welded box section to steel plate

huangyhg

welded box section to steel plate.
without going in to much detail on the application i am attempting to weld a shs onto a plate which is welded ot two angles 6" apart. there is a moment applied at the connection and the shs is located in the 6" 'gap'. any advice on how to prove plate will not be overstressed due to this moment? would imagine moment would put a equal and opposite reaction force in the top and bottom 'legs' of the box section?  
can you be a little more descriptive in how the connection is detailed?  i am having a hard time visualizing which way the moment is bending.
probably not the best descrription i know. the moment could potetionally bend in both directions which is why ive gone for a box section. the two angles form part of a column system and there is a plate welded across the angles. in one direction im not to concerned as the legs of the shs are welded at the angles but for moment in vertical direction, that's where im more concerned.