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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) Bring out the difference between concentric and eccentric prestressing.
b) What is the minimum concrete strength requirements prescribed for prestressed
concrete members in IS : 1343 code?
2. a) Explain with sketch ‘Hoyer’s long line system of pretensioning ’.
b) A concrete beam of rectangular section having a width of 300 mm and depth 500mm,
is prestressed by a cable carrying a force of 750 KN at an eccentricity of 100mm. If the
beam supports a live load of 20KN/m over an effective span of 7m, estimate the resultant
stress at the top and bottom fibres at mid span section due to the effective of prestress,
dead and live loads, assume unit weight of concrete as 24KN/m3.
3. a) What are the factors influencing the loss of stress due creep of concrete?
b) A pretensioned prestressed concrete sleeper 300mm wide by 250mm deep is
prestressed using 9 wires of 7mm diameter. Four wires are located at top and 5 wires near
the soffit. The effective cover being 40mm. The intial stress in the wires is 1256N/mm2.
Assuming the modular ratio as 6, estimate the percentage loss of stress in the top and
bottom wires due to elastic deformation of concrete.
4. A pretensioned beam of rectangular section 400mm wide by 900mm deep is stressed by
1400mm2 of high-tensile steel at an effective depth of 800mm. The beam is also
reinforced with supplementary reinforcement consisting of 2 bars each of 25mm diameter
located 100mm from the soffit of the beam. Given fc' = 40N/mm2, fpu =1600/mm2
fy =460N/mm2 and (fpy/fpu) = 0.90, estimate the moment capacity of the section using
American code provisions.
5. How do you estimate the ultimate shear strength of prestressed concrete sections with
web shear cracks?
6. The end block of a prestressed beam 500mm wide and 1050 mm deep contains 6
freyssinet cables, each carrying a force of 266 KN anchored through 100mm diameter
anchorages, Which are spaced 150mm apart at the end of the beam. Calculate the
maximum tensile stress and the bursting tension and design the reinforcement for the end
block using Rowe’s method.Adopt yield stress in mild steel reinforcement as 260N/mm2.
7. a) Explain, with sketches the stresses developed due to differential shrinkage in structural
elements comprising precast prestressed and cast in situ concrete elements.
b) How do you compute the shrinkage and resultant stresses in composite members?
8. a) Explain with examples the effect of tendon profile on deflections of prestressed
concrete members.
b) How do you compute long-term deflections of uncracked prestressed concrete beams?
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