# Prestressed Concrete Structures Questions and Answers – Cracking Moments

This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Cracking Moments”.

1. The soffit of the beam after the transfer of prestress to concrete will be under ____________
a) Tension
b) Compression
c) Breakage
d) Bondage

Explanation: The bending moment at which visible cracks developed in prestressed concrete members is generally referred to as the cracking moment after the transfer of prestress to concrete, the soffit of the beam will be under compression.

2. The compressive and tensile stresses developed in cracking moments are due to ____________

Explanation: These compressive stresses are balanced by the tensile stresses developed due to the transverse loads on the beam, so that the resultant stress at the bottom fiber is zero, a further increase in loading results in the development of tensile stresses at the soffit of beam.

3. The micro cracks develop as soon as the tensile strain in concrete exceeds about ____________
a) 80-100×10-6 units
b) 100-150×10-6 units
c) 150-300×10-6 units
d) 300-500×10-6 units

Explanation: As concrete is weak in tension, micro cracks develop as soon as the tensile strain in concrete exceeds about 80-100×10-6 units and the parameters in concrete are influenced by various materials and their permissible values according to Indian standard codes.

4. When the loads in a concrete member are further increased than permitted, the crack widths are of an order of limit?
a) 0.01-0.02mm5
b) 0.05-0.10mm5
c) 0.03-0.05mm5
d) 0.07-0.08mm5

Explanation: If the loads are further increased, than 80-100×10-6 units visible cracks appear in the tension zone and at this stage, it is estimated that the crack widths are of an order of 0.01-0.02mm5 and these width of cracks are considered from a value of minimum to maximum not excess till permitted.

5. A rectangular concrete beam of cross section 120mm wide and 300mm deep is prestressed by a straight cable, effective force of 180kn at an eccentricity 50, area of 36×103mm2(z=18×105mm3). Find the total stress due to prestress?
a) 10
b) 25
c) 35
d) 45

Explanation: P = 180kn, A = 36×103mm2, e = 50mm, b = 120mm, d = 300mm, z = 18×105mm3
Stresses due to prestress = (p/a) = (180×103/36×103) = 5n/mm2, (pe/z) = (180×103×50/18×105) = 5n/mm2

Total stress = ((p/a)+(pe/z))=(5+5) = 10n/mm2.

6. A rectangular concrete beam of cross section 100mm wide and 400mm deep is prestressed by a straight cable of span 6m, imposed load is 3.14kn/m, area is 36×103mm2(Z=18×105mm3). Calculate working moment assuming the self weight of concrete as 24kn/m3?
a) 10.25
b) 2.25
c) 3.25
d) 4.25

Explanation: Z = 18×105mm3, A = 36×103mm2, G = (0.1×0.4×24) = 0.96kn/m
Total load W = (g+q) = (0.96+3.14) = 4.1kn/m, Maximum working moment = (0.125×4×62) = 18.45knm
(M/Z) = (18.45×106/18×105) = 10.25n/mm2.

7. When the tensile stresses are developed in the cracks, they are visible at ____________
a) Hoop stress of beams
b) Soffit of beams
c) Sagging of beams
d) Hogging of beams

Explanation: The tensile stresses are developed when cracks become visible at the soffit of beams depend upon the type and distribution of steel reinforcement and the quality of concrete in beam, at the soffit of the beam concrete behaves according to influencing parameters of steel reinforcement.

8. The cracks appear when the tensile stresses at the soffit are equal to ____________
a) Modulus of elasticity
b) Modulus of rupture
c) Tension modulus
d) Reinforcement modulus

Explanation: It is generally considered that visible cracks appear when the tensile stresses at the soffit are approximately equal to modulus of rupture of the material, it is an ultimate strength pertaining to failure of beams by flexure equal to the bending moment at rupture divided by the section modulus of beams.

9. The widths of the cracks are influenced by ____________
a) Degree of bond
b) Stress
c) Strain
d) Tension

Explanation: The widths of the cracks are highly influenced by the degree of bond developed between concrete and steel and stress corrosion cracking results from the combined action of corrosion and static tensile stress which may be either residual or externally applied.

10. The formulae for load factor against cracking is ____________
a) Cracking moment/Working moment
b) Cracking moment/Bending moment
c) Cracking moment/Tensile moment
d) Cracking moment/Aerial moment

Explanation: The beam at which visible cracks developed in prestressed concrete members is generally referred to as the “cracking moment” and their formula for load factor against cracking is cracking moment/working moment.

Sanfoundry Global Education & Learning Series – Prestressed Concrete Structures.

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