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VTU Civil Engineering (Semester 7)
Design of Pre-stressed Concrete Structures
June 2014
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

1 (a) Distinguish between RCC and PSC.
8 M
1 (b) Distinguish between pre-tensioning and post tensioning.
6 M
1 (c) Describe with neat sketches the Fryssinet system of post tensioning.
6 M

2 A rectangular beam 240mm × 500mm in section is simply supported over a span of 10mtr and is pre-stressed with an initial pre-stressing of 600 kN which is located at 100mm from the soffit. The beam is required to carry a load of 8 kN/m in addition to its own weight assume the loss ration as 80%. Determine the stress distribution under the following conditions: i) At transfer of pre-stress ii) At working load condition. Assume density of concrete is 25 kN/m3.
20 M

3 (a) List the various losses of pre-stress that occur in pre-tensioned and post-tensioned beams.
6 M
3 (b) A post - tensioned beam 150mm wide and 300 mm deep is having parabolic cable consisting of 12 wires of 5mm diameter. The cable having an eccentricity of 50mm at mid span and zero at the ends, the initial pre-stress in steel is 1000 N/mm2. Find the total percentage loss of pre-stress length of beam = 12mtr, μ=0.35, K=0.015/mtr, relaxation of steel = 8%, anchorage slop δa=5mm, Ec=3.5×104 N/mm2, Es=2.1×105 N/mm2 creep coefficient is 2.5, Shrinkage of concrete ess=2×10-4.
14 M

4 (a) Distinguish between short term and long term deflection of PSC beams.
6 M
4 (b) A concrete beam with a cross-sectional area of 32×103 mm2 and radius of gyration of 72mm is pre-stressed by a parabolic cable carrying an effective stress of 1000 N/mm2. The span of the beam is 8m. The cable, composed of 6-7 mm diameter, has an eccentricity of 50mm at the centre and zero at the supports. Neglecting all losses, find the central deflection of the beam as follows: i) Self weight+ pre-stress and ii) Self weight + pre-stress + live load of 2 kN/m, Take E=38 kN/mm2 and density of concrete = 24 kN/m3.
14 M

5 (a) Explain the various types of failure occur in PSC beam due to flexure.
8 M
5 (b) A pre-stressed concrete simply supported beam rectangular in section of 200mm wide and 500mm deep is prestressed by tendons having an area of 600mm2 located at 100mm from the soffit of the beam, given fck=40N/mm2, fb=1600 N/mm2 estimate the ultimate flexural strength of the beam for the following cases as per following cases as per IS-code recommendation i) If the beam is pre-tensioned; ii) If the beam is post tensioned with effective bond; iii) Post-tensioned unbounded tendon.
12 M

6 (a) Explain the factors responsible for improvement of shear resistance in PSC beams.
8 M
6 (b) The support section of a PSC beam 120mm wide 240mm deep is required to support an ultimate shear force of 75kN. The compressive pre-stress at centroidal axis is 5MPa fck=40 Mpa, fy=415 Mpa concrete cover for shear reinforcement is 50mm. Design a suitable shear reinforcement as per IS 1343-1980.
12 M

7 (a) Explain the anchorage stress in post-tensioned member and end block reinforcement.
10 M
7 (b) A PSC beam 300mm wide and 500mm deep has two anchorage of 1500mm diameter with the centres at 125mm from top and bottom of the beam. The forces transmitted by each cable is 800 kN. Estimate the maximum tension and brushing force.
10 M

8 (a) Write briefly about the limiting zone for cables in PSC members.
6 M
8 (b) A pre-tensioned PSC beam of rectangular cross-section is required to support a design ultimate moment of 120 kN-m. Design the section. Take fck=50N/mm2 and fb=1600 N/mm2.
14 M

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