Permissible Stresses:

As stated, the permissible stress in axial tension (σ_at) in steel is obtained by applying a factor of safety of the 1.67 to the minimum yield strength σ_y.

Thus, we get

σ_at  = σ _y/1.67 = 0.6 σ _y

For common structural mild steel (IS: 226 -1975), the minimum yield strength (σ_y) is 250 MPa. Hence, σ_at = 0.6 f_y = 150 MPa.

This is used for members with thickness less than 20 mm. For thicker sections this value is slightly reduced.

For      20 < t ≤ 40 mm          : σ_at = 144 MPa, and

For      t > 40 mm      : σ_at = 138 MPa

However, for ordinary structural sections t ≤ 20 mm hence σ_at can be taken as 150 MPa.