VARIATION IN WORKMANSHIP OR QUALITY CONTROL MAY REDUCE THE CHARACTERISTIC STRENGTH TO LOWER VALUE (DESIGN STRENGTH)

DESIGN VALUES (MATERIALS) :-- A factor is used to allow for the possible differences between the material strength obtained in the actual Structure and the characteristic strength. It allows for variation in workmanship or quality control in the manufacture of material and reduces the characteristic strength to lower values known as design strength and may be have from characteristic strength of material divided by the partial safety factor appropriate to material and limit state being considered. Load factors are used to allow for the possible differences in the loads that may actually come on a structure as compared to their characteristic values and the design load may be obtained from the characteristic load divided by the partial safety factor appropriate to the nature of loading and limit state being considered. PARTIAL SAFETY FACTORS :-- MATERIALS - The value of partial safety factor for material strength should account for the following parameters: (a) possibility of deviation of the strength of material.(b) deviation of the sectional dimensions.(c) accuracy of the calculation procedure,and (d) risk to life and economic consequences. When assessing the strength of a structure or structural member for the limit state of collapse, values of partial safety factors are taken as 1.5 for concrete and 1.15 for steel .It suggests that a greater variation is expected in the strength of concrete than that in the steel reinforcement .When a structure is designed for wind or earthquake,the permissible stresses in concrete and steel may be  increased by 33.33 percent.This increase is applicable only in the elastic method of design and not in the limit state method .The value of partial safety factor for loads should account for the following parameters (a) unusual increase in loads beyond that used for deriving characteristic values (b) unforseen stress redistribution (c) inaccurate assessment of the effects of loading,and (d) importance of the limit state considered .A factored load is obtained by multiplying a characteristic load by an appropriate partial safety factor. This factored load is used to calculate factored bending moment, factored shear force, factored thrust etc . Alternatively bending moment, shear force and thrust can be calculated based on the characteristic load values and then factored values can be obtained by multiplying them by an appropriate partial safety factor.