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 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
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
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
Here we have a detailed article on Different Types of Limit State of Design
Assumptions of Limit State of Collapse: Flexure
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.
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