In this paper, a dynamic interaction factor between a raft and a pile group that constituted a piled raft foundation subjected to vertical loading is investigated. Because, if the dynamic interaction factor can be evaluated, the dynamic impedance of the piled raft foundation can be easily calculated from the dynamic impedance of a spread foundation and a pile foundation. The dynamic interaction factor is formulated as an arbitrary function in the frequency domain, based on the relationship of

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... e dynamic impedance between the piled raft, the spread and the pile foundation obtained by parametric numerical analyses with various changes in the foundation configuration and the ground condition. Moreover, the coefficients used in the function have related the width of the foundation or the stiffness ratio of the spread and pile foundations. It is confirmed that the results of the simplified method using the interaction factor are in good agreement even for the dynamic impedance of piled raft foundation with conditions similar to the spread foundation or the pile foundation.