Various design philosophies have been invented in the different parts of the world to design RCC structures. In 1900 theory by Coignet and Tedesco was accepted and codified as Working Stress Method. The Working Stress Method was in use for several years until the revision of IS 456 in 2000.

IS 456 : 2000 gives more importance to Limit State Method as it designs more economical structures. WSM also included in IS 456 : 2000 but regulated from main content of code to Annex B i.e. at the end of code., with only few pages devoted.

Whereas Ultimate Load Method was developed in the 1950s as a alternative to Working Stress Method. ULM got accepted as an alternative to WSM in ACI code in 1956 and in British code in 1957. ULM was based on the ultimate load carrying capacity of reinforced concrete on collapse.

Now standing in 21st Century we have three design philosophies of RCC structure those are Limit State Method, Working Stress Method, Ultimate Load Method which are described in detail below

Here are methods of structural design

- Working Stress Method
- Ultimate Load Design
- Limit State Method

Working Stress Method was the traditional method of design for RCC structures, steel structures, and timber structures also.

The basic concept of working stress method is adequate safety can be achieved by restricting the stresses in the material due to working loads on the structures, if we assume that structural materials in a linear elastic manner.

Working Stress Method assumes that concrete and steel reinforcement in a structure acts together and are perfectly elastic and that’s why modular ratio can be used to determine the stresses in concrete and steel.

**Also Read: Differentiate between WSM and LSM**

- The plane section before bending remains plane after bending at any cross-section.
- The stress-strain behavior between steel and concrete is linear.
- All the tensile stresses are taken only by the reinforcement and not by the concrete.
- Modular Ratio (m = 280/3σ
_{cbc}) is taken in order to consider creep effects.

- The assumption that the stresses under working loads can be kept within permissible stresses is unrealistic or not practical.
- Using modular ratio (m) a imaginary factor ultimately results in large amount of compression steel and large member sizes than designs done by ultimate load or limit state method.
- Stress Strain behavior for concrete is non linear and time dependent thus modular ratio is not constant, and WSM does not considers consequences of this non-linearity of material.

Despite having these much amount of limitations, structures designed with this method have performed well for long period of time. Also this method is very simple in understanding and in application.

The ultimate load design method also termed as load factor method or the ultimate strength method. This method was developed in the 1950s with overcoming shortcomings of WSM and introduced in the USA in 1956, the UK in 1957, and also in 1964 revision of IS 456.

In the ultimate load method, the nonlinear stress-strain curves of steel and concrete also the stress conditions just before the collapse are considered so as to avoid the problems arisen due to the imaginary concept of modular ratio (m).

**Also Read: Differentiate between WSM and LSM**

- Stress in reinforcement varies linearly with strain up to specified yield strength.
- Strain hardening of steel is ignored.
- Tensile strength of concrete is neglected.

- Performance under normal service load is not considered, that is why it was realized that the design approach that combines the very best features of the working stress method and the ultimate load method will result in better structural stability and performance.
- Sections designed by using the Ultimate load method are too thin that they can easily fail under excessive deformations and cracking.
- No factor of safety chosen for material stresses.

In the limit state method of design, the structure should be designed to withstand safely all the loads acting on it throughout its life. Also, the structure should satisfy the serviceability requirements, like limitations of deflection and cracking.

WSM was based on calculations on service load conditions only, ULM was based on calculations on ultimate load conditions only and now LSM provides comprehensive and rational solutions to the design problems by considering safety at ultimate loads and serviceability at working loads.

Any structure should be designed for the worst-case scenario i.e. most critical limit state., and checked for other limit states.

**Also Read: Differentiate between WSM and LSM**

- Limit State of Collapse
- Limit State of Collapse: Flexure
- Limit State of Collapse: Compression
- Limit State of Collapse: Shear
- Limit State of Collapse: Torsion

- Limit State of Serviceability
- Limit State of Serviceability: Deflection
- Limit State of Serviceability: Cracking

- Limit State of Durability

Here we have a detailed article on Different Types of Limit State of Design

**Assumptions of Limit State of Collapse: Flexure**

- A section that is plane normal to the axis of structural components remains plane after bending.
- The maximum strain in concrete at the outermost compression fiber is taken as 0.0035 in bending.
- The tensile strength of concrete is ignored.

In the limit state method of design, the structure should be designed to withstand safely all the loads acting on it throughout its life. Also, the structure should satisfy the serviceability requirements, like limitations of deflection and cracking.

6 months ago

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