On 2008-04-22, ransley wrote:

I would like to construct a very simple footbridge over our creek. The
distance is around 12-14 feet from bank to bank.

a 4x4 will hold a truck

A 4x4 spanning 12-14 feet won't hold a truck.

To the OP, the stiffness of a rectangular beam of depth d and width
varies as bd^3 (that's b*d*d*d). For a 4x6, that's 3.5 * 5.5^3 = 582,
and for a 2x8 that's 1.5 * 7.25^3 = 571. So they are almost
equivalent in stiffness.

Cheers, Wayne