[mura_gadi (Steve)]

The material is less import than the type of beam/support and distance of loading.

You get a board/beam and it will sag at a calculated distance by weight, for the strength, put it in a tube and your dynamics change completely.

Take a A4 sheet, a 5 cents piece placed on the opposite side width wise and the paper/span will fail. Roll that paper into a tube(single thickness) and you can place a couple of dollar coins in the end and the span will hold.

Tubes are great, better than squares, diameter is a big factor and your starting at around 12". Hence guessing 5mm and 8mm options being fine.

Sub's are tubes for a reason...

*Things like stress to strain numbers can be found in the full product pdfs occasionally, US sites seems to be very good for full pdfs on wood ... (Modules of Elasticity is it???)