# Prestressed Concrete Structures Questions and Answers – Design for Flexure and Axial Tension

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This set of Prestressed Concrete Structures Questions and Answers for Freshers focuses on “Design for Flexure and Axial Tension”.

1. Which of failure is considered in the design of prestressed concrete members for the limit states of collapse?
a) Total failure
b) Shear failure
c) Ultimate failure
d) Collapse failure

Explanation: The distribution of compressive stress in the section at the ultimate failure stage is considered and the effective depth required is estimated by equating the total ultimate moment with the internal resisting angle.

2. The maximum design value of the moment of resistance of rectangular flanged section varies from ____________
a) 0.08fckbd2 to 0.2fckbd2
b) 0.11fcckd2 to 0.5fckbd2
c) 0.15fcckd2 to 0.8fckbd2
d) 0.21fcckd2 to 0.9fckbd2

Explanation: A comparative analysis of the various code recommendations indicate that the maximum design value of the moment of resistance of rectangular and flange of sections vary from 0.08fckbd2 to 0.2fckbd2, depending upon the recommendations of the stress block parameters.

3. It is often preferable to use a larger section, because it means a saving on ____________
a) Deflection
b) Cost
c) Stress
d) Strain

Explanation: The maximum ultimate moment of resistance of a resistance of a rectangular section rectangular section according to the Indian standard code IS: 1343-1980 is given by:
Mud = 0.21fckbd2, the dimension based on this expression are the minimum values and it often preferable to use a larger section because it means a saving on the costly prestressing tendons.

4. The area of high tensile and un tensioned reinforcement required to mobilize the desired flexural strength is computed using?
a) Tension equilibrium
b) Force equilibrium
c) Transverse equilibrium
d) Rotational equilibrium

Explanation: The area of high tensile and un tensioned reinforcement required to mobilize the desired flexural strength is computed using the force equilibrium at the limit state of collapse and the force equilibrium rather depends upon the pressures exerted in different conditions.

5. A pretensioned prestressed concrete beam of rectangular section is required to support a design ultimate moment of 100knm. Design the section if fck is 50n/mm2 and fp 1600n/mm2, xu/d is 0.5?
a) 300
b) 500
c) 600
d) 200

Explanation: Mu = 0.36fckbxu(d-0.42xu) = 0.14fckbd2, b = 0.5d, d3 = (mu/0.14fck0.5) = (100×106/0.15x50x0.5), d = 300mm, b = 150mm, xu = (0.5×300) = 150mm for (xu/d) = 0.5, fpu = 0.87fp, Ap = (mu/0.87fp(d-0.42xu)) = (100×1006/0.87×1600(300-0.42×150)) = 300mm2, adopt a section 150mm wide by 350mm deep with 300mm2 of high tensile steel located at an effective depth of 300mm.

6. A post tensioned beam of unsymmetrical I section is required to support a design ultimate moment of 1200 knm and determine the overall depth and thickness of the compression flange required if fck is 35n/mm2 and fp is 1500n/mm2?
a) 1000mm2
b) 1015mm2
c) 1030mm2
d) 1003mm2

Explanation: For flanged sections, mud = 0.08fckbd2 assuming b = 0.5d and bw = 0.25b, d3 = ((1200×106)/0.10x35x0.5)), d = 1000mm, b = 500mm, thickness of top flange = hf = 0.2d = (0.2×10000) = 200mm, thickness of web = bw = 0.25b = (0.25×500) = 125mm
Assuming the neutral axis depth, xu = hf = 200mm, mu = 0.87fpAp (d-0.42xu), Ap = (mu/0.87fp (d-0.42xu)) = (1200×106/0.87×1500(1000-0.42×200)) = 1003mm2.

7. Due to presence of precompression, prestressed concrete is ideally suited for the design of members subjected to ____________
a) Axial tension
b) Prestressed tension
c) Principle stress
d) Bonded stress

Explanation: Due to presence of precompression, prestressed concrete is ideally suited for the design of members subjected to axial tension and the axial tension in a member depends upon the direction of tension acting on a member and its reaction.

8. The design essentially considered for axial tension is to determine the ____________
a) Deflection
b) Eccentricity
c) Cross section

Explanation: The design essentially consists of determining the cross section area of the member and the required prestressing force to safely support the axial tensile load conforming to the limit state of serviceability and collapse.

9. According to Indian standard code IS: 3370 the load factors against cracking and collapse should not be less than?
a) 15
b) 1.5
c) 3
d) 0.3

Explanation: According to Indian standard code IS: 3370 1967 the load factors against cracking and collapse should be not less than 1.2 and 2 respectively and IS: 3370 code is used for design factors of load factors.

10. Design a suitable section for the tie member of a truss to support a maximum design tensile force of 500kn. The permissible compressive stress in concrete at transfer is 15n/mm2.
a) 41500
b) 4000
c) 22000
d) 1500 