Concrete design methods or concrete design philosophies such as working stress method, ultimate strength method, and limit state method are used for the design of reinforced concrete, Steel, and timber structures.
These design methods are based on certain assumptions and procedures which are used to meet the conditions of safety, functionality, serviceability, and economy of the structure.
Several design philosophies have been introduced from different parts of the world. These design philosophies are commonly accepted by various codes, for instance, the ultimate strength method is established in the 1950s and accepted by ACI code in 1956, British codes in 1957, and Indian codes in 1964. Similarly, the limit state method is the most recent one which is accepted by the ACI Code, British Code, and Indian Standard.
Finally, the stress diagram and strain diagram for each design philosophies are presented in Fig.1, Fig.2, and Fig.3. Different stresses and strains in concrete sections under imposed load based on each design methods are shown as well.
Table 1 Difference Between Different Design Methods; Working Stress Method, Ultimate Strength Method, and Limit State Method
| Working Stress Method | Ultimate Strength Method | Limit State Method |
| It is also known as elastic method of design or modular ratio method or alternate design method. | It is also called ultimate load design method or load factor method | It is also termed as plastic design method. |
| It is based on the elastic theory which assumes that concrete and steel are elastic and the stress strain curve is linear for both. | It is based on the ultimate strength, when the design member would fail. It uses the actual stress-strain curve. | This method is based on the actual stress-strain curves of steel and concrete. For concrete the stress-strain curve is non-linear. |
| It is stress based method of RCC design. | It is strain based method of design. | It is strain based method of RCC design. |
| Limits the structural usefulness of the material of the structure up to a certain load at which the maximum stress in extreme fiber reaches the characteristic strength of material in bending. | Limits the structural usefulness of the material of the structure up to ultimate load. | Limits the structural usefulness of the material of the structure up to a certain load at which acceptable limit of safety and serviceability are applied so that the failure of structure does not occur. |
| It uses factors of safety for stresses only (to get permissible stresses) and not for loads. As a result, it does not give true margin of safety with respect to loads because the failure load is not known. | Load factors are used on service loads and strength reduction factors are employed on materials. | Partial factor of safety is considered on both loads and stresses (to get design values of stresses). |
| Working stress method only deals with serviceability such as crack, vibration, deflection etc. | Ultimate load method only deals with on safety such as strength, overturning, and sliding, buckling, fatigue. | Limit state method advances than other two methods. it takes into account the ultimate strength of the structure and also the serviceability requirements |
| Designed sections are uneconomical since they are oversized and use larger reinforcement ratio compare with other two design philosophy. | Designed sections are economical. | Designed sections are economical from both ultimate limit state and serviceability limit state point of view. |
| - | Despite the fact that the nonlinear stress strain behavior of materials is considered, the nonlinear analysis of the structural has not been carried out for the load effects. Consequently, the stress distribution at ultimate load is just the magnification of service load by load factor following the linear elastic theory. | The process of stress redistribution and moment redistribution are considered in the analysis and more realistic factor of safety values are used in the design |
| It provides better serviceability performance under service loads compare with ultimate strength method since designed sections are large. | It does not take into consideration the serviceability criteria of deflection and cracking. | Designed sections perform satisfactorily in terms of serviceability requirements |
| It is only the method available when one has to investigate the reinforced concrete section for service stresses and for the serviceability state of deflection and cracking. | - | - |
| It fails to differentiate between different types of loads that act simultaneously but have different uncertainties. | It considers the effects of different loads acting simultaneously but have different uncertainties. | - |
| Exact margin of safety is not known. | The load factor gives the exact margin of safety against collapse. | Exact margin of safety is known. |
| It does not account for shrinkage and creep which are time dependent and plastic | The method does not take into consideration the effects of creep and shrinkage. | - |
| It pays no attention to the conditions that arise at the time of collapse. | The method is based on the ultimate strain as the failure criteria. | The acceptable limits of safety and serviceability requirements before failure occurs is called a limit state. This method is based on the concept of safety at ultimate loads (ultimate load method) and serviceability at working loads (working stress method). |
| Material strength is not fully utilized in the designed member | Material strength is fully utilized in the designed member | Material strength is fully utilized in the designed member |
