Method of RCC Design
The following methods are used for the design of RCC structure-
- The Working Stress Method
- The Ultimate Load Method
- The Limit State Method
The Working Stress Method
This method is based on linear elastic theory or the classical elastic theory. This method ensured adequate safety by suitably restricting the stress in the materials (i.e. concrete and steel) induced by the expected working leads on the structures. The assumption of linear elastic behavior considered justifiable since the specified permissible stresses are kept well below the ultimate strength of the material.
The ratio of the yield stress of the steel reinforcement or the cube strength of the concrete to the corresponding permissible or working stress is usually called a factor of safety.
The WSM uses a factor of safety of about 3 with respect to the cube strength of concrete and a factor of safety of about 1.8 with respect to the yield strength of steel.
Ultimate Load Method
The method is based on the ultimate strength of reinforced concrete at the ultimate load is obtained by enhancing the service load by some factor called a load factor for giving a desired margin of safety. Hence the method is also referred to as the load factor method or the ultimate strength method.
In the ULM, stress condition at the state of in pending collapse of the structure is analyzed, thus using, the non-linear stress-strain curves of concrete and steel. The safety measure in the design is obtained by the use of proper load factor. The satisfactory strength performance at ultimate loads does not guarantee satisfactory serviceability performance at normal service loads.
Limit State Method
Limit states are the acceptable limits for the safety and serviceability requirements of the structure before failure occurs. The design of structures by this method will thus ensure that they will not reach limit states and will not become unfit for the use for which they are intended. It is worth mentioning that structures will not just fail or collapse by violating (exceeding) the limit states. Failure, therefore, implies that clearly defined limit states of structural usefulness have been exceeded.
Limit state are two types
i) Limit state of collapse
ii) Limit state of serviceability.
Limit states of collapse
The limit state of collapse of the structure or part of the structure could be assessed from rupture of one or more critical sections and from bucking due to elastic bending, shear, torsion and axial loads at every section shall not be less than the appropriate value at that section produced by the probable most unfavorable combination of loads on the structure using the appropriate factor of safety.
Limit State of Serviceability
Limit state of serviceability deals with deflection and cracking of structures under service loads, durability under working environment during their anticipated exposure conditions during service, the stability of structures as a whole, fire resistance, etc.
Factor of Safety for Limit State Method
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