INTRODUCTION TO REINFORCED CONCRETE DESIGN.ppt

1
Reinforced Concrete Structure-I
College of Architecture and Civil Engineering
Department of Civil Engineering
Lecture-1
Instructor: Alemu F.(Structural Engineering)

Concrete
• Concrete is a construction material composed of cement,
fine aggregates (sand), coarse aggregates(gravel), water
and sometimes Admixtures.
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Concrete
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• When the ingredients (concrete making materials) are
mixed, they form a plastic mass which can be poured into
suitable molds (forms) and then hardens to a solid mass.

• The hardening of concrete is due to chemical reaction
between cement and water (Hydration).
• The properties as well as the proportions of concrete
ingredients have a major influence on the fresh and
hardened concrete
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Concrete

Properties of Concrete
Strength properties of concrete depends on:
 Proportions of constituent materials
 Degree of compaction
 Temperature and moisture during placing and
curing
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Compressive strength of Concrete
• Compressive strength of concrete is one of the most
important and useful property.
• As a construction material, concrete is employed to
resist compressive stresses.
• In reinforced concrete, concrete takes up most of the
compressive forces of the structure, while ‘reinforcing
bars (rebars) take up most of the tensile forces.
• Therefore, it is of utter importance to know exactly
what the compressive strength of concrete is.
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•the compressive strength of concrete is obtained from
standard test specimens , at the age of 28days;
 150mm cube or
 150mm diameter and 300mm height cylinder.
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• Concrete compressive strength Testing Machine
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Compressive Strength of Concrete at Various Ages
• The strength of concrete increases with age.
• The strength of concrete at different ages in comparison
with the strength at 28 days after casting is shown below.
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• The friction between the loading plate and the contact
faces of the test specimen has more effect on cube
strength than the cylinder strength.
• Because of this, the cube strength gives more strength
than the true compressive strength of concrete,
whereas, cylinder strength gives reasonably the true
compressive strength.
• On average, cube strength is taken as 1.25 times
cylinder strength.
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Stress-Strain Diagram for Concrete
•is a graphical representation of concrete behavior under
load.
•It is produced by plotting concrete strain at various interval
of concrete loading (stress).
•Since concrete is mostly used in compression, its
compressive stress strain curve is of major interest.
•It is obtained by testing concrete specimen (cube or
cylinder) at age of 28days, using compressive test machine.
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Modulus of Elasticity of Concrete
• It can be defined as the ratio of the applied stress to the
corresponding strain within the elastic range.
• It indicates a material's resistance to being deformed
(material’s stiffness) when a stress is applied to it.
• It is the slope of the stress-strain curve of concrete.
• It can be Initial, Tangent, or Secant modulus.
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Modulus of Elasticity of Concrete
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Tensile strength of concrete
• concrete is weak in tension and strong in compression,
The tensile strength being only 10-15% of its
compressive strength.
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• Even though concrete is weak in tension, its tensile
strength is important in a variety of items.
• Shear and torsion resistance of RC members primarily
depend on tensile strength of concrete.
• Further, the conditions under which cracks form and
propagate on tension zone of RC flexural members depend
strongly on the tensile strength of concrete.
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•The direct determination of tensile strength of concrete is
difficult due to handling problem.
•So, the tensile strength of concrete is obtained indirectly
from tests such as;
 Beam-Test (Flexural Strength-Test) or
 Split-Cylinder Test.
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Beam-test method
• Tensile strength is obtained by loading plain concrete
beam having a size of 10*10*50cm or 15*15*70cm,
laterally by two point loads at the third points of test-
beam until the tension zone of the beam fracture.
• Tensile strength of concrete is then computed using
flexural stress formula:
Where fr = flexural tensile strength/modulus of rupture
M=moment
C = h/2, I = bh3
/12
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I
c
M
fr
.


Split-cylinder test method
• Tensile strength of concrete is obtained by loading
standard plain concrete cylinder (150mm diameter and
300mm length) along the length until the cylinder splits
in to two pieces.
• The tensile strength of concrete is then computed by the
formula;
Where; P = maximum applied load on the test
l = length of specimen
d = diameter of specimen
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l
d
P
.
.
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

Shrinkage and Thermal mov’t
• Shrinkage is the reduction in volume of fresh or
hardened concrete without application of any load.
• Shrinkage can be caused by;

Absorption of water by cement and
aggregate. (Initial shrinkage)

Evaporation of the water that rises to the
surface due to capillary action. (Drying
shrinkage)

Change in temperature (Thermal shrinkage)
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• As the drying of concrete continues with time,
shrinkage will also increase with time at a
decreasing rate.
• Restrained shrinkage is liable to cause tensile
stresses which in turn cause cracking of concrete.
• Tensile stresses caused by restrained shrinkage can
be controlled by;
 Casting concrete using a system of
constructing successive bays.
 Correct positioning of expansion joints.
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• When tensile stresses caused by restrained
shrinkage exceeds tensile strength of concrete,
cracks will occur.
• To control this cracks, steel reinforcement is
provided close to the concrete surface.
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Creep of concrete
• Creep is increased deformation of a member under
sustained compressive load over considerable length
of time.
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Creep of concrete
• It is associated with brittle materials.
• It is most important in beams and slabs because
increased deformation leads to opening of cracks
which in turn results in damage to finishes and
equipment's.
• Creep has a little effect on strength of concrete.
• To reduce creep deformations, nominal
reinforcement is provided in compression zone of
structural members.
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Reinforcing Steel
• rebars are used to improve the tensile strength of the
concrete, since concrete is very weak in tension.
• are available in the form of round bars and welded
wire fabric.
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Reinforcing Steel
• The most commonly used bars have projected ribs on
their surface. Such bars are called deformed bars.
• The bars are ribbed in order to improve the bond
between steel and the surrounding concrete.
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Reinforcing Steel
• A wide range of reinforcing bars are available
with nominal diameter ranging from 6mm to
50mm.
• Most bars except 6mm diameter are deformed
one.
• Some of the common bar size with their
application in concrete works are given in table
below.
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Strength of reinforcing steel
• Reinforcing steel is capable of resisting both tension
and compression.
• Compared with concrete, it is a high strength
material.
• For instance, the strength of ordinary reinforcing
steel is about 10 & 100 times, the compressive &
tensile strength of common structural concrete.
• Mostly made up of mild steel and high yield steel.
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Strength of reinforcing steel
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Reinforced Concrete
(as a composite material)
• RC is a construction material that is formed by a logical
combination of plain concrete and reinforcement steel.
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Reinforced Concrete
• Therefore, the two materials are more or less
complementary. i.e
• when the steel is embedded in tension zone, it can
provide tensile and shear strength. While concrete
which is strong in compression protect the steel from
fire and corrosion.
• Hence, by doing so the structure will be durable and
resist both tension and compression loads.
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Reinforced Concrete
• When plain concrete & reinforcing steel bar
together assumed to act as one composite Unit,
it is termed as Reinforced concrete (RC).
• In all RC members, strength design is made on
the assumption that concrete does not resist any
tensile stresses.
• All the tensile stresses are assumed to be resisted
by the reinforcing steel imbedded in tension
zone.
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Reinforced Concrete
• The tensile stresses developed in the section are
transferred to reinforcing steel by the bond
between the interfaces of the two materials.
• Some times if necessary, reinforcing steel is
provided in compression zone to assist the
concrete resisting compression in addition to
reducing creep deformation.
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Advantages of Reinforced Concrete
• It is monolithic. This gives it more rigidity.
• While it is plastic, it can be moldable into any
desired shape
• It is fire, weather and corrosion resistant.
• It is durable. It does not deteriorate with time.
• Its maintenance cost is practically nil.
• By proper proportioning of mix, concrete can be
made water-tight.
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Disadvantages of Reinforced Concrete:
• It is difficult to demolish in case of repair or
modification.
• It is too difficult to inspect after the concrete has
been poured.
• It requires formwork.
• Large section not efficiently used due to cracking.
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Design Methods (Philosophies)
 Design problems consists of the following steps:
1) Idealization of structure for analysis
2) Estimation of loadings.
3) Analysis of idealized structural model to determine stress-
resultants and their effects (deformations).
4) Design of structural elements
5) Detailed structural drawings and schedule of reinforcing bars.
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Design Methods (Philosophies)
• Design of RC member is;
 Fixing the x-sectional size of the member and
 Determining the amount (area) of steel required
in the member.
• The design of any member should be safe and
economical.
• The design methods are based on determination of
design load and design material strength.
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Design Philosophies (Methods)
• To achieve safe and economical structures, three
philosophies of design had been adopted by
codes of practices. These are:
1.Working Stress Design (WSD) or Elastic
Design Method
2.Ultimate Strength Design (USD) Method,
and
3.Limit State Design (LSD) Method.
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Working Stress Design (WSD) method
 WSD is the oldest and simplest method of
design used for reinforced concrete structures.
 It is based on the assumption that concrete is
elastic, steel & concrete together act elastically.
 The design stresses of materials also known as
allowable/permissible/elastic stresses are
determined by dividing material strengths by a
factor of safety.
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Working Stress Design (WSD) method
 the stresses developed in concrete & steel are
not exceeded the respective allowable stresses
 The safety factors specified by codes are
assumed to cover all uncertainties existing in
estimations of service loads and material
strengths.
 The distribution of strain and stress over x-
section is assumed to be linear/straight.
 WSD is also known as
allowable/permissible/elastic stress design
method. 44

Drawbacks of WSD method
 Concrete is not elastic material rather it is semi-
plastic material having nonlinear strain-stress
distribution.
 factor of safety is applied on the strength of
materials, there is no way to account for different
degrees of uncertainty associated with different
types of loadings.
 difficult to account for creep and shrinkage by
computations of elastic stresses.
 It is conservative (uneconomical section
designed). 45

Ultimate Strength Design (USD) method
 Also known as load-factor method.
 Design loads are obtained by multiplying
service/working loads by an appropriate factor of
safety.
 Strength of materials are not factored. i.e., the
ultimate material strengths are used as design
strength.
 It is based on ultimate load theory, i.e., collapse loads
are resisted by nonlinear stress block.
 Less reinforcement required compared to WSD.
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Drawbacks of USD method
 Uncertainties associated with material
strengths are not accounted.
 There is no way to control excessive
deflections.
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Limit State Design (LSD) method
 Limit state design method has developed
from ultimate strength design method in
order to apply in service load and ultimate
load conditions.
 Design of structure in limit state is made to
achieve an acceptable probability that
structure or part of it will not become unfit
for use for which it is intended during
expected life.
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 The design of structures must ensure that;

Under the worst loading (ultimate
load), the structure should be safe. and

Under normal working conditions,
deformations should not be excessive.
 Working loads are multiplied by partial safety
factors and material strengths are divided by
further partial safety factors.
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 Overcomes many of the disadvantages of
WSD and USD methods.
 There are two principal or major types of limit
states;
 Ultimate strength limit states
 Serviceability limit states
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Ultimate limit states (ULS)
 Deals with strength and stability of structures
under ultimate/collapse loads.
 Includes ULS for flexure, shear,
compression, torsion, tension, overturning
and sliding.
 Ultimate load theory is applied.
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Serviceability limit states (SLS)
 Deals with conditions such as deflection,
cracking under working loads, durability,
excessive vibration, fire resistance, fatigue etc.
 At SLS, functional use of structure is disrupted
but collapse will not occur.
 Elastic stress theory is applied.
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Loads on structures
 Loads that act on structures can be divided
into three categories:
 Dead loads
 Live loads
 Environmental loads
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Dead Loads
 are constant in magnitude and fixed in location
throughout the lifetime of the structure.
 major part of the dead load is the weight of the structure
itself.
 Calculated from dimensions of the structure, and
density of the material.
 For buildings the dead loads include;
 Weight of structural components
 Weight of non - structural components
 Weight of finishing materials
 Weight of permanently fixed objects
 Weight of sanitary and lightning equipment 54

Live Loads
 Have temporary action.
 Mainly of occupancy loads in buildings.
 Variable in both magnitude and location.
 Have higher uncertainties compared to dead
loads.
 For design of floors and roofs of buildings,
values of live loads are specified in design
codes.
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Environmental Loads
Consist mainly of ;
 Snow loads.
 Wind pressure and suction.
 Earthquake loads.
 Soil pressures on subsurface portion.
 Forces caused by temperature changes.
 Loads from ponding of rainwater on flat
surfaces.
Like live loads, at any given time they are
uncertain both in magnitude and distribution.
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Provision of Building Design Codes
 Code is a written document about the best practice by
engineers and experienced researchers.
 Current code of practice (Design code) in Ethiopia is
designated by ES EN.
 For reinforced concrete design, the codes of practice
being used are :
 ES EN 1990: 2015, Basis of Structural Design
 ES EN 1991: 2015, Actions on structures
 ES EN 1992: 2015, Design of concrete structures
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Characteristic Strength of
Materials
 The characteristic strength( fk ) is defined as the value of
the cube strength( fck,cube ) or the cylinder strength( fck ) of
concrete , the yield or proof stress of reinforcement(fyk),
below which 5% of all possible test results would be
expected to fall.
 Simply, it is a test result value that has 95% of probability
of exceedance or 5% of probability of non exceedance.
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Characteristic Strength of
Materials
 It is given by; fk = fm – 1.64s
where: fm - mean strength of actual test results
S - the standard deviation
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Characteristic Loads
 The characteristic load would be that value of loading such that
not more than 5% of the spectrum of loading throughout the
life of structure will lie above it.
 Simply, it is value of a load that has only 5% of probability of
exceedance.
 It is given by; Lk = Lm +1.64s
where: Lm – the mean Load
S - the standard deviation
 It is defined ideally in statistical terms.so it is not possible
to determine statistically in the absence of sufficient load
data.
 Therefore, nominal values given by design codes can be
taken as characteristics values. 60

Grades of Concrete
 Compressive strength of concrete is denoted by concrete strength
class which relate to the characteristic cylinder strength(fck) or cube
strength (fck,cube). The strength class in ES EN are based on the
characteristic cylinder or cube strength determined at 28 days.
 Example: Concrete class C25/30 means characteristic cylinder
compressive strength of 25MPa and characteristic cube compressive
strength of 30 MPa. The characteristic strength and corresponding
properties are listed in table below of ES EN 1992:2015.
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 The relation between σc and εc for short term uniaxial
loading is described by the following diagram.
where;
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εc1 - the strain at peak stress
εcu1 the nominal ultimate
strain

Stress-Strain Diagram for the
design of x-section
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 Parabola-rectangle diagram for concrete under compression.

Stress-Strain Diagram for the design
of x-section (Simplified)
 Bi-linear stress-strain relation
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Distribution of Strain and Stress
over x-section
 Equivalent Rectangular stress distribution
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Stress-Strain Diagram for Rebars
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Stress-Strain Diagram for Rebars
 The value of k= (ft/fy)k is given in ES EN as below
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Design Compressive and Design
Tensile Strengths
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Partial safety factor for materials
 These are factors used for allowance of
possible variations in material strengths.
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Partial safety factor for Actions
 Used to provide for errors/inaccuracies due to:
 Design assumptions and inaccuracy of
calculation.
 Possible unusual load increment .
 Unforeseen stress redistributions.
 Constructional errors.
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Partial safety factor for Actions
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Design Working Life (Design Period)
 Assumed period for which a structure or part of it is to be
used for its intended purpose with anticipated maintenance
but without major repair being necessary.
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INTRODUCTION TO REINFORCED CONCRETE DESIGN.ppt

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