CE 6702 PRESTRESSED CONCRETE STRUCTURES, Final year, Department of Civil Engineering, Model Exam Questions

Answer ALL Questions

PART A – (10 x 2 = 20 Mark )

1.      Define Partial Prestressing ?
2.      Define Unbondedtendon ?
3.      How will you improve the shear resistance of concrete beam using prestressingtechniques ?
4.      What are the different types of flexural failure modes observed in prestressed concrete beams ?
5.      What is meant by End block in a post tensioned member ?
6.      Sketch the loop reinforcement, hair-pin bars in end blocks.
7.      Define the concept of composite in PSC.
8.      Sketch some typical c/s of composite bridge decks with precast prestressed elements.
9.      What is Circular prestressing ?
10.  Draw the section of a prestressing concrete tank wall with fixed base.

PART B(5X16)                                                                        
1.      (a)     Explain with neat sketches the various methods of prestressing?
( or )
(b)      A post tension concrete beam 100mm wide and 300mm deep spacing over a 10cm is stressed by successive tensioning and anchoring of 3cable 1,2 and 3 respectively. The c/s of each cable is 200mm2 and the initial stress in the cable 1200N/mm2α = 6. The 1st cable is parabolic with an eccentricity of 50mm below the centroidal axis at the centre of span and 50mm above the centroidal axis in the support section the 2nd cable is parabolic with zero eccentricity @ support and 50mm below at the centre span. Third cable is straight line with a uniform eccentricity of 50mm below the centroidal axis. Estimate the % of loss of stress due to each cable.

2.      (a)     A pretension beam has T section has a flange 1200mm wide, 150mm thick the width and depth of web are 300mm and 1500mm. The high tensile steel has an area of 4700mm2 and is located at an effective depth of 1600mm if the characteristic cube strength of concrete and steel are 40 and 1600N/mm2. Calculate the flexural strength of T section.
( or )
(b)       A post tension bridge grider with unbounded tendons is box section of overall dimension 1200mm wide and 1800mm deep with wall thickness of 150mm. The high tensile steel has an area of 4000mm2 and is located at an effective depth of 1600mm. The effective prestress after all losses is 1000N/mm2 (fpo) and the effective span of girder is 24m. If fck is 40N/mm2 and fp=1600N/mm2. Calculate the ultimate moment resistance of the girder.

3.      (a)    (i)    The end block of a post tensioned prestressed concrete beam 300mm wide and 300mm deep is subjected to a concentric anchoring force of 832800N by a freyssinet anchorage of area 11720mm2. Design and detail the anchorage reinforcement for the end block.
(ii)      The end block of a prestress concrete beam rectangular in section is 100mm wide and 200mm deep. The prestressing force of 100KN is transmitted to concrete by distribution plate 100mm wide and 50mm deep concentrically located at the ends. Estimate the position and magnitude of the maximum transverse tensile stress and bursting tension for the end block.
( or )
( b )    Design a post tensioned bridge girder of span 30m and carries an imposed load of 30KN/m is prestressing cable each of 7mm diameter wires 12nos in freyssinet system and characteristic strength of steel 1600Mpa and M50 grade concrete used. Assumed loss of prestress 15%


4.      ( a )        A proposed pretension beam of  rectangular section has a breadth of 150mm and depth of 250mm. A beam with an effective span of 6m. The initial force of tendon is 200KN. The loss of prestressmay be assume 15%. The beam is incorporated in a composite T beam by casting a top flange of breadth of 450mm and thick 50mm. If the composite beam support a live load of 8KN/m2. Calculate the resultant stress develop in the composite beam. Assume modulus of elasticity of concrete beam and slab is same.
( or )
( b )       A composite T beam is made up of a pretensioned rib 150mm wide and 250mm deep and a cast in situ slab 450mm wide and 50mm thick having a modulus of elasticity is 28KN/mm2. If the different shrinkage is 100x10-6  units. Determine the shrinkage stresses developed in precast cast insitu units.

5.      ( a )       Design a Non-cylinder prestressed concrete pipe of 600mm internal diameter to withstand a working load hydrostatic pressure of 1.05N/mm2. Using a 2.5mm high tensile wire stressed to 1000N/mm2 at transfer permissible maximum and minimum stresses at transfer  and service loads of 14 and 0.7N/mm2. The loss ratio is 0.8 Calculate also the test pressure required to produce a tensile stress of 0.7N/mm2 in concrete when applied immediately after tensioning  and also the winding stress in steel if Es=210KN/mm2 and Ec=35KN/mm2.
( or )
( b )       A cylindrical prestressed concrete water tank of internal diameter 30m is required to store water over a depth of 7.5m. The permissible compressive stress in concrete at transfer is 13N/mm2 and the minimum compressive stress under working pressure is 1N/mm2. The loss ratio is 0.75. Wires of 5mm dia with an initial stress of 1000N/mm2 are available for circumferential winding and fressynet cables made up of 12wires of 8mm  stressed to 1200N/mm2 are to be used for vertical prestressing . Design the tank walls. Assuming the base as fixed. The cube strength of concrete is 40N/mm2