This paper proposed an assembled underpinning joint (UPJ) based on the interfacial friction action between the underpinned concrete member and the underpinning steel beams (UPSBs). To investigate the mechanical performance and failure modes of the proposed UPJ, 20 specimens with different concrete strengths were fabricated and tested with varied preloading levels. Experimental results showed that the proposed UPJs could effectively transfer the vertical loads in the concrete columns through the friction action and all specimens failed due to the slip between the underpinned concrete column and the UPSBs. The failure loads of the specimens were in a positively linear relationship with the preloading levels and an interfacial friction coefficient of 0.46 was found. Numerical models were also created to further examine the detailed structural behaviours of each components in the UPJs, which confirmed that, the UPSBs were subjected to a composite action mainly combining both bending and torsion. The frictional stresses between the UPSBs and the concrete column were concentrated in the regions of the web of UPSBs near flanges and stiffeners. The load transfer mechanisms of the proposed UPJs under complex loading conditions were clarified. The damages, occurred at the corners of concrete columns due to high stress and strain concentrations, should be alleviated or prevented by protective measures in practice.