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

本研究旨在建立一种在地基无破坏和桩基无破坏的条件下，利用地基-桩体系的自振周期和单自由度上部结构体系的自振周期两轴反应谱，使反应谱最小的抗震优化设计方法。首先从地震烈度、桩土参数等方面对基本地基-结构模型进行了探讨，最后将其应用于1995年神户地震中软弱地基上发生重大破坏的桩基大型建筑物的实例。结果表明：1.在现有的液化分析程序中加入吸收边界条件，可以在PC机上准确地实现自由场。结论1.挤密砂桩(SCP)法地基处理程序适用于从砂土到粘土的各种土体和施工条件。采用SCP施工中每一阶段的最小压实时间作为防止液化和桩身破坏的最佳准则。结果表明，通过延长击实时间，神户的大部分破坏实例都可以安全地设计为540伽拉地震事件。然而，桩损伤位置的准确估计将取决于考虑液化引起的土体的大侧向流动。对1995年日本南部地震作用下原软土地基或改良地基上的单自由度体系结构模型进行了室内试验，实验反应谱与计算反应谱吻合较好。改良后地基的压实率也与模拟结果吻合较好。结果表明，该方法在实验上也得到了实现。

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.