backing plate in MEPs .

If placed between the flange and the end plate and its not connected via welding to the face of the column flange it not actually going to do anything, it just moves with the end plate and you'll only get yielding in the original column flange.

Because they work to increase the effective plate thickness on the tension bolt group my understanding is that they don't even need to be welded/fixed on the back side, however if on the outside I can see the welds and the forces they are subject to needing quite careful consideration to ensure they force the additional yieldlines and hence capacity assumed by Eurocode.

Having said that I've never ever seen them put on the outside of the flange, and I'd like to see some research that considered this specific arrangement before departing from the Eurocode requirements which are pretty tried and tested.

Malikasal:
The whole idea of the backing pls. is to improve the way the col. flgs. react to the loads that the bolts of the moment end pl. impart on the col. flg. With the backer pls. on the inside the load distribution to the col. flg. is improved. While with the backer pls. on the outside, the bolt loads to the col. flg. are really unchanged (not improved) and additionally, the pls. must be extended to fill the gap at the compression bolts. In the basic configuration, the bolt loads essentially use the col. flg. as a cantilever (canti.) off the col. “k” area (flg./web radius area), with canti. length equal to the bolt gage dimension +/-; and these bolt forces are distributed over some col. height (length) greater than the vert. bolt spacing. The backer pls. improve the above picture. The backer pls. on the outside do none of these improvements, they are just spacers.