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Second Semester M.E. (Civil) Degree Examination, January/February 2014 (2K8 Scheme) (Common to PSC/Str) PC – 201/SE – 206 : ADVANCED PRESTRESSED CONCRETE Time : 3 Hours

Max. Marks :100

Note : 1) Answer any five full questions. 2) Use of IS-1343 is permitted. 3) Any missing data may be suitably assumed. 1. a) Discuss briefly different types of prestressed concrete floor slabs, with neat sketches.

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b) The floor slab of an industrial structure covers an area of 25 m × 50 m. For economy the span of the slab along 50 m is reduced by providing 4 equally spaced intermediate beams. The specified floor loading consists of a live load of 10 kN/m2 and a dead load (due to floor finish, partition etc.,) of 2.0 kN/m2 in addition to the self weight. Design anyone typical intermediate slab as a one way prestressed concrete slab with parallel posttensioned cables of 12#5 mm φ diameter initially stressed to 1200 N/mm2. The permissible compressive and tensile stresses in concrete at any stage not exceeding 15 N/mm 2 and zero N/mm2 respectively. Calculate the horizontal spacing of the cables and their positions at the midspan section. Assume the loss ratio as 0.80. Check for the safety of the slab against collapse. Assume fck = 40 N/mm2 and fp =1600 N/mm2. 14 2. A simple flat slab 14 m × 10 m is supported by four columns so placed as to form a symmetrical rectangular grid 10 m × 8 m. The live load on the slab is 3 kN/m2. Prestressing cables consisting of four wires of 5 mm φ are initially stressed to 1200 N/mm2 are available for use. Design the No. of cables required and arrange them suitably in the two principal directions and check if max. and minimum stresses are within the safe permissible limits of 14 N/mm2 in compression and 20 zero N/mm2 in tension.

P.T.O.

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3. a) Briefly discuss the advantages of prestressed concrete pipes.

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b) A prestressed concrete pipe is to be designed to withstand a liquid pressure of 2 N/mm2 internally. The diameter of the pipe is 1200 mm and wall thickness of 100 mm. The maximum permissible compressive stress in concrete is 16 N/mm2 at transfer. A residual compression of 2 N/mm2 is expected to be maintained at service load. The possible losses in prestress can be expected to be 20% 5 mm φ and 7 mm φ high tension wires are available. The maximum level to which they are stressed is 1000 N/mm2 at the time of prestress. Permissible longitudinal tensile stress is 0.8 fci . The strength of concrete at transfer is 50 N/mm2. Check the safety of the designed pipe against 14 longitudinal stresses and bursting as per IS 78 4′ provisions. 4. a) Write a brief note on tank floors.

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b) Design of prestressed concrete balancing reservoir having the junction of the reservoir wall and the floor slab as hinged. The floor level of the reservoir is 80 m and maximum water level is 100 m. The volume of the tank is 15000 m3. The permissible compressive stress in concrete at transfer is 14 N/mm2 and the minimum compressive stress under working pressure is 2 N/mm2. The loss ratio is 0.85. Wires of 4 # 5 mm φ diameter and 12 # 8 mm φ diameter initial stressed to 1200 N/mm2 are available for circumferential and vertical prestressing respectively. The Grade of concrete used is of M40. (Reference to table 16.2 to 16.5 can be made). 14 5. Design a circular cylindrical shell with prestressed edge beams to cover an area of 20 m by 60 m and to support an imposed load of 1.5 kN/m2. Estimate the number of cables consisting of 12 wires of 8 mm diameter required in the edge beam. The dimensions of the shell with edge beams are as follows : Semi central angle α = 40°. Radius of shell = R = 10 m Thickness of shell = 100 mm Width of edge beam, B = 0.20 m and depth of edge beam H = 2.5 m.

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6. a) Discuss briefly the design aspects of prestressed concrete Domes.

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b) A reinforced concrete domes of 30 m base diameter and a rise of 3.5 m is to be designed for a prestressed concrete cylindrical tank. The shell dome is to be provided with a prestressed concrete ring beam. Design the dome and the ring beam for a super imposed load of 1.5 kN/m2. The 5 mm. diameter high tensile wires, initially stressed to 1200 N/mm2 are available for prestressing the ring beam. The loss ratio is 0.75. The compressive stress in concrete at transfer is 14 N/mm2. Radius of the shell dome R = 32 m, thickness of shell = 75 mm, semi central angle, α = 28° 4′ .

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7. a) Briefly explain prestressed concrete sheet piles.

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b) Design an oblique cable system for an airport runway, 150 m long and 20 m wide using

cables of 12#5mm φ diameter. The coefficient of

friction between the pavement and the subgrade may be taken as 1.5. Determine the obliquity and spacing of cables. The thickness of pavement is 200 mm and the minimum longitudinal and transverse prestress should not be less than 2 mm2 and 1.5 N/mm2 respectively. 8. Write brief notes on : a) Prestressed concrete poles. (advantages and design consideration) b) Merits and demerits of different shape of prestressed concrete piles.

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