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The design formula is    v = 1.5 x V / A   or  A = 3V / 2Fv
in which  v = maximum unit horizontal shear stress in psi
  V = total vertical shear in pounds
  A = area of member in square inches (breadth x depth)
Allowable horizontal unit shear stress is a given for different species and grades, as Fv (in psi) and must be adjusted for different conditions of use.  v must equal or be less than the Fv adj. in psi.

4.  DEFLECTION
The tendency of the member to bend under a load.  Expressed as D. Excessive deflection in members can damage plaster ceilings, visually sag, or have a tendency to bounce when walked on.  General limits for some uses are specified in the building code, expressed as L (length in inches) divided by a numeric limit such as 180, 240, or 360.  Example:  "L / 240"  if L = 120 in. then 120 / 240 = 0.5 inch limit.
Different formulas for different loading conditions are used to find an approximation of the actual deflection. These formulas use the load, the member's length, the given attribute for a species and grade called the modulus of elasticity expressed as E, and the member's Moment of Inertia expressed as I.

5.  BEARING AREA
The tendency for the weight of a load or reaction at a support to crush the wood fibers if the area (in sq. inches) is not large enough.
The total force W in pounds at a bearing location, divided by the given allowable compression perpendicular to grain, expressed as Fc will equal the minimum required bearing area. 
The formula is  W / Fc adj = Minimum bearing area in square inches.