Shear, bond bearing,camber & deflection in prestressed concrete


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This Presentation was presented as a partial fulfillment of Prestressed Concrete Design Lab Course. Behavior & Design of Prestress on above topic is shortly discussed on the presentation. The part "Shear & Shear Design in Prestressed" Concrete was prepared by me. Other topics were prepared by other members of my group. Thanks to all my teachers & friends who helped us in different stages during preparation of the total presentation.

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  • Types of prestress con.
  • How prestress force transfer in prestressed concrete
  • Which members transfer load through bond
  • Bond stress reduce by reduction of tendon area
  • Transmission length function as anchorage when mechanical anchorage is not present….
  • The stress field in the end zone of a post-tensioned member is complicated. The compressive stress trajectories are not parallel at the ends. The trajectories diverge from the anchorage block till they become parallel. The larger transverse dimension ofthe end zone is represented as y0. Thecorresponding dimension of the bearing plate is represented as yp0. For analysis, the end zone is divided into a local zoneand a general zoneas shown in the following sketch
  • The local zone is the region behind the bearing plate and is subjected to high bearing stress and internal stresses. The behaviour of the local zone is influenced by the anchorage device and the additional confining spiral reinforcement. The general zone is the end zone region which is subjected to spalling of concrete. The zone is strengthened by end zone reinforcement.
  • The variation of the transverse stress (σt) at the CGC along the length of the end zoneis shown in the next figure. The stress is compressive for a distance 0.1y0fromthe end. Beyond that it is tensile. The tensile stress increases and then drops down to zero within a distance y0 from the end.
  • The end zone reinforcement is provided in several forms, some of which are proprietary of the construction firms. The forms are closed stirrups, mats or links with loops. A few types of end zone reinforcement is shown in the following sketches.
  • The expression of allowable bearing stress takes advantage of the dispersion of the bearing stress in the concrete. The following sketch illustrates the dispersion of bearing stress in concrete
  • The performance of anchorage blocks and end zone reinforcement is critical during the post-tensioning operation. The performance can be evaluated by testing end blockspecimens under compression. The strength of an end block specimen should exceed the design strength of the prestressing tendons.
  • The effect of deflection in a structure variesaccording to the use of the structure. Excessive deflections may lead to saggingfloors, to roof that do not drain properly, tofloors, to roof that do not drain properly, todamage partitions and finishes, to the creationof pools of water on road surface of bridges, andto other associated troubles
  • Deflections of prestress-concrete beams are different from reinforced-concrete beams,
  • The magnitude and distribution of the load, the length of the span, the size and the configuration of the cross section and the quality of the concrete affect the short time deflection a of PSC beam
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  • Shear, bond bearing,camber & deflection in prestressed concrete

    1. 1. 1
    2. 2. Course No: CE-416 Course Name: PRESTRESS CONCRETE DESIGN SESSIONAL 2
    3. 3. 3
    4. 4. Name Student ID Md. Zahidul Islam Shaikh Mahfuzur Rahman Rifath Ara Rimi MD. Jahirul Islam MD. Rakibul Islam Md. Neshar Ahmed Raiyan Fardous Ratul Md. Shahadat Hossain Md. Ridwan-Ur-Rahman Group : 4 4
    5. 5. 5
    6. 6. What Is Shear Force Shear forces are unaligned forces pushing one part of a body in one direction, and another part the body in the opposite direction. Shear force acting on a substance in a direction perpendicular to the extension of the substance. 6
    7. 7. Shear Mechanism In a simply supported rectangular beam, self weight & super imposed loads act downward, reaction acts upward. Resultants of all these vertical forces generates vertical shear in a member. 7
    8. 8. Shear Normal Concrete Vs Pre-stressed Concrete • Comparatively smaller sectioned member needed for load carrying, so less self weight i.e. less shear. RCC BEAM Prestressed Concrete Member D1 D2 D1>D2 i.e. for same load carrying 8
    9. 9. Shear Normal Concrete Vs Pre-stressed Concrete • Sagged tendon in most case provide additional shear but opposite direction. 9
    10. 10. Shear Normal Concrete Vs Pre-stressed Concrete • Prestressing prevents the occurrence of shrinkage cracks which could conceivably destroy the shear resistance. 10
    11. 11. Modes of Failure in Prestressed Beam 11
    12. 12. Flexure-Compression (FC): Flexure compression failures are the result of having a beam with higher shear strength than flexural strength. Failure occurs at the point of maximum flexural stress where the compressive strain exceeds its capacity. 12
    13. 13. Flexure-Shear Failure A flexure-shear failure, is the result of a crack which begins as a flexural crack, but as shear increases, the crack begins to “turn over” and incline towards the loading point. Failure finally occurs when the concrete separates and the two planes of concrete slide past one another. This mode of failure is common in beams which do not contain web reinforcement. 13
    14. 14. Shear-Compression Failure Shear compression failures, shown in Figure, typically occur in beams which contain adequate web reinforcement. In this mode, the crack propagates through the section until it begins to penetrate the compression zone. This crack causes a redistribution of compressive forces in the compression zone onto a smaller area. When the compressive strength is exceeded, a shear compression failure occurs. This type of failure is common in deep beams, where arch action is prevalent. The compressive strut caused by arch action prevents a diagonal tension crack from propagating into the compression zone. 14
    15. 15. Web-shear Failure Before a section cracks from flexure, it is possible to exceed the tensile strength of the concrete at the point of maximum shear stress. This mode is primarily observed in sections with thin webs. Failure occurs at the location of peak shear stress, as shown in Figure. While, the mechanics of this failure are identical to flexure- shear, failure is brittle and occurs with little or no warning. 15
    16. 16. Factors Influencing Shear Strength • Axial Force: Shear failures are commonly due to tensile failure of the concrete. Axial compression can delay the onset of critical tension in the section, axial tension can hasten the failure. Compression, such as provided by an axial force or prestressing tendons, provides an increase in shear strength. • Tensile Strength of Concrete: As the tensile strength of the concrete is increased, there is a corresponding increase in the shear strength of the section. • Longitudinal Reinforcement Ratio: Low amount of steel may result in wider flexural cracks, resulting in reduced dowel action and aggregate interlock. • Shear Span-to-Depth Ratio: High values of require a larger compression zone, raising the amount of shear which can be transferred by the uncracked concrete shear transfer mechanism, thus increasing shear strength 16
    17. 17. Shear Carrying of Concrete & Tendon on Different Tendon Profile 17
    18. 18. Some Important Notes about Shear in Prestressed Concrete • Prestressed beam never fail under direct shear or punching shear. They fail as a result of tensile stress produced by shear. • In some rare instance the transverse component of prestress increases the shear in concrete. • By following load balancing approach, it is theoretically possible to design a beam with no shear in concrete under a given condition of loading. 18
    19. 19. Development of Shear Cracking 19
    20. 20. Steps of Shear Design For a Simply Supported Beam Section with UDL loading • Step -1: Calculate the moment of inertia of the section. • Step -2: Calculate Support reaction. • Step -3: Calculate Moment at desire beam section from x distance from support. • Step -4: Calculate ‘a’ and then the eccentricity of tendon at desire (x) distance from support i.e. ex 20
    21. 21. For Flexural Shear Crack • Calculate • Calculate • Calculate Flexural Cracking Moment • Calculation of cracking flexural shear • Calculation of Nominal flexural shear 21
    22. 22. For Web Shear Crack • Calculate • Calculation of Nominal web shear • Calculate ultimate load • Calculate factored shear at a section x distance from support 22
    23. 23. Shear Reinforcement Spacing Smallest spacing among S1, S2, S3 should be chosen as stirrup spacing. 23
    24. 24. End of topic Shear in Prestressed Concrete 24
    25. 25. “BOND” in Prestressed Concrete 25
    26. 26. Definition Interlocking between two properties e.g. pre- stressed tendon and concrete. 26
    27. 27. Main Types of Internal Prestressed Concrete • Pre-Tension Concrete: Pre-stressing steel is tension stressed prior to the placement of the concrete and unloaded after concrete has harden to required strength. • Bonded post-tensioned concrete: Unstressed pre- stressing steel is placed with in the concrete and then tension stressed after concrete has harden to required strength • Un-bonded post-tensioned concrete: Differs from bonded post-tensioning by providing the pre-stressing steel permanent freedom of movement relative to the concrete. 27
    28. 28. Transfer of Prestressing Force: Bond between concrete and prestressing steel. Bearing at end anchorages. 28
    29. 29. Existence of Bond in Prestressed concrete 1.Pre-Tension Concrete 2.Bonded post- tensioned concrete 29
    30. 30. “Bond ” effects in Prestressed concrete Bond exists on two different basis: 1. Pre-tensioning system  Used as a means of transferring the prestressing force of tendon to the concrete section. 2. Post-tensioning system  In this, bond is necessary for two purposes, -Protection against corrosion -Increase in ultimate strength 30
    31. 31. Bond effect in Pre-tensioned construction 1.It is furnished by two factors, -Reduction in area of cross section of steel -Adhesive property 2.The phenomenon of recovery of lateral contraction develops a wedge action at the end of the cable by which prestressing force is transferred. 3.This property was discussed detail by Hoyer and is called “HOYER EFFECT”. 4.Transverse reinforcement has to be provided to resist tensile force. 31
    32. 32. Bond mechanisms in the prestressing concrete : 1) Adhesion between concrete and steel 2) Mechanical bond at the concrete and steel interface 3) Friction in presence of transverse compression. 32
    33. 33. Hoyer Effect After stretching the tendon, the diameter reduces from the original value due to the Poisson’s effect. When the prestress is transferred after the hardening of concrete, the ends of the tendon sink in concrete. The prestress at the ends of the tendon is zero. The diameter of the tendon regains its original value towards the end over the transmission length. The change of diameter from the original value (at the end) to the reduced value (after the transmission length), creates a wedge effect in concrete. This helps in the transfer of prestress from the tendon to the concrete. This is known as the “Hoyer effect”. 33
    34. 34. Development length(Ld): The development length (Ld) is the sum of the transmission length (Lt) and the bond length (Lb). 34
    35. 35. Transmission length:  The bond needed to transmit the complete prestressing force is called transmission length(Lt).  The stress in the tendon is zero at the ends of the members. It increases over the transmission length to the effective prestress (fpe) under service loads and remains practically constant beyond it. Fig : Variation of prestress in tendon along transmission length 35
    36. 36. Factors that influence the transmission length: 1) Type of tendon  ¾ wire, strand or bar 2) Size of tendon 3) Stress in tendon 4) Surface deformations of the tendon  ¾ Plain, indented, twisted or deformed 5) Strength of concrete at transfer 6) Pace of cutting of tendons  ¾ Abrupt flame cutting or slow release of jack 7) Presence of confining reinforcement 8) Effect of creep 9) Compaction of concrete 10) Amount of concrete cover. 36
    37. 37. The bond length: Fig : Variation of prestress in tendon at ultimate The bond length (Lb) is the minimum length over which, the stress in the tendons can increase from the effective prestress(fpe) to ultimate prestress(fpu) at critical location. The expression of the bond length is derived as, 37
    38. 38. The bond length depends on the following factors: 1) Surface condition of the tendon 2) Size of tendon 3) Stress in tendon 4) Depth of concrete below tendon 38
    39. 39. End zone reinforcement The prestress and the Hoyer effect cause transverse tensile stress (σt). This is largest during the transfer of prestress. To resist the splitting of concrete, transverse reinforcement need to be provided at each end of a member along the transmission length. This reinforcement is known as “End zone reinforcement’’. The minimum amount of end zone reinforcement is given as, h = total depth of the section M= moment at the horizontal plane at the level of CGC due to the compressive stress block above CGC fs = allowable stress in end zone reinforcement 39
    40. 40. Bond in Post-tensioned construction  Effect of bond in post-tensioned construction has two distinct purposes; 1.Protection against stress corrosion -Moisture enters into duct -Cause corrosion to high tension steel -Rusting reduces effective area of steel -This causes splitting of wires called stress corrosion 40
    41. 41. 2.Increase in ultimate strength ● In bonded construction -Crack at the critical section does not affect the strain in steel -Because of this, the compressive area is not reduced considerably 41
    42. 42. Process – Concrete is casted around a curved duct (usually corrugated), to allow room for the Tendon to be inserted. – After the concrete has hardened the tendons are pulled in tension and then wedged. – The duct is then injected with grout  There are 2 layers of bonding media in post-tensioned construct -Bond between the steel and the sheath or duct -Bond between the sheath and the concrete 42
    43. 43. End of this topic 43
    44. 44. Bearing or Bearing plate 44
    45. 45. Definition A bearing plate is a specially-designed metal plate used to spread the force of a load- bearing wall or column out over a larger area Fig: Bearing plates 45
    46. 46. The end zone (or end block) of a post-tensioned member is a flared region which is subjected to high stress from the bearing plate next to the anchorage block. It needs special design of transverse reinforcement. The design considerations are bursting force and bearing stress. Some Important things to know 46
    47. 47. Behavior of the local zone • The behavior of the local zone is influenced by the anchorage device and the additional confining spiral reinforcement 47
    48. 48. The transverse tensile stress is known as splitting tensile stress. The resultant of the tensile stress in a transverse direction is known as the bursting force(Fbst). Compared to pre-tensioned members, the transverse tensile stress in post-tensioned members is much higher. Behavior of the local zone (Contd.) 48
    49. 49. For calculating bursting force (Fbst) an individual square end zone loaded bearing plate. Calculating bursting force 49
    50. 50. End Zone Reinforcement • The amount of end zone reinforcement in each direction (Ast) can be calculated from the following equation. 50
    51. 51. 51
    52. 52. • The bearing stress in the local zone should be limited to the following allowable bearing stress (fbr,all) 52
    53. 53. Dispersion of bearing stress in concrete 53
    54. 54. Manufacturing of an end block specimen Fabrication of end zone reinforcement Anchorage block and guide 54
    55. 55. Manufacturing of an end block specimen (Contd.) End zone reinforcement with guide and duct End block after casting 55
    56. 56. End of this topic 56
    57. 57. Camber & Deflection 57
    58. 58. Camber  Camber is the upward deflection in the beam after release of the prestressing strands due to the eccentricity of the force in the strands. The camber of the beam is usually the largest contribution to hunch. 58
    59. 59. Factors of camber The ability to predict camber accurately is critical for the design and constructions . However, this is a complex task, since the camber is dependent on many variables, some of which are interdependent and change over time. Four of the most significant variables are the properties of the concrete , 1. creep of the concrete, 2. concrete temperature 3. the magnitude 4. location of the prestress 59
    60. 60. Deflection 60
    61. 61. Definition  In general, Deflection is the degree to which a structural element is displaced under a load. 61
    62. 62. Types of Deflection  Short-term deflection occurs immediately upon the application of a load.  Long-term deflection takes into account the long-term shrinkage and creep movements. 62
    63. 63. Causes of Deflection in PSC Beams Due to external loads Due to prestress force 63
    64. 64. Tendon Profile The deflection due to prestress depends on the profile of the c.g.s. line 64
    65. 65. Methods of Calculation  Double Integration Method  Moment Area Method  Conjugate Beam Method  Principle of Virtual Load 65
    66. 66. Calculations of the Short-term Deflection The usual loading which should be investigated in calculating deflections are: Prestress plus dead load Prestress plus maximum service load Prestress plus minimum service load 66
    67. 67. 67
    68. 68. 68
    69. 69. ANY QUESTION 69