JNTU
JNTU B.Tech III PRESTRESSED CONCRETE Previous Question Paper (February - 2023 - R18)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions. iv) Use of only IS 1343 – 2012 is allowed.
PART - A
Define prestressed concrete.
Answer
Prestressed concrete is a form of concrete construction in which internal stresses are introduced by high-strength steel tendons or bars to counteract the tensile stresses resulting from external loads. By applying a permanent compressive force, the concrete is kept in a state of compression, which effectively prevents or minimizes cracking and allows for longer spans and thinner structural sections.
Write the advantages of prestressed concrete.
Answer
Prestressed concrete offers several significant advantages over conventional reinforced concrete:
- Improved Serviceability: By keeping the entire cross-section in compression under service loads, cracking is minimized or eliminated, which enhances durability and resistance to corrosion.
- Efficiency of Materials: It allows for the use of high-strength concrete and high-tensile steel, resulting in smaller, lighter sections and reduced self-weight, which is particularly beneficial for long-span structures.
- Enhanced Structural Performance: The active prestressing force provides better control over deflections and increases the load-carrying capacity, allowing for greater versatility in architectural design and structural span lengths.
Why loss due shrinkage is more for pre tensioned member compared to post tensioned member and describe about friction loss in a post tensioned member?
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What are the factors influencing the loss of stress due to creep of concrete?
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What are the the ways by which shear resistance of structural concrete members can be improved?
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What is meant by Type – I, Type – II and Type – III structures in pre-stressed concrete?
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Enumerate the requirements of end blocks in post-tentioned pre-stressed concrete element.
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Explain the term busting tension with reference to post tensioned pre stressed members.
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What is the main factor that influences the deflection of a prestressed concrete member?
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What is meant by composite beam?
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PART - B
What is the necessity of using high strength concrete and high strength tensile steel in prestressed concrete?
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Briefly explain the general principles of pre-tensioned and post-tensioned concrete structures.
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Explain the advantages of prestressed concrete over reinforced concrete construction.
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State different types of prestressing with their important features.
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A pretensioned beam, 250mm wide and 350mm deep, is prestressed by 10 wires of 7mm diameter initially stressed to 1200N/mm2, with their centroids located 100mm from the soffit. Find the maximum stress in concrete immediately after transfer, allowing only for elastic shortening of concrete. If the concrete undergoes a further shortening due to creep and shrinkage while there is a relaxation of seven percent of steel stress, estimate the find percentage loss of stress in the wires using IS:1343 regulations, and by assuming Es=210KN/mm2, Ec=35KN/mm2, creep coefficient =1.6 and total residual shrinkage strain=3×10-4.
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Explain about various anchorages systems used in pre-tensioing and post-tensioning of prestressed concrete with neat sketches.
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A prestressed concrete beam of rectangular cross section 230 mm wide × 600 mm deep supports a live load of 8 KN/m. spanning over 8m. Find the effective pre stressing force in the parabolic cable having an eccentricity of 80 mm at the centre of span and concentric at the supports for the following loading conditions.
If the bending effect of the stressing force is nullified by the imposed load for the mid span section (neglecting self weight of the beam).
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If the resultant stress due to self weight, live load, and pre stressing force is zero at the soffit of the beam at centre of span section. Take density of concrete is 25 KN/m3.
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The support section of a prestressed concrete beam 150mm wide and 300mm deep is required to support an ultimate shear force of 100 KN. The compressive prestress at the centroidal axis is 5 N/mm2. The characteristic cube strength of concrete is 40 N/mm2. The cover to the tension reinforcement is 30mm. If the characteristic tensile strength of stirrups is 500 N/mm2, design suitable shear reinforcements in the section using IS code recommendations.
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Explain the stress distribution in end block with neat sketches.
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Describe Magnels method for end block design.
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What are the advantages of end blocks? Explain in detail the stepwise design procedure by any method.
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i) Explain transmission length
ii) Explain anchorage zone.
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A rectangular beam 300mm×300mm in section is simply supported over a span of 15m. It is prestressed with a parabolic cable which has a miximum eccentricity of 150mm at mid span and 35mm at supports sections. Effective prestressing force is 1500KN. Concrete grade is M40. Determine the deflection due to prestress and self-weight.
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Explain the step wise design considerations in the analysis of composite beams.
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