The accurate dynamic analysis of concrete arch dams relies heavily on employing a three-dimensional semi-infinite fluid element. The usual method for calculating the impedance matrix of this fluid hyper-element is dependent on the solution of a complex eigenvalue problem for each frequency. In the present study, a modified efficient procedure is proposed, which simplifies this procedure significantly, and results in great computational time savings. Moreover, the accuracy of this technique is examined thoroughly and it is concluded that modified efficient procedure is also extremely accurate under all practical conditions. It is also shown that the original efficient method proposed in a previous study can be considered as a special case of the present more general procedure.

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