[TowerTalk] square vs round booms

[Jim Thomson]

The section modulus of a 4 inch OD  x .125 inch  wall thickness  tube is greater than  a 3 inch square boom, also with a .125 inch wall thickness. 
Both have a 12 inch circumference.   The 4 inch OD tube is a hair more.   They would both weigh aprx the same.   Assume both are 6061-T6 and both are 40 ksi yield strength. 

For the  round tube.....  bending moment  =  section modulus X  yield strength . 

Does that also apply to rectangular tubing ??      If it does, square tubing offers no advantage at all..in terms of wind survival. 
Wait, it gets worse. The 3 inch sq boom has a smaller projected area vs the 4 inch round boom, but,  per the latest wind specs,
you have to multiply the  projected area of the square tubing, with its flat surface... by  2.0      The round tubing is multiplied by 1.2    So the 4 inch
round tubing still ends  up with less effective  area  vs the square tubing.   

So now the square tubing has lost...twice.    For the same circumference and wall thickness, and yield strength,  the square tubing has a smaller sectional modulus, plus more effective area. 
Am I on track here ?   So why are we seeing square booms used, other than perhaps a slight ease in mounting eles.     What am I missing ? 

Jim   VE7RF