In article ,
Mike Reed wrote:
The simple answer is round.

The reason is that for a given cross-sectional surface area, a square
has narrow spans and wide spans through the center. A circle only has
one span through the center (which is the diameter). If you don't know
what direction a load will be applied, then a circle gives you even
coverage in all directions, where a square is stronger along the
diagonals, but weaker than a circle of the same area, perpendicular to
the edges.

As intuitive as this may seem to you, it is, quite simply, very wrong.

A square, with sides of 1 in. and cross-sectional area of 1 in^2, has a
sectional modulus of 0.333 in^3.

A round, with diameter of 1.128 in, has a cross-sectional area of 1
in^2, and a sectional modulus of 0.141 in^2.

That means that in bending, a square is over twice as strong as a
round of the same cross-sectional area.

Have you noticed that, for example when building frames out of steel,
square tubing is much preferred over round tubing, despite the fact
that round tubing is much cheaper for a given size?

Intuition often misleads, engineering calculations rarely do.

Kelly