Development of the optimal aseismic design method for upper structures due to improvement of the ground and pile foundation

本研究旨在利用地基-桩体系的自然周期和上部结构体系的一自由度自然周期的两轴响应谱，建立一种使地基和桩基不破坏条件下的响应谱最小化的抗震优化设计方法。本文首先从地震烈度、桩、土参数等方面对基础地基结构模型进行了研究，最后将该设计方法应用于1995年神户小谷南部地震中软土地基桩基大建筑物的致命破坏实例。实验结果表明，采用振动台作为随机输入的比例模型，该方法是可行的。所得结果总结如下：1。在现有的液化分析程序中加入吸收边界条件，可以在个人计算机上准确地实现自由场。砂压桩法地基整治方案适用于从砂土到粘土的各种土质和施工条件。采用单阶段最小压实时间作为防止液化和桩破坏的最优准则。结果表明，通过延长压实时间，神户震区的大部分破坏样都可以设计为540gal地震的安全事件。然而，对桩基损伤位置的准确估计需要考虑液化引起的土体横向大流动。1995年小谷县南部地震对原软土地基和改良土地基上的一自由度体系结构模型进行了室内试验，试验反应谱与计算反应谱吻合较好。改良地基的压实率与模拟结果吻合较好。实验结果表明，该方法是可行的。

This study aims to establish a optimal aseismic design method which minimizes the response spectra under the condition of non failure of ground and non failure of pile foundations, using two axis response spectra of natural period of ground-pile system and natural period of 1 DOF upper structure system. This design method is investingated first for the fundamental ground-structure models in terms of seismic intensity, pile and soil parameters, and finally applied to the examples of fatal damaged pile-supported massive buildings resting on the soft ground at Kobe in 1995 Hyogoken Nanbu earthquake. It is also confirmed that this proposed method is realized in the laboratory for scaled models using the shaking table for random inputs.Results obtained are summarized as follows :1. By equipping the absorbing boundary conditions into existing program of liquefaction analysis, free field can be realized exactly in the personal computer. The program of ground improvement by sand compaction pile (SCP) method become applicable to every soil and construction condition from sand to clay.2. Minimum compaction time per one stage in the construction of SCP is adopted as the optimal criteria for prevention of liquefaction and pile failure. As a result, it is shown that most damaged examples at Kobe can be designed safely event for the earthquake of 540gal by extending the compaction time. However exact estimation of positions of pile damages will depend on consideration of large lateral flow of soil due to liquefaction.3. After the laboratory test of one-degree-of-freedom system structural model resting on the original soft ground or improved ground subjected to the 1995 Hyogoken Nanbu earthquake, experimental response spectra agrees well with the computational one. Compaction rate of improved grounds also agree well with simulation results. Hence it is shown that the proposed method is also realized experimentally.