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不破坏、桩基不破坏的条件下，利用地-桩体系自振周期和单自由度上部结构体系自振周期的双轴反应谱，以反应谱最小为目标的抗震优化设计方法。首先根据地震烈度、桩和土参数对基本地基-结构模型进行了研究，最后将该设计方法应用于1995年兵库县南部地震中科比软土地基上严重受损的桩基大型建筑物的实例。本文还证实了所提出的方法可以在实验室中实现，并对缩尺模型进行了振动台随机输入试验。在现有液化分析程序中加入吸收边界条件，可在微机上准确地模拟自由场。挤密砂桩（SCP）加固地基方案适用于从砂土到粘土的各种土质和施工条件.采用单桩施工阶段的最小压实时间作为防止液化和桩破坏的最佳标准。结果表明，科比大部分受损的例子可以通过延长压实时间来安全设计540加仑的地震事件。然而，对桩身损伤位置的准确估计将取决于对由于膨胀引起的大的侧向土流的考虑.在1995年兵库县南部地震中，对原软土地基和加固地基上的单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。加固后的地基压实率与模拟结果吻合较好。因此，它表明，所提出的方法也实现了实验。

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.