The extreme fiber stress in bending for a rectangular timber beam is

f=6M / bh²
=M/S

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The allowable unit stresses given for compression perpendicular to the grain apply to bearings of any length at the ends of beams and to all bearings 6 in (152.4 mm) or more in length at other locations. Read the rest of this entry »

Stress-grade lumber consists of three classifications:

1. Beams and Stringers. Lumber of rectangular cross section, 5 in (127 mm) or more thick and 8 in (203 mm) or more wide, graded with respect to its strength in bending when loaded on the narrow face.

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