Aluminium frames are very strong. Much stronger than wood of a similar profile. Very large frames however can sag sometimes, so when frames are over one meter in diameter it is best to cross- brace them with wire. This prevents them sagging and also prevents the possibility of a corners slipping (rare but can happen with additional weight). We often supply large frames with 3mm perspex (rather than 2mm) and with MDF backboards (rather than foamboard) so they can be quite a lot heavier than frames under a metre wide. X bracing is simple, we just supply extra hangers and wire that takes a minute or two to put in place and it gives you (and me) peace of mind.