A Study on Seismic Load Distribution Applied to Pile Foundation

在桩基支撑建筑抗震设计中，将水平集中荷载作用于桩头作为上部结构的地震惯性力，对桩的水平抗力进行了分析。由于桩的计算应力随着深度的增加而迅速减小，桩的下部截面容易设计为具有相应的较低承载力。但根据近期桩基建筑的地震反应分析或地震观测等研究工作的结果，桩的下部和桩头也产生了较大的应力，即土层的反应主要控制了桩的性状。因此，本文提出了一种新的分析方法，通过引入土的地震响应效应来获得桩的水平荷载分布的抗力。首先，给出了无限长桩挠度曲线的一般解…从弹性介质上梁的弯曲理论出发，通过与M.Hetenyi解相同的方法，进一步推导出了三种载荷：单一集中载荷、均布载荷和三角分布载荷。其次，通过数值分析得到了实际有限长桩的解，引入了桩头和桩端的边界条件，以及无限长桩各层边界的连续性条件。换言之，通过数值分析计算修正解，其和无限长桩的解满足各层的边界条件和连续性条件。对4种土体模型和3种建筑模型(5层、10层和15层建筑)进行了地震反应分析。结果表明，上部结构惯性力对桩顶的影响较大，而桩下部土体的响应占主导地位。其次，将各深度处的桩身弯矩时程分解为土体响应分量和上部结构惯性力分量。对于后者，桩的弯矩分布与现有的设计方法非常相似，而对于另一种情况，如果存在一个与其他层刚度相差很大的中间土层，则至少要计算到第二阶固有振型才能模拟桩的弯矩分布。最后，研究了土体反应分量与上部结构惯性力的合成方法，得到了桩的设计弯矩。在此基础上，得到了以下结果。1)绝对值之和很好地解释了T_s和T_b(T_s：土的第一自然周期，T_b：建筑物的第一自然周期)情况下桩的弯矩分布。另一方面，对于ts=Tb和ts&lt；Tb两种情况，平方和的根都是较好的。较少

In earthquake resistant design of buildings supported by pile foundation, horizontal resistance of pile has been analyzed under the condition that a concentrated horizontal load is subjected to pile head as a seismic inertia force of superstructure. Because computed stresses of pile are decreasing rapidly with depth, section of lower part of piles is apt to be designed to have a corresponding lower capacity. But according to the results of recent research works such as sesmic response analysis or earthquake observation of pile-supported buildings, it has been clarified that comparatively large stresses are also produced in lower part of piles as well as pile head, i.e., response of soil deposits controls mainly behavior of piles. A new analytical method, therefore, is proposed in this study to get horizontal resistance of piles to static load distribution by introducing effects of seismic response of soil deposits.Firstly, general solutions for deflection curve of infinite piles subjec … More ted to three types of loading, such as a single concentrated load, uniformly distributed load and triangularly distributed load were derived from bending theory of beams on elastic medium by the same procedure as M. Hetenyi's solutions. Secondly, solutions for actual finite piles were getting by numerical analysis introducing the boundary conditions at pile head and pile tip, and the continuity conditions at the boundaries of each layer of infinite piles. In other words, correcting solutions were computed by numerical analysis, the summation of which and solutions of infinite piles satisfies the boundary conditions and continuity conditions of each layer.Seismic response analysis was carried out for combination of 4 models of soil deposits and 3 models of buildings, i.e., 5 storied, 10 storied and 15 storied buildings. From the results on stresses of piles, it is seen that effects of inertia force of superstructure are distinctive at pile head but response of soil deposits is dominant at lower part of piles. Next, time histories of bending moment of pile at each depth were separated into the components of response of soil deposits and of inertia force of superstructure. As for the latter, distribution of bending moment of pile is very similar to the one by the existing design method, while the other, it is necessary to evaluate up to second natural mode at least to simulate distribution of bending moment of pile, if exists an intermediate soil layer which has extremely different rigidity to other layers. Lastly, composition method for components of response of soil deposits and inertia force of superstructure was investigated to get the design bending moment of piles. And then, following results were obtained. 1) Summation of absolute values explains well bending moment distribution of piles for the case of Ts>Tb (Ts:the first natural period of soil deposits, Tb:the first natural period of buildings). 2) On the other hand, root of summation of square is better for the both cases of Ts=Tb and Ts<Tb. Less