Theory of Structures
For calculating the allowable stress of long columns σ0 = σy/n [1 - a (1/r)²]is the empirical formula, known as

Straight line formula
Perry
Rankine
Parabolic formula

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Theory of Structures
Slenderness ratio of a long column, is

Area of cross-section divided by least radius of gyration
Area of cross-section divided by radius of gyration
Length of column divided by least radius of gyration
Radius of gyration divided by area of cross-section

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