Methods of rcc design

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.

Methods of design of concrete structures
• Modular ratio method or Working stress method
• Load factor method
• Limit State method

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.

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

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

[ ReferTable -21, Pg.81, IS 456]

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.

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

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.

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.

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.

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.

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

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]

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.

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.

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

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’.

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

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.

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]

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

Limit state method
DESIGN STRENGTH
• The ratio between Characteristic
strength and partial safety factor
is called as design strength.
IS 456, Pg:58

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]

• Partial safety factors for load [IS 456, Pg: 68, Table – 18]
Limit state method

• Partial safety factors for material strength [IS 456, Pg: 68, cl:36.4.2]
Limit state method

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

Methods of rcc design

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Methods of rcc design

Similar to Methods of rcc design (20)

More from Selvakumar Palanisamy

More from Selvakumar Palanisamy (20)

Recently uploaded

Recently uploaded (20)

Methods of rcc design