c4ce01 Pre Stressed Concrete

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Code No: C4CE01

IV B.Tech (CCC) Regular Examinations, December 2008 PRESTRESSED CONCRETE (Civil Engineering) Time: 3 hours Max Marks:100 Answer any FIVE Questions All Questions carry equal marks ????? 1. (a) Explain in detail the principles of pre tensioning and post tensioning? (b) Explain the necessity of wing high strength concrete and high tensile steel in pre stressing. [10+10] 2. (a) Explain the hoyer systems of pre-stressing? (b) Differentiate between hoyer system and presinet system in detail.

[10+10]

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3. (a) Explain different losses in pre-tensioning?

(b) A rectangular concrete been 200mm × 250 mm deep was pre-stressed with 14 nos of 5 mm diameter wires located at 60 mm from the bottom of the beam and five 5 mm wires located at 25 mm from the top of the beam. If the wires are initially tensioned to a stress of 800 N/mm2 , calculate the percentage loss of sheir on steal immediately after transfer allowing the loss of prestress due to elastic shortening of concrete only. [20]

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4. A prestressed concrete beam of 400 mm × 600 mm is prestressd with tendons as shown in the figure 1. It is subjected to a UDL of 10 KN/M along with a concentrated load of 150 KN. Calculate the extreme stress in concrete for mid span section. [20]

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Figure 1: 5. A post tensioned beam of 200 × 400 mm deep is 12 m long and canes a U DL of 9 KN / m on the entire beam. The effective pre-stressing force is 550 KN in the cable which is having a parabolic provide with zero eccentricity at the supports 15 and 150 mm eccentricity at the centre. Calculate the principal stresses at the supports 1 of 2

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Code No: C4CE01 (a) with pre shearing force (b) with out pre shearing force.

[20]

6. (a) With neat sketches explain the stress distributor in end blocks. (b) With neat sketches explain the Magnel’s method of analysis of end block stresses. [20] 7. A composite the T beam shown in the figure 2 is having a modulus of elasticity of 28 KN/nm2 of the differential shrinkage is 110 × 10−6 units, determine the shrinkage stresses developed on the pre cast and cast in situ units. [20]

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8. (a) What are the factors influencing the deflections of pre-stressed concrete members? (b) Explain short team and long term deflections as per IS code? ?????

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Code No: C4CE01

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech (CCC) IV Year Examinations February - 2011 PRESTRESSED CONCRETE (Civil Engineering) Time: 3 Hours

Max.Marks:100 Answer Any Five Questions All Questions Carry Equal Marks ---

1. a) b)

Explain with sketches Hoyer Long Line System of Pretensioning. What are supplementary anchoring devices?

2.

A post tensioned concrete beam 200 mm wide and 500 mm deep, is prestressed by a circular cable (Total Area=800 mm2) with zero eccentricity at the ends and 150 mm at the centre. The span of the beam is 10 m. The cable is to be stressed from one end such that an initial stress of 840 N/MM2 is available in the unjacked end immediately after anchoring. Determine the stress in the wires at the jacking end and the percentage loss of stress due to friction. Coefficient of friction for curvature effect = 0.6. Friction coefficient for wave effect = 0.003/m. [20]

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A prestressed concrete beam 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 5 kN/m, which includes the self-weight of the beam. The beam is prestressed by a straight cable carrying a force of 200 kn and located at an eccentricity of 500 mm. Determine the location of the thrust-Line in the beam and plot its position at quarter and central span sections. [20]

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A rectangular concrete beam of cross-section 150 mm wide and 300 mm deep is simply supported over a span of 8 m and is prestressed by mean of a symmetric parabolic cable at a distance of 75 mm from the bottom of the beam at mid span and 125 mm from the top of the beam at supported sections. If the force in the cable is 350 KN and the modulus of elasticity of concrete is 38 KN/mm2, calculate: a) The deflection at mid-span when the beam is supporting its own weight and b) The concentrated load which must be applied at mid-span to restore it to the level supports. [20]

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5.

An unsymmetrical I-section has an overall depth of 2000 mm. The top flangewidth and depth are equal to 1200 and 300 mm respectively and the bottom flange width and depth are equal to 750 and 200 mm respectively. The thickness of the web is 300 mm. The tendons having a cross-sectional area of 7000 mm2 are located 200 mm from the soffit. If the ultimate compressive strength of concrete and the tensile strength of steel are 42 and 1750 N/mm2 respectively and the tendons are effectively bonded to concrete, estimate the flexural strength of the section. [20]

6. a) b)

What is transmission length? List the various influencing transmission length. Discuss briefly the IS: 1343 code provisions regarding bond and transmission length. [20]

7.

The end block of a prestressed concrete beam, rectangular in section is 100 mm wide and 200 mm deep. The prestressing force of 1000 KN is transmitted to concrete by a distributed plate, 100 mm wide and 50 mm deep, concentrically located at the ends. Calculate the position and magnitude of the maximum tensile stress on the horizontal section through the centre and edge of the anchor plate. Compute the bursting tension on these horizontal planes. [20]

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A post tensioned beam of rectangular cross section 200 mm wide and 400 mm deep is 10 m long and carries an applied load of 8 KN/m, uniformly distributed on the beam. The effective prestressing force is in the cable is 500 Kn. The cable is parabolic with zero eccentricity at the supports and a maximum eccentricity of 140 mm at the centre of span. a) Calculate the principal stresses at the supports. b) What will be the magnitude of the principal stresses at the supports in the absence of prestress? [20]

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Code.No: C4CE01 JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech (CCC) IV Year Examinations January 2010 PRE-STRESSED CONCRETE (CIVIL ENGINEERING) Time: 3 Hours Max. Marks:100 Answer Any Five Questions All Questions Carry Equal Marks --1.

A simply supported prestressed concrete beam of rectangular cross-section 300 mm x 500 mm is loaded with a total uniformly distributed load of 20 kN over a span of 5 m. Sketch the distribution of stresses at the mid-span and end sections if the prestressing force is 1500 kN and the tendon is eccentric, located at 150 mm above the bottom fiber. [20]

2.

A pretensioned prestress concrete beam of 7 m span has a cross-section of 300 mm x 500 mm, and is prestressed with 1500 kN force at transfer. The cable has cross-sectional area of 1500 mm2 of steel, and has a parabolic profile with a maximum eccentricity of 1500 mm at the mid-span. Determine the loss of prestress, given that Es = 2.1 x 105 N/mm2 and Ec = 3 x 104 N/mm2. [20]

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Write notes on (i)Kern distance (ii) Efficiency factor Find the kern distance, and compare the efficiency factors of (i) Rectangular section (ii) Symmetrical I-section, (iii) T-section, if all the three have the same cross-sectional area of 2800 cm2, and same depth of 70 cm. Assume suitable thickness of web and flange. [20]

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A rectangular beam of prestressed concrete is required to support a dead load moment of 12 x 106 N-mm (inclusive of its own weight) and a live load moment of 30 x 106 N-mm at its mid-section. Determine the initial prestressing force and its eccentricity at the mid-span section. Adopt the following values: Allowable initial compressive stress = 16 N/mm2 Allowable final compressive stress = 13 N/mm2 Allowable initial or final tension = 1N/mm2 Ultimate stress in steel = 1500 N/mm2 Assume losses as 15%. [20]

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6.

Design a prestressed concrete beam to the following requirements: a) Span = 10m b) Superimposed load = 30kN/m c) Cube strength of concrete at 28 days = 35N/mm2 d) Safe stress in concrete at transfer of prerstress fr = 0.5 fck e) Safe stress nconcrete due to final prestress = fc = 0.4fck f) Total loss of prestress = 20% g) Allowable tensile stress in concrete = 0.219vfck h) Ultimate stress in steel = 1500 N/mm2 i) Safe stress in steel = 60% of ultimate stress.

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A prestressed concrete beam 300 mm wide and 650 mm deep is subjected to an effective prestressing force of 1350 kN along the longitudinal centroidal axis. The cables may be assumed to be symmetrically placed over mild steel anchor plate in an area 150 mm x 350 mm. Design the end block. Take cube strength of concrete at transfer fci = 30 N/mm2 characteristic strength of concrete fck = 30 N/mm2. Assume initial prestressing force = 1.5 times the effective prestressing force. [20]

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A composite beam of rectangular section is made up of a pre-tensioned inverted T-beam having a slab thickness and width of 180 and 1000 mm respectively. The rib size is 150 mm by 850 mm. The cast in situ concrete has a thickness and width of 800 mm with a modulus of elasticity of 28 kN/mm2. If the differential shrinkage is 100 x 10-6 units, estimate the shrinkage stresses developed in the precast and cast in-situ units. [20]

8.

A rectangular concrete beam of cross-section 180 mm wide and 300 mm deep is simply supported over a span of 10 m and is prestressed by means of a symmetric parabolic cable, at a distance of 50 mm from the bottom of the beam at mid span and 125 mm from the top of the beam at support sections. If the force in the cable is 400 kN and the modulus of elasticity of concrete is 38 kN/mm2, calculate the deflection at mid-span when the beam is supporting its own weight. [20]

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Code. No: C4CE01 JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech (CCC) IV Year Supplementary Examinations July/August - 2010 PRESTRESSED CONCRETE (Civil Engineering) Time: 3 Hours

Max.Marks:100 Answer Any Five Questions All Questions Carry Equal Marks ---

1. a) b) 2.

3.

What is the minimum concrete strength requirements prescribed for prestressed concrete members in IS: 1343 code? What are post-tensioning anchorages? [20]

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A post-tensioned concrete beam, 100mm wide and 300mm deep is prestressed by three cables, each with a cross-sectional area 50 mm2 and with an initial stress of 120Kn/mm2. All the cables are straight and located 100mm from the soffit of the beam. If the modular ratio is 6, calculate the loss off stress in the cables due to elastic deformation of concrete for the only the following cases: a) Simultaneous tensioning and anchoring of all three cables; and b) Successive tensioning of the three cables, one at a time. [20]

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A rectangular concrete beam 250mm wide by 300mm deep is prestressed by a force of 540KN at a constant eccentricity of 60mm. The beam supports a concentrated load of 68KN at the centre of a span of 3m. Determine the location of the pressure line at the centre, quarter span and support sections of the beam. Neglect the self weight of the beam. [20]

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A concrete beam with a cross-section area of 32 * 103 mm2 and radius of gyration of 72mm is prestressed by a parabolic cable an effective stress of 1000 N/mm2. The span of the beam is 8m. The cable composed of 6 wires of 7mm diameter has an eccentricity of 500mm at the centre and zero at the supports, neglecting all losses; find the central deflection of the beam as follows: a) Self-weight + prestress, and b) Self-weight + prestress + live load of 2KN/m. [20]

5.

The cross-section of a symmetrical-sectional prestressed beam is 300mm by 750mm (overall), with flanges and web 100mm thick. The beam is post tensioned by cables containing 48 wires of 5mm diameter high –tensile steel wires at an eccentricity of 250mm. The day strength of concrete in compression is 40N/m2 and the ultimate tensile strength of concrete in compression is 40N/mm2 and the ultimate tensile strength of wires is 1700n/mm2. Assuming that the grouping of the tendons is 100 percent effective, determine the ultimate moment of the section (adopt IS: 1343 provisions). [20] Contd…2

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6.

A pre-tensioned beam of 8m span has a symmetrical –section. The flanges are 200mm wide and 60mm thick. The web thickness is 80mm and the overall depth of girder is 400mm. The member is prestressed by 8 wires of 5mm diameter located on the tension side such that the effective eccentricity is 90mm. The initial stress in the wires is 1280N/mm2 and the cube strength of concrete at transfer is 42N/mm2 a) Determine the maximum vertical tensile stress developed in the transfer zone; and b) Design suitable mild steel reinforcement, assuming the permissible stress in steel as 140N/mm2. [20]

7.

The end block of a prestressed concrete beam 100mm wide and 200mm deep support an eccentricity prestressing force of 100KN, line of action of which coincides with the bottom kern of the section. The depth of the anchor plate is 50mm, Estimate the magnitude and position of the principal tensile stress on a horizontal plane passing through the central of the anchorage plate. [20]

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A cantilevered portion of a prestressed concrete bridge with a rectangular crosssection 600mm wide and 1650mm deep is 8m long and carries a reaction of 350KN from the suspended span at the free end, together with a uniformly distributed load of 60KN/m inclusive of its own weight. The beam is prestressed by 7 cables each carrying a force of 1000KN, of which 3 are located at 150mm, 3 at 400mm and 1at 750mm from the top edge. Calculate the magnitude of the principal stresses at a point 550mm from the top of cantilever at the support section. [20]

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Code No: C4CE01

R05

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD R05 IV B.Tech. (CCC) Supplementary Examinations June – 2009 PRE-STRESSED CONCRETE (Civil Engineering) Time: 3 hours Max Marks: 100 Answer any FIVE Questions. All Questions carries equal marks. -----

1 a) b)

Explain the strength and durability limits for materials in prestressed concrete? Explain the basic concepts of prestressing? What are its advantages? [10+10]

2 a) b)

Explain in detail with neat sketches, the freyssinet system of Post-Tensioning? Explain the Permissible stresses in steel and stress – strain behavior for steels in pre – stressing? [10+10]

3.

A prestressed concrete beam 250mm wide and 360mm deep has a span of 13m. The beam is prestressed by steel wires of area 350 mm2 provided at a uniform eccentricity of 50mm with an initial prestress of 1200 N/mm2. Determine the percentage loss of pre-stress in the wires

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If the beam is a pretensioned beam If the beam is a post tensioned beam. ES=200 kN/mm2; EC=32 kN/mm2

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Ultimate creep: 45 × 10-6 mm/mm per N/mm2 per pretensioned beam : 22 × 10-6 mm/mm per N/mm2 per post tensioned beam Shrinkage of concrete : pre tensioned: 300 × 10-6 post tensioned: 215 × 10-6 Relaxation of steel stress : 5% of the initial stress Anchorage slip: 1.25mm

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Code No: C4CE01

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4.

A Pre stressed concrete T beam is to be designed for a super imposed load of 4 kN/m over a span of 6m. The stress in the concrete should not exceed 15 N/mm2 in compression and Zero in tension. Check for the adequacies of the section and calculate the minimum pre-stressing necessary and corresponding eccentricity assuring 15% loss of Pre-stress after transfer? [20]

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A prestressed concrete beam of 12m span and 130mm × 300mm deep is axially prestressed by an effective force of 200kN. The beam is subjected to a total ULD of 5kN/m including self weight. Calculate the magnitude of principal tension developed in the beam with and with out axial Pre-stress? [20]

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A Prestressed concrete beam 120mm × 250mm deep is transmitting 100 kN prestressing force by 100mm × 60mm deep distribution plate, concentrically located at the ends. Calculate the position and magnitude of the maximum tensile stress on the horizontal section through the center and edge of anchor plate. Complete the bursting tension on these horizontal planes? [20]

7 a) What are the advantages of composite structural prestressed members? b) Explain with neat sketches the types of composite construction? [10+10] 8 a) Explain the effect of tension profile on deflections? b) Explain the short term deflections of uncracked members? *****

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[10+10]

Available B.Tech(CCC) Previous Question Papers with Solutions

Code. No: C4CE01 JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech (CCC) IV Year Supplementary Examinations July/August - 2010 PRESTRESSED CONCRETE (Civil Engineering) Time: 3 Hours

Max.Marks:100 Answer Any Five Questions All Questions Carry Equal Marks ---

1. a) b) 2.

3.

What is the minimum concrete strength requirements prescribed for prestressed concrete members in IS: 1343 code? What are post-tensioning anchorages? [20]

i. n c

A post-tensioned concrete beam, 100mm wide and 300mm deep is prestressed by three cables, each with a cross-sectional area 50 mm2 and with an initial stress of 120Kn/mm2. All the cables are straight and located 100mm from the soffit of the beam. If the modular ratio is 6, calculate the loss off stress in the cables due to elastic deformation of concrete for the only the following cases: a) Simultaneous tensioning and anchoring of all three cables; and b) Successive tensioning of the three cables, one at a time. [20]

a . i n e

A rectangular concrete beam 250mm wide by 300mm deep is prestressed by a force of 540KN at a constant eccentricity of 60mm. The beam supports a concentrated load of 68KN at the centre of a span of 3m. Determine the location of the pressure line at the centre, quarter span and support sections of the beam. Neglect the self weight of the beam. [20]

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4.

A concrete beam with a cross-section area of 32 * 103 mm2 and radius of gyration of 72mm is prestressed by a parabolic cable an effective stress of 1000 N/mm2. The span of the beam is 8m. The cable composed of 6 wires of 7mm diameter has an eccentricity of 500mm at the centre and zero at the supports, neglecting all losses; find the central deflection of the beam as follows: a) Self-weight + prestress, and b) Self-weight + prestress + live load of 2KN/m. [20]

5.

The cross-section of a symmetrical-sectional prestressed beam is 300mm by 750mm (overall), with flanges and web 100mm thick. The beam is post tensioned by cables containing 48 wires of 5mm diameter high –tensile steel wires at an eccentricity of 250mm. The day strength of concrete in compression is 40N/m2 and the ultimate tensile strength of concrete in compression is 40N/mm2 and the ultimate tensile strength of wires is 1700n/mm2. Assuming that the grouping of the tendons is 100 percent effective, determine the ultimate moment of the section (adopt IS: 1343 provisions). [20] Contd…2

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Available B.Tech(CCC) Previous Question Papers with Solutions

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6.

A pre-tensioned beam of 8m span has a symmetrical –section. The flanges are 200mm wide and 60mm thick. The web thickness is 80mm and the overall depth of girder is 400mm. The member is prestressed by 8 wires of 5mm diameter located on the tension side such that the effective eccentricity is 90mm. The initial stress in the wires is 1280N/mm2 and the cube strength of concrete at transfer is 42N/mm2 a) Determine the maximum vertical tensile stress developed in the transfer zone; and b) Design suitable mild steel reinforcement, assuming the permissible stress in steel as 140N/mm2. [20]

7.

The end block of a prestressed concrete beam 100mm wide and 200mm deep support an eccentricity prestressing force of 100KN, line of action of which coincides with the bottom kern of the section. The depth of the anchor plate is 50mm, Estimate the magnitude and position of the principal tensile stress on a horizontal plane passing through the central of the anchorage plate. [20]

8.

A cantilevered portion of a prestressed concrete bridge with a rectangular crosssection 600mm wide and 1650mm deep is 8m long and carries a reaction of 350KN from the suspended span at the free end, together with a uniformly distributed load of 60KN/m inclusive of its own weight. The beam is prestressed by 7 cables each carrying a force of 1000KN, of which 3 are located at 150mm, 3 at 400mm and 1at 750mm from the top edge. Calculate the magnitude of the principal stresses at a point 550mm from the top of cantilever at the support section. [20]

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