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CE-415 (prestressed concrete) (1)-civil engineering.pptx
- 1. CE 415 Pre-stressed Concrete ImonHassan Bhuiyan Lecturer Dept. of Civil Engineering Dhaka International University
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- 3. Reinforced Concrete ❖Concrete isstrong in compression and weak in tension. ❖Steel is strong in tension. ❖Reinforced Concrete uses concrete to resist compression and to hold bars in position and uses steel to resist tension. ❖Tensile strength of concrete is neglected (i.e. zero). ❖R.C. beams allows cracks under service load.
- 4. Pre-stressed Concrete ❖Pre-stressed concreteis a method for overcoming concrete’s natural weakness in tension. ❖In 1904, Freysinet attempted to introduce permanent acting forces in concrete to resist elastic forces under loads and called ‘pre-stressing’. ❖It can be used to produce beams, floors or bridges with a longer span than in practical with ordinary reinforced concrete.
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- 8. Principle of pre-stressing Cross section Pre-stressing force Stress fromDL Stress from LL Resultant Stress Large compression Very little or zero tension Stress in concrete when pre-stressing is applied at the c.g. of the section.
- 9. Principle of pre-stressing Cross section Pre-stressing force Stress fromDL Stress from LL Resultant Stress Large compression Very little or zero tension Stress in concrete when pre-stressing is applied eccentrically w.r.to the c.g. of the section. Pre-stressing Force due to eccentricity
- 10. Basic concept ofpre-stressing There are three basic concepts of pre-stressing: o First concept: Pre-stressing to transform concrete into an elastic material. o Second concept: Pre-stressing for combination of high strength steel with concrete. o Third concept: Pre-stressing to achieve load balancing
- 11. Advantages of pre-stressing ❑Fullsection is utilized. ❑Reduction in steel corrosion. ❑Improved performance under dynamic and fatigue loading. ❑High span to depth ratio. ❑Rapid construction. ❑Better quality control. ❑Reduced Maintenance. ❑Availability of standard shapes.
- 12. Limitations for pre-stressing ✔Pre-stressingneeds skilled technology. ✔The use of high strength material is costly. ✔There is additional cost in auxiliary equipment. ✔Harder to recycle. ✔There is need for quality control and inspection.
- 13. Pre-stressing Pre-tensioning: The tendons aretensioned against some abutments before the concrete is placed. After the concrete hardened, the tension force is released. The tendon tries to shrink back its initial length but concrete resists it through its bond between them. Thus compressive force is induced in concrete. Post-tensioning: In post- tensioning, the tendons are tensioned after the concrete has hardened. Commonly metal or plastic ducts are placed inside the concrete before casting. After the concrete hardened and had enough strength, the tendon was placed inside the duct, stressed and anchored against concrete. This can be done either as pre-cast or cast-in-place.
- 14. Pre-tensioning Post-tensioning 1. Tensionis applied on tendons before concrete placement. 1. Tensions is applied after the concrete placement. 2. Pre-stress loss due to anchorage slip and friction loss is zero. 2. Pre-stress loss occur due to anchorage slip and friction. 3. Pre-stress loss due to elastic deformation occur. 3. Pre-stress loss occur due to elastic deformation if all the tendons are not equally tensioned. 4. Pre-stress is applied against the abutment. 4. Pre-tension is provided against the concrete. 5. Use- Prefabricated element. 5. For cast-in-situ element (slab & beam).
- 15. Application ❖Bridges. ❖Slab in buildings. ❖Watertank. ❖Repair and rehabilitations. ❖Nuclear power plant. ❖Off shore platform. ❖Thin sheet structure. ❖Concrete piles, etc.
- 16. Concrete strain characteristics Inpre-stressed concrete, strain are produced as well as stresses. Such strain can be classified in 4 types: ❖ Elastic strain ❖ Lateral strain ❖ Creep strain ❖ Shrinkage strain
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- 18. Lateral strain ❖ Lateralstrains are computed by poison's ratio. ❖ Poison's ratio varies from 0.15 to 0.22 avg.=0.17
- 19. Creep strain ❖ Definedas time dependent deformation resulting from the presence of stress. ❖ Creep continued over the entire period of the total creep. ❖ Creep increase with a larger w/c ratio and with a lower aggregate cement ratio. ✔18-35% occurred in the first 2 weeks of loading. ✔40-70% with in 3 months. ✔60-83% with in one year.
- 20. Shrinkage strain ❖As distinguishedfrom creep, shrinkage in concrete is the contraction due to drying and chemical changes dependent on time and on moisture condition, but not on stresses. ❖It may be ranges from 0.0000 to 0.0010 and beyond under vary dry condition 0.0010 can be expected.
- 21. Compaction ❖Compacting the concreteby vibration is desirable and necessary. ❖Usually without using an excessive amount of mortar, a low w/c ratio and a low slump concrete must be chosen.
- 22. Curing ❖To early curingmay results shrinkage cracks before applying pre-stress. ❖Only by the careful curing the specified high strength concrete an be attained.
- 23. Pre-stressing steel ❖The developmentof pre-stressed concrete was introduced by the invention of high strength steel. ❖It is an alloy of iron, carbon, manganese and optimal materials.
- 24. Wires ❖A pre-stressing wireis a single unit made of steel. The nominal diameter of the wires are 2.5, 3.0, 4.5, 5.0, 7.0 & 8.0 mm. Two types: 1. Indented wire: There are circular or indentation on the surface. 2. Plain wire: No indentation on the surface.
- 26. Strands ❖A few wiresare spun together in a helical form to form a pre-stressing stand. There are different types of stands are as follows: 1. Two wire strands. 2. Three wire strands.
- 27. Tendons: ❖A group ofstrands or wire are placed together to form a pre- stressing tendon. The tendons are used in post tensioned member. Cables: ❖A group of tendon form a pre-stressing cable. The cables are used in bridges.
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- 32. 5.0 5.0 5.0 5.0 2.16 2.16 9.0 9.0 11.16 1.16 Analysis of stressesat Mid-Span Prestress Self- weight stress Live load stress Resultant stress
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- 35. Problem-3: An unsymmetrical I-sectionbeam is used to support an imposed load of 2 kN/m over a span of 8 m. The effective pre-stressing force is 100 kN. Estimate the stresses at the center of the span section of the beam. 60 mm 60 mm 80 mm 300 mm 400 mm 50 mm 100 mm
- 36. 60 mm 60 mm 80mm 300 mm e = 194 mm 400 mm 50 mm 100 mm y
- 37. 60 mm 60 mm 80mm 300 mm e = 194 mm 400 mm 50 mm 100 mm y = 156 mm
- 38. 60 mm 60 mm 80mm 300 mm e = 194 mm 400 mm 50 mm 100 mm y = 156 mm 244 mm
- 39. Type of stress Pre-stress Self-weightstress Live load stress Resultant -3.3 -0.35
- 40. 60 mm 60 mm 80mm 300 mm 400 mm 50 mm 260 mm 60 mm 300 mm 400 mm 50 mm 300 mm 80 mm 60 mm 60 mm 300 mm 400 mm 50 mm 300 mm 80 mm Assignment-1
- 41. 80 mm 70 mm 300mm 400 mm 50 mm 300 mm 80 mm 70 mm 80 mm
- 42. 60 mm 60 mm 80mm 300 mm 400 mm 50 mm 260 mm 300 mm 400 mm 50 mm 300 mm 80 mm 80 mm 60 mm 300 mm 400 mm 50 mm 300 mm 60 mm 60 mm 80 mm 60 mm 80 mm 70 mm
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- 44. Pressure line/ Thrustline: At any given section of a pre-stressed concrete beam, the combined effect of the pre-stressing force and the externally applied load will result in a distribution of concrete stresses that can be resolved into a single force. The locus of the point of application of this resultant force in any structure is termed as the pressure line or thrust line.
- 45. Problem-4: A pre-stressed concretebeam of section 120 mm wide by 300 mm deep is used over an effective span of 6 m to support a uniformly distributed load of 4 kN/m, which includes the self weight of the beam. The beam is pre-stressed by a straight cable carrying a force of 180 kN and located at an eccentricity of 50 mm. Determine the location of the thrust line in the beam and plot its position at quarter and central span locations.
- 46. 100 mm 75 mm P= 180 kN P = 180 kN 1.5 m 3 m
- 47. Problem-5: A pre-stressed concretebeam of section 250 mm wide by 400 mm deep is used over an effective span of 4 m to support a point load of 70 kN. The beam is pre- stressed by a straight cable carrying a force of 600 kN and located at an eccentricity of 65 mm. Determine the location of the thrust line in the beam and plot its position at quarter, central and support sections for the concentrated load only.
- 48. 70 KN 35 KN35 KN 2 m 2 m
- 49. 70 kN 35 kN35 kN 2 m 2 m 116.67 mm 58.33 mm P = 600 kN P = 600 kN 1 m 2 m
- 50. Problem-6: A pre-stressed concretebeam of section 250 mm wide by 400 mm deep is used over an effective span of 4 m to support a point load of 70 kN at quarter span from left support. The beam is pre-stressed by a straight cable carrying a force of 600 kN and located at an eccentricity of 65 mm. Determine the location of the thrust line in the beam and plot its position at quarter, central and support sections for the concentrated load only.
- 51. 70 kN 52.5 kN17.5 kN 1 m 3 m
- 52. 58.33 mm 29.17 mm P= 600 kN P = 600 kN 1 m 2 m 87.5 mm
- 53. Assignment-2 70 kN 2 m2 m 35 kN 70 kN 2 m 2 m 35 kN 2 m 2 m 5 kN/m 2 m 2 m 5 kN/m 5 kN/m
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- 55. Pre-stress Loss Pre-stress lossis the difference between initial pre-stress and effective pre-stress that remains in a member. Pre-stress loss Short term or immediate losses: ❖ Elastic shortening of concrete. ❖ Slip at anchorages immediately after pre-stressing. ❖ Friction between tendon and cable effect. Long term or time dependent losses: ❖ Creep and shrinkage of concrete. ❖ Relaxation of pre-stressing steel.
- 56. Pre-stress loss Concrete Steel Elastic Shortening CreepShrinkage Friction Anchorage slip Relaxation Pre-stress Loss According to Material
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- 64. a) Under simultaneoustensioning and anchoring of all the three cables, there will be no loss due to the elastic deformation of concrete. b) When the cables are successively tensioned: Cable-1 Cable-2 Cable-3 When cable-1 tensioned 0 0 0 When cable-2 tensioned 16.2 0 0 When cable-3 tensioned 16.2 16.2 0 Total loss 32.4 16.2 0
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- 78. Ultimate Moment Problem-16: Find thenominal & ultimate moment capacity of the beam section.
- 81. 175 mm 200 mm 100mm 500 mm 875 mm 100 mm 100 mm
- 82. 175 mm 200 mm 100mm 500 mm 875 mm 100 mm 100 mm
- 83. 175 mm 200 mm 100mm 500 mm 875 mm 100 mm 100 mm
- 84. 175 mm 200 mm 100mm 500 mm 875 mm 100 mm 100 mm
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- 86. Cracking Moment The bendingmoment at which visible cracks develop in pre-stressed concrete members is generally referred to as cracking moment.
- 87. Cracking Moment 160 mm 160mm 160 mm 400 mm e = 275 mm 800 mm
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- 90. Effective Stress 8 in 12in 4 in
- 92. 8 in 12 in 3in 3 in 2 in
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- 95. Problem-21: The liveload on the beam is 2.5 k/ft and L is 8 m. a) Determine the effective force in the cable for balancing the dead and live loads on the beam. b) Calculate the shift of the pressure line from the tendon center line. 80 mm 80 mm 80 mm 250 mm e = 150 mm 450 mm
- 96. 80 mm 80 mm 80mm 250 mm e = 150 mm 450 mm
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- 98. Problem-22: Calculate theresultant thrust by Internal resisting couple method. F = 7000 KN, eccentricity at mid span is 800 mm and concentric in support. The wall is of uniform thickness of 200 mm and the live load moment at mid span of 40 m span is 2000 KN-m. 200 mm e = 800 mm 200 mm 1200 mm 1800 mm
- 99. 200 mm e = 800 mm 200 mm 1200mm 1800 mm