Lumber Stress Grades and Design Properties

Lumber sawn from a log, regardless of species and
size, is quite variable in mechanical properties.
Pieces may differ in strength by several hundred
percent. For simplicity and economy in use, pieces of lumber
of similar mechanical properties are placed in categories
called stress grades, which are characterized by (a) one or
more sorting criteria, (b) a set of properties for engineering
design, and (c) a unique grade name.