I once came accross the factory for Knoll Furniture while in South America, and they showed me plans of how they built a bent tube chair designed by Le Corbusier. The tube was stainless steel, and they inserted a solid piece of aluminum round bar (which was the size of the inside diameter of the tube) in the areas that were carrying the load and therefore most likely to bend. This could be done to reinforce the areas that are bending on your trapseat design without having to increase the diameter of the tube. You can also increase the wall thickness on the tube.