Uh... I guess that b is the breadth of a square and l the length of a cuboid with a square face...
We know that the volume is
Hence, we get:
I don't know what you mean by Fy and sy...
Is that it?
well I am in that point , Fy = failure load of a beam equation , sy=yield strees .
the final Merit index is = sy^2/3 / ρ . I am in the point Fy=b^4/12 / b/2 * σy/(m/b^2*ρ) .
So the thing is , how you get to that σy^2/3 / ρ ?
To make it a bit clearer it will be in the end Fy = b^3 *6 * m σy/2^3/ρ . So this σy/2^3/ρ will be in the final equation as it is now . without any mass or length in its function.
I hope I am clear now .