Answer:
The arrangement of backing plates in the column only increases the resistance of a bolt row, for which Failure Mode 1 (complete yielding of the flange without bolt failure) is governing. This is probably not the case in your example; that is, Failure Mode 2 is governing: In this case, the backing plates do not result in an increased joint resistance of the frame corner.
Further information on this topic can be found in the corresponding section of DIN EN 1993‑1‑8, which is the basis for the calculation in FRAME‑JOINT Pro.
When using backing plates, it is also necessary to pay attention to the geometry specifications according to Section 6.2.4.3 of the above-mentioned standard.
