Extraction of free body frequency response functions from flexibly constrained test

•The best identification formulae of the ‘negative structure’ method are found.•Requirements on the decoupling DOFs are given mathematically.•The pseudo-force method is extended to the cases of lacking interface information.•A criterion is given to design the support and place the measurement points.

The traditional way of low stiffness suspension may no longer work for very large scale structures with extremely low first flexible natural frequency. In this paper, we replace the low stiffness suspension with a group of supports. After separate Frequency Response Function tests of the whole supported structure and the support structures, the influence of the supports could be removed from a later computation. To this end, the identification method and support design are critical. The existing substructure decoupling methods are briefly reviewed and some incomplete issues in the earlier works are discussed. Then the relationship among the methods is investigated. With these studies, some improvements on the pseudo-force method are made. The proper design of supports and measurement locations are also studied. Both numerical and experimental examples are described in the paper.