WOOD BEAM SIZINGTM   Fiber stress

BENDING

When a beam is loaded, the maximum bending stress occurs in the outer fibers of the beam: the top fibers are compressed and the bottom fibers are stretched. As the load increases, the beam will eventually break as the fibers give way.

We can calculate what the fiber stress will be under a given load for a given size beam. For example, a nominal 4x12 simple span beam, supporting a load of w = 480 plf, spanning 10 feet, will stress the outer fibers of the beam to
932 psi.

WBS FIBER STRESS PANEL

In this example, with wood with a maximum permitted adjusted value
Fb = 1,224 psi, the beam is stressed to 947 psi or 78% of its maximum allowable load.