E0-190-2008_(4)Chapter_3

In order to express this tendency of the imaginary

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Unformatted text preview: il. In order to express this tendency of the imaginary part, the first natural frequency fg of the surface stratum is particularly termed “cut-off frequency”. The frequency characteristic of the imaginary part is depicted in the figure: Both the real and the imaginary parts of the horizontal D.I.F are fluctuating in higher frequencies over 2.5Hz. The fluctuations are particular for the layered strata. The fluctuations for the rotational D.I.F are also observed in far higher frequencies. Comparing the raft foundation to the pile foundation, slight differences in the horizontal D.I.F are observed between both foundations. Distinct differences are recognized in the real parts of the rotational D.I.F. The real parts of the pile foundation are much larger than that of the raft foundation. The rotational stiffness of the pile group foundation is composed of: (1) The rotational stiffness at each pile caps (2)The stiffness estimated by a product of the vertical stiffness at the each pile caps and the distance of the caps from a neutral axis of the foundation. In a case of the pile group foundation, the latter (2) becomes significant. Consequently, the rotational stiffness of the pile foundation becomes larger than the raft foundation. We can understand the differences in D.I.F between both the foundation types arise distinctly in the real parts of the rotational and the other components exhibit only trivial differences. 3.4 Damping Factor of Dynamic Impedance Function A damping factor, which is evaluated from the hysterics loops of the structural components, is employed for a dynamic analysis of a superstructu...
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