The used design shall ascertain structural safety and accounts for the uncertainty of the loads, material strength, workmanship, and any other factors may affect structural strength and stability. structural safety of reinforced concrete structure can be calculated using two methods.
First, the method for working stress design. in this method, the stress for the structure is calculated and compared to allowable stresses. safety of the structure for this design method is the smaller of fc’/fc or fy’/fs.
In strength design method. the loads multiplied by a load factor greater than one to account for the uncertainty in the actual load magnitude. load factor will vary depends on type and combination of loads. the factored load will be used for the design of the structure.
To account for the uncertainty in material strength, dimension, and workmanship. nominal strength or ultimate strength of the member multiplied by the strenght reduction factor.
The reduction factor will usually vary from 0.65 to 0.90. in strength design method. the reduced ultimate strength is equal or more than the services loads multiplied by their factors. the estimated member capacity by ultimate strength method is more accurate than in working stress design method.