Also, dont under estimate the 'bendyness' of steel

Just think about the title of this thread. What the round rails are made of, and what you need to do to stop them bending.

It's exactly the same for steel box section. It will need supporting and bracing too.

not knocking it, just reminding you and anyone else reading this, to think about the design, as much as your chosen material.

As a very extreme example, pick up that piece of 50 x 50 x 7m long box section Jazz mentioned, in the middle, and it will bend and wobble like a bendy wobbly thing !