ok maybe i've mis-understood the whole thing... i was under the impression that if he used box section then he would have to do some welding or atleast joining 2-3 pieces where as the rsj was a solution that didnt need any work other than attaching other parts?

my thought are that he is using 2 pieces of box to make what would be the top and bottom of the beam... but i can now see you are talking about one big piece of box the same size of the rsj... sorry my bad