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DESIGN STEPS TO SIZE A WOOD MEMBER

1.  BENDING STRENGTH
The relationship of the allowable extreme fiber bending stress Fb to the maximum bending moment M, equals the required section modulus S. 
The design equation is    S = M / Fb adj.
Fb is a given (in psi)  depending upon the species and grade.
The Fb must be adjusted for various conditions such as wet use, duration of load, size factor, repetitive use, flat use, and temperature, etc. to find the value for Fb adj.

2.  VERTICAL SHEAR
The tendency of the member to break and fall between the supports.
Expressed as V.  Since a particular member will fail first by horizontal shear (see below), the investigation of vertical shear is not deemed necessary.

3.  HORIZONTAL SHEAR
The tendency for the wood fibers to slide past each other at the center (neutral axis) of the member and fracture along its length. The maximum horizontal shearing stress for rectangular sections is one and one half times the average vertical unit shear stress.