DSK wrote:

gonefishiing wrote:

refresh my memory:
i'm looking for the section modulus for a boom section to understand
allowable bending stress.
sx= bd(squared) ?


No, IIRC it's the integral of the solid cross section area distance from
the axis.

Perhaps term you are groping for is "section moment"? You can look it up
on the web.

That's why triangular sections have the greatest rigidty for
their cross section area, and square sections are more rigid than oval
or round.

I think you may be confusing a truss with a section. For bending in
plane, the most rigid section per unit weight is close to a T
section with the top under compression and the bulb (or smaller bottom
plate) at the bottom in tension. If the direction can be up and down
this becomes the familiar I beam. For compression, a circle (tube) is
generally best as again, it places most material away from the axis for
all planes containing the axis. This important as the failure mode is
almost always buckling and not material compression. For a combination
of compression and bending the best solution lies between these cases so
we see egg shapes and ovals but the object is always to place as much
material from the axis as needed to contain the stress well below a
yield stress.

Hope this helps.

Cheers