8/11/2019 Design of Prestressed Concrete Structures QP-02

1/2

Seventh Semester B.E. Degree Examination, Dec. 07/ Jan. 08

Design of Pre-stressed Concrete Structures

Time: 3 hrs. Max. Marks:100

Note:1- Answer any five full questions.

2- Use of IS: 1343 permitted.

1a. Explain why high strength steel is needed in PSC.(04Marks)

1b. Explain with neat sketches Freyssinets system or Giffard udall system of prestressing.(08Marks)

1c. A simply supported PSC beam is 300mm wide and 800mm deep. The beam has an effective span of

8m and is subjected to a superimposed load of 6kN/m.

Suggest a suitable cable profile and the prestressing force using gross sectional properties. Alsosketch the stress distribution at the middle span for

i-

Prestress+Self weight

ii- Prestress+Self weight+Live load.(08Marks)

2a. What are the assumptions made in the analysis of PSC members.(06Marks)

2b. How does equivalent concrete section influence the stress calculations as compared to gross cross

section.(04Marks)

2c. A PSC beam supports a live load of 6kN/m over a simply supported span of 8m. The beam has an I

section having an overall depth of 600mm and 200mm wide flanges at top and bottom. The thicknessof web and flanges are 80mm. Find the magnitude of the prestressing force with an eccentricity of

80mm, which balances the stresses due to dead and live loads at the bottom fibre of the central

section of the beam(10Marks)

3a. How do you estimate the loss of prestress due to

i- Elastic deformation

ii-

Shrinkage and

iii- Creep of concrete.(12Marks)

3b. A post tensioned simply supported beam of 8m span is provided with curved cables of area 800mm2

with slope of 1 in 20 at each end and is initially stressed to 1200 N/mm2, Es = 210 kN/mm2.

i-

Calculate the loss of prestress if the coefficient of friction between duct and cable= 0.50 a

friction coefficient for wave effect =0.0015/m.

ii- Calculate the loss if there is a slip of 2mm at the jacking end during anchoring.

iii- Calculate the final force force in the cable and the percentage loss of prestress due to friction

and slip.(08Marks)

4a. Discuss the importance of deflection control for PSC beams.(05Marks)

4b. What are the long term deflection due to(05Marks)

4c. A simplysupported PSC beam of span 8m is provided with a parabolic cable of area 300mm2havingan eccentricity of 80mm at centre and zero at the end, and has an effective prestress of 1200 N/mm2.

If the beam is 250mm wide and 400mm deep,

i- Find the central deflection due to self weight and prestress and

8/11/2019 Design of Prestressed Concrete Structures QP-02

2/2

ii- The central concentrated load that can be applied to limit the downward deflection to l/360.

Neglect all losses.(10Marks)

5a. Explain the concept of pressure linewith neat sketches.(04Marks)

5b. Design a post tensioned PSC beam with minimum possible depth for the following data:

Span=8m, Live load=30kN/mm, Width=300mm, Losses=12%, Compressive and tensile stresses in

concrete not to exceed 18 and 1.5N/mm2. Calculate the prestressing force and the corresponding

eccentricity.(16Marks)

6a. What is a balanced beam.(05Marks)

6b. Write an expression for minimum percentage of prestressing steel.(05Marks)

6c. A post tensioned simply supported beam of 20m span is provided with unbounded cable of area

2400mm2with centre of steel at 1000mm from top. The steel is initially stresses to 1000N/mm2after

all losses. If the beam is 400mm wide, calculate the ultimate moment of resistance of the beam using

IS recommendations. Assume M40 grade concrete and ultimate tensile strength of steel as

1500N/mm2.(10Marks)

7a. Discuss briefly the modes of failure due to shear.(06Marks)

7b. Write a note on limit state of serviceability.(06Marks)

7c. Using Fe415 reinforcement, design the shear reinforcement, using IS recommendations for the

following data:

B=200, D=300 Vu=180kN, fck=40N/mm2, effective cover=50mm, Compressive

stress=5N/mm2.(08Marks)

8a. What is transmission length as applicable to PSC members.(06Marks)

8b. Write a note on end zone reinforcement or secondary reinforcement.(06Marks)

8c. A PSC beam 300mm wide and 500 deep has two anchorages of 150mm dia with their centres at

125mm from the top and bottom of the beam. If the force transmitted by each cable is 800kN

estimate the

i- Max tension and

ii- Bursting force.(08Marks)

*****