JNTU
JNTU B.Tech III PRESTRESSED CONCRETE Previous Question Paper (February/March - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Distinguish between linear and circumferential prestressing.
Answer
Linear prestressing involves the application of prestress to straight or curved members, such as beams, slabs, or columns, where the prestressing force is applied along the longitudinal axis. This is typically achieved using tendons or bars that are tensioned and anchored at the ends of the member to counteract bending moments and shear forces.
Circumferential prestressing is primarily used for circular structures like tanks, pipes, and silos. The prestressing force is applied in a circumferential direction, usually by wrapping high-tensile wire or strands around the perimeter of the structure. This creates a compressive hoop stress that counteracts the tensile stresses induced by internal fluid or gas pressure.
Write about:
i) Permissible stress
ii) Relaxation of stress
iii) Stress corrosion
iv) Prestressed Concrete.
Answer
i) Permissible stress refers to the maximum allowable stress levels in the concrete and steel as specified by design codes (such as IS 1343). These limits are set to ensure safety against cracking, crushing, and fatigue during both the transfer stage and the service life of the structure.
ii) Relaxation of stress is the loss of stress in prestressing steel that occurs when it is maintained at a constant strain over time. This time-dependent phenomenon results in a gradual reduction of the prestressing force, which must be accounted for in the design calculations to ensure the required effective prestress is maintained.
iii) Stress corrosion is a form of brittle failure that occurs when high-tensile steel is subjected to sustained tensile stress in a corrosive environment. It leads to the formation of cracks and eventual rupture of the tendons, necessitating proper protection methods like grouting or coating to prevent exposure to moisture and aggressive agents.
iv) Prestressed Concrete is a structural material where internal stresses are intentionally introduced during construction to counteract the stresses resulting from external loads. By applying a compressive force to the concrete—typically through tensioned steel tendons—the section is kept in a state of compression, which enhances its load-carrying capacity, controls deflection, and prevents cracking under service conditions.
Explain about any three post tensioned anchorage systems.
Answer
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A prestressed concrete pile 250 mm square, contains 60 pre-tensioned wires, each of 3mm diameter, uniformly distributed over the section. The wires are initially tensioned on the prestresssing bed with a total force of 500 kN. Calculate the final stress in concrete and the percentage loss of stress in steel after all losses, given the following data:
Shortening due to creep = 30 × 10-6 mm/mm per N/mm2 of stress
Total shrinkage = 200 × 10-6 per unit length
Relaxation of steel stress = 5 per cent of initial stress
Prestressing force, P = 400 kN
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A prestressed concrete beam having unsymmetrical I-section has a fibre stress distribution of 13 N/mm2 (compression) at the top edge linearly reducing to zero at the bottom. The top flange width and thickness are 2400 mm and 400 mm respectively. The bottom flange width and thickness are 1200 mm and 900 mm respectively and the depth and web thickness are 1000 mm and 600 mm respectively. The total shear force is 2350 kN. Compute and compare the principal stresses at the centroidal axis and the junction of web with lower flange.
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Explain stress concept.
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A prestressed concrete beam is of size 500 mm × 900 mm. The beam is simply supported on a span of 8 m. It is subjected to a central concentrated load of 1500 kN. The beam carries a dead load of 10 kN/m. A bent tendon is provided at an eccentricity of 100 mm below the centroidal axis and 50 mm below the centroidal axis at the ends. Compute the extreme stresses at midspan of the beam.
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Explain in detail about the designing Type I and Type II PSC beams.
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The end block of a post tensioned concrete beam 300 mm×300 mm is subjected to a concentric anchorage force of 800 kN by a freyssinet anchorage system of area 1100mm2. Design, Discuss and detail the anchorage reinforcement for the end block.
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A concrete beam having a rectangular section 100 × 300 mm is prestressed by a parabolic cable with an initial prestressing force of 240 kN. The cable has an eccentricity of 50 mm at the centre and concentric at the supports. If the span of the beam is 10 m and subjected to a live load of 2 kN/m. Calculate the short-term deflection at midspan. Assume Ec = 38 kN/mm2, creep coefficient = 2, loss of prestress = 20%. Estimate the long-term deflection.
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