ce 575-lecture-1(course description and introduction)1

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LECTURE 1

Course Descript ion , Principle and Methods of

Prestressing

2016CE 407-Prestressed Concrete Structures

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PRESTRESSED CONCRETE STRUCTURES

(CE 407)

سم ا لرحن لرحيم

By

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Contents

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Objectives of the present lecture

Course Description

Text Book

Outcome Assessment

description of prestressing prestressing with concrete structures

prestressing compared to ordinary reinforced concrete

Advantages and Disadvantages of Prestressed Concrete

Methods of Prestressing: Pretensioning and Posttensioning

Material Requirements

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Objectives of the Present lecture


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To provide an overview of the course contents

To discuss the methods of Prestressing

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Course Description

Principle and Methods of Prestressing

Prestressing Material

The Philosophy of Design

Flexure: Working Stress Analysis and Design Flexure: Ultimate Strength Analysis and Design

Design for Shear and Torsion

Computation of Prestress Losses

Relevant code provisions

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Text Book


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Prestressed concrete analysis anddesign: Fundamentals

by

Antoine E. Naaman

Publisher: Techno Press 3000

Edition: 2nd Edition

Publication Date: 2010

Int. Standard Book No(ISBN): 0-9674939-1-9

Pages: 1108

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Outcome Assessment

First Midterm Exams 15% Second Midterm Exams 15

Home Work and Quizes 10%

Presentation (10 minutes) 10%

Final Exam 50%


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Midterm Exams

First Midterm After lec Flexure: Working Stress Analysis and Design

Second Midterm

After lec Computation of Prestress Losses


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What is Prestressed concrete


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Prestress is a method in which compression force is appliedto the reinforced concrete section prior dead and live load isacting.

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The prestressing usually improves the behaviour of thestructure by combining two materials which arecomplementary to each other such as concrete and steel Concrete is relatively strong in compression, but weak in

tension. This weakness can be compensated by prestressedsteel partly or totally.

Weaknesses of pure steel structure are price of material,resistance in fire and stability of compressed slender steelstructures


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Examples of prestressing


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Bicycle wheel Spokes are strong in tension but weak in

compression. Spokes must be put intension between outer and inner rims

A load to the axle compresses the lowerspokes and increase the tension in theupper spokes.

The prestressing of the spokes shallexceed the compression forces created in

normal or short term situations such as braking


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Plug or screw anchor in concrete ormasonry structure

The screw driven into plug

squeezes the plug against thesides of the hole, generatingcompressive stresses in the plugand in the wall around it

Compressive prestressing

generates frictional resistance topulling out the screw


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Air-supported or air-inflated structure

The prestressing is produced by internal pressure exceeding anyexternal pressure being applied to the structure

The membrane shall be in tension all the time


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Concrete beam Plain concrete beam brittle

local failure

Reinforced concrete beamcracking occurs when tensilestrength of concrete will beexceeded ductility depends onreinforcement

Prestressed concrete beam• Cracking can be avoided• Ductile behaviour is usually

achievable


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Prestressed Concrete


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In reinforced concrete, concrete and steel are combined such that concrete resistscompression and steel resists tension. This is a passive combination of the twomaterials. In prestressed concrete high strength concrete and high strength steel arecombined such that the full section is effective in resisting tension and compression.This is an active combination of the two materials.

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WHAT IS DIFFERENT FROM REINFORCED CONCRETE


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In Materials• High strength concrete, High strength steel

In Behavior and Performance• Less cracking, less deflection, better durability • Longer spans, thinner depth, higher loads

In Design• Consider new concepts: Losses, Load Balancing, Pre‐stressing• Important: Shrinkage, Creep, Support settlement, Loading

sequence,

In Construction• Special techniques and equipment needed• Special construction procedures and sequence


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PRESTRESSED VERSUS REINFORCED CONCRETE


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In ordinary reinforced members, cracking is essential totransfer loads from concrete to steel. (Steel can only contributeto strength if cracking occurs)

In prestressed members, cracking is considered as an onset of failure. i.e there is not a big difference between cracking stressand yield stress.

Pre-stressing is a successfulattempt of overcoming concrete's

natural weakness in tension


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PRESTRESSED VERSUS REINFORCEDCONCRETE


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In the plain concrete beam, the momentsabout point O due to applied loads are

resisted by an internal tension-compressioncouple involving tension in the concrete.

Such a beam failsvery suddenly and

completely when the first crack forms.

In a reinforced concrete bam, reinforcing bars areembedded in the concrete in such a way that the

tension forces needed for moment equilibrium after

the concrete cracks can be developed in the bars

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Role of tension steel in RC beam


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The external loads cause tension in the bottom fibers which maylead to cracking.

Practical reinforced concrete beams are thus usually crackedunder the day-to-day service loads.

On a cracked cross-section, the applied moment is resisted by compression in the concrete and tension in the bondedreinforcing steel.

Although the steel reinforcement provides the cracked concretebeam with flexural strength, it does not prevent cracking anddoes not prevent the loss of stiffness caused by cracking.

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Prestressed Concrete Beam


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When the loadscause a positivemoment in a beam,it is possible byprestressing tointroduce a negativemoment that cancounteract part orall of the positivemoment.

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Advantages of Prestressed Concrete


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The entire cross sections ofmembers resist the loads. Thussmaller members can be used tosupport the same loads, or the

same-size members can be usedfor longer spans.

Prestressed members are crack-free under working loads and ,

as a result, are more watertight,providing better corrosion

protection for the steel.

The negative moments caused by prestressing produce camberin the members, with the result

the total deflections arereduced.

Other advantages: greaterstiffnesses under working loads, better fatigue and impactresistance as compared to

ordinary reinforced concrete.

Advantages

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Disadvantages of Prestressed Concrete


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Better quality control isrequired

Losses in the initial prestressing forces

Additional stress conditions mustbe checked in design, such as the

stresses occurring when prestress

forces are first applied and thenafter prestress losses have taken

place, as well as the stressesoccurring for different loading

conditions

Additional costs of endanchorage devices and end-beam plates

Disadvantages

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Methods of Prestressing

Two different procedures for prestressing concrete were developed:

a) Pretensioning

In pretensioning the prestress tendons are placed before the

concrete was placed. After the concrete has hardened sufficiently, the tendons are

cut and the prestress force is transmitted to the concrete by bond.

Posttensioning In posttensioned construction, the tendons are stressed after

the concrete is placed and has gained the desired strength.


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Pretensioned concrete (procedure)

One end of the reinforcement (i.e. tendon) is secured to an abutment while

the other end of the reinforcement is pulled by using a jack and this end isthen fixed to another abutment.

The concrete is now poured. After the concrete has cured and hardened, the

ends of the reinforcement are released from the abutments.


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How prestressing force is transmitted to concrete ?

The reinforcement which tends to res u m e i ts o r ig in al l e n g t h compress the concrete surrounding it by bond

action. The prestress is thus transmitted to concrete

entirely by the action of bond between the reinforcement

and the surrounding concrete.


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Pretensioning


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See this video on you tube.http://youtu.be/jSWN9GHCnY

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Advantage of Pretensioning

This method is particularly well suited for massproduction because the casting beds can beconstructed several hundred meters long.

The tendons can be run for the entire bed lengthsand used for casting several beams in a line at thesame time.


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Prestress bed

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Posttensioning Procedure

When the concrete has hardened and developed its strength, the tendon ispassed through the duct.

One end is provided with an anchor and the other end of the tendon is pulled by a jack to desired stress and then properly anchored to the concrete.

After stressing and anchoring, the void between each tendon and its duct isfilled with a mortar grout which subsequently hardens "bonded tendons.“ orfilled with grease "unbonded tendons.".


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Partial Prestressing


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Full Prestressed concrete

Structure is uncracked under dead load withpossible deflection upwards

Structure is uncracked (or almost uncracked) underfull service load


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Partial Prestressing

Partial prestressing

Uncracked during dead load

Cracked during full service load


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Terminology


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Forms of Prestressing Steel Wires : Prestressing wire is a single unit made of steel.Strands : Two, three or seven wires are wound to form a prestressing strand.Tendon : A group of strands or wires are wound to form a prestressing tendon.Cable : A group of tendons form a prestressing cable.Bars : A tendon can be made up of a single steel bar. The diameter of a bar is much

larger than that of a wire.Nature of Concrete-Steel InterfaceBonded tendon When there is adequate bond between the prestressing tendon and concrete, it iscalled a bonded tendon. Pre-tensioned and grouted post-tensioned tendons are bonded tendons.

Unbonded tendonWhen there is no bond between the prestressing tendon and concrete, it is called

unbonded tendon. When grout is not applied after post-tensioning, the tendon is an

unbonded tendon.

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Terminology


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Full Prestressing When the level of prestressing is such that no tensile stress is allowed in concreteunder service loads, it is called Full PrestressingPartial Prestressing When the level of prestressing is such that under tensile stresses due to service loads,the crack width is within the allowable limit, it is called Partial Prestressing

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Material Requirements

In order to get the maximum advantage of a pre-stressedconcrete member, it is necessary to use not only high strengthconcrete but also high tensile steel wires.

Concrete used for prestressed work should have minimum

cube strength of 35 MPa for post-tensioned system and 45 MPafor pre-tensioned system.

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Steel used for Prestressing


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There are three basic types of high-strength steel commonly usedas tendons in modern prestressed concrete construction

1. round wires2. (strand3. steel bars

Round wires Available sizes of wires vary from country to country, with diameters of5-7 mm being the most often used.

For design purposes, the yield strength of stress-relieved wires may be

taken as 0.85 times the minimum tensile strength (i.e. 0.85 f p) and themodulus of elasticity of the wires may be taken as E p= 200 Gpa.Note: In recent years, the use of wires in prestressed concreteconstruction has declined, with 7-wire strand being preferred in mostapplications.

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Strands


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Stress-relieved strand is the most commonlyused prestressing steel. Strand is fabricatedfrom a number of prestressing wires, usuallyseven. Seven wire strand consists of six wirestightly wound around a seventh, slightly large

diameter, central wire.

Diameters ranging from 7.9 to 15.2 mm aretypical.

For design purposes, the yield strength ofstress-relieved strand may be taken as 0.85times the minimum tensile strength (i.e.0.85 f p) and the modulus of elasticity of thestrand may be taken as E p= 195 Gpa.

Note: The mechanicalproperties of the strandare slightly differentfrom those of the wire

from which it is made.This is because thestranded wires tend tostraighten slightly whensubjected to tension.

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Bars


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The high strength of alloy steel bars is also used as prestressing steel. In theUSA, both plain and deformed bars are available in two grades (ultimatestress f p= 1000 and 1100 MPa) with diameters range from 12.7 (1/2 in) mmto 33 mm (1 3/8 in).

The elastic modulus for bars is generally lower than those for strand and wire. For design purposes E p may be taken to be 170 Gpa and the yieldstress (0.2% offset) may be taken to be 0.85 f p.

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Building Codes


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SBC 304. Concrete Structures Requirements, 2007.

ACI 318 (American Concrete Institute 318, 2011), Building Code Requirements for Structural

Concrete , Farmington Hills, Michigan.

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Thank You

6CE 407 P t d C t St t

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ce 575-lecture-1(course description and introduction)1

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