ultimate stress design method(strength design)
ultimate stress design method become popular after 1960s. working stress design used extensively before that to design reinforced concrete members. ultimate stress design method produces a more rational and economical design compared to working stress design method. also, the ultimate stress design method utilizes a more realistic factor of safety. in this method, the reinforced concrete structure is designed beyond the elastic region. the working dead load and live load are multiplied by a factor of safety. the section designed to fail at factored load. failure at factored load means the section exceeds the elastic region to ultimate strength then failure.
advantages of strength design:
- strength design method considers the non-linear linear shape of stress-strain design which resulted in a better estimation of load carrying capacity.
- strength design methods utilize different factor of safety for a different type of load. dead load and live load will have the different factor of safety. dead load for structure can be estimated better than live load. therefore, the factor of safety for the dead load will be lesser than the factor of safety for live load
- strength design method produce more economical structures. strength design method utilizes the high strength of steel. working stress design method utilizes part of steel strength.
- the strength design method is more flexible compared to working stress design method. for example, a large section can be used with a small amount of steel or a small section can be used with a large amount of steel.