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

View Answer

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) 0.08f_{ck}bd^{2} to 0.2f_{ck}bd^{2}

b) 0.11fc_{ck}d^{2} to 0.5f_{ck}bd^{2}

c) 0.15fc_{ck}d^{2} to 0.8f_{ck}bd^{2}

d) 0.21fc_{ck}d^{2} to 0.9f_{ck}bd^{2}

View Answer

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 0.08f

_{ck}bd

^{2}to 0.2f

_{ck}bd

^{2}, 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

View Answer

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:

M

_{ud}= 0.21f

_{ck}bd

^{2}, 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

View Answer

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 f_{ck} is 50n/mm^{2} and f_{p} 1600n/mm^{2}, x_{u}/d is 0.5?

a) 300

b) 500

c) 600

d) 200

View Answer

Explanation: M

_{u}= 0.36f

_{ck}bx

_{u}(d-0.42x

_{u}) = 0.14f

_{ck}bd

^{2}, b = 0.5d, d

^{3}= (m

_{u}/0.14f

_{ck}0.5) = (100×10

^{6}/0.15x50x0.5), d = 300mm, b = 150mm, x

_{u}= (0.5×300) = 150mm for (x

_{u}/d) = 0.5, f

_{pu}= 0.87f

_{p}, A

_{p}= (m

_{u}/0.87f

_{p}(d-0.42x

_{u})) = (100×100

^{6}/0.87×1600(300-0.42×150)) = 300mm

^{2}, adopt a section 150mm wide by 350mm deep with 300mm

^{2}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 f_{ck} is 35n/mm^{2} and f_{p} is 1500n/mm^{2}?

a) 1000mm^{2}

b) 1015mm^{2}

c) 1030mm^{2}

d) 1003mm^{2}

View Answer

Explanation: For flanged sections, m

_{ud}= 0.08f

_{ck}bd

^{2}assuming b = 0.5d and b

_{w}= 0.25b, d

^{3}= ((1200×106)/0.10x35x0.5)), d = 1000mm, b = 500mm, thickness of top flange = h

_{f}= 0.2d = (0.2×10000) = 200mm, thickness of web = b

_{w}= 0.25b = (0.25×500) = 125mm

Assuming the neutral axis depth, x

_{u }= hf = 200mm, m

_{u}= 0.87f

_{p}A

_{p}(d-0.42xu), A

_{p}= (m

_{u}/0.87f

_{p}(d-0.42xu)) = (1200×10

^{6}/0.87×1500(1000-0.42×200)) = 100

^{3}mm

^{2}.

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

View Answer

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

d) Loads

View Answer

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

View Answer

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/mm^{2}.

a) 41500

b) 4000

c) 22000

d) 1500

View Answer

Explanation: Design tensile load, n

_{d}= 500kn, f

_{ct}= 15n/mm2, f

_{tw}= 0, ɳ = 0.80

Area of concrete section = (N

_{d}/ ɳ f

_{ct}) = (500×10

^{3}/0.8×15) = 41500mm

^{2}.

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