soil-structure interaction is considered, the range of soil size in FEM modeling is not specified but related to the distribution of soil stiffness. When the stiffness of soil increases, the range selected can be decreased. The stiffness of soil around foundation affects low-order frequency of structure more and the stiffness of deep-layer soil affects high-order frequency of structure more.FEM computational results of traction excitation show that when the vibration amplitude of upper structure is not large and soil bonds to foundation concrete well, theupper-lower structure can transfer vibration to soil smoothly. The acceleration amplitude of model without foundation is larger than model with foundation. The acceleration amplitude of upper structure is larger than lower. The acceleration amplitude of points on soil near foundation is close to that of points on foundation. The amplitude of acceleration parallel to tractional direction is about ten times as that of acceleration in vertical direction for test points on soil.更多还原