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Posted Date: 01 Jul 2009 Posted By: Sunil Reddy Member Level: Platinum
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2009 Jawaharlal Nehru Technological University Civil Engineering IV B.Tech II Semester Regular Examinations, April/May 2009 JNTU Kakinada, PRESTRESSED CONCRETE Question paper
Code No: 2420105
IV B. Tech II Semester Regular Examinations, April/May 2009
PRESTRESSED CONCRETE
(Civil Engineering )
Time: 3 Hours Max. Marks 80
Answer any FIVE questions
All questions carry equal marks
********
1. (a) What is the necessity of using high strength concrete and high tensile steel in
prestressed concrete?
(b) What type of aggregates do you recommended for the production of high strength
concrete?
2. (a) Distinguish between pre-tensioned and post-tensioned members.
(b) A concrete beam, 120 mm wide and 300 mm deep, is prestressed by a straight cable
carrying an effective force of 180 KN at an eccentricity of 50 mm. The beam spanning
over 6 m supports a total uniformly distributed load of 4 KN/m, which includes the selfweight
of the beam. The Initial stress in the tendons is 1000 N/mm2. Determine the
percentage increase of stress in the tendons due to the loading on the beam.
Es=210 KN/mm2, Ec=35 KN/mm2
3. (a) How do you compute the loss of stress due to elastic deformation of concrete?
(b) A post –tensioned concrete beam with cable of 24 parallel wires (total area = 800
mm2) is tensioned with 2 wires at a time. The cable with zero eccentricity at the ends
and 150 mm at the centre is on a circular curve. The span of the beam is 10m. The cross-
Section is 200mm wide and 450mm deep. The wires are to be stressed from one end to a
value of f1 to overcome frictional loss and then released to a value of f2 so that
immediately after anchoring, an Initial prestress of 840N/mm2 would be obtained.
Compute f1 and f2 and the final design stress in steel after all losses, given the following
data;
Coefficient of friction for curvature effect = 0.6 friction coefficient for ‘wave’
effect=0.003/m Deformation and slip of anchorage =1.25mm
Es = 210KN/mm2 Ec = 28KN/mm2 shrinkage of concrete = 0.0002
Relaxation of steel stress = 3 percent of the Initial stress.
4. A bonded prestressed concrete beam is of rectangular section of width 400mm and
overall depth 1200mm. The tendons consisting of 3300mm2 of standard strands with a
characteristic strength of 1700N/mm2 , are stressed to an effective prestress of
910N/mm2. The strands are located 870mm from the top face of the beam. The
characteristic cube strength of concrete is 60N/mm2. Estimate the ultimate moment
capacity of the section using British Code(BS:8110-1985) provisions.
5. Distinguish between web shear, flexural and flexure- shear cracks in concrete beams with
sketches.
6. A prestressing force of 250KN is transmitted through a distribution plate 120mm wide
and 120mm deep, the centre of which is located at 100mm from the bottom of an end
block having a section 120mm wide and 300mm deep. Evaluate the position and
magnitude of the maximum tensile stress on a horizontal section passing through the
centre of the distribution plate using the methods of
(a) Magnel (b) Guyon and (c) Rowe. Find the area of steel necessary to resist the largest
tensile force resulting from any of these methods. Yield stress in steel= 260N/mm2.
7. (a) Distinguish between propped and un propped construction methods in composite
construction using stress diagrams at various stages of construction.
(b) What is the different shrinkage? Explain its importance in composite construction.
8. (a) List the various factors influencing the deflections of prestressed concrete members.
(b) Discuss the various methods of predicting long-term deflections of uncracked
prestressed concrete members.
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