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AHSANULLAH UNIVERSITY OF
SCIENCE & TECHNOLOGY

Course No : CE-416.
Course Title : Prestress Concrete Design
Sessional.

...
2
Welcome To Our Presentation

3
Group Members
10.01.03.062
10.01.03.063
10.01.03.065
10.01.03.066
10.01.03.069
10.01.03.072
10.01.03.075
10.01.03....
What is Pre-stressed Concrete?
–

–

Internal stresses are
induced to counteract
external stresses.
In 1904, Freyssinet
at...
Introduction
• In prestressed concrete applications, most important
variable is the prestress.
• Prestress does not remain...
Prestress Loss
• loss in prestress is the difference between initial prestress and
the effective prestress.
• Loss of pres...
Types
Loss of prestress is classified into two types:
1. Immediate Losses
 immediate

losses

occur

during

prestressing...
Types According To Time
Prestress Losses

Time
Dependent

Immediate

Elastic
Shortening

Friction

Anchorage
Slip

Creep

...
Types According To Material
Prestress Losses

Concrete

Elastic
Shortening

Creep

Steel

Shrinkage

Friction

Anchorage
S...
Types of Prestressing Systems
I . Pre-tensioning:
 In Pre-tension, the tendons are tensioned before the
concrete is place...
Example of Pre-tensioning

Fig : Pre-tensioned electric pole
12
Example of Post-tensioning

Fig : Post-tensioning of a box girder
13
Losses in Various Prestressing Systems
Type of Loss

Pre-tensioning

Post-tensioning
i.

No, if all the cables are
simulta...
Immediate Loss

15
Elastic Shortening
• It is the shorten of concrete member, when the prestress is transferred
to concrete, the member short...
Elastic Shortening at Pre-tensioned Members
 When the tendons are cut and the prestressing
force is transferred to the me...
Elastic Shortening at Pre-tensioned Members

Prestressing bed
Pre-tensioning of a member
18
Elastic Shortening at Post-tensioned Members
 If there is only one tendon, there is no loss because

the applied prestres...
Elastic Shortening at Post-tensioned Members

Duct
Anchorage

jack

Casting bed
Post-tensioning of a member
20
Anchorage Slip
• In most Post-tensioning systems when the prestress force is

transferred from the jack to the anchoring e...
Force variation diagrams for various
stages
a) The initial tension at the right
end is high to compensate for the
anchorag...
Force variation diagrams for various
stages
c) The initial tension at the left end
also corresponds to about initial
prest...
Typical values of anchorage slip

Anchorage System
Freyssinet system
12 - 5mm Φ strands
12 - 8mm Φ strands
Magnel system
D...
Frictional Loss
•

The friction generated at the interface of
concrete and steel during the stretching
of a curved tendon ...
Frictional loss occurs only in Posttensioned Members
• The loss due to friction does not occur in pretensioned members bec...
Frictional Loss
Frictional Loss is the summation of
– Friction Loss Due to length Effect.
– Friction Loss Due to Curvature...
Length & Curvature Effects

28
Methods available to “Reduce” the
frictional losses
1. Cables should pass through metal tubes.
2. The bends should be thro...
Time Dependent Losses

30
Creep of Concrete
• The Continuous deformation of concrete with time under
sustained load.
 Factors affecting creep of co...
Condition for calculating the loss of
prestress due to creep.

• Creep is due to sustained (permanent) loads only. Tempora...
Shrinkage of Concrete
• Shrinkage of
defined
as

concrete
the

is

contraction due to loss of
moisture.

• Due to the shri...
Shrinkage of Concrete
• For pre-tensioned members, transfer
commonly takes place after 24 hours after
casting and nearly a...
Relaxation
 Relaxation is the reduction in stress with time at

constant strain.
– decrease in the stress is due to the f...
Factors effecting Relaxation :
• Time
• Initial stress
• Temperature and

• Type of steel.

36
Method Available to Reduce The Loss
due to Relaxation
• Choice of proper steel helps to reduce this
loss.
• Prestressed wi...
Calculation of Total Amount of Loss

38
Loss of Prestress=Initial StressEffective Stress

39
Initial Prestress
• Deducting the loss due to anchorage take-up and
friction, initial prestress is obtained.
• If prestres...
Effective Prestress
• Initial Prestress in steel minus the losses is
known as the effective or design prestress.
Effective...
Total Amount Of Losses According To
Tensioning System
• Total pretension losses=Loss due to creep +
Elastic shortening + S...
Total Losses
• It is difficult to generalize the amount of loss
of prestress, because it is dependent on so
many Factors :...
Method of Loss Estimation
• There are two methods that can be used to
estimate losses in prestressed concrete:
(a)lump sum...
Lump Sum Estimation For Prestress
Loss
• First introduce by the ACI-ASCE Committee 423 in
1958.
Table : AASHTO Lump Sum Lo...
AASHTO-LRFD Specifications
For Lump-Sum estimates following conditions should met :
1. Members that are post-tensioned mus...
Thumb rule of Losses
• For average steel and concrete properties ,the tabulated
percentages may be taken as representative...
Comparison Between RCC & Prestress
Concrete

48
Thank You

49
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Prestress loss

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Prestress loss

  1. 1. AHSANULLAH UNIVERSITY OF SCIENCE & TECHNOLOGY Course No : CE-416. Course Title : Prestress Concrete Design Sessional. 1
  2. 2. 2
  3. 3. Welcome To Our Presentation 3
  4. 4. Group Members 10.01.03.062 10.01.03.063 10.01.03.065 10.01.03.066 10.01.03.069 10.01.03.072 10.01.03.075 10.01.03.076 - Shahriar Mujthahid Hossain. - H.M.Suruzzaman. - Sabbir Bin Delwar. - Md. Imran Islam. - Tasfia Ahmed. - Mohammad Shakib Rahman. - Munshi Md. Rasel. - Md.Moinul Islam. 4
  5. 5. What is Pre-stressed Concrete? – – Internal stresses are induced to counteract external stresses. In 1904, Freyssinet attempted to introduce permanent acting force in conc. to resist elastic forces under loads and was named “Pre stressing”. 5
  6. 6. Introduction • In prestressed concrete applications, most important variable is the prestress. • Prestress does not remain constant with time. • Even during prestressing of tendons, and transfer of prestress, there is a drop of prestress from the initially applied stress. • Reduction of prestress is nothing but the loss in prestress. 6
  7. 7. Prestress Loss • loss in prestress is the difference between initial prestress and the effective prestress. • Loss of prestress affects – the strength of member and – member’s serviceability [ Stresses in Concrete, Cracking, Camber and Deflection ] 7
  8. 8. Types Loss of prestress is classified into two types: 1. Immediate Losses  immediate losses occur during prestressing of tendons, and transfer of prestress to concrete member. 2. Time Dependent Losses  Time dependent losses occur during service life of structure. 8
  9. 9. Types According To Time Prestress Losses Time Dependent Immediate Elastic Shortening Friction Anchorage Slip Creep Shrinkage Relaxation 9
  10. 10. Types According To Material Prestress Losses Concrete Elastic Shortening Creep Steel Shrinkage Friction Anchorage Slip Relaxation 10
  11. 11. Types of Prestressing Systems I . Pre-tensioning:  In Pre-tension, the tendons are tensioned before the concrete is place. After the concrete hardened, the tension force is released. II . Post tensioning:  In Post tension, the tendons are tensioned after the concrete has hardened. 11
  12. 12. Example of Pre-tensioning Fig : Pre-tensioned electric pole 12
  13. 13. Example of Post-tensioning Fig : Post-tensioning of a box girder 13
  14. 14. Losses in Various Prestressing Systems Type of Loss Pre-tensioning Post-tensioning i. No, if all the cables are simultaneously tensioned. If the wires are tensioned in stages loss will exist. 1. Elastic Shortening Yes 2. Anchorage Slip 3. Friction Loss No No Yes Yes 4. Creep and Shrinkage of Concrete Yes Yes 5. Relaxation of Steel Yes Yes ii. 14
  15. 15. Immediate Loss 15
  16. 16. Elastic Shortening • It is the shorten of concrete member, when the prestress is transferred to concrete, the member shortens and the prestressing steel also shortens in it. Hence there is a loss of prestress. Original length of member at transfer of prestress Pi Length after elastic shortening P0 16
  17. 17. Elastic Shortening at Pre-tensioned Members  When the tendons are cut and the prestressing force is transferred to the member, concrete undergoes immediate shortening due to prestress.  Tendon also shortens by same amount, which leads to the loss of prestress. 17
  18. 18. Elastic Shortening at Pre-tensioned Members Prestressing bed Pre-tensioning of a member 18
  19. 19. Elastic Shortening at Post-tensioned Members  If there is only one tendon, there is no loss because the applied prestress is recorded after the elastic shortening of the member.  For more than one tendon, if the tendons are stretched sequentially, there is loss in a tendon during subsequent stretching of the other tendons. 19
  20. 20. Elastic Shortening at Post-tensioned Members Duct Anchorage jack Casting bed Post-tensioning of a member 20
  21. 21. Anchorage Slip • In most Post-tensioning systems when the prestress force is transferred from the jack to the anchoring ends, the wedges slip over a small distance. • Loss of prestress is due to the consequent reduction in the length of the tendon. • Amount of slip depends on type of anchorage system. 21
  22. 22. Force variation diagrams for various stages a) The initial tension at the right end is high to compensate for the anchorage slip. It corresponds to about initial prestress. The force variation diagram (FVD) is linear. b) After the anchorage slip, the FVD drops near the right end till the length lset. Note : Effect of anchorage slip is present up to a certain length, called the setting length lset. 22
  23. 23. Force variation diagrams for various stages c) The initial tension at the left end also corresponds to about initial prestress. The FVD is linear up to the centre line of the beam. d) After the anchorage slip, the FVD drops near the left end till the length lset. It is observed that after two stages, the variation of the prestressing force over the length of the beam is less than after the first stage. 23
  24. 24. Typical values of anchorage slip Anchorage System Freyssinet system 12 - 5mm Φ strands 12 - 8mm Φ strands Magnel system Dywidag system Anchorage Slip (Δs) 4 mm 6 mm 8 mm 1 mm 24
  25. 25. Frictional Loss • The friction generated at the interface of concrete and steel during the stretching of a curved tendon in a post-tensioned member. • The friction in the jacking anchoring system is generally small. • More serious frictional loss occurs between the tendon and its surrounding material. 25
  26. 26. Frictional loss occurs only in Posttensioned Members • The loss due to friction does not occur in pretensioned members because there is no concrete during the stretching of the tendons. • Friction is generated due to curvature of tendon, and vertical component of the prestressing force. A typical continuous post-tensioned member 26
  27. 27. Frictional Loss Frictional Loss is the summation of – Friction Loss Due to length Effect. – Friction Loss Due to Curvature Effect. • Length Effect: If the profile of cable is linear, the loss will be due to straightening or stretching of the cables. • Curvature Effect: If the profile is curved, there will be loss in stress due to friction between tendon and the duct or between the tendons themselves. 27
  28. 28. Length & Curvature Effects 28
  29. 29. Methods available to “Reduce” the frictional losses 1. Cables should pass through metal tubes. 2. The bends should be through as small an angle as possible. 3. Radius of curvature for bends should be large. 4. Prestressing the wire from both ends. 5. Over-tensioning the wires. 29
  30. 30. Time Dependent Losses 30
  31. 31. Creep of Concrete • The Continuous deformation of concrete with time under sustained load.  Factors affecting creep of concrete • Age • Applied Stress level • Density of concrete • Cement Content in concrete • Water-Cement Ratio • Relative Humidity and • Temperature 31
  32. 32. Condition for calculating the loss of prestress due to creep. • Creep is due to sustained (permanent) loads only. Temporary loads are not considered in calculation of creep. • Since the prestress may vary along the length of the member, an average value of the prestress is considered. 32
  33. 33. Shrinkage of Concrete • Shrinkage of defined as concrete the is contraction due to loss of moisture. • Due to the shrinkage of concrete, the prestress in the tendon is reduced with time. 33
  34. 34. Shrinkage of Concrete • For pre-tensioned members, transfer commonly takes place after 24 hours after casting and nearly all shrinkage takes place after that. • For post-tensioned members, stressing may takes place after one day or much later, thus a large percentage of shrinkage may already taken place by them. 34
  35. 35. Relaxation  Relaxation is the reduction in stress with time at constant strain. – decrease in the stress is due to the fact that some of the initial elastic strain is transformed in to inelastic strain under constant strain. – Percentage of relaxation varies from 1 to 5%. 35
  36. 36. Factors effecting Relaxation : • Time • Initial stress • Temperature and • Type of steel. 36
  37. 37. Method Available to Reduce The Loss due to Relaxation • Choice of proper steel helps to reduce this loss. • Prestressed wires have lesser creep. • Galvanised wires also have no creep. • overstressing steel about 10% above its initial stress and then releasing it to the initial stress 37
  38. 38. Calculation of Total Amount of Loss 38
  39. 39. Loss of Prestress=Initial StressEffective Stress 39
  40. 40. Initial Prestress • Deducting the loss due to anchorage take-up and friction, initial prestress is obtained. • If prestress is measured at the time of pulling the wire, the stress is termed as the jacking stress. • if jacketing stress is treated as the initial stress, effective stress is jacketing stress minus all losses. 40
  41. 41. Effective Prestress • Initial Prestress in steel minus the losses is known as the effective or design prestress. Effective prestress=Initial prestress-Losses Note: For Pre-Tension system , Pretension Losses are used instead of losses. For Post-Tension system , Post-Tension Losses are used instead of losses. 41
  42. 42. Total Amount Of Losses According To Tensioning System • Total pretension losses=Loss due to creep + Elastic shortening + Shrinkage + Steel Relaxation. • Total post-Tension Loss=Loss due to creep + Elastic shortening + Shrinkage + Steel Relaxation +Anchorage slip + Friction. 42
  43. 43. Total Losses • It is difficult to generalize the amount of loss of prestress, because it is dependent on so many Factors : The properties of concrete & steel. Curing & moisture condition. Magnitude & time of application of prestress. Process of prestress. 43
  44. 44. Method of Loss Estimation • There are two methods that can be used to estimate losses in prestressed concrete: (a)lump sum approximations; (b)refined estimations. One should keep in mind that all estimates for prestress loss are just that – ESTIMATIONS . 44
  45. 45. Lump Sum Estimation For Prestress Loss • First introduce by the ACI-ASCE Committee 423 in 1958. Table : AASHTO Lump Sum Losses. Total Loss Types of prestressing steel Pretensioning strand Posttensioning wire or strand Bars f’c=28 MPa f’c=35 MPa 310 MPa 220 MPa 230 Mpa 150 MPa 160 Mpa 45
  46. 46. AASHTO-LRFD Specifications For Lump-Sum estimates following conditions should met : 1. Members that are post-tensioned must be non-segmental members with spans less than 160 feet and concrete stressed an age of 10-30 days. 2. Members that are pretensioned must be stressed at an age where the concrete strength is not less than 3,500-psi. 3. Members must be made from normal weight concrete. 4. Members cannot be steam-cured, nor moist-cured. 5. The prestressing steel must be normal or low-relaxation. 6. There must be Average exposure conditions at the site. 46
  47. 47. Thumb rule of Losses • For average steel and concrete properties ,the tabulated percentages may be taken as representative of the average losses. Pretensioning ,% Posttensioning. % Elastic shortening & bending of concrete 4 1 Creep of concrete 6 5 Shrinkage of concrete 7 6 Steel relaxation 8 8 25 20 Total Loss 47
  48. 48. Comparison Between RCC & Prestress Concrete 48
  49. 49. Thank You 49

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