Dude said:1 inch squre tubing--and no way would I use anything thicker. It is VERY solid. It is almost too heavy to be considered "portable". The poor little caster wheels are really stressed by the weight.
Probably a couple hundred pounds of steel there I'd estimate. Plenty of safety factor I'd say.
Are the shelves plywood and the side panels sheet metal or aluminum? Do you have a shear?