Chapter 1 Basic Concepts (PART I) (1).ppt
- 2. DEFINATION Prestressing can be defined in general terms as the preloading of a structure, before application of the service loads, so as to improve its performance in specific ways. A prestressed concrete member can also be defined as one in which, there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from the given external loading are counteracted to a desired degree.
- 3. INTRODUCTION In Conventional Reinforced Concrete, the high tensile strength of steel is combined with concrete's great compressive strength to form a structural material that is strong in both compression and tension. The principle behind Prestressed Concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.
- 4. INTRODUCTION Prestressing removes a number of design limitations that conventional concrete places on span and load and permits the building of roofs, floors, bridges, and walls with longer unsupported spans. This allows architects and engineers to design and build lighter and shallower concrete structures without sacrificing strength.
- 5. Use of precast "double-T" beams carrying a floor with clear span about 20 ft.
- 6. Twin box-girder bridge under construction using the segmentally cast cantilever method
- 7. Highway crossing in Switzerland, continuous over three spans
- 8. Segmentally precast post-tensioned rigid frames for the Olymic stadium in Montreal
- 9. INTRODUCTION Prestressed concrete has experienced greatest growth in the field of commercial buildings. For buildings such as shopping centers, Prestressed Concrete is an ideal choice because it provides the span length necessary for flexibility and alteration of the internal structure.
- 13. INTRODUCTION Prestressed concrete is also used in school auditoriums, gymnasiums, and cafeterias because of its properties and its ability to provide long, open spaces. One of the most widespread uses of prestressed concrete is parking garages.
- 17. INTRODUCTION Although prestressed concrete was patented by a San Francisco engineer in 1886, it did not emerge as an accepted building material until a half-century later. North America's first prestressed concrete structure, the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania, however, was not completed until 1951.
- 18. The Linn Cove Viaduct, North Carolina, U.S. 1997
- 20. INTRODUCTION Prestressing means the internal creation of permanent stresses in a structure or assembly, for the purpose of improving its behavior and strength under various service conditions. The concept of Prestressing Concrete was to introduce sufficient axial pre-compression in beams so that all tension in the concrete was eliminated in the member at service load.
- 21. The Basic Idea The principle behind prestressing is applied when a row of books is moved from place to place. Instead of stacking the books vertically and carrying them, the books may be moved in a horizontal position by applying pressure to the books at the end of the row.
- 22. The Basic Idea When sufficient pressure is applied, compressive stresses are induced throughout the entire row, and the whole row can be lifted and carried horizontally at once.
- 23. METHODS OF PRESTRESSING Although many methods have been used to produce the desired state of pre-compression in concrete members, all prestressed concrete members can be placed in one of two categories: (a) Pre-tensioned concrete (b) Post-tensioned concrete
- 24. PRETENSIONED CONCRETE In Pretensioning, the steel is stretched before the concrete is placed. High-strength steel tendons are placed between two abutments and stretched to 70 to 80 percent of their ultimate strength. Concrete is poured into molds around the tendons and allowed to cure. Once the concrete reaches the required strength, the stretching forces are released. As the steel reacts to regain its original length, its tensile stresses are translated into compressive stresses in the concrete.
- 25. PRETENSIONED CONCRETE Typical products for Pretensioned concrete are roof slabs, piles, poles, bridge girders, wall panels, and railroad ties.
- 26. PRETENSIONED CONCRETE This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. It requires heavy anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size.
- 27. Prestressing strand, Made of high strength steel PRETENSIONED CONCRETE
- 28. The Prestressing strand is stretched across the casting bed. About 30,000 pounds of tension will be applied to the cable before it's surrounded by concrete. PRETENSIONED CONCRETE
- 29. The ends are cleaned and the prestressing strands are sealed with a protective coating. PRETENSIONED CONCRETE
- 30. PRETENSIONED CONCRETE Pretensioning is well suited to the mass production of beams using the long-line method of prestressing. In present practice anchorage and jacking abutments may be as much as 800 ft apart. The strands are tensioned over the full length of the casting bed at one time, after which a number of individual members are cast along the stressed tendon. When the jacking force is released, the prestress force is transferred to each member by bond, and the strands are cut free between members.
- 32. POST TENSIONING In Post-tensioning, the steel is stretched after the concrete hardens. Concrete is cast around, but not in contact with unstretched steel. In many cases, ducts are formed in the concrete unit using thin walled steel forms. Once the concrete has hardened to the required strength, the steel tendons are inserted and stretched against the ends of the unit and anchored off externally, placing the concrete into compression.
- 33. POST TENSIONING Post-tensioned concrete is used for cast-in-place concrete and for bridges, large girders, floor slabs, shells, roofs, and pavements. Tendons are normally grouted in their conduits after they are stressed. A cement paste grout is forced into the conduit at one end under high pressure, and pumping is continued until the grout appears at the far end of the tube. When it hardens, the grout bonds to the tendon and to the inner wall of the conduit, permitting transfer of force.
- 35. POST TENSIONING Although the anchorage fittings remain in place to transfer the main prestressing force to the concrete, grouting improves the performance of the member should it be overloaded and increases its ultimate flexural strength.
- 36. Post-tensioned beam under construction, showing inserted tendon conduits and anchorages in position prior to placing side forms and pouring concrete POST TENSIONING
- 39. POST TENSIONING Post Tensioning of a Slab
- 40. TENDON PROFILES Different types of tendon profiles are used for load balancing in prestressed concrete. Load balancing concept sees prestressed concrete as primarily an attempt to balance a portion of the load on the structure. It has no significant advantage for statically determinate structure while this method offers tremendous advantages for statically indeterminate structures.
- 41. TENDON PROFILES Simply supported beams with concentrated load Fig illustrates how to balance a concentrated load by sharply bending the tendon at mid span thus creating an upward component. F F θ θ L Parabolic Tendon Concrete Centroid P = V V = 2FSinθ FSinθ FCosθ c.g.s. c.g.c.
- 42. TENDON PROFILES Simply supported beams with UDL Fig illustrates the balancing of a uniformly distributed load by means of a parabolic cable with an upward component v (lb/ft). F P θ θ L v h Parabolic Tendon Concrete Centroid Uniform load w
- 43. TENDON PROFILES Simply Supported Cantilever Beams Fig illustrates the balancing of a uniformly distributed load for a cantilever beam by means of a parabolic cable. F L1 Parabolic Tendon Concrete Centroid h h/4 h1 Uniform load w . A C B L
- 44. TENDON PROFILES Simply Supported Beam with curved centroid. Fig illustrates the balancing of a uniformly distributed load for Simply Supported Beam with curved centroid. F F L h Parabolic Tendon Concrete Centroid Uniform load w . .
- 45. TENDON PROFILES Continuous Beams Fig illustrates the balancing of a uniformly distributed load for Continuous Beam F F θ θ L Parabolic Tendon Concrete Centroid Uniform load w . L
- 46. ADVANTAGES OF PRESTRESSING Prestressing the steel and anchoring it against concrete produces desirable strains and stresses which serve to reduce or eliminate cracks in concrete. The entire section of the concrete is effective in prestressed concrete, whereas only the portion of section above the neutral axis is supposed to act in the case of reinforced concrete.
- 47. ADVANTAGES The use of curved tendons help to carry some of the shear in the member. Pre-compression in the concrete tends to reduce the diagonal tension. So, it is possible to use smaller section in prestressed concrete to carry the same amount of external shear in the beam. Prestressed concrete is more suitable for structures of long spans and those carrying heavy loads.
- 48. ADVANTAGES Prestressed structures are more slender and hence more adaptable to artistic treatment. Under dead load, the deflection is reduced, owing to the cambering effect of prestress. Under live loads, the deflection is smaller because of the effectiveness of the entire uncracked concrete section, which has a moment of inertia two to three times that of the cracked section.
- 49. ADVANTAGES Prestressed elements are more adaptable to precasting because of the lighter weight. The resistance to corrosion is better than that of reinforced concrete for the same amount of cover, owing to the nonexistence of cracks.
- 50. ADVANTAGES Regarding fire resistance, high-tensile steel is more sensitive to high temperatures, but for the same amount of minimum cover, prestressed tendons can have a greater average cover because of the spread and curvature of the individual tendons. Reduction of weight saves handling and transportation costs.
- 51. DISADVANTAGES Prestressed concrete is not suitable for structures of shorter spans. Prestressed concrete cannot be used in such situations where weight and mass are desired instead of strength.
- 52. DISADVANTAGES Prestressed concrete members require more care in design, construction and erection than those of ordinary concrete. Cost increases due to high strength concrete and high strength steel. Cost increases due to highly skilled labour.
- 53. DISADVANTAGES Complicated form work is required due to nonrectangular shapes. Losses in prestress force due to slip, creep, friction etc. Anchorages, conduits & Jacks are required.
- 54. INSTANTANEOUS LOSSES IN PRESTRESSING There is an Instantaneous Stress Loss because of the elastic shortening of the concrete as the prestress force is transferred to it. In Pretensioned concrete, as the prestress is transferred to the concrete, the member shortens and the prestressed steel shortens with it. In Post-tensioned concrete member, the concrete shortens as the tendons are jacked against the concrete. Hence, there is a loss of prestress force.
- 55. INSTANTANEOUS LOSSES IN PRESTRESSING When the tendon is tensioned to its full value, the jack is released and the prestress is transferred to anchorage. The Anchorage Fixtures that are subjected to stress at the time of transfer, will tend to deform, thus allowing the tendon to slacken slightly. Friction wedges employed to hold the wires will slip a little distance before the wires can be firmly gripped. The amount of slippage depends on type of wedge and the stress in the wires. However, in beams pre-tensioned by the long-line method, slip loss is apt to be insignificant because of the great length of tendon over which the slip is distributed.
- 56. INSTANTANEOUS LOSSES IN PRESTRESSING Change in Prestress force of member also occur, when member is subjected to bending. There may be a loss or a gain , depends on the direction of bending and the location of the tendon. Another source of immediate loss of prestress force is the friction between the steel and the conduit through which it passes as the tendon is stretched.
- 57. TIME-DEPENDENT LOSSES IN PRESTRESSING The Time-dependent Losses are shrinkage of concrete and concrete creep under sustained compressive stress. Creep is one of the main sources of loss and a serious one because the amount of creep ranges from 1 to 5 times than the elastic shortening, if the prestress is low and the compression in the concrete is high.
- 58. TIME-DEPENDENT LOSSES IN PRESTRESSING The percentage of creep increases with increasing stress, and when steel is under low stress, the creep is negligible. Stress relaxation in steel is the loss of its stress when it is prestressed and maintained at a constant strain for a period of time.
- 59. TOTAL LOSSES IN PRESTRESSING The sum of all type of losses i.e., instantaneous and Time dependent, may be of the order of 20 to 35 percent of the original jacketing force.