Basic R C C structurs

Basic Reinforced Concrete
Design of Structures

Design Concrete Structures(D C S)
• What do you mean by
STRUCTURE ?
Design of Concrete Structures(D C S)

Design of Concrete Structures
• A ‘STRUCTURE’ is an element that which
receives load at a point and transfer it at
another point.
• A beam receives load from slab and transfer it to column
• A Column receives the load from beam and transfer it to foundation
• A foundation receives the load from column and tranferit soil uniformly
• [ In that process the structural elements are subjected to stresses known
as STRESS RESULANTS]

• What is
CONCRETE?

• Good concrete is one that has the desired
qualities of strength, impermeability,
durability, etc., in the hardened state. To
achieve this, the concrete has to be
‘satisfactory’ in the fresh state (which includes
mixing, handling, placing, compacting and
curing).

Design of concrete Structures
• Broadly, this means that the mix must be of
the right proportions, and must be cohesive
enough to be transported and placed without
segregation by the means available,
• and its consistency must be such that it is
workable and can be compacted by the means
that are actually available for the job.

• Same raw materials in concrete always give
same quality of concrete
• True
• False

• Structure need not take LOAD
• True
• False

• What is
DESIGN?

Analysis and Design
• The purpose of analysis is to determine the stress
resultants and displacements in the various
members of a structure under any loading (static or
dynamic).
• The purpose of design is to provide adequate
member sizes, reinforcement and connection
details, so as to enable the structure to withstand
safely the calculated load effects.

• The aim of structural design is to design a
structure so that it fulfils its intended purpose
during its intended lifetime with
• adequate safety (in terms of strength,
stability and structural integrity),
• adequate serviceability (in terms of stiffness,
durability, etc.) and economy.

Design of Reinforced concrete
Structures
• In this subject we discuss about the structural
elements(like beams, slabs, columns, footings)
made up of with concrete with proper design
(i.e for their safety and serviceability
requirements.)

Reinforced Cement Concrete
• What is Cement mortar?
• What is Cement concrete?
• [Grades of concrete;
M10,M15,M20,M25,M30.....]
• What is R C C?
• [When steel is embedded into concrete, we
get a new composite material known as R C C]

• Why R C C?
• Concrete may be remarkably strong in
compression, but it is equally weak in
tension!.
• Its tensile ‘strength’ is approximately one-
tenth of its compressive ‘strength’.

• fig1

• So, the important assumption in R C C is
• The total tension is taken by STEEL
• And the total compression is taken by
CONCRETE

• Hence, the use of plain concrete as a
structural material is limited to situations
where significant tensile stresses and strains
do not develop, as in hollow (or solid) block
wall construction, small pedestals and ‘mass
concrete’ applications (in dams, etc.).

• In the analysis the dimensions of the
beam are to be calculated
• True
• false

• In cantilever beam the steel is provided
in the top profile of the beam
• True
• false

• The aim of the design is to achieve only
strength .
• True
• false

• Why only STEEL is used in R C C?

• Why only STEEL is used in R C C?
• 1. very good bond between concrete and steel
• 2. The coefficient of expansion both materials
nearly same
• 3. Cheaply available
• 4. Ductility
• Mild steel: Fe250. HYSD bars: Fe415, Fe500.

Characteristic strength of reinforcing steel =
yield strength of steel– for those with well-defined yield point
(Fe250)
0.20% Proof Stress – for those without well-defined yield point
(Fe415&Fe500)

Classification
Ordinary concrete – M10 to M20 [for R C C min M20]
Standard concrete – M25 to M55
High Strength concrete – M60 and above; Ec = 5000√fck

Stress-Strain curve of CONCRETE
• Maximum stress is attained by concrete at an
approximate strain of 0.002
• The strain at failure is in the range 0.003 to 0.005. The
max strain in concrete is taken as 0.0035
• For higher grades of concrete, the initial portion of the
stress-strain curve is steeper, but the failure strain is
low. For low strength concrete, the initial slope of curve
is gentle but has high failure strain. (observe the above
figure)
•

DESIGN CODES AND HANDBOOKS
• National building codes have been formulated in
different countries to provide guidelines for the
design and construction of structures.
• The codes have evolved from the collective wisdom
of expert structural engineers, gained over the years.
• These codes are periodically revised to bring them
in line with current research, and often, current
trends.

• The codes serve at least four distinct functions.
• Firstly, they ensure adequate structural safety, by
specifying certain essential minimum requirements for
design.
• Secondly, they render the task of the designer relatively
simple; by providing simple formula or charts for the
tedious calulations.
• Thirdly, the codes ensure a measure of consistency among
different designers.
• Finally, they have legal validity, in that they protect the
structural designer from any liability due to structural
failures that are caused by inadequate supervision and/or
faulty material and construction.

• IS 456 : 2000 — Plain and reinforced concrete – Code of
practice (fourth revision)
• IS 875 (Parts 1-5) : 1987 — Code of practice for design loads
– Part 1 : Dead loads(CC:24kN/m3; RCC: 25kN/m3; Brick: 19kN/m3)
– Part 2 : Imposed (live) loads (on slabs:Residential: 2kN/m2; Official: 3-5kN/m2)
– Part 3 : Wind loads
– Part 4 : Snow loads
– Part 5 : Special loads and load combinations
• IS 1893 : 2002 — Criteria for earthquake resistant design of
structures
• SP 16 : 1980 — Design Aids (for Reinforced Concrete) to IS 456 : 1978
• SP 24 : 1983 — Explanatory Handbook on IS 456 : 1978
• SP 34 : 1987 — Handbook on Concrete Reinforcement and Detailing
• SP 23 : 1982 — Design of Concrete Mixes
•

• The stain in concrete is constant irrespective
of Grade of Concrete
• True
• False

• The young’s modulus of concrete is constant
irrespective of Grade of Concrete
• True
• False

• The code books are legal documents
• True
• False

Design Philosophies
• A ‘design philosophy’ is built up on a few
fundamental assumptions in its method to be
adopted
• 1. Working Stress Method(WSM)
• 2. Ultimate Load Method.
• 3. Limit State Method(LSM)

Design Philosophies(WSM)
• 1.This was the traditional method of design,
basically assumes that the structural material
behaves in a linear elastic manner.
• 2. Adequate safety can be ensured by suitably
restricting the stresses known as PERMISSIBLE
Stresses in the material by introducing a factor of
safety
• 3. The ratio of the yield strength of the material
to the permissible stress is often referred to as
the factor of safety. For CC-3; steel-2

Design Philosophies (WSM)
• Modular ratio concept is introduced to achive
strain compatibility
• Drawbacks of WSM:
• It is uneconomical, as it is not utilising the full
strength of the material.
• It unable to explain the long term effects like
creep and shrinkage.
• It unable to address the serviceability
requirements

Design Philosophies(ULM)
• 1.In this method, the non-linear stress-strain
curves of concrete and steel are made use of.
• 2. The concept of ‘modular ratio’ and its
associated problems are avoided entirely in this
method.
• 3. The safety measure in the design is introduced
by an appropriate choice of the load factor,
defined as the ratio of the ultimate load (design
load) to the working load.
• 4. The full strength of the material is utilised.

(ULM)
• This method generally results in more slender
sections, of beams and columns (compared to
WSM),
• Unable to give satisfactory ‘serviceability’
performance at the normal service loads. The
designs sometimes result in excessive deflections
and crack-widths under service loads, owing to
the slender sections resulting from the use of
high strength reinforcing steel and concrete.

Design Philosophies(LSM)
• LIMIT STATE METHOD:
• The acceptable limit for safety and serviceability
requirements before failure occurs is known as
Limit State.
• SAFETY: With due consideration to strength,
stability & structural integrity.
• Collapse may occur due to:
– If the load exceeds the strength of the materials.
– Sliding
– Overturning
– Buckling
– Fatigue
– Fracture

• Limit states involved in collapse are called “Limit State
of Collapse” or “Ultimate Limit State”, which are
defined for the following,
– Flexure
– Compression
– Shear
– Torsion
• SERVICEABILITY: Satisfactory performance of
structure in its life time under service loads. Ensures
no discomfort to the user

– User discomfort may occur due to:
• Deflection
• Cracking
• Vibrations
• Durability
• Impermeability
• Limit states involved in user comfort are called
“Limit state of serviceability”,

• IMPORTANT TERMINOLOGIES:
• Characteristic Strength:
• It is the strength of the material below which not
more than 5% of the test results are expected to fall.
• Characteristic strength of concrete = 28-day characteristic
compressive strength of 150mm concrete cubes (fck). For the
design of structures, only 67% of fck is considered.
Characteristic strength of reinforcing steel = yield strength of
steel (fy) (mild steel) or 0.20% Proof Stress (HYSD).
•

• Characteristic Loads:
• Loads which have 95% probability of not being
exceeded during the life of the structure
• IS 875 (Parts 1-5) : 1987 — Code of practice for design loads
– Part 1 : Dead loads(CC:24kN/m3; RCC: 25kN/m3; Brick:
19kN/m3)
– Part 2 : Imposed (live) loads (on slabs:Residential: 2kN/m2;
Official: 3-5kN/m2)
– Part 3 : Wind loads
– Part 4 : Snow loads
– Part 5 : Special loads and load combinations

• Design strength:
• Design strength = characteristic strength of
material/ Partial safety factor for material
• Partial safety factor for material:
• = 1.5 (concrete)
• = 1.15(steel)
• Characteristic strength of material
• Concrete= 0.67 fck
• Steel = fy

Design strength of concrete = 0.67 fck/1.5 =0.447=0.45
• Design strength of steel = fy/1.15 = 0.87 fy
• Design Loads:
• Design Load = Characteristic Load x Partial safety factor for
corresponding load
• Partial safety factor for different loading conditions is given in Table 18 of
IS456 (Page 68)
• For structures subjected to Dead loads & Live loads Design load on the
structure = 1.5 DL + 1.5 LL
•
For structures subjected to Dead loads, Live loads & Wind loads Design
load on the structure = 1.2 DL + 1.2 LL + 1.2 WL
•

LIMIT STATE OF COLLAPSE – FLEXURE

• Assumptions:
• Plane sections normal to the beam axis
remain plane even after bending, i.e., in an initially
straight beam, normal strain varies linearly over the depth of
the section.
• The maximum compressive strain in concrete
(at the outermost fibre) εcu = 0.0035.
• The tensile strength of the concrete is ignored.

• The stress block of concrete in compression may be assumed
to be rectangle, trapezium, parabola or any other shape which
results in prediction of strength in substantial agreement with
test results.
• Stress – strain curve for steel bar with definite
yield point and for cold worked deformed bars
is shown in Fig 23B and Fig 23A respectively.
• To ensure ductility, maximum strain in tension
reinforcement shall not be less than
(fy/1.15Es)+ 0.002.

• Balanced section:
• Both concrete & steel in the given section
reaches ultimate stress simultaneously.
• Yielding of steel and crushing of concrete
occurs simultaneously.
TYPES OF R.C.C. SECTIONS

– Therefore, strain in
• Concrete (at the extreme compression
fibre) = εcu = 0.0035
• Steel (on tension side) = εst = εy = 0.87fy/ Es + 0.002
• Depth of Neutral axis (where the strain is zero)
for a balanced section is designated as Xu,max.

• Under-reinforced section:
• Steel reaches ultimate stress before concrete.
• Here, Yielding of steel occurs first. This is
accompanied by wider tensile cracks and
increased curvatures and deflection of the beam.
Thus it gives clear warning to the user about
failure. Finally, failure occurs by crushing of
concrete.
• In other words, the steel would already have
yielded by the time the concrete crushes

–Concrete (at the extreme compression
fibre) = εcu = 0.0035
• Steel (on tension side) εst > 0.87fy/ Es +0.002
– Depth of Neutral axis (Xu) for an under-reinforced section is less than
Xu,max
– This type of failure is called “Ductile Failure”.
•
•

• Over-reinforced section:
• Concrete reaches ultimate stress first.
• Here, crushing of concrete occurs first. It gives
no warning to the user about failure. Hence,
it is a catastrophic or disastrous failure
• In other words, when the concrete at extreme
compression fibre crushes, the steel would
not have reached the yield point.

• Concrete (at the extreme compression fibre) = εcu =
0.0035
• Steel (on tension side) εst < 0.87fy/ Es +0.002
– Depth of Neutral axis (Xu) for an over-reinforced
section is greater than Xu,max
– This type of failure is called “Brittle Failure”.

Stress-Strain Block
• The general expression for Depth of Neutral
axis (Xu) is obtained from strain diagram,
considering similar triangles
• The limiting value of depth of Neutral axis
(Xu,max)

Stress-Strain Block
• The limiting value of depth of Neutral axis
(Xu,max)

MOMENT OF RESISTANCE
• What is
MOMENT OF RESISTANCE?

• It is the RESISTANCE offered by the beam
against BENDING MOMENT
• It is calculated as the COUPLE generated by
the COMPRESSION and TENSION
• It is denoted as Mu
• For balanced section ,Mu limit

DIMENSIONS OF THE BEAM
b = width of beam
D = overall depth of beam
d’ = effective cover to the tension
steel d = effective depth of beam
(= D – d’)
xu = depth of neutral axis
(measured from the extreme
compression fibre)
Ast = Area of steel on the tension
side of cross- section

• Find the limiting moment of resistance of
singly reinforce section 200×450mm. Use M25
concrete, Fe415 steel, effective cover to
reinforcement is 40mm. Also find the limiting
percentage of steel.
• b = 200mm; d = 450-40 = 410mm;
• fck = 25N/mm2 ; fy = 415 N/mm2
• Xu max/d = 0.48 for Fe415 steel.
• Use Mulimit formula in page96

• Find the moment of resistance of singly
reinforced concrete beam 200mm wide and
400mm effective depth reinforce with 3-
16mm dia of Fe415 steel. Use M20 concrete.
• b = 200mm; d = 400mm;
• Ast = 3x Π x 162/4 = 603mm2

• Find the moment of resistance of singly
reinforced concrete beam 200mm wide and
400mm effective depth reinforce with 4-
16mm dia of Fe415 steel. Use M20 concrete
• b = 200mm; d = 400mm
• Ast = 4x Π x 162/4 = 804mm2

Doubly reinforced sections
Doubly reinforced beams are resorted to under
following circumstances:
• Where cross sectional dimensions of the beam
are restricted, due to architectural or other
considerations
• Where singly reinforced beam is inadequate
to resist moment
• Where reversal stresses are likely to occur

• Compressive force in Concrete Cc = 0.36 fck b xu
• Compressive force in steel Cs = fsc Asc - 0.45 fck Asc
• Tensile force in steel T = 0.87 fy Ast
• Value of fsc is given below (in N/mm2)
•

• Depth of neutral axis Xu is obtained from Cc + Cs = T,
• Ultimate moment of resistance of this section is:
• Mu = Cc (d - 0.42 Xu) + Cs (d – d’)
•

• A doubly reinforced beam 230 x 500mm
overall depth is reinforced withn4-16mm dia
in compression and 6-20mm dia in tension.
Effective cover is 40mm. Find the safe UDL
that beam can carry if the beam is simply
supported over a span of 6m. Use M25
concrete and Fe500 Steel

• As per IS 456 (Cl. 23.1.2),
• bf = l0/6 + bw + 6Df (for T-beams)
• bf = l0/12 + bw + 3Df (for L-beams)
• lo = effective distance between points of zero moments in
beam
• = 0.7 x effective span (for continuous beams)
• bw = breadth of web
• Df = overall depth of flange

Basic R C C structurs

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