Before nailed-on bridging or blocking was used commonly between floor joists, mortise and tenon joinery was employed, at least in higher class construction. ‘Strutting’ are strong pieces, usually in 2″ stock, tenoned through joists, and so forth, and wedged with a tapered key on each end:
From Corkhill’s The Complete Dictionary of Wood (1982). Click on the image for a larger picture.
It appears that the struts are meant to be below the floor joist’s upper surface. If, however, the strut is placed level with the floor joists, and thereby carries a greater portion of the flooring load above, then a tusk tenon joint would be a more appropriate connection to employ. In either case, the pair of joists may have to have the struts pre-fitted prior to their placement, depending upon how the joists attach to the sills or plates.
Maybe it’s time more of us, ahem, ‘strutted our stuff’? har-har…