Methods of rcc design
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The document discusses different methods of designing reinforced concrete elements: 1. Modular ratio (working stress) method, which assumes elastic behavior and uses factors of safety. It was the first accepted method but has limitations. 2. Load factor method, which avoids modular ratio and uses load factors to account for ultimate loads. However, it does not consider serviceability. 3. Limit state method, adopted in modern codes, which considers both ultimate and serviceability limit states using partial safety factors applied to loads and material strengths. It provides a comprehensive solution for safety and serviceability.
![Modular ratio method or Working stress method
Modular ratio
• Modular ratio =
Modulus of Elasticity of steel
Modulus of Elasticity of concrete
• IS code recommendation
• σcbc – Permissible Compressive stress [ Refer Table -21, Pg.81, IS 456]
m= 280/3 σcbc](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-7-320.jpg)
![[ ReferTable -21, Pg.81, IS 456]](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-8-320.jpg)
![Load factor Method
Design philosophies
• The strength of a RC section at working load is estimated
from the ultimate strength of the section.
• This method used the ultimate load principles for design.
The strength of the member must be more than the ultimate
load acting on the member.
Design resistance(Rn) ≥ Design load effect
[Strength of the section] [Ultimate load acting]](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-16-320.jpg)
![Limit state method
Characteristic load (F)
• Structures have to carry dead and live loads. The maximum
working load that the structure has to withstand and for which it is
to be designed is called the characteristic load. [IS 456, clause.36.2]
Characteristic strength (f )
• The strengths that one can safely assume for the materials (steel
and concrete) are called their characteristic strengths.
[IS 456, clause.36.1]](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-23-320.jpg)
![Limit state method
Partial safety factors
• Having obtained the characteristic load and characteristic strength,
the design loads and design strengths are obtained by the concept
of partial safety factors.
• Partial safety factors are applied both to loads on the structure and
to strength of materials.
[Refer IS 456, Cl:36.4, Pg: 68]](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-26-320.jpg)
![• Partial safety factors for load [IS 456, Pg: 68, Table – 18]
Limit state method](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-27-320.jpg)
![• Partial safety factors for material strength [IS 456, Pg: 68, cl:36.4.2]
Limit state method](https://image.slidesharecdn.com/methodsofrccdesign-200731082731/85/Methods-of-rcc-design-28-320.jpg)
Methods of rcc design
- 1. CE8501 - DESIGN OF REINFORCED CEMENT CONCRETE ELEMENTS Unit 1 Introduction Methods of RCC design Presentation By P.Selvakumar.,B.E.,M.E. Assistant Professor, Department Of Civil Engineering, Knowledge Institute Of Technology, Salem.
- 3. Methods of design of concrete structures • Modular ratio method or Working stress method • Load factor method • Limit State method
- 4. Modular ratio method or Working stress method • This method of design was evolved around 1900. • This is the first theoretical method accepted by the National code of practice for design of reinforced concrete sections. • It assumes that both steel and concrete act together and perfectly elastic at all stages so that the modular ratio can be used to determine the stresses in steel and concrete.
- 6. Modular ratio method or Working stress method Permissible stress (or) Allowable Stress • Permissible stress = Ultimate or Yield stress Factor of Safety Factor of Safety • Factor of safety = Yield stress Working stress Working Stress • Working stress ≤ Permissible stress
- 7. Modular ratio method or Working stress method Modular ratio • Modular ratio = Modulus of Elasticity of steel Modulus of Elasticity of concrete • IS code recommendation • σcbc – Permissible Compressive stress [ Refer Table -21, Pg.81, IS 456] m= 280/3 σcbc
- 8. [ ReferTable -21, Pg.81, IS 456]
- 9. Working stress method Service load • The term service load (or working load) refers to the expected load to which the structure is subjected to its normal use/service. • It does not include any possible overloading or any safety factors. Ultimate load • The term ultimate load refers to the maximum load the member or structure, as the case may be, can carry before failure.
- 10. Working stress method • Design Load • The term design load refers to the estimated load of a structure by using Factor of safety or Partial safety factor. • Ex: Design load = Factor of safety x Service load
- 11. Modular ratio method or Working stress method Design philosophies • Design moment and shears in the structure are calculated by elastic analysis with the characteristic loads (service loads). • The stress in concrete and steel are calculated on the basis of Elastic behaviour of the composite section.
- 12. Modular ratio method or Working stress method Design philosophies • This method adopts permissible stresses which are obtained by applying specific safety on material strength for design. • It uses a factor of safety of about 3 with respect to cube strength of concrete and a factor of safety of about 1.8 for Steel.
- 13. Modular ratio method or Working stress method Limitations of working stress method (Drawbacks) • Since the method deals only with the elastic behaviour of the member, it neither shows its real strength nor gives the true factor of safety of the structure against failure. • Modular ratio design results in a larger percentage of compression steels than the limit state design, thus leading to uneconomical sections in the compression zone. • The modular ratio itself is an imaginary quantity, it does not consider the creep and shrinkage in concrete. Because the concrete does not have a definite modulus of elasticity as in steel.
- 14. Load factor Method Introduction • This method of design was evolved around 1957 and become alternative to WSM. • The concept of ‘modular ratio’ and its associated problems are avoided entirely in this method. • The safety measure design is introduced by an appropriate choice of the load factor.
- 15. Load factor Method • Load factor • In the ultimate load method, the working loads are increased by suitable factor to obtain ultimate loads. These factor are called load factors. • The structure is then designed to resist the desired ultimate loads. • This method takes into account the non-linear stress- strain behaviour of concrete. • Load factor = Ultimate load Working load
- 16. Load factor Method Design philosophies • The strength of a RC section at working load is estimated from the ultimate strength of the section. • This method used the ultimate load principles for design. The strength of the member must be more than the ultimate load acting on the member. Design resistance(Rn) ≥ Design load effect [Strength of the section] [Ultimate load acting]
- 17. Load factor Method Limitations of load factor method • The performance at the normal service loads is not considered. • This method does not take into consideration for serviceability criteria of deflections and Cracking. • This method does not take into consideration for effect of creep and shrinkage.
- 18. Limit state method Introduction • The designs based on limit state principles are nowadays internationally accepted for design of reinforced concrete structures. • The Indian code IS 456 : 2000 has adopted limit state method for design of RC members.
- 19. Limit state method Working stress Method (Service Load) (Factor of Safety) 1900 Ultimate Load Method (Ultimate load) (Load factor) 1957 Limit state Method (Service Load & ultimate load) (Partial safety factor) 2000
- 20. Limit state method • Design philosophy • Unlike WSM which based calculations on service load conditions alone, and unlike ULM, which based calculations on ultimate load conditions alone. • LSM aims for a comprehensive and rational solution to the design problem, by considering safety at ultimate loads and serviceability at working loads. • The LSM philosophy uses a multiple safety factor format which attempts to provide adequate safety at ultimate loads as well as adequate serviceability at service loads, by considering all possible ‘Limit State’.
- 21. Limit state method • Limits State - Definition • A limit state is a state of impending failure, beyond which a structure ceases to perform its intended function satisfactorily, in terms of either safety of serviceability i.e. it either collapses or becomes unserviceable. • There are two types of limit states: 1. Limit states of collapse 2. Serviceability limit states
- 22. Limit state method • Limit states of collapse (Ultimate limit states):- Which deal with strength, overturning, sliding, buckling, fatigue fracture etc. • Serviceability limit states: – which deals with discomfort to occupancy and/ or malfunction, caused by excessive deflection, crack width, vibration, leakage etc., and also loss of durability, etc.
- 23. Limit state method Characteristic load (F) • Structures have to carry dead and live loads. The maximum working load that the structure has to withstand and for which it is to be designed is called the characteristic load. [IS 456, clause.36.2] Characteristic strength (f ) • The strengths that one can safely assume for the materials (steel and concrete) are called their characteristic strengths. [IS 456, clause.36.1]
- 24. Limit state method DESIGN LOAD • The load obtained by multiplying the characteristic load by the partial safety factor is called the factored load or design load. IS 456, Pg:58
- 25. Limit state method DESIGN STRENGTH • The ratio between Characteristic strength and partial safety factor is called as design strength. IS 456, Pg:58
- 26. Limit state method Partial safety factors • Having obtained the characteristic load and characteristic strength, the design loads and design strengths are obtained by the concept of partial safety factors. • Partial safety factors are applied both to loads on the structure and to strength of materials. [Refer IS 456, Cl:36.4, Pg: 68]
- 27. • Partial safety factors for load [IS 456, Pg: 68, Table – 18] Limit state method
- 28. • Partial safety factors for material strength [IS 456, Pg: 68, cl:36.4.2] Limit state method
- 29. Limit state method Limit states of collapse Refer IS 456 (page) Serviceability limit states Refer IS 456 (page) Flexure 69 Durability 17 Compression 70 Stability 33 Shear 72 Fire resistance 33 Torsion 74 Deflection 75 Tension Cracking 76
- 30. Thank You