Prestressed Introduction

Introduction to Prestressed Concrete2nd Part of Reinforced Concrete II

LB3 Civil Engineering ITS

Fakultas Teknik Sipil dan PerencanaanInstitut Teknologi Sepuluh Nopember Surabaya

Last Edited Feb-2015

(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 1 / 39

References

Design of Prestressed Concrete Structures, T.Y. Lin, Ned H.Burns, John Wiley & Sons, 1982.

Prestressed Concrete, a Fundamental Approach, Edward G.Nawy, 5th Ed, Prentice Hall, 2006.

Prestressed Concrete Analysis and Design, Fundamentals,Antoine E. Naaman, 2nd Ed, Tecno Press 3000, 2004.


Objectives

Upon completion of this topic, student will be able to:Understand basic concept of prestressed concrete.Understand the effect of low strength steel and high strength steelto prestressing.Distinguish three principle of prestressed concrete.Understand the advantages and disadvantages of prestressedconcrete.Understand the loading stages of prestressing.


Introduction

Developement of prestressed concrete.General principle of prestressed concrete.Classification and Types.Stages of Loading.Reinforced vs. Prestressed vs. Partial Prestressed Concrete.Design Codes for Prestressed Concrete.


Definitions:

Leonhard F.The basic idea of prestressing is that the concrete should,beforeexternal loading is applied, be put under compression in all partswhere the loading produces tensile stresses, so that on the tensile sidethese compressive prestresses will first have to be cancelled before anytension actually occurs in the concrete.

Naaman A.E.Prestressing is the deliberate creation of permanent internal stresses ina structure or system in order to improve its performance. Suchstresses are designed to counteract those induced by external loading.


Definitions:(cont.)

Abeles,P.W. ;Bardhan-Roy,B.K; Turner,F.H.Prestressing may be defined as the purposeful creation and controlledcreation of permanent stresses in a structures member, before the fulldead and live loads are opplied, so as to counteract all of part of theseloads. It serves two main purposes: to inprove the resistence of themember to the dead load and live loads(service load) and to modifythe behavior of the members or structure in such a way as to make itmore suitable for its intended purpose.

Guyon Y from FreyssinetPrecontraindre est une construction, cest y crer artificiellement avantapplication des charges exterieures ou simultanement avec celle-ci, descontraintes permanentes tellque composes avec les conterintes dueaux charge exterieur les contraint totales restent en tout point, et pourtous les cas envisage de compris entre les limites au contraintes que lamatiere peut uporter infinement.


Concept

Basic ConceptPrestressed concrete is basically concrete in which internal stresses of asuitable magnitude and distribution are introduced so that the stressesresulting from the external loads are counteracted to a desired degree.


History

The application of pre-stressing in concrete structures is not the onlyinstance. There were some earlier attempts made.Two of the instancesare provided below

Force-fitting of metal bands on woodenbarrels.The metal bands around the barrel induce astate of initial hoop compression tocounteract the hoop tension caused by fillingof liquid in the barrels.

Pre-tensioning of spokes in a bicycle wheel.The pre-tension is applied in the spoke tosuch an extent that there will always be aresidual tension in the spoke


Prestressed History

1886 Jackson, P. H., (USA)Introduced the concept of tightening steel tie rods in artifi-cial stone and concrete arches.

1888 Doehring, C. E. W., (Germany)Manufactured concrete slabs and small beams with embed-ded tensioned steel.

1908 Stainer, C. R., (USA)Recognised losses due to shrinkage and creep, and sug-gested retightening the rods to recover lost prestress.

1923 Emperger, F., (Austria)Developed a method of winding and pretensioning hightensile steel wires around concrete pipes.

1924 Hewett, W. H., (USA)Introduced hoop-stressed horizontal reinforcement aroundwalls of concrete tanks through the use of turnbuckles.


1925 Dill, R. H., (USA)Used high strength unbonded steel rods. The rods weretensioned and anchored after hardening of the concrete.

1926 Eugene Freyssinet (France)Used high tensile steel wires, with ultimate strength as highas 1725 MPa and yield stress over 1240 MPa. In 1939,he developed conical wedges for end anchorages for post-tensioning and developed double-acting jacks. He is oftenreferred to as the Father of Prestressed concrete.

1938 Hoyer, E., (Germany)Developed long line pre-tensioning method.

1940 Magnel, G., (Belgium)Developed an anchoring system for post-tensioning, usingflat wedges.


Development of Building Materials

MATERIALRESISTING

COMPRESSION

STONES

BRICKS

MATERIALRESISTINGTENSION

BAMBOOS

ROPES TIMBER

MATERIAL

RESISTING TENSIONAND COMPRESSION

CONCRETEIRON BAR

STEEL WIRE

STRUCTURAL

STEEL

REINFORCED

CONCRETE

HIGH

STRENGTHSTEEL

HIGH

STRENGTH

CONCRETE

CONCRETE

PRESTRESSED

PASSIVE

COMBINATION

ACTIVE

COMBINATION

Reference: Lin, T. Y. and Burns, N. H., Design of Prestressed Concrete Structures(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 11 / 39

First Patent

Original Length of Steel = L

Steel is Prestressed

Prestressed is Lost

lengthening of steel

Shrinkage and creepof concrete

= 0.00062L

= -0.00062L

The first patented method were notsuccessful because the low tensileprestress in the steel was soon lostas result of the shrinkage and creepof concrete.Consider an ordinary steel barprestressed to a working stress of124 MPa. If modulus of elasticity ofsteel approximately 200 103 MPa,the unit lengthening of the bar isgiven by:

=fE

=124

200000= 0.00062

Shrinkage and creep of concrete induce comparable amount ofshortening in concrete.


Modern Prestressed

Original Length of Steel = L

Steel is Prestressed

Prestressed is Lost

lengthening of steel

Shrinkage and creepof concrete = -0.0008L

= 0.0050L

effective strain insteel = 0.0042L

Using high strength steel wire forprestressing. Such wire, withultimate strength as high as 1725MPa, and yield point over 1240MPa, are prestressed to about 1000MPa, creating strain of:

=fE

=100

200000= 0.005

Assuming the total lost due to shrinkage and creep is 0.0008, the netstrain of 0.0050-0.0008=0.0042 would still be left in the wires, which isequaivalent to a stress of:

f = E = 200000 0.0042 = 840MPa


General Principles

Three different concepts may be applied to explain and analyze thebasic behavior of prestressed concrete.

Prestressed to transform concrete into an Elastic Material.Prestressing for combination of High-strength Steel with Concrete.Prestressing to archive Load Balancing.


General Principles: Elastic Material

First concept Prestressing to transform concrete into elasticmaterial.

concentric tendon

(Force F )

Beam Prestressed and Loaded

FA

McI

MyI

FA McI

FA +

McI

FA McI

The stress distribution is given by:

f =FA My

I


Example : First Concept

A beam of 200x300mm with the span of 8m, prestress with forceF = 240 kN placed at c.g of beam, Concrete weight= 25 kN/m3, thedistributed dead load qDL = 1.5 kN/m

Moment at mid span MDL,mid =18qDLL2 = 12 kN.m

Moment at 1/4 span MDL,L/4 =332

qDLL2 = 9 kN.mStresses:caused by Compression force:

FA

=240000

200 300 = 4MPa

caused by moment at mid span:

MDLcI

=12 106 150112 200 3003

= 4MPa, (3MPa ,for L/4)


concentric tendon

(Force F )


4 -4

48

0

4 -3 1

7

Stress at mid section

Stress at L/4 section

Stress due to F Stress due to M Stress combination


Example : First Concept, add excentricity

Put the prestressed 100mm below concrete center of gravity (c.g.c), thestresses are:


4 -4

4 0

Stress at mid section


e = 100c.g.c

c.g.sF F

b = 200

h = 300

f = FA FeyI MyI

FA

8

-8

8FeyI

MDLyI f =

FA FeyI MyI

4 -3

3 -1

FA

8

-8

7FeyI

MDLyI f =

FA FeyI MyI

stress at any section

capable of additional load :

MLL = fIy = 8

1122003003

150

MLL = 24kN.m or

qLL = 3 kN/m

there is tension (-1 MPa)at top fiber on 14L section


Parabolic tendon

The ideal tendon geometry is the same shape of the moment diagram,which is:

Mx =(

12qL)x 1

2qx2 or Mx =

12q(l x2)

The ideal curve set by parabolic equation:

y = ax2 + bx+ c

by finding a, b, and c we get:

y =4fh(l x)

l2

where h is depth of parabola. (our case f = e = 100)by putting x = 14 l = 2000mm will get:

y =4 100 2000(8000 2000)

80002= 75mm


the y = 75mm is excentricity of prestressed at 14 l of the section.the stress at 14 l section is:

f =FecI

=240 103 75 150

112 200 3003

= 6MPa.


4 -3

3 1

FA

6

-6

7FeyI

MDLyI f =

FA FeyI MyI

the advantages of parabola curve that there is no tension at any sectionof the beam.


General Principles: C-line Method

Second Concept Prestressing for Combination of High-StrengthSteel with Concrete.This concept consider prestressed concrete as a combination ofsteel and concrete, similar to reinforced concrete. with steel takingtension and concrete taking compression so that the two materialsform a resisting couple against external moment.From figure it is evident that the C-line is varying distance a fromthe T-line. The moment given by:

M = Ca = Ta

while excentricity e is known, so e = a e, since C = T, a = M/Te =

MT e

from figure:

f t =CAc

+CectIc

and fb =CAc Ce

ctIc


aC = 0

T = 0

w = 0

l/2

a

C = C1

T = T1

w = w1

l/2

a

C = C2

T = T2

w = w

l/2

T = p

w = 0

l/2

C = p

e

a = 0

T = p

w = w1

l/2

a = a1 = e

C = p

T = p

w = w

l/2

e

C = p

ea

(a) (b)

(d)(c)

(e) (f)


Example: C-line Method

From previous parabolic tendon with excentrivity 100mm, at midlesection M = 12 kN.m , T = C = 240 kN

a =MT

=12 106240 103 = 50mm

e = a e = 50 100 = 50mm below cgc

ft =FA Fe

ctIc

=240 103200 300

240 103 50 150112 200 3003

= 0

fb =FA

+FecbIc

=240 103200 300 +

240 103 50 150112 200 3003

= 8

We get similar result compared to previous example.


General Principles: Load Balancing Method

This technique is based on utilizing the vertical force of the draped orharped prestressing tendon to counteract or balance the imposedgravity loading to which a beam is subjected. Hence, it is applicable tononstraight prestressing tendons.

F R

w

F wb

w

(a) (b)

Parabolic Tendon Profile.:Let the parabolic function

y = Ax2 + Bx+ C

represent the tendon drape; the force F denotes the pull to which thetendon is subjected.


Then for x = 0, we have.

y = 0, C = 0dydx

= 0, B = 0

for x = l/2,

y = a, A =4al2

But from calculus, the load intensity is

q = F2yx2

This will yield:

q = F4al2 2 = 8Fa

l2Hence, if the tendon has a parabolic profile in the prestressed beamand the prestressing force is denoted by F, the balanced-load intensity

wb =8Fal2


Example: Load Balancing Method

As previous example, calculate balancing load:

wb =8Fal2

=8 240 0.1

82= 3 kN/m

This upward balancing load will hold the dead load from structure,the balancing load it self have twice the value of dead load (1.5kN/m).The unbalance load (reserved capacity):

wub = 1.5 3 = 1.5 kN/mor equivalent to:

M =18

1.5 82 = 12 kN.mThis potentially used to carry live load and supper imposed dead load.


Classification and Type

Based on method ofprestressing

pre-tensioningPost-tensioningSelf stressing

Based on position of tendons.External prestressingInternal prestressing

Based on tendon shapesLinearCircular

Based on concretingIn-situPrecast

Based on level of prestressingFull prestressingPartial prestressing

Based on concrete steelinterface

BondedUnbonded


Advantage of Prestressed Concrete

1 Section remains uncracked under service loadsReduction of steel corrosion: Increase in durability.Full section is utilised: Higher moment of inertia, Lessdeformations, Increase in shear capacity, Suitable for use inpressure vessels, Improved performance (resilience) underdynamic and fatigue loading

2 High span-to-depth ratiosReduction in self weight.More aesthetic appeal due to slender sections.More economical sections.

3 Suitable for precast constructionRapid constructionBetter quality controlReduced maintenanceSuitable for repetitive constructionMultiple use of formworkAvailability of standard shapes.


Disadvantage of Prestressed Concrete

1 Prestressing needs skilled technology. Hence, it is not as commonas reinforced concrete.

2 The use of high strength materials is costly.3 There is additional cost in auxiliary equipments.4 There is need for quality control and inspection.


Stages of Loading

Initial stageBefore Prestressing:

Support yield should be prevented,Controlled curing is important,Shrinkage crack should be prevented.

At transfer of Prestress:

Maximum tendon stress is limited (0.8fpu or 0.95fpy)Crushing of concrete at the anchorage is prevented

Decentering and Retensioning:

False work maybe removed after prestressingThe stresses at various stages of tensioning must be studied


Stages of Loading

Intermediate stageThis is the stage during transportation and erection. It is occur only forprecast member when they are transported to the site and erected inposition. It highly important to ensure

The member are properly supportedThe member are properly handledproper support condition and loading.


Stages of Loading

Final stageThis is the stage when actual working loads come on stuuctures.

Sustained loadControl of camber and deflection.Working LoadCheck for excessive stresses and strins.Cracking LoadInvestigate the cracking load due to use of structures.Ultimate LoadFor code requirement, ultimate load should be calculated usingload factor and strength reduction factor.


Reinforced vs Prestressed vs Partial Prestressed

Typical Load

ReinforcedConcrete

Cracked withdeflection underdeadload andfull service load

Reinforcing bar

Dead Load

Full Service Load

Prestressing TendonPrestressedConcrete

Uncracked withlikely camberunder dead loadand prestressed

Dead Load

PrestressedConcrete

Partial

Full Service Load

Uncracked underdead load

Cracked underService Load


Design Code for Prestressed Concrete

SNI 2847-2013 or ACI 318-11 already integrate the design of reinforcedand prestressed concrete design.SNI 2847-2013 or ACI 318-11 already use unified provision, meaningsthat there is no difference in strength reduction factors for reinforcedconcrete or prestressed concrete.Prestressed concrete sections shall be classified as eithertension-controlled, transition, or compression-controlled sections, inaccordance with 10.3.3 and 10.3.4. The appropriate strength reductionfactors, , from 9.3.2 shall apply.The provisions of Chapter 18 of SNI/ACI were developed primarilyfor structural members such as slabs, beams, and columns that arecommonly used in buildings.For bridges structures AASHTO-LRFD Bridge Design Specificationsnormally used or for indonesia RSNI T12 2004.


Terminology

Tendon:stretched element used in a concrete member of structure toimpart prestress to the concrete.Anchorage:A device generally used to enable the tendon to impart andmaintain prestress in concrete.Pretensioning:A method of prestressing concrete in which the tendons aretensioned before the concrete is placed. In this method, theconcrete is introduced by bond between steel & concrete.Post-tensioning:A method of prestressing concrete by tensioning the tendonsagainst hardened concrete. In this method, the prestress isimparted to concrete by bearing.


Posttensioned Beam


Pretensioned Beam


External Prestressing


Home Work

Find the prestressed structures that you think it is most attractive andmost innovative use of prestressing.Pictures/ figures/ skets is good.


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Prestressed Introduction

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