Rcc Details

8/10/2019 Rcc Details

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Reinforced Cement Concrete (RCC)


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Introductory Lectures - 1

Design of Structures

- determine forces acting on the structure

using structural analysis- proportion different elements economically,

stability, safety, serviceability functionality

Structural concrete is commonly used fordifferent civil engineering structures


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Introductory Lectures

Structural concreteconcrete and steel

Complimentary properties

- Concreteresists compression- Steel resists tension in most cases

Structural concreteplain, reinforced,

prestressed.


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Code of Practice

Designers guided by guidelines andspecifications called Code of Practice

Codes specified by different organizations to

ensure public safety Codes specify design loads, allowable stresses,

materials , construction types and other

details US American Concrete Institute Code 318-

ACI 318 or ACI code


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Code of PracticeACI code

Unified Design Method (UDM) is based on

strength of structural members assuming

failure conditioncrushing strength of

concrete or yield of reinforcing bars

Actual loads or working loads are multiplied

by load factors to obtained factored design

loads


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Limit State design

3 limit states have to be analyzed in this

method

- Load carrying capacity(safety, stability

- deformation ( deflections, vibrations)

- crack formation


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Units

SI system(System International)

W= m g = 1 kg. x 9.81 m/s2

= 9.81 N

1 kN = 1000 N

1 m. = 100 cm.


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Loads

Members design to resist loads

Two types of loads

- Dead Loadsweight of structure and otherelements placed on ittiles, roofing , walls

- Live Loadssteady / unsteady , slowly or

/rapidly, laterally or vertically - weight ofpeople, furniture, wind, temperature,

earthquake etc.


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Loads to be specified

ACI does not specify loads

American National Standards Institute

specifies loads

AASHTO specifies highway and railway loading

for bridges and highways


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Safety Provisions

Structural Members designed for higher loadsthan actual to have margin / safety against failure

Multiply actual loads by load factors to get

factored loads Load factors depend on how accurately the loads

can be estimated eg. Dead loads lower loadfactors compared to Live loads.

Several load combinations have to be alsoconsidered to design the structure for differentload combinations


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Strength Reduction Factor

ACI provides for

strength reduction factor to reduce the

strength to account for degree of accuracy to

which strength is estimated, variations on

materials and dimensions and other factors


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Part -2 Basics of Concrete and

Steel


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Basics of Cement Concrete

Cement Concrete made up of

- Coarse Aggregate

- Fine Aggregate

- Cement

- Water

- Admixture

Water- Cement mix produces a paste filling voidsof aggregates producing a uniform denseconcrete


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Concrete Casting

Plastic Concrete placed in a mold

Cured

Left to set, harden and gain strength with time

Strength of concrete depends on many factors

a. Water Cement ratio

b. Properties and proportions of constituents

c. Method of mixing and curing

d. Age of Concrete

e. Loading Conditions

f. Shape and Dimensions of tested specimens


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Concrete Mix

Proper proportioning of different componentsand well graded sound aggregates give strengthto concrete

Admixtures give concrete desired strength andquality

Concrete is subsequently poured using mixers,vibrated to get a dense mix at site and then curedto get concrete of desired strength and properties

Concrete strength increases with age about 70% in 7 days and 85-90 % in 14 days (28 daysstrength is a benchmark of design)


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Concrete Strength

Concrete strength is measured by testing cubes

(6)or cylinders (6x 12)

Performance of RCC depends on relative

strength of concrete and steel

Stress-strain behavior of both materials is

important.

Stress-strain behavior is assessed using 6x12

cylinders


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Initial straight lineportion

Max. stress at

about 0.002 strain

Rupture at about

0.003 strain

Concrete strength3000-6000 psi (21-

42 N/mm.2)

High strength

concrete recently

used


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Concrete Strength and Modulus

Tensile strength of concrete is low compared tocompressive strength

Flexural strength is 10-20 % of compressivestrength

ModularRatio , Es / Ecplays an important rolein design of RCC elements

The ACI Code allows the use of

Ec= 57,000 fc (psi) = 4700 fc Mpa

Poissonsratio:- transverse to longitudinal strain0.15 to 0.20 average 0.18


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Stress Strain Curves for Steel


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Modulus of Elasticity of Reinforcement

Modulus of elasticity is constant for all types

of steel

The ACI Code has adopted a value of Es= 29 X

106psi (2.0 x 105MPa)


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Part -3

Flexure Analysis of RCC Beams


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Flexure Analysis of RCC Beams

The analysis and design of a structural member may beregarded as the process of selecting the proper materials

and determining the member dimensions such that the

design strength is equal or greater than the required

strength

The required strength is determined by multiplying the

actual applied loads, the dead load, the assumed live load,

and other loads, such as wind, seismic, earth pressure, fluid

pressure, snow, and rain loads, by load factors

These loads develop internal forces / stresses such asbending moments, shear, torsion, or axial forces depending

on how these loads are applied to the structure


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Flexure Analysis

In proportioning reinforced concrete structuralmembers, three main items can be investigated:

1. The safety of the structure, which is maintained by

providing adequate internal design strength

2. Deflection of the structural member under service

loads. The maximum value of deflection must be

limited and is usually specified as a factor of the

span, to preserve the appearance of the structure


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Flexure Analysis

3. Control of cracking conditions under service loads.

Visible cracks spoil the appearance of the structure andalso permit humidity to penetrate the concrete,causing corrosion of steel and consequently weakening

the reinforced concrete member.

The ACI Code implicitly limits crack widths to 0.016 in.(0.40mm) for interior members and 0.013 in. (0.33mm) for exterior members.

Control of cracking is achieved by adopting and limitingthe spacing of the tension bars


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Assumptions

RCC sections are non-homogenous since thesection is made up of two materialsconcreteand steel

Proportioning ( determining sizes and areasof each component) by ultimate strength isbased on assumptions

These assumptions make the design simpler-but their validity needs to be checked andkept in mind


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Assumptions

1. Strain in Concrete is the same as that inreinforcing steel at that level this willhappen provided the bond is adequate

2. Strain in concrete is proportional to thedistance from the neutral axis

3. The modulus of elasticity of all grades of steelis taken as E, = 29 x106lb./in2( 200,000 MPaor N/mm.2 )The stress in the elastic range isequal to the strain multiplied by Es
http://en.wikipedia.org/wiki/File:Beam_bending.png


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The neutral axis is an axis in the cross section of a beam or shaft along which

there are no longitudinal stresses or strains. If the section is symmetric, isotropic

and is not curved before a bend occurs, then the neutral axis is at the geometric

centroid. All fibers on one side of the neutral axis are in a state of tension, while

those on the opposite side are in compression
http://en.wikipedia.org/wiki/File:Beam_bending.pnghttp://en.wikipedia.org/wiki/File:Beam_bending.png


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Assumptions

4. Plane cross-sections continue to be plane afterbending.

5. Tensile strength of concrete is neglected because

a. concrete's tensile strength is only about 10% of its

compressive strength,

b. cracked concrete is assumed to be not effective,

and

c. before cracking, the entire concrete section iseffective in resisting the external moment.


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Assumptions

6. At failure the maximum strain at the extremecompression fibres is assumed equal to 0.003- ACI Code provision

7. For design strength, the shape of thecompressive concrete stress distribution maybe assumed to be rectangular, parabolic, ortrapezoidal. In this course, a rectangular

shape will be assumed (ACI Code, Section10.2)


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Behavior of Simply Supported RCC

Beam Loaded to Failure

Concrete being weakest in tension, a concrete beamunder an assumed working load will definitely crack atthe tension side, and the beam will collapse if tensilereinforcement is not provided

Concrete cracks occur at a loading stage when itsmaximum tensile stress reaches the modulus ofrupture of concrete

Therefore, steel bars are used to increase the moment

resisting capacity of the beam; the steel bars resist thetensile force, and the concrete resists the compressiveforce


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Behavior of a RC beam to failure

To study the behaviour of a reinforced concrete beamunder increasing load, let us examine how two beams weretested to failure. Details of the beams are shown in Fig.

Both beams had a section of 4.5 in. by 8 in. (110 mm. by200 mm), reinforced only on the tension side by two no. 5bars. They were made of the same concrete mix. Beam 1had no stirrups, whereas beam 2 was provided withreinforcement no. 3, stirrups, spaced at 3 in

The loading system and testing procedure were the samefor both beams. To determine the compressive strength ofthe concrete and its modulus of elasticity, Ec, a standardconcrete cylinder was tested, and strain was measured atdifferent load increments


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Behavior of RC beam to Failure

Stage 1

At zero external load, each beam carried its own weight in addition to thatof the loading system, which consisted of an I-beam and some plates.

Both beams behaved similarly at this stage

At any section, the entire concrete section, in addition to the steelreinforcement, resisted the bending moment and shearing forces.

Maximum stress occurred at the section of maximum bending moment-thatis, at midspan. Maximum tension stress at the bottom fibers was much lessthan the modulus of rupture of concrete.

Compressive stress at the top fibers was much less than the ultimateconcrete compressive stress, fc. No cracks were observed at this stage.


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Behavior of RC beam to failure Stage 3 contd. Load increase beyond P1

In general, the development of cracks and the spacing andmaximum width of cracks depend on many factors, such asthe level of stress in the steel bars, distribution of steel barsin the section, concrete cover, and grade of steel used.

At this stage, the deflection of the beams increased clearly,because the moment of inertia of the cracked section was

less than that of the uncracked section. Cracks started about the midspan of the beam, but other

parts along the length of the beam did not crack. When loadwas again increased, new cracks developed, extendingtoward the supports.

The spacing of these cracks depends on the concrete coverand the level of steel stress. The width of cracks alsoincreased.

One or two of the central cracks were most affected by theload, and their crack widths increased appreciably, whereas

the other crack widths increased much less.


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Behavior of RC beam to failure

Stage 3 contd.

At high compressive stresses, the strain of the concrete

increased rapidly, and the stress of concrete at any strain

level was estimated from a stress-strain graph obtained

by testing a standard cylinder to failure for the same

concrete.

As for the steel, the stresses were still below the yield

stress, and the stress at any level of strain was obtainedby multiplying the strain of steel, by Es, the modulus of

elasticity of steel.


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Types of failures and Strain Limits

Three types of flexural failure of a structural

member can be expected / designed depending

on the percentage of steel used in the section

1. Tension Controlled section (also called

Under-Reinforced)

2. BalancedSection

3. Compression controlled (also called Over

Reinforced)


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Tension Controlled Section

Steel may reach its yield strength before theconcrete reaches its maximum strength, Fig.3.3 a.

In this case, the failure is due to the yielding ofsteel reaching a high strain equal to or greaterthan 0.005.

The section contains a relatively small amountof steel and is called a tension-controlledsection.


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Stress Strain Curves for Steel


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Fig. 3.3 a


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Balanced Section

Steel may reach its yield strength at the same

time as concrete reaches its ultimate strength,

Fig. 3.3b. The section is called a balanced

section.


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Fig. 3.3 b


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Compression Controlled Section

Concrete may fail before the yield of steel, Fig.3.3 c, due to the presence of a high percentage ofsteel in the section. In this case, the concretestrength and its maximum strain of 0.003 arereached, but the steel stress is less than the yieldstrength, that is, fsis less than fy.

The strain in the steel is equal to or less than

0.002. This section is called a compression-controlled

section.


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Compression Controlled Section

Fig. 3c


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Choice of Type of Section

In beams designed as tension-controlled sections,steel yields before the crushing of concrete.Cracks widen extensively, giving warning beforethe concrete crushes and the structure collapses.The ACI Code adopts this type of design.

In beams designed as balanced or compression-controlled sections, the concrete fails suddenly,

and the beam collapses immediately withoutwarning. The ACI Code does not allow this type ofdesign.

Strain Limits for Tension and Tension


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Strain Limits for Tension and Tension

Controlled Sections The design provisions for both reinforced concrete

members are based on the concept of tension orcompression-controlled sections, ACI Code, Section 10.3.

Both are defined in terms of tensile strain (TS), (, in theextreme tension steel at nominal strength)

Moreover, two other conditions may develop:

(1) the balanced strain condition and ,

(2) the transition region condition.

These four conditions are defined in the next few slides:

Strain Limits in Tension and Tension


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Strain Limits in Tension and Tension

Controlled Sections

Compression-controlled sections are those sections in whichthe Tensile strain, TS, in the extreme tension steel atnominal strength is equal to or less than the compressioncontrolled strain limit at the time when concrete incompression reaches its assumed strain limit of 0.003, (

concrete= 0.003).

For grade 60 steel, (fy= 60 ksi), the compression-controlledstrain limit may be taken as a net strain of 0.002, Fig. 3.4a.

This case occurs mainly in columns subjected to axial forcesand moments.

Strain in Tension and Tension


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Strain in Tension and Tension

Controlled Sections

Tension-controlled sections are those sections inwhich the TS, tensile , is equal to or greater than0.005 just as the concrete in the compressionreaches its assumed strain limit of 0.003, Fig. 3.4 c

Sections in which the TS in the extreme tensionsteel lies between the compression controlled

strain limit (0.002 for f , = 60 ksi) and the tension-controlled strain limit of 0.005 constitute thetransition region, Fig. 3.4b.


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Strain levels


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Balanced Section

The balanced strain condition develops in the

section when the tension steel, with the first

yield, reaches a strain corresponding to its yield

strength, fy , or s = fy/Es just as the maximum

strain in concrete at the extreme compression

fibers reaches 0.003, Fig. 3.5.


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Load Factors

Based on historical studies of various structures, experience, andthe principles of probability, the ACI Code adopts a load factor of1.2 for dead loads and 1.6 for live loads. The dead load factor issmaller, because the dead load can be computed with a greaterdegree of certainty than the live load.

The choice of factors reflects the degree of the economical designas well as the degree of safety and serviceability of the structure. Itis also based on the fact that the performance of the structureunder actual loads must be satisfactorily within specific limits.

If the required strength is denoted by U (ACI Code, Section 9.2), andthose due to wind and seismic forces are W and E, respectively,according to the ACI Code, the required strength U, shall be themost critical of the following factors (based on the ASCE 7-05)


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Load Combinations


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Strength Reduction Factor


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Strength Reduction Factors


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Analysis and Design

C i St Di t ib ti


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Compressive Stress Distribution The distribution of compressive concrete stresses at failure

may be assumed to be a rectangle, trapezoid, parabola, orany other shape that is in good agreement with test results.

When a beam is about to fail, the steel will yield first if the

section is under-reinforced, and in this case the stress insteel is equal to the yield stress.

If the section is over-reinforced, concrete crushes first and

the strain is assumed to be equal to 0.003

B l d S ti


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Balanced Section

In Fig. 3.7, if concrete fails, c, = 0.003, and if

steel yields, as in the case of a balanced

section, fs= fy


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Compressive Stress Distribution

A compressive force, C, develops in the compression zoneand a tension force, T, develops in the tension zone at thelevel of the steel bars.

The position of force T is known, because its line of

application coincides with the center of gravity of the steelbars.

The position of compressive force C is not knownunless thecompressive volume is known and its center of gravity is

located.

If that is done, the moment arm, which is the verticaldistance between C and T, will consequently be known.

B l d S ti


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Balanced Section

In Fig. 3.7, if concrete fails, c, = 0.003, and if

steel yields, as in the case of a balanced

section, fs= fy

f l


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Compressive force location

The compression force, C, is represented bythe volume of the stress block, which has the

non uniform shape of stress over the

rectangular hatched area of bc. This volume may be considered equal to

C = bc(1fc), where 1fcis an assumed

average stress of the non uniform stress

block.


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Compressive Stress Distribution BalancedS ti


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Section


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Singly Reinforced Beam Balanced


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Singly Reinforced Beam Balanced

Section

This value is equal to the area of steel, As, dividedby the effective cross-section, b* d:

b= As / b *d (For % multiply by 100)

whereb = width of the compression face of the

member

d = distance from the extreme compression

fiber to the centroid of the longitudinal

tension reinforcement

E i f A l i d D i


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Equations for Analysis and Design

Two basic equations for the analysis and design ofstructural members are the two equations ofequilibrium that are valid for any load and any section:

1. The compression force should be equal to the

tension force

2. The internal bending moment, Mn, is equal to eitherthe compressive force, C, multiplied by its arm or the

tension force, T , multiplied by the same lever arm:

Internal Moment Mn and


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Internal Moment Mn and

Bending Moment Mu

Internal Moment


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Step 1.

From the strain diagram of Fig. 3.11,

B l d S ti


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Balanced Section

B l d S ti


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Balanced Section

B l d S ti


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Balanced Section

B l d S ti St l %


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Balanced Section Steel %

Because Balanced section steel is used



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Internal Moment


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Internal Moment

Design Moment


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Design Moment


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Ratio of a to d


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Ratio of a to d

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