Characteristic strength and loads
Limit state method is based on statistical concepts. Strength of materials and loads are highly variable in a range of values. The test in laboratory on compressive strength of concrete has indicated coefficient of variation of ±10%. Hence in reinforced concrete construction, If is not practicable to specify a precise cube strength. Hence in limit state design uses the concept of 'characteristic strength' fck indicates characteristics strength of concrete & by characteristic strength of steel. In general fk indicates the characteristic strength of material.
? fk = fm -1.646 (2.6) here fm = mean strength.
Similarly 'characteristic load' is that value of load which has an accepted probability of not being exceeded during the life span of structure. In practice the load specified by IS875 - 1987 is considered as characteristic load. Equation for characteristic load is
Lk = Lm + 1.64_.