Reinforced Concrete Design Method
A reinforced concrete structure is designed to fulfill the following parameters -:-
- Proper safety, with regard to stiffness and durability
- Justifiable economy.
Reinforced concrete design is based on the following methods :-
- The working stress method (WSM)
- The ultimate load method (ULM)
- The limit state method (LSM)
Working Stress Method (WSM)
This method is implemented on the basis of linear elastic theory or the classical elastic theory. This method ascertains tolerable safety by correctly constraining the stress in the materials (i.e. concrete and steel) caused by the projected working loads on the structures. The proposition of linear elastic behaviour legitimate as the specified allowable stresses are retained well under the ultimate strength of the material. The ratio of yield stress of the steel reinforcement or the cube strength of the concrete to the related allowable or working stress is normally known as factor of safety.
The WSM applies a factor of safety of about 3 relating to the cube strength of concrete and a factor of safety of about 1.8 relating to the yield strength of steel.
Ultimate Load Method (ULM)
This method is implemented on the basis of the ultimate strength of reinforced concrete since ultimate load is achieved by improving the service load with some factor known as load factor for providing a desired margin of safety. Therefore, the method is also called the load factor method or the ultimate strength method.
In the ULM, stress condition at the state of pending collapse of the structure is examined by applying the non-linear stress – strain curves of concrete and steel. The safely measure in the design is acquired by the application of proper load factor. The satisfactory strength performance at ultimate loads does not ensure the tolerable serviceability performance at normal service loads.
Limit State Method (LSM)
Limit states stand for the satisfactory limits for the safety and serviceability requirements of the structure prior to failure takes place. The design of structures by this method will make sure that they will not attain limit states and will not become unsuitable for the use for which they are projected. The structures will not just fail or collapse by breaching (exceeding) the limit states. So, failure assumes that clearly defined limit states of structural usefulness has been surpassed.
Limit state are categorized in the following types :-
- Limit state of collapse
- Limit state of serviceability.
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Limit States Of Collapse
The limit state of collapse of the structure or part of the structure is evaluated from the rupture of one or more critical sections and from bucking caused by elastic bending, shear, torsion and axial loads at every section shall not be under the correct value at that section created the probable most unfavourable combination of loads on the structure with the proper factor of safely.
Limit State Of Serviceability
Limit state of serviceability addresses the deflection and cracking of structures under service loads, durability under working environment throughout their projected exposure conditions during service, stability of structures as a whole, fire resistance etc.