PSC Design and construction.ppt

N
Naresh Prasad KeshariEngineer at Department of Road, Nepal à Goverment of Nepal, Department of Road
PSC BASICS, DESIGN ASPECT, CONCRETE
BRIDGE CODE, CONSTRUCTION ASPECT
Avinash Kumar
Prof Track-II
CONTENTS
I. Basic Concept of prestressing
II. Basic Design concept
III.Losses in Prestressed structure
IV.Provisions of IRS:CBC and IS 1343-2012
V. Construction Aspect
What is Pre Stressing ?
It is application of a predetermined force to introduce stresses of suitable
magnitude and distribution so that the stresses resulting from External
Loads can be counteracted to a desired degree.
P
P
Force-fitting of metal bands on wooden barrels.
The metal bands induce a state of initial hoop compression, to counteract
the hoop tension caused by filling of liquid in the barrels.
Pre-tensioning the spokes in a bicycle wheel
The pre-tension of a spoke in a bicycle wheel is applied to such an extent
that there will always be a residual tension in the spoke.
• Concrete has established itself a universal building
material because of its high compressive strength and
capability to take any form & shape,
But
• It has low tensile strength, which is generally
compensated by reinforcement and the resultant
composite mass is known as RCC – Reinforced cement
concrete.
.
RCC What’s bad about it
• Cracks are Inevitable
[Moment resisted by concrete in Compression zone above
Neutral axis and by reinforcement in tension (Cracked) zone]
• Crack widths proportional to strain of steel and therefore the
steel stresses to be limited to low value in service.
• Loss of stiffness due to cracking [Net effective concrete section]
• Large deflections [Reduced Stiffness]
• Unsuitable for large spans as Dead Load becomes very high
INITIAL EFFORTS OF ACHIEVING PSC FAILED
Mainly because of Shrinkage & Creep losses of concrete were not known
Case-I If Mild Steel bar is Used,
Stress = 124 Mpa,
Elongation = Stress/Modulus of Elasticity
= 140/200000 = 0.0007
Loss due to Creep and Shrinkage = 0.0005
Stress after loss = Negligible
Case-II If HSS is used, Stress = 1200 Mpa
Elongation = 1200/200000 = 0.006
Loss due to Creep and Shrinkage = 0.0005
Stress after loss = More than 90%
Hence HSS became necessary for prestressed concrete.
Why High Strength Concrete Needed in PSC structures ?
I. In anchorage area, stresses are very high, so high grade
concrete is a necessity
II. Less shrinkage and Creep in Concrete
III. More Modulus of Elasticity, Less Elastic Shortening
Advantages of Prestressing
I. Improved durability – No corrosion as no cracks in concrete
II. Full section utilized hence effective saving in material
(RCC – only part of section uncracked carries compression).
III. Less Deflection (More stiffness as entire section uncracked and
effective)
IV. Better shear resistance (more shear for uncracked section and due to
vertical component of curved tendon)
V. Lighter & slender members due to high strength concrete and
steel.
Advantages Contd…..
VI. Economical for high spans
VII.Better fatigue strength for dynamically loaded structures (as
section remains un-cracked )
VIII.Lower maintenance compared with steel structures
IX. Suitable for pre-cast construction, hence faster execution is
possible
X. Steel and concrete are tested in service (as they are subjected to
considerable loading conditions before application of external
loads)
Disadvantages of Prestressed Concrete
I. The unit cost of high strength materials being used is higher.
II. Extra initial cost is incurred due to use of prestressing
equipment and its installation.
III. Extra labour cost for prestressing is also there.
IV. Prestressing is uneconomical for short spans and light loads.
Case-I [No Prestress]
Case-II [Concentric prestress, e=0]
Stress Distribution
Case-III [Eccentric Prestress]
When e = d/6
Stress Distribution
Type of Prestressing
Source
Based
Location
Based
Stressing
Sequence
Based
Direction
Based
Shape
Based
Mechanical Internal Pre Tensioned Uni-axial Linear
Hydraulic External Post Tensioned Biaxial Circular
Electrical Multi-axial
Chemical
Definitions: Stages of Loading
The analysis of prestressed members can be different for the
different stages of loadings.
1) Initial : It can be subdivided into two stages.
a) During tensioning of steel
b) At transfer of prestress to concrete.
2) Intermediate : This includes the loads during transportation and
erection of the prestressed members.
3) Final : It can be subdivided into two stages.
a) At service, during operation.
b) At ultimate Load
Definitions: Nature of Concrete-Steel Interface
• Bonded tendon: When there is adequate bond between the
prestressing tendon and concrete, it is called a bonded tendon. Pre-
tensioned and grouted post-tensioned tendons are bonded tendons.
• Unbonded tendon: When 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.
Pre-tensioning or Post-tensioning
• Pre-tensioning: The tension is applied to the tendons
before casting of the concrete. The pre-compression is
transmitted from steel to concrete through bond over the
transmission length near the ends.
• Post-tensioning: The tension is applied to the tendons
(located in a duct) after hardening of the concrete. The
pre-compression is transmitted from steel to concrete by the
anchorage device (at the end blocks).
Pre - Tensioning
• The abutments are fixed at the ends of a prestressing bed.
• The high-strength steel tendons are pulled between two end
abutments prior to the casting of concrete. After that concrete is
cast.
• Once the concrete attains the desired strength for prestressing, the
tendons are cut loose from the abutments.
• The prestress is transferred to the concrete from the tendons, due
to the bond between them.
• During the transfer of prestress, the member undergoes elastic
shortening.
• If the tendons are located eccentrically, the member is likely to
bend and deflect.
Stages of pre-tensioning
Various stages of the post-tensioning operation
 Placement of reinforcement cage and sheathing duct
 Casting of concrete.
 Placement of the tendons.
 Placement of the anchorage block and jack.
 Applying tension to the tendons.
 Seating of the wedges.
 Cutting of the tendons and grouting.
Stages of post-tensioning
PSC Design and construction.ppt
PSC Design and construction.ppt
PSC Design and construction.ppt
PSC Design and construction.ppt
Internal Prestressing
External Prestressing
Cable outside
concrete
Circular Prestressing
Biaxial Prestressing
Loss of Prestressing
Immediate
Elastic Shortening
Anchorage slip
N
o
Creep
Y
e
s
Ye
s
Friction
Shrinkage
Relaxation
N
o
o
Yes
Time dependent
Loss due to Elastic Shortening
Pre-tensioned Members
When tendons are cut and PS force is transferred to the member, the concrete
undergoes immediate shortening due to the pre-stress. The tendon also shortens
by the same amount, which leads to the loss of pre-stress due to elastic
shortening.
Post-tensioned Members
If there is only One tendon, No Loss because applied pre-stress is recorded after
elastic shortening of member.
For more than one tendon, if tendons are stretched sequentially, there is loss
recorded in a tendon during the subsequent stretching of other tendons
Loss Due to Friction
Pre-tensioned Members
No Loss because there is no concrete present during stretching of the tendons
Post-tensioned Members
• Friction generated at interface of Concrete and steel during stretching of a curved tendon
leads to a drop in Prestressing force along the member from stretching end.
• The friction is generated due to the curvature of the tendon and the vertical component
of the PS force.
Loss Due to Friction
• In addition to friction, stretching has to overcome wobble effect of tendon
which refers to change in position of the tendon along the duct.
• The losses due to friction and wobble are grouped together under friction
Loss Due to Friction
Pre-stressing force variation diagram after stretching
Approximately
;
Pressure at a distance x from stretching end;
Loss in PS force due to friction at a distance x from stretching end; ΔP = Po(μα + kx)
x
PSC Design and construction.ppt
Anchorage Slip
In most Post-tensioning systems when the tendon force is
transferred from the jack to the anchoring ends, the friction
wedges slip over a small distance.
Anchorage block also moves before it settles on concrete.
Loss of prestress is due to the consequent reduction in the
length of the tendon.
Certain quantity of prestress is released due to this slip of wire
through the anchorages. – Amount of slip depends on type of
wedge and stress in the wire.
Loss due to Creep
Time-dependent increase of deformation under sustained load.
Due to creep, the prestress in tendons decreases with time.
Factors affecting creep and shrinkage of concrete
• Constituent of Concrete
• Size of the member
• Environmental condition
• Total amount of water in concrete
• Cement Content in concrete
• Stress in Concrete
• Age of Loading
• Duration of Loading
For Creep only
Loss due to Creep
Shrinkage loss
Time-dependent strain measured in an unloaded and unrestrained specimen
at constant temperature.
Loss of prestress (Δfp ) due to shrinkage is as follows
Δfp = Ep x ε,sh
Ep is the modulus of prestressing steel.
The factors responsible for creep of concrete will have influence on
shrinkage of concrete also except the loading conditions
Total shrinkage strain in plain concrete, reinforced concrete and pre-
tensioned prestressed concrete: 0.0003.
Shrinkage Loss
Relaxation in steel
Change in stress with time in steel under a constant strain (elongation) or
a plastic flow
• Relaxation do not occur below 0.5fy
• Relaxation at 1000 hr at 300C
Relaxation in steel ..contd
• 1000 hr value obtained from manufacturer
• It is 4% for normal steel & 2.5 % for low relaxation steel
• This is used to obtain value at about 0.5 x 106 hr (~ 57 yrs)
• It is = 2.5 times the 1000 hr value for normal steel & 3.0 times for
low relaxation steel
• The above value is at initial stress level of 70% of the
characteristic strength
• It reduces to 0 at 50%. In between values are obtained from
interpolation.
Average losses
Type of loss Pre-
tensioning
(%)
Post-
tensioning
(%)
Elastic Shortening of Concrete 4 1
Creep of concrete 6 5
Shrinkage of Concrete 7 6
Steel Relaxation 8 8
Total Loss 25 20
Loss due Friction and Anchorages have been considered to be
overcome by over tensioning
Some Relevant Clauses of CBC
Some Relevant Clauses of CBC
Some Relevant Clauses of CBC
Some Relevant Clauses of CBC
Some Relevant Clauses of CBC
Some Relevant Clauses of CBC
Some Relevant Clauses of CBC
PSC Design and construction.ppt
Prestressing Steel
• SEVEN WIRE STRAND
• Outer wires enclose inner wire in a helix with a uniform pitch
of 12 to 16 times nominal diameter
DIA. OF CENTRAL WIRE IS 1.5%
MORE THAN THE SURRONDING
WIRE
IS 14268-2017 provisions
( Uncoated Stress Relieved Low Relaxation Seven Wire(Ply) Strand for
Prestressed Concrete)
Cont..
7.2.6.5.6 Measurements of Prestressing Force
The force induced in the prestressing tendon shall be determined by means of gauges
attached to the tensioning apparatus as well as by measuring the extension of the steel and
relating it to its stress-strain curve.
The variation between the two measurements should be within +-5%. It is essential that
both methods are used jointly so that the inaccuracies to which each is singly susceptible
are minimized
If the variation of two measurements exceeds 5% then
I. the cause shall be ascertained.
II. the cable should be released and re-stressed.
III. even then, if the variation does not come within 5% then the cable is to be rejected.
Difference between pressure and elongation
The difference
between the
elongation and
the pressure
should not be
more than 5%
Measurement of Prestressing Force
( Clause 13.2.1.3 of IS 1343-2012)
• In practice, the force and elongation of tendon may not exactly match with
the expected values given in stressing schedule.
• In such cases either the force (or the elongation) will be achieved first and
the other value lag behind. In such cases the force (or elongation) shall be
further increased, but not exceeding 5 percent of the design value till the
elongation (or force), which has lagged behind reaches the design value.
• If, even after reaching 5 percent extra value of the force (or elongation),
the other lagged quantity does not reach the design value, reference
should be made to the designer for review and corrective action.
PSC Design and construction.ppt
0
16
30
46
60
76
78
15.33
31.33
45.33
61.33
75.33
91.33
93.33
Correction as per IS 1343-2012 and CBC
0
30
16
60
46
76
15.33
78
93.33
Post-tensioning Systems and Devices
Wedge action: Anchoring Devices
Sequence of Anchoring
Sequence of Anchoring
PSC Design and construction.ppt
CASTING OF PSC GIRDER
CASTING OF PSC GIRDER
VIDE
O
CURING OF PSC GIRDER
CASTING YARD OF PSC GIRDER
Post Tensioning Work
PSC Slab
HDPE Sheathing for Duct
Measurement of Elongation of Cables
EMERGENCY CABLES
• Should be symmetrically placed
• Should be capable of generating additional pre-stressing force
of about 4% of design value
• Stressed only those required to make up deficiency
• Remaining pulled out & hole grouted
FUTURE CABLES
• For easy installation at later date
• Made in girders to cater for increased pre-stress force required
in future due to revision of loading standard, strengthening etc.
• Provision of 15% (minimum) of design pre-stressing force.
Cable Layout
• Cable layout means
– Deciding about the location of cable at various section
• Vertical profile
• Horizontal profile
– The locations between which the cable will be in straight and on curve
• Working out the ordinates at every meter and at every change
of curvature from curved to straight and vice versa in vertical as
well horizontal plane
PSC Design and construction.ppt
How Proper Positioning of Cable is ensured
• Cable tends to sag due to its self weight if not supported properly on
reinforcement chairs and supports
• Cable tends to float and move upwards due to buoyancy effect when
concrete is poured (and is in liquid form), if not tied down properly
• So cable has to be secured against downward as well as upward
movement.
Why Reinforcement is required in PSC?
In the end block
End Block is a highly stressed zone Extending from point of application of
Prestress to the section where linear distribution of stress is assumed to
occur.
Reinforcement is provided to
To take the local transverse tension around the tendon behind the
anchorage
To cater for the tension developed between two or more anchorages,
which tends to split the member
PSC Slab
Cracks in End Zone
PSC Design and construction.ppt
END BLOCK DEATAILS
PSC Design and construction.ppt
Shear Reinforcement in PSC Girders
Load Test of Structures or Parts of Structures [CBC-18.2]
The tests described in this clause are intended as a check on structures where
there is doubt regarding serviceability or strength.
The test should be carried out as soon as possible after the expiry of 28 days
from the time of placing the concrete.
The test load should be equal to the sum of the characteristic dead load plus
1.25 times the characteristic imposed load and should be maintained for a
period of 24 h.
Measurements of deflection and crack width should be taken immediately after
the application of load, at the end of 24 hours loaded period, just after
removal of load and after the 24 hours recovery period.
Acceptance Criteria
For prestressed concrete structures, no visible cracks should occur under the test
load for local damage
For members spanning between two supports, the deflection immediately after
application of the test load should be not more than 1/500 of the effective span.
If the maximum deflection shown during the 24 h under load is less than
40(L²/h) it is not necessary for the recovery to be measured. (L in m and h in
mm)
If ,within 24 h of the removal of the test load the concrete structures does not
have a recovery of at least 85% of the maximum deflection shown during the 24
h under load, The loading should be repeated.
The structure should be considered to have failed to pass the test if the recovery
after the second loading is not at least 85% of the maximum deflection shown
during the second loading.
PSC Design and construction.ppt
1 sur 92

Recommandé

Chemical attack in concrete par
Chemical attack in concreteChemical attack in concrete
Chemical attack in concreteer_kamal
25K vues25 diapositives
Durability of concrete par
Durability of concreteDurability of concrete
Durability of concreteSelva Prakash
12.4K vues47 diapositives
4.corrosion of reinforcement in concrete par
4.corrosion of reinforcement in concrete4.corrosion of reinforcement in concrete
4.corrosion of reinforcement in concreteAqib Ahmed
4.3K vues12 diapositives
CT 3 +5 prestressing b+ par
CT 3 +5 prestressing b+CT 3 +5 prestressing b+
CT 3 +5 prestressing b+Est
2.5K vues108 diapositives
Concrete abrasion-Adam Baba Abdulai par
Concrete abrasion-Adam Baba AbdulaiConcrete abrasion-Adam Baba Abdulai
Concrete abrasion-Adam Baba AbdulaiAdam Baba Abdulai
816 vues13 diapositives
Bridge rehabilitation par
Bridge rehabilitationBridge rehabilitation
Bridge rehabilitationHendro Widagdo
10.9K vues44 diapositives

Contenu connexe

Tendances

Acid attack on concrete par
Acid attack on concreteAcid attack on concrete
Acid attack on concreteAyaz Malik
30.3K vues33 diapositives
Losses in prestressed concrete par
Losses in prestressed concreteLosses in prestressed concrete
Losses in prestressed concreteShubham Singh [NITK]
4.5K vues13 diapositives
Deterioration of concrete par
Deterioration of concreteDeterioration of concrete
Deterioration of concreteHarikrishna M.S
3.7K vues19 diapositives
Deterioration of concrete ppt par
Deterioration of concrete pptDeterioration of concrete ppt
Deterioration of concrete pptwasim shaikh
10K vues34 diapositives
Design & Construction Errors- Building Maintenance and Repairs par
Design & Construction Errors- Building Maintenance and Repairs Design & Construction Errors- Building Maintenance and Repairs
Design & Construction Errors- Building Maintenance and Repairs Srishti Wakhloo
12.7K vues43 diapositives
Design philosophies par
Design philosophiesDesign philosophies
Design philosophiesValmik Mahajan
205 vues7 diapositives

Tendances(20)

Acid attack on concrete par Ayaz Malik
Acid attack on concreteAcid attack on concrete
Acid attack on concrete
Ayaz Malik30.3K vues
Deterioration of concrete ppt par wasim shaikh
Deterioration of concrete pptDeterioration of concrete ppt
Deterioration of concrete ppt
wasim shaikh10K vues
Design & Construction Errors- Building Maintenance and Repairs par Srishti Wakhloo
Design & Construction Errors- Building Maintenance and Repairs Design & Construction Errors- Building Maintenance and Repairs
Design & Construction Errors- Building Maintenance and Repairs
Srishti Wakhloo12.7K vues
Loss of pre stress in pre-stressed concrete par Abdulrahman Salah
Loss of pre stress in pre-stressed concrete Loss of pre stress in pre-stressed concrete
Loss of pre stress in pre-stressed concrete
Abdulrahman Salah2.1K vues
Chemical attack on concrete 2017 par Sagar Vekariya
Chemical attack on concrete 2017Chemical attack on concrete 2017
Chemical attack on concrete 2017
Sagar Vekariya7.2K vues
Cable Layout, Continuous Beam & Load Balancing Method par Md Tanvir Alam
 Cable Layout, Continuous Beam & Load Balancing Method Cable Layout, Continuous Beam & Load Balancing Method
Cable Layout, Continuous Beam & Load Balancing Method
Md Tanvir Alam24.2K vues
Cracks on Fresh and Hardened Concrete par Leema Margret A
Cracks on Fresh and Hardened ConcreteCracks on Fresh and Hardened Concrete
Cracks on Fresh and Hardened Concrete
Leema Margret A135 vues
What is post tensioning par Charlie Gupta
What is post tensioningWhat is post tensioning
What is post tensioning
Charlie Gupta10.9K vues
Presentation on construction of cable stay bridge - a modern technique for su... par Rajesh Prasad
Presentation on construction of cable stay bridge - a modern technique for su...Presentation on construction of cable stay bridge - a modern technique for su...
Presentation on construction of cable stay bridge - a modern technique for su...
Rajesh Prasad32.2K vues
DFI india 2014 defects in piles par Infocengrs63
DFI india 2014 defects in pilesDFI india 2014 defects in piles
DFI india 2014 defects in piles
Infocengrs63312 vues
Effect of temp. ppt Amit Payal par AMIT PAYAL
Effect of temp. ppt Amit PayalEffect of temp. ppt Amit Payal
Effect of temp. ppt Amit Payal
AMIT PAYAL591 vues

Similaire à PSC Design and construction.ppt

PSC Lecture PPT.pptx par
PSC  Lecture PPT.pptxPSC  Lecture PPT.pptx
PSC Lecture PPT.pptxabhinavbharat9
89 vues47 diapositives
PSC Lecture PPT.pdf par
PSC  Lecture PPT.pdfPSC  Lecture PPT.pdf
PSC Lecture PPT.pdfabhinavbharat9
32 vues47 diapositives
Prestressing Concept, Materilas and Prestressing System par
Prestressing Concept, Materilas and Prestressing SystemPrestressing Concept, Materilas and Prestressing System
Prestressing Concept, Materilas and Prestressing SystemLatif Hyder Wadho
12.6K vues95 diapositives
PrestressedConcrete.pptx par
PrestressedConcrete.pptxPrestressedConcrete.pptx
PrestressedConcrete.pptxChristopherArce4
22 vues19 diapositives
Prestressed concrete par
Prestressed concretePrestressed concrete
Prestressed concreteDhanya Kodungallur
2.8K vues19 diapositives
Unit I Introduction theory and behaviour par
Unit I Introduction theory and behaviourUnit I Introduction theory and behaviour
Unit I Introduction theory and behaviourSelvakumar Palanisamy
31 vues23 diapositives

Similaire à PSC Design and construction.ppt(20)

Prestressing Concept, Materilas and Prestressing System par Latif Hyder Wadho
Prestressing Concept, Materilas and Prestressing SystemPrestressing Concept, Materilas and Prestressing System
Prestressing Concept, Materilas and Prestressing System
Latif Hyder Wadho12.6K vues
Pre stressed concrete construction par JenilPatel22
Pre stressed concrete constructionPre stressed concrete construction
Pre stressed concrete construction
JenilPatel225.3K vues
PRESTRESSED CONCRETE STRUCTURES par Tarun kumar
PRESTRESSED CONCRETE STRUCTURESPRESTRESSED CONCRETE STRUCTURES
PRESTRESSED CONCRETE STRUCTURES
Tarun kumar637 vues
UNIT-1 PSC.pptx par jairam131
UNIT-1 PSC.pptxUNIT-1 PSC.pptx
UNIT-1 PSC.pptx
jairam131530 vues
Pre stressed concrete Bridge par Shubham Das
Pre stressed concrete BridgePre stressed concrete Bridge
Pre stressed concrete Bridge
Shubham Das1.6K vues
Prestressed concrete par Udisha15
Prestressed concretePrestressed concrete
Prestressed concrete
Udisha1522.6K vues
Prestressed Concrete par Ravi Savani
Prestressed ConcretePrestressed Concrete
Prestressed Concrete
Ravi Savani61.8K vues

Plus de Naresh Prasad Keshari

21204 - Track Management System (TMS).pptx par
21204 - Track Management System (TMS).pptx21204 - Track Management System (TMS).pptx
21204 - Track Management System (TMS).pptxNaresh Prasad Keshari
103 vues46 diapositives
6. CMG_Durability _ concrete.pptx par
6. CMG_Durability _ concrete.pptx6. CMG_Durability _ concrete.pptx
6. CMG_Durability _ concrete.pptxNaresh Prasad Keshari
37 vues95 diapositives
5. Certification of speed on bridges April 20.pptx par
5. Certification of speed on bridges April 20.pptx5. Certification of speed on bridges April 20.pptx
5. Certification of speed on bridges April 20.pptxNaresh Prasad Keshari
2 vues66 diapositives
1.Pile foundations 2021 (1).pptx par
1.Pile foundations 2021 (1).pptx1.Pile foundations 2021 (1).pptx
1.Pile foundations 2021 (1).pptxNaresh Prasad Keshari
13 vues75 diapositives
Masonry Arch Bridges_May_2021.pptx par
Masonry Arch Bridges_May_2021.pptxMasonry Arch Bridges_May_2021.pptx
Masonry Arch Bridges_May_2021.pptxNaresh Prasad Keshari
111 vues149 diapositives
webinar on speed raising to 130or 160 kmph.pptx par
webinar on speed raising to 130or 160 kmph.pptxwebinar on speed raising to 130or 160 kmph.pptx
webinar on speed raising to 130or 160 kmph.pptxNaresh Prasad Keshari
15 vues70 diapositives

Dernier

Open Access Publishing in Astrophysics par
Open Access Publishing in AstrophysicsOpen Access Publishing in Astrophysics
Open Access Publishing in AstrophysicsPeter Coles
808 vues26 diapositives
TF-FAIR.pdf par
TF-FAIR.pdfTF-FAIR.pdf
TF-FAIR.pdfDirk Roorda
6 vues120 diapositives
DATABASE MANAGEMENT SYSTEM par
DATABASE MANAGEMENT SYSTEMDATABASE MANAGEMENT SYSTEM
DATABASE MANAGEMENT SYSTEMDr. GOPINATH D
7 vues50 diapositives
plasmids par
plasmidsplasmids
plasmidsscribddarkened352
9 vues2 diapositives
Pollination By Nagapradheesh.M.pptx par
Pollination By Nagapradheesh.M.pptxPollination By Nagapradheesh.M.pptx
Pollination By Nagapradheesh.M.pptxMNAGAPRADHEESH
16 vues9 diapositives
CSF -SHEEBA.D presentation.pptx par
CSF -SHEEBA.D presentation.pptxCSF -SHEEBA.D presentation.pptx
CSF -SHEEBA.D presentation.pptxSheebaD7
11 vues13 diapositives

Dernier(20)

Open Access Publishing in Astrophysics par Peter Coles
Open Access Publishing in AstrophysicsOpen Access Publishing in Astrophysics
Open Access Publishing in Astrophysics
Peter Coles808 vues
CSF -SHEEBA.D presentation.pptx par SheebaD7
CSF -SHEEBA.D presentation.pptxCSF -SHEEBA.D presentation.pptx
CSF -SHEEBA.D presentation.pptx
SheebaD711 vues
A training, certification and marketing scheme for informal dairy vendors in ... par ILRI
A training, certification and marketing scheme for informal dairy vendors in ...A training, certification and marketing scheme for informal dairy vendors in ...
A training, certification and marketing scheme for informal dairy vendors in ...
ILRI13 vues
himalay baruah acid fast staining.pptx par HimalayBaruah
himalay baruah acid fast staining.pptxhimalay baruah acid fast staining.pptx
himalay baruah acid fast staining.pptx
HimalayBaruah7 vues
Conventional and non-conventional methods for improvement of cucurbits.pptx par gandhi976
Conventional and non-conventional methods for improvement of cucurbits.pptxConventional and non-conventional methods for improvement of cucurbits.pptx
Conventional and non-conventional methods for improvement of cucurbits.pptx
gandhi97618 vues
How to be(come) a successful PhD student par Tom Mens
How to be(come) a successful PhD studentHow to be(come) a successful PhD student
How to be(come) a successful PhD student
Tom Mens473 vues
Small ruminant keepers’ knowledge, attitudes and practices towards peste des ... par ILRI
Small ruminant keepers’ knowledge, attitudes and practices towards peste des ...Small ruminant keepers’ knowledge, attitudes and practices towards peste des ...
Small ruminant keepers’ knowledge, attitudes and practices towards peste des ...
ILRI5 vues
Distinct distributions of elliptical and disk galaxies across the Local Super... par Sérgio Sacani
Distinct distributions of elliptical and disk galaxies across the Local Super...Distinct distributions of elliptical and disk galaxies across the Local Super...
Distinct distributions of elliptical and disk galaxies across the Local Super...
Sérgio Sacani31 vues
Experimental animal Guinea pigs.pptx par Mansee Arya
Experimental animal Guinea pigs.pptxExperimental animal Guinea pigs.pptx
Experimental animal Guinea pigs.pptx
Mansee Arya15 vues
Metatheoretical Panda-Samaneh Borji.pdf par samanehborji
Metatheoretical Panda-Samaneh Borji.pdfMetatheoretical Panda-Samaneh Borji.pdf
Metatheoretical Panda-Samaneh Borji.pdf
samanehborji16 vues

PSC Design and construction.ppt

  • 1. PSC BASICS, DESIGN ASPECT, CONCRETE BRIDGE CODE, CONSTRUCTION ASPECT Avinash Kumar Prof Track-II
  • 2. CONTENTS I. Basic Concept of prestressing II. Basic Design concept III.Losses in Prestressed structure IV.Provisions of IRS:CBC and IS 1343-2012 V. Construction Aspect
  • 3. What is Pre Stressing ? It is application of a predetermined force to introduce stresses of suitable magnitude and distribution so that the stresses resulting from External Loads can be counteracted to a desired degree. P P
  • 4. Force-fitting of metal bands on wooden barrels. The metal bands induce a state of initial hoop compression, to counteract the hoop tension caused by filling of liquid in the barrels.
  • 5. Pre-tensioning the spokes in a bicycle wheel The pre-tension of a spoke in a bicycle wheel is applied to such an extent that there will always be a residual tension in the spoke.
  • 6. • Concrete has established itself a universal building material because of its high compressive strength and capability to take any form & shape, But • It has low tensile strength, which is generally compensated by reinforcement and the resultant composite mass is known as RCC – Reinforced cement concrete.
  • 7. .
  • 8. RCC What’s bad about it • Cracks are Inevitable [Moment resisted by concrete in Compression zone above Neutral axis and by reinforcement in tension (Cracked) zone] • Crack widths proportional to strain of steel and therefore the steel stresses to be limited to low value in service. • Loss of stiffness due to cracking [Net effective concrete section] • Large deflections [Reduced Stiffness] • Unsuitable for large spans as Dead Load becomes very high
  • 9. INITIAL EFFORTS OF ACHIEVING PSC FAILED Mainly because of Shrinkage & Creep losses of concrete were not known Case-I If Mild Steel bar is Used, Stress = 124 Mpa, Elongation = Stress/Modulus of Elasticity = 140/200000 = 0.0007 Loss due to Creep and Shrinkage = 0.0005 Stress after loss = Negligible Case-II If HSS is used, Stress = 1200 Mpa Elongation = 1200/200000 = 0.006 Loss due to Creep and Shrinkage = 0.0005 Stress after loss = More than 90% Hence HSS became necessary for prestressed concrete.
  • 10. Why High Strength Concrete Needed in PSC structures ? I. In anchorage area, stresses are very high, so high grade concrete is a necessity II. Less shrinkage and Creep in Concrete III. More Modulus of Elasticity, Less Elastic Shortening
  • 11. Advantages of Prestressing I. Improved durability – No corrosion as no cracks in concrete II. Full section utilized hence effective saving in material (RCC – only part of section uncracked carries compression). III. Less Deflection (More stiffness as entire section uncracked and effective) IV. Better shear resistance (more shear for uncracked section and due to vertical component of curved tendon) V. Lighter & slender members due to high strength concrete and steel.
  • 12. Advantages Contd….. VI. Economical for high spans VII.Better fatigue strength for dynamically loaded structures (as section remains un-cracked ) VIII.Lower maintenance compared with steel structures IX. Suitable for pre-cast construction, hence faster execution is possible X. Steel and concrete are tested in service (as they are subjected to considerable loading conditions before application of external loads)
  • 13. Disadvantages of Prestressed Concrete I. The unit cost of high strength materials being used is higher. II. Extra initial cost is incurred due to use of prestressing equipment and its installation. III. Extra labour cost for prestressing is also there. IV. Prestressing is uneconomical for short spans and light loads.
  • 15. Case-II [Concentric prestress, e=0] Stress Distribution
  • 16. Case-III [Eccentric Prestress] When e = d/6 Stress Distribution
  • 17. Type of Prestressing Source Based Location Based Stressing Sequence Based Direction Based Shape Based Mechanical Internal Pre Tensioned Uni-axial Linear Hydraulic External Post Tensioned Biaxial Circular Electrical Multi-axial Chemical
  • 18. Definitions: Stages of Loading The analysis of prestressed members can be different for the different stages of loadings. 1) Initial : It can be subdivided into two stages. a) During tensioning of steel b) At transfer of prestress to concrete. 2) Intermediate : This includes the loads during transportation and erection of the prestressed members. 3) Final : It can be subdivided into two stages. a) At service, during operation. b) At ultimate Load
  • 19. Definitions: Nature of Concrete-Steel Interface • Bonded tendon: When there is adequate bond between the prestressing tendon and concrete, it is called a bonded tendon. Pre- tensioned and grouted post-tensioned tendons are bonded tendons. • Unbonded tendon: When 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.
  • 20. Pre-tensioning or Post-tensioning • Pre-tensioning: The tension is applied to the tendons before casting of the concrete. The pre-compression is transmitted from steel to concrete through bond over the transmission length near the ends. • Post-tensioning: The tension is applied to the tendons (located in a duct) after hardening of the concrete. The pre-compression is transmitted from steel to concrete by the anchorage device (at the end blocks).
  • 21. Pre - Tensioning • The abutments are fixed at the ends of a prestressing bed. • The high-strength steel tendons are pulled between two end abutments prior to the casting of concrete. After that concrete is cast. • Once the concrete attains the desired strength for prestressing, the tendons are cut loose from the abutments. • The prestress is transferred to the concrete from the tendons, due to the bond between them. • During the transfer of prestress, the member undergoes elastic shortening. • If the tendons are located eccentrically, the member is likely to bend and deflect.
  • 23. Various stages of the post-tensioning operation  Placement of reinforcement cage and sheathing duct  Casting of concrete.  Placement of the tendons.  Placement of the anchorage block and jack.  Applying tension to the tendons.  Seating of the wedges.  Cutting of the tendons and grouting.
  • 33. Loss of Prestressing Immediate Elastic Shortening Anchorage slip N o Creep Y e s Ye s Friction Shrinkage Relaxation N o o Yes Time dependent
  • 34. Loss due to Elastic Shortening Pre-tensioned Members When tendons are cut and PS force is transferred to the member, the concrete undergoes immediate shortening due to the pre-stress. The tendon also shortens by the same amount, which leads to the loss of pre-stress due to elastic shortening. Post-tensioned Members If there is only One tendon, No Loss because applied pre-stress is recorded after elastic shortening of member. For more than one tendon, if tendons are stretched sequentially, there is loss recorded in a tendon during the subsequent stretching of other tendons
  • 35. Loss Due to Friction Pre-tensioned Members No Loss because there is no concrete present during stretching of the tendons Post-tensioned Members • Friction generated at interface of Concrete and steel during stretching of a curved tendon leads to a drop in Prestressing force along the member from stretching end. • The friction is generated due to the curvature of the tendon and the vertical component of the PS force.
  • 36. Loss Due to Friction • In addition to friction, stretching has to overcome wobble effect of tendon which refers to change in position of the tendon along the duct. • The losses due to friction and wobble are grouped together under friction
  • 37. Loss Due to Friction Pre-stressing force variation diagram after stretching Approximately ; Pressure at a distance x from stretching end; Loss in PS force due to friction at a distance x from stretching end; ΔP = Po(μα + kx) x
  • 39. Anchorage Slip In most Post-tensioning systems when the tendon force is transferred from the jack to the anchoring ends, the friction wedges slip over a small distance. Anchorage block also moves before it settles on concrete. Loss of prestress is due to the consequent reduction in the length of the tendon. Certain quantity of prestress is released due to this slip of wire through the anchorages. – Amount of slip depends on type of wedge and stress in the wire.
  • 40. Loss due to Creep Time-dependent increase of deformation under sustained load. Due to creep, the prestress in tendons decreases with time. Factors affecting creep and shrinkage of concrete • Constituent of Concrete • Size of the member • Environmental condition • Total amount of water in concrete • Cement Content in concrete • Stress in Concrete • Age of Loading • Duration of Loading For Creep only
  • 41. Loss due to Creep
  • 42. Shrinkage loss Time-dependent strain measured in an unloaded and unrestrained specimen at constant temperature. Loss of prestress (Δfp ) due to shrinkage is as follows Δfp = Ep x ε,sh Ep is the modulus of prestressing steel. The factors responsible for creep of concrete will have influence on shrinkage of concrete also except the loading conditions Total shrinkage strain in plain concrete, reinforced concrete and pre- tensioned prestressed concrete: 0.0003.
  • 44. Relaxation in steel Change in stress with time in steel under a constant strain (elongation) or a plastic flow • Relaxation do not occur below 0.5fy • Relaxation at 1000 hr at 300C
  • 45. Relaxation in steel ..contd • 1000 hr value obtained from manufacturer • It is 4% for normal steel & 2.5 % for low relaxation steel • This is used to obtain value at about 0.5 x 106 hr (~ 57 yrs) • It is = 2.5 times the 1000 hr value for normal steel & 3.0 times for low relaxation steel • The above value is at initial stress level of 70% of the characteristic strength • It reduces to 0 at 50%. In between values are obtained from interpolation.
  • 46. Average losses Type of loss Pre- tensioning (%) Post- tensioning (%) Elastic Shortening of Concrete 4 1 Creep of concrete 6 5 Shrinkage of Concrete 7 6 Steel Relaxation 8 8 Total Loss 25 20 Loss due Friction and Anchorages have been considered to be overcome by over tensioning
  • 56. • SEVEN WIRE STRAND • Outer wires enclose inner wire in a helix with a uniform pitch of 12 to 16 times nominal diameter DIA. OF CENTRAL WIRE IS 1.5% MORE THAN THE SURRONDING WIRE
  • 57. IS 14268-2017 provisions ( Uncoated Stress Relieved Low Relaxation Seven Wire(Ply) Strand for Prestressed Concrete)
  • 59. 7.2.6.5.6 Measurements of Prestressing Force The force induced in the prestressing tendon shall be determined by means of gauges attached to the tensioning apparatus as well as by measuring the extension of the steel and relating it to its stress-strain curve. The variation between the two measurements should be within +-5%. It is essential that both methods are used jointly so that the inaccuracies to which each is singly susceptible are minimized If the variation of two measurements exceeds 5% then I. the cause shall be ascertained. II. the cable should be released and re-stressed. III. even then, if the variation does not come within 5% then the cable is to be rejected.
  • 60. Difference between pressure and elongation The difference between the elongation and the pressure should not be more than 5%
  • 61. Measurement of Prestressing Force ( Clause 13.2.1.3 of IS 1343-2012) • In practice, the force and elongation of tendon may not exactly match with the expected values given in stressing schedule. • In such cases either the force (or the elongation) will be achieved first and the other value lag behind. In such cases the force (or elongation) shall be further increased, but not exceeding 5 percent of the design value till the elongation (or force), which has lagged behind reaches the design value. • If, even after reaching 5 percent extra value of the force (or elongation), the other lagged quantity does not reach the design value, reference should be made to the designer for review and corrective action.
  • 64. Correction as per IS 1343-2012 and CBC 0 30 16 60 46 76 15.33 78 93.33
  • 70. CASTING OF PSC GIRDER
  • 71. CASTING OF PSC GIRDER VIDE O
  • 72. CURING OF PSC GIRDER
  • 73. CASTING YARD OF PSC GIRDER
  • 78. EMERGENCY CABLES • Should be symmetrically placed • Should be capable of generating additional pre-stressing force of about 4% of design value • Stressed only those required to make up deficiency • Remaining pulled out & hole grouted
  • 79. FUTURE CABLES • For easy installation at later date • Made in girders to cater for increased pre-stress force required in future due to revision of loading standard, strengthening etc. • Provision of 15% (minimum) of design pre-stressing force.
  • 80. Cable Layout • Cable layout means – Deciding about the location of cable at various section • Vertical profile • Horizontal profile – The locations between which the cable will be in straight and on curve • Working out the ordinates at every meter and at every change of curvature from curved to straight and vice versa in vertical as well horizontal plane
  • 82. How Proper Positioning of Cable is ensured • Cable tends to sag due to its self weight if not supported properly on reinforcement chairs and supports • Cable tends to float and move upwards due to buoyancy effect when concrete is poured (and is in liquid form), if not tied down properly • So cable has to be secured against downward as well as upward movement.
  • 83. Why Reinforcement is required in PSC? In the end block End Block is a highly stressed zone Extending from point of application of Prestress to the section where linear distribution of stress is assumed to occur. Reinforcement is provided to To take the local transverse tension around the tendon behind the anchorage To cater for the tension developed between two or more anchorages, which tends to split the member
  • 89. Shear Reinforcement in PSC Girders
  • 90. Load Test of Structures or Parts of Structures [CBC-18.2] The tests described in this clause are intended as a check on structures where there is doubt regarding serviceability or strength. The test should be carried out as soon as possible after the expiry of 28 days from the time of placing the concrete. The test load should be equal to the sum of the characteristic dead load plus 1.25 times the characteristic imposed load and should be maintained for a period of 24 h. Measurements of deflection and crack width should be taken immediately after the application of load, at the end of 24 hours loaded period, just after removal of load and after the 24 hours recovery period.
  • 91. Acceptance Criteria For prestressed concrete structures, no visible cracks should occur under the test load for local damage For members spanning between two supports, the deflection immediately after application of the test load should be not more than 1/500 of the effective span. If the maximum deflection shown during the 24 h under load is less than 40(L²/h) it is not necessary for the recovery to be measured. (L in m and h in mm) If ,within 24 h of the removal of the test load the concrete structures does not have a recovery of at least 85% of the maximum deflection shown during the 24 h under load, The loading should be repeated. The structure should be considered to have failed to pass the test if the recovery after the second loading is not at least 85% of the maximum deflection shown during the second loading.