The allowable unit stress on timber columns consisting of a single piece of lumber or a group of pieces glued together to form a single member is

P/A=3.619E/(l/r)2

For columns of square or rectangular cross section, this formula becomes

P/A=0.30E/(l/d)2

For columns of circular cross section, the formula becomes

P/A=0.22E(l/d)2

The allowable unit stress, P/A, may not exceed the allowable compressive stress, c. The ratio, 1/d, must not exceed 50. Values of P/A are subject to the duration of loading adjustment given previously.

P= total allowable load, lb (N)

A= area of column cross section, in2 (mm2)

c= allowable unit stress in compression parallel to grain, lb/in2 (MPa)

d= dimension of least side of column, in (mm)

l= unsupported length of column between points of lateral support, in (mm)

E= modulus of elasticity, lb/in2 (MPa)

r= least radius of gyration of column, in (mm)

For members loaded as columns, the allowable unit stresses for bearing on end grain (parallel to grain) are given in data published by lumber associations. These allowable stresses apply provided there is adequate lateral support and end cuts are accurately squared and parallel. When stresses exceed 75 percent of values given, bearing must be on a snug-fitting metal plate. These stresses apply under conditions continuously dry, and must be reduced by 27 percent for glued-laminated lumber and lumber 4 in (102 mm) or less in thickness and by 9 percent for sawn lumber more than 4 in

(102 mm) in thickness, for lumber exposed to weather.