Seventh Semester B.E. Degree Examination, Dec-06/Jan-07,

Civil Engineering

Design of Prestressed Concrete Structures

Time: 3hrs. Max. Marks: 100

 Note: 1- Answer any FIVE questions.

2- Use of IS 1343-permitted.

1a. Explain why high strength concrete and high strength steel are essential for prestressed concrete.

Indicate grades of concrete and steel used in prestressed concrete construction.(06marks)

1b. A simply supported beam of span 10m and section 500 x 750 is prestressed by a parabolic cable

having an eccentricity of 100mm at centre of span and zero at supports with a prestressing force of

1600 kN. If the beam supports an u.d.l. of 40kN/m, find the extreme stresses at mid span

section.(14marks)

2a. Explain the terms pretensioning and post tensioning and indicate the differences between

them.(06marks)

2b. A prestressed concrete beam with I-section has flanges 200 x 60 and web 280 x 80 (overall depth

400). It supports a live load of 4kN/m over a simply supported span of 8m. The beam is to be

 prestressed by an effective prestressing force of 235kN. Find the eccentricity at the centre such that

the bottom soffit stresses are zero. What would be the prestressing force if the prestressingg force is

concentric.(14marks)

3a. List the various types of losses in PSC beams and write the equations used to determine

them.(06marks)

3b. A prestressed concrete beam 300 x 600 is prestressed by tendons of area 250mm2 at a constant

eccentricity of 100mm with an initial stress of 1050N/mm2. Span of beam is 10.5m. Data regarding

losses are

Modular ratio: 6, Friction coefficient = 0.0015/m, Anchorage slip: 1.5mm,

Ultimate creep strain: 40 x 10-6 for pretensioned

20 x 10-6 for post tensioned

Shrinkage of concrete: 300 x 10-6 for pretensioned

200 x 10-6 for post tensioned

Relaxation of steel: 2.5%. Calculate the losses for both pre tensioning and post tensioning

cases.(14marks)

4a. Explain the significance of long term deflections in PSC beams and indicate how it is

calculated.(06marks)

 

4b. A concrete beam having a rectangular section 150 x 300 is prestressed by a parabolic cable at an

eccentricity of 75mm at mild span towards bottom soffit and at eccentricity of 25mm towards top at

support sections. The effective prestressing force is 350kN. The beam supports a concentrated load

of 20kN at centre of span in addition to the self weight with a span at 8m. Find the short term

deflection at centre of span under prestress, self weight and live load. Find also the long term

deflection if the loss ratio is 0.8 and the creep coefficient is 1.6. Ec = 38kN/mm 2.(14marks)

5a. Write the formula for finding the ultimate strength of rectangular and flanged PSC beams as per IS

1343.(06marks)

5b. A pretensioned PSC beam of I-section with 160 x 70 flanges, with thickness of web 50mm and

overall depth of I section is 320mm. The beam is prestressed with 4 high strength steel wires of 7mm

at an effective depth of 265mmm f CR  = 50N/mm2, fy= 1600N/mm2. Find the ultimate flexural

strength.(14marks)

6a. How does prestressing increase the shear strength of beams? Explain.(06marks)

6b. The support section of a PSC beam 120 x 250 is subjected to an ultimate shear of 60kN and has a

 prestress of 5N/mm2 acting at centroid f CK  = 40N/mm2. Cover to tension reinforcement= 50mm fy,

stumps = 250N/mm2. Design suitable shear reinforcement using IS 1343 procedure.(14marks)

7. Design a post tensioned PSC I-section beam for a bridge girder with an effective span of 12m and a

live load of 10kN/m2. Use f CK  = 40N/mm2.(20marks)

8. Write short notes on any four:

a. Centre of thrust and thrust line in PSC beams.

 b. Anchorage zone stresses.

c. Post tensioning systems.

d. Pretensioning methods.

e. Limiting zone for cables in PSC beams.

f. Transmission length in pretensioned beams.(20marks)

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