Permissible Stresses in Compression Members:

As per IS 800 : 1984, the direct stress in compression on the gross-sectional area of axially loaded compression members shall not exceed 0.6 fy nor the permissible stress σac, calculated using the following formula :

σ_ac = 0.6 (f_cc. f _y/[(f_cc)ⁿ +(f_y)ⁿ]^1/n)(MPa)

where f_cc = Elastic stress in compression (MPa) = π² E/λ²                          

f_y = Yield stress of steel (MPa),

E = Modulus of elasticity of steel = 2 × 10⁵ MPa,

λ= l/r_min = Slenderness ratio of member, and

n = A factor assumed as 1.4.

Values of σ_ac for some of Indian Standard structural steels are given in Table 2. It may be observed that for steel having f_y = 250 MPa, the value of allowable σ_ac becomes 33 MPa just, for the maximum permissible slenderness ratio of 180, being thus reduced by 87%.