Dimentions for cross section

From previous calculations, i worked out that the available cross sectional area to ensure that each 3m section of the beam only weighed 30kg was 1282.05mm2.

With this in hand, the dimentions for the cross section can be found.Where a is the outer square side length, b is the inner square side length and c is the wall thickness.

Now, i would think that a 80mm - 120mm size for a would be substantial enough to take a load of 1000kg therefore using a as 120mm,

a2 - b2 = 1282.05mm2
(120^2) - b2 = 1282.05
b2 = 13117.95mm2
b = 114.534mm

therefore the wall thickness c would equal (120 - 114.534)/2 = 2.733mm

I do not think that this wall thickness would be large enough so take a as 80mm

a2 - b2 = 1282.05mm2
(80^2) - b2 = 1282.05mm2
b2 = 5117.95mm2
b = 71.54mm

therefore the wall thickness c would equal (80 - 71.54)/2 = 4.23mm

this wall thickness is more substantial and might give us a strong structure for our purpose.