Inclusion of incoherency effects in soil-structure interaction (SSI) analysis provides a more realistic determination of earthquake induced building structural responses than those obtained considering coherent ground motion. Separately, industry demand for high-fidelity, comprehensive coupled structural models that often directly include structure-soil-structure interaction (SSSI) effects continues to increase. Combining incoherency and SSSI effects has raised questions about whether the implementation of incoherency effects in the popular System for Analysis of Soil Structure Interaction (SASSI) software may cause incoherency effects to be inappropriately overemphasized for individual separate structures within a comprehensive, coupled SSSI model of multiple structures, due to the larger effective foundation area of the coupled structure model. If true, this could cause incoherent SSSI analysis to unintentionally predict un-conservative response in the high frequency range.

This paper provides a study example demonstrating that the spatial configuration of multiple foundations for a particular SSSI model has little influence on the magnitude of incoherent effects on building structural responses. Evaluation of foundation transfer function (TF) and incoherency transfer function (ITF) responses between a comprehensive, coupled SSSI model and analogous individual models shows little difference in magnitude of incoherency effects and demonstrates that the total effective coupled foundation area is not a driver of incoherent response. In addition, when the individual foundations of the SSSI model are integrated into a single, continuous foundation, incoherency effects are shown to be influenced by the dynamic foundation characteristics. It can therefore be concluded that for this study, incoherency effects appear to be mainly a function of the foundation area of each individual structure that comprise a coupled SSSI model. Derivation of the underlying mathematics associated with these observations needs further study.

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