Irving makes the bottom line point. The bridge/boom should have added section as far as possible from the shear center of the beam axis. That's why here is an "I" in I-beams. From a practical standpoint, a thickwall tube would be my suggestion for a boom candidate. Then add the gussets and shear walls.
If you are going to dig soft ground, pull up small trees for replanting, or bury a water line, you may not need too much added metal. Rocks, clay and concrete busting are another matter.
That's the engineer's opinion....