"The Medway Handyman" wrote

Ok great, so what do Z, B, D & I stand for?



Z is called the section modulus (b*d^2)/6 for a rectangular section as noted
earlier
B is the width of the section
D is the depth of the section
I is the second moment of area of the section (b*d^3)/12

The key starting point is that the stress due to bending (f) = [Bending
Moment (M)] / Z

To understand how this lot relates, you would need information on basic
structural engineering either steel or in the case of this thread timber.
Timbers a bit nasty cos there's dodgy variables which affect strength like
moisture content, grain direction etc.



HTH

Phil