Thanks for the info Wayne,

A friend has a bridge of similar length (but just over a marshy
depression) that is built using 4x4x16s, and that thing would b a bit
too bouncy for comfort when the drop is closer to 2.5'.

I'm not sure what the units are (distance^4 ?) or how the length of the
span figures in, but from the equation, it looks like using 4x6s should
provide almost four times the stiffness of 4x4s (3.5 * 3.5^2 = 150).

Anyone know how to calculate the weight that can be supported if the
weight is centered over a 14' span?

Thanks.

-Jonathan


Wayne Whitney wrote:

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