B.E. (Civil Engineering) Eighth Semester (C.B.S.

)
Elective-II : Pre-Stressed Concrete
P. Pages : 3 AHK/KW/19/2378
Time : Three Hours *1213* Max. Marks : 80
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Notes : 1. All questions carry marks as indicated.
2. Solve Question 1 OR Questions No. 2.
3. Solve Question 3 OR Questions No. 4.
4. Solve Question 5 OR Questions No. 6.
5. Solve Question 7 OR Questions No. 8.
6. Solve Question 9 OR Questions No. 10.
7. Solve Question 11 OR Questions No. 12.
8. Assume suitable data whenever necessary.
9. Illustrate your answers whenever necessary with the help of neat sketches.
10. Use of non programmable calculator is permitted.
11. Use of IS 1343 and IS 3370 is permitted.
1. a) A pre tension concrete beam 150 mm wide and 300 mm deep is prestressed by a straight 9
wires carrying on initial force 150 kN at an eccentricity of 40 mm. The modulus of
elasticity of steel and concrete are 210 and 35 kN/mm2 respectively. Estimate the
percentage loss of stress in steel due to elastic deformation of concrete if the area of steel
wires is 188 mm2.

b) List the various types of loss of prestress in pretensioned and post tensioned members? 4
OR
2. The end block of post tensioned prestressed concrete beam, rectangular in section is 100 13
mm wide and 200 mm deep. The prestressing force of 100 kN is transmitted to concrete
by a distribution plate 100 mm wide and 50 mm deep, concentrically located at ends.
Calculate the position & magnitude of max. tensile stress on horizontal section
through the centre and edge of the anchor plate. Calculate the bursting tension on these
horizontal planes.

3. A post-tensioned prestressed beam of rectangular section 250 mm wide is to be designed 13

for an imposed load of 15 kN/m, uniformly distributed on a span of 10 m. The stress in
the concrete must not exceed 17 MPa in compression and 1 MPa in tension at any time
and loss of prestress may be assumed to be 15%.
Calculate
1) The minimum possible depth of beam.
2) The minimum prestressing force and the corresponding eccentricity.
OR
4. A prestressed concrete beam of span 8 m having a rectangular section of 150 mm x 300 13
mm, the beam is prestressed by a parabolic cable having an eccentricity of 75 mm below
the centroidal axis at the centre of span and eccentricity of 25 mm above the centroidal
axis at support sections. The initial force in cable is 350 kN. The beam supports three
concentrated loads of 10 kN each at intervals of 2 m. EC = 38kN / mm2
i) Neglect the losses of prestress, Estimate short term deflection due to
(prestress + self wt).
ii) Allowing for 20% loss in prestress, Estimate long term deflection under
(prestress + self wt + live load) assume creep coefficient as 1.8.

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5. The cross section of a bridge girder is made up of a T-section with the following details : 14
Top flange width = 600 mm and thickness = 230 mm, Thickness of web = 150 mm,
Distance of the centroidal axis from the top of the section = 550 mm, Area of c/s = 328500
mm2 and second moment of area = 665 108 mm4 . The girder is used over an effective
span of 25 m. The tendons with a c/s of 2300 mm2 are parabolic with an eccentricity of
670 mm at the centre of span and 285 mm at the support section. The effective prestress
in the tendons is 900 N/mm2 after all losses. If tensile strength of concrete is 1.6 N/mm2.
Estimate the ultimate shear resistance of the support section and maximum
permissible uniformly distributed working load on the beam using an overall load factor
of 2.

OR

6. a) Distinguish between web shear, flexural & flexural shear cracks in concrete beams with 7
sketches.

b) What the requirements of longitudinal and Transverse reinforcements if the prestressed 7

beam is subjected to combined bending, shear and torsional stresses.

7. A prestressed beam having a rectangular c/s with a width of 120 mm and a depth of 300 14
mm is continuous over two span AB = BC = 8 m. The cable with zero eccentricity at the
ends and an eccentricity of 60 mm towards the top fibres of the beam over the central
support, carries an effective force of 600 kN.
a) Calculate the secondary moment developed at B.
b) If the beam supports concentrated loads of 20 kN each at mid points of span, evaluate
the resultant stress at the central support section B.
c) Locate also position of the pressure line at section.

OR

8. A continuous beam ABC, AB = BC = 10 m has a uniform rectangular section with a width 14

150 mm and depth of 300 mm. The cable carrying effective prestressing force of 400 kN
parallel to axis of the beam located at 125 mm from the soffit.
a) Determine the secondary and resultant moment at central support B.
b) If the supports and imposed load of 1.5 kN/m, calculate the resultant stresses at top
& bottom of a beam at B.
c) Locate the resultant line of thrust through beam AB.

9. A prestressed concrete cylinder pipe is formed by lining a steel cylinder of diameter 700 13
mm and thickness 2.5 mm with a layer of spun concrete 25 mm thick. If the pipe is required
to with stand a hydraulic pressure of 0.85 N/mm2 without developing any tensile stresses
in concrete, Calculate
i) The required pitch of 4 mm wires, wound round the cylinder at tensile stress of
1.4 N/mm2 in the concrete immediately after the winding and
ii) The approximate bursting pressure modular ratio = 6 Tensile strength of wire
= 1700 N/mm2, Loss ratio = 0.85 Yield stress of cylinder = 280 N/mm2.

OR

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10. A prestressed concrete circular cylinder tank is required to store 20,000 million liters of 13
water. The permissible compressive stresses in concrete at transfer should not exceed 13
N/mm2 and the minimum compressive stress under working pressure not less than 1
N/mm2. The loss ratio is 0.7 High tensile wires of 7 mm diameter with an initial stress of
1000 N/mm2 are available for winding round the tank. Freyssy net cable of 12 mm wires
of 8 mm diameter which are stressed to 1000 N/mm2 are available for vertical prestressing.
The cube strength of concrete is 40 N/mm2. Design the tank walls supported on
elastometric pads.
Assume the coefficient of friction as 0.5.

11. Explain any three. 13

i) Explain in brief the behavior of prestressed concrete elements under dynamic loads.

ii) Behaviour of prestressed concrete elements under fire.

iii) Prestressed concrete elements under fatigue loads.

iv) Effect of corrosion on prestressed concrete elements.

OR

12. a) What are grid floors? What is the advantage of prestressing such floor? Sketch a typical 7
grid floor showing the cables in the principal directions.

b) Explain the advantages of using prestressed concrete floor slabs mentioning their common 6
applications.

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