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Timber coloums are seldom used singly. Mostly they are a group of lumber which are stick together to form a single member. The allowable unit stress on timber is calculated by using the following formula
When the Columns have square or rectangular cross section
When the Columns have circular cross section
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)
Specific Conditions to be kept in mind
1) The allowable unit stress(P/A) may not exceed the allowable compressive stress c.
2) 1/d must not exceed 50.
Note: The allowable unit stresses for bearing on end grain are published by lumber associations.