Method of RCC design

 Method of RCC design

Aim of Design

 As per IS 456-2000, the aim of design is the achievement of an acceptable probability that structures being designed will perform satisfactorily during their intended life. With an appropriate degree of safety, they should sustain all the loads and deformations of normal construction and use and have adequate durability and adequate resistance to the effects of misuse and fire. 

Method of RCC design
A reinforced concrete structure should be designed to satisfy the following criteria

i) Adequate safety, in items stiffness and durability 

ii) Reasonable economy. 

The following design methods are used for the design of RCC Structures. 

a) The working stress method (WSM)

 b) The ultimate load method (ULM) 

c) The limit state method (LSM) 

Working Stress Method (WSM) 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 behaviour considered justifiable since the specified permissible stresses are kept well below the ultimate strength of the material. The ratio of yield stress of the steel reinforcement or the cube strength of the concrete to the corresponding permissible or working stress is usually called 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 (ULM) The method is based on the ultimate strength of reinforced concrete at ultimate load is obtained by enhancing the service load by some factor called as 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 analysed, thus using, the non-linear stress – strain curves of concrete and steel. The safely measure in the design is obtained by the use of proper load factor. The satisfactory strength performance at ultimate loads does not guarantee satisfactory strength performance at ultimate loads does not guarantee satisfactory serviceability performance at normal service loads. 

 Limit state method (LSM) 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 has 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 unfavourable combination of loads on the structure using the appropriate factor of safely. 

Limit state of serviceability-   Limit state of serviceability deals with deflection and crocking of structures under service loads, durability under working environment during their anticipated exposure conditions during service, stability of structures as a whole, fire resistance etc.

References -IS 456 (2000): Plain and Reinforced Concrete - Code of Practice [CED 2: Cement and Concrete] July 2000 IS. 456 : 2000 (R••fflrmed2005) Indian Standard PLAIN AND REINFORCED CONCRETE ­ CODE OF PRACTICE (Fourth Revision)