8/11/2019 Design of Prestressed Concrete Structures QP-03

1/2

Seventh Semester B.E. Degree Examination, January/February 2005

Civil Engineering/Transportation Engineering

Prestressed Concrete (Elective)

Time: 3 hrs. Max. Marks: 100

Note: 1- Answer any FIVE full questions.

2- Use of IS 1343 is permitted.

1a- Explain the necessity of high strength steel and concrete in prestressed concrete members.(05Marks)

1b- A prestressed concrete beam of 200mm wide and 300mm deep is used over an effective span of 5m

to support an imposed load of 5kN/m. The density of concrete is 24kN/m3. At the quarter span

section of the beam, find the magnitude of

i- The concentric prestressing force necessary for zero fibre stress at the soffit when the beam is

fully loaded.

ii- The eccentric prestressing force located 100mm from the bottom of the beam, which would

modify the bottom fibre stress due to loading.(15Marks)

2a- What are the factors influencing the loss of prestress in prestressed concrete beams.(5Marks)

2b- A prestressed concrete beam 150 x 300mm deep is prestressed by 10 wires of 8mm diameter with an

initial stress of 1200N/mm2with their centroid at 80mm from the soffit. Find the maximum stress in

the concrete immediately after transfer allowing only for elastic shortening of the concrete. If thebeam undergoes a further shortening due to creep and shrinkage and there is a relaxation of steel of

4%, estimate the final percentage loss of stress. Total shrinkage strain = 3(10)-4, ultimate creep strain

= 4(10)-6/ unit stress, modulus of elasticity of concrete = 35kN/mm2, Modulus of elasticity of steel =

210kN/mm2.(15Marks)

3- A prestressed concrete simply supported beam of rectangular section 250 x 500mm deep has an

effective span of 8m. It is subjected to a live load of 3kN/m. It is prestressed with a straight cable

carrying an effective represtressing force of 250kN at an eccentricity of 80mm. Find the short term

deflection and long term deflection at the centre of the span. Assume live loads and dead loads are

applied simultaneously after the release of prestress. Ec= 40kN/mm2creep coefficient = 2.0, loss of

prestress = 20%.(20Marks)

4a- Explain briefly the term transmission length as applicable to pretensioned members.(5Marks)

4b- A pretensioned beam of 8m span has a symmetrical I section. The flanges are 200mm wide and

60mm thick. The web thickness is 80mm and the overall depth of girder is 400mm. The member is

prestressed by 8 wires of 5mm diameter located on the tension side such that the effective

eccentricity is 91mm. The initial stress in the wires is 1280N/mm2and the cube strength of concrete

at transfer is 42N/mm2.

i- Determine the maximum vertical tensile stress developed in the transfer of zone and

ii- Design suitable mild steel reinforcement assuming the permissible stress in steel is

140N/mm2(15Marks)

8/11/2019 Design of Prestressed Concrete Structures QP-03

2/2

5a- What is an end block in a post tensioned prestressed concrete beams.(5Marks)

5b- A Freysinelt anchorage 125mm diameter carrying 12 wires of 7mm diameter, stressed to 950N/mm2

is embedded concentrically in the web of an I section. The thickness of web is 225mm.

Evaluate the maximum tensile stress and bursting tensile force in the end block. Design the

reinforcement for the block.(15Marks)

6a- Explain how the shear resistance of the beam is computed.(5Marks)

6b- The support section of a prestressed concrete beam 120mm wide and 240mm deep is required to

support an ultimate shear force of 75kN. The compressive prestress at the centroidal axis is 5MPa,

fck= 40MPa, fy= 415MPa. Concrete cover to shear reinforcement is 50mm. Design suitable shear

reinforcement as per IS1343.(15Marks)

7a- Discuss the IS code method of determining the ultimate movement of resistance of rectangular and

flanged sections.(5Marks)

7b- A double Tee section having a flange of 1200mm wide and 150mm thick is prestressed by 500mm2

each. If the cube strength of concrete is 40N/mm2and tensile strength of steel is 1600N/mm2,

determine the flexural strength of the above section.(15Marks)

8- A pretensioned beam 80mm wide and 120mm deep is to be designed to support working loads 5kN

each concentrated at the third points over a span of 3m. If the permissible tensile stress in tension is

zero at transfer and 1.4N/mm2under working loads, design the number of 3mm wires and the

corresponding eccentricity at midspan section. Permissible tensile stress in the wires is 1400N/mm2.

The loss of prestress is 20 percent and the density of concrete is 24kN/m 3.(20Marks)

*****