WOOD BEAM SIZINGTM   Shear stress

In order for the load on the beam to be transferred to the end supports, the beam must hold together. (A 12x12 beam is a great deal stronger than twelve 1x12 planks in a stack - each layer of the stack just slides along the others).

Horizontal shear stress is the measure of how well the layers of the wood hold together before splitting apart. Generally, shear stress is seldom a problem with lightly loaded beams or joists or rafters.

Heavily loaded beams with short spans may fail first in shear, however.

WBS SHEAR PANEL

In this example, with wood selected with a maximum allowable Fv = 150 psi, the beam is shown stressed to 46 psi or 31% of allowable stress.