Prestressed Introduction
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Transcript of Prestressed Introduction
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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
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 2 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 3 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 4 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 5 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 6 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 7 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 8 / 39
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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 pre- tensioning 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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 9 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 10 / 39
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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
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 12 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 13 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 14 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 15 / 39
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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)
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 16 / 39
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concentric tendon
(Force F )
Beam Prestressed and Loaded
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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 17 / 39
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Example : First Concept, add excentricity
Put the prestressed 100mm below concrete center of gravity (c.g.c), thestresses are:
Beam Prestressed and Loaded
4 -4
4 0
Stress at mid section
Stress at L/4 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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 18 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 19 / 39
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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.
Stress at L/4 section
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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 20 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 21 / 39
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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)
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 22 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 23 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 24 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 25 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 26 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 27 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 28 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 29 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 30 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 31 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 32 / 39
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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
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 33 / 39
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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.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 34 / 39
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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.
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Posttensioned Beam
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 36 / 39
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Pretensioned Beam
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 37 / 39
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External Prestressing
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 38 / 39
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Home Work
Find the prestressed structures that you think it is most attractive andmost innovative use of prestressing.Pictures/ figures/ skets is good.
(ITS Surabaya) Reinforced Concrete II First Ed Feb 2015 39 / 39