This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
As we have already noticed, the stress should exceed the strain as many times as the adopted factor-of-safety, or: Streess/Strain = factor-of-safety.
Or, stress = strain x factor-of-safety.
This holds good for all calculations, and can be expressed by the following simple fundamental formula: v = s.f (1)
Where v = the ultimate stress in pounds.
s= strain in pounds.
And where f= the factor-of-safety.