Because of soil-foundation-structure interaction (SFSI), the measured foundation motion (FM) diverges from the free-field motion (FFM). In the substructure SFSI analysis, a motion for a massless rigid foundation is defined as the foundation input motion (FIM), which cannot be physically measured. This paper describes a set of seismic centrifuge experiments to compare the FM motions in single foundations with responses closer to FIM. Three tests were performed on a soil-foundation-structure model (SFSM), a steel foundation model (SFM), and an acrylic foundation model (AFM). Results show that FM in SFSM is different from the ones in single foundation models and in a frequency range around the flexible base natural frequency of the structure. While the lateral component of FM in SFSM is smaller than those of SFM and AFM, the rocking component is greater. Therefore, using the measured FM instead of FIM specially in the translational direction would be non-conservative.
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