In limit state Method, design compressive stress is taken as

(Adopting standard notations) 

Explanation:

The permissible compressive stress in concrete is taken as 0.67 fck and a partial factor of safety of 1.5 is applied to it. So, final stress comes out to be 0.45 fck.

• For design purposes, the compressive strength of concrete is assumed to be 0.67 times the characteristic strength of concrete.
• The partial factor of safety of 1.5 is also to be applied in addition to this.
• Characteristic Strength of concrete Cube = fck
• Characteristic strength of concrete in actual structure = 0.67 fck
• Design strength of concrete in flexural compression = 0.67 fck / partial FOS = 0.67fck / 1.5 = 0.45f­ck