| Research Abstract |
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
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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
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