This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Stresses in Tendons”.
1. The prestressed member undergoes deformation due to the action of ______________
a) Prestressing force and flexural loads
b) Prestressing force and combined loads
c) Prestressing force and transverse loads
d) Prestressing force and tangential loads
View Answer
Explanation: A prestressed member undergoes deformation due to the action of the prestressing force and transverse loads acting on the member and the curvature of the cable changes which results in a slight variation of stresses in the tendons.
2. The rotation equation obtained by applying Mohr’s theorem considering a concrete beam of span l, force p, eccentricity e is?
a) θp = PeL/2EI
b) θp = PeL/4EI
c) θp = PeL/16EI
d) θp = PeL/20EI
View Answer
Explanation: A concrete beam of span l is prestressed by a cable carrying an effective force p at an eccentricity e the rotation at the supports due to hogging of the beam is obtained by applying Mohr’s theorem as θp = Area of bending moment/flexural rigidity = PeL/2EI.
3. The cross section of a prestressed concrete beam is 100mm wide and 300mm deep and the initial stress in tendons are located at a eccentricity of 50mm is 1000n/mm2, the sectional area is 100mm2. Find rotation due to prestress (hogging moment)?
a) 0.00155
b) 0.00165
c) 0.00175
d) 0.00185
View Answer
Explanation: Moment of area I = (100×3003/12) = 225×106mm4
Prestressing force p = (1000×100) = 105 = 100kn
Rotation due to prestressing force θp = PeL/2EI = (100×50×6×103/2×36×225×106)
Hogging moment = 0.00185radians.
4. In the elastic range, any increase in prestressed member does not show any effect on ____________
a) Steel stress
b) Compressive stress
c) Bending stress
d) Flexural stress
View Answer
Explanation: In the elastic range any increase in loading on a prestressed member does not result in any significant change in the steel stress and in other words, the stress in steel is more or less constant in elastic range of prestressed concrete.
5. The rate of increase in stress in the tendons of a prestress concrete member depends upon ____________
a) Bond and breakage
b) Bond and surrounding concrete
c) Bond and elasticity
d) Bond and anchorage
View Answer
Explanation: The rate of increase of stress in the tendons of a prestressed concrete member under loads depends upon the degree of bond between the high tensile steel wires and the surrounding concrete, the increase of stress in steel depends on the average strain in concrete at the level of steel.
6. A prestressed concrete beam used over a span of 6m is 100mm wide and 300mm deep, live load of 4kn/m, density of concrete is 24kn/m3, modulus of elasticity of concrete is 36 and steel is 210kn/mm2. Find rotation due to loads(sagging moment)?
a) 0.005
b) 0.00525
c) 0.0024
d) 0.0045
View Answer
Explanation: E = 210kn/mm2, Self weight of the beam, g = (0.1×0.3×24) = 0.72kn/m
Live load on the beam (q) = 4kn/m, Total load on the beam, Wd = (q+g) = (4+0.72) = 0.00472kn/mm
Rotation due to prestressing force θp = PeL/24EI = (100×50×6×103/24×36×210×106)
Sagging moment = 0.00525radians.
7. The stress in tendons of bonded beams is ____________
a) αe (My/I)
b) αe (My/R)
c) αe (My/L)
d) αe (My/20)
View Answer
Explanation: The stresses in tendons of bonded beams is αe (My/I), αe = Modular ratio of steel to concrete, y = vertical distance of a point from centroid of concrete section, M = bending moment, I = moment of area of the section, in cases of bonded members such as pretensioned elements or post tensioned grouted members, the composite action between steel and concrete prevails and the stresses in steel are computed using the theory of composite sections up to stage of cracking.
8. The rate of increase of stress is larger in case of ____________
a) Bonded beams
b) Un bonded beams
c) Tensioned beams
d) Anchorage beams
View Answer
Explanation: The rate of increase of stress is larger in case of bonded beams than in un bonded beams both in the pre cracking and post cracking stages, in case of unbounded beams, the tendons are free to elongate independently throughout their length under the action of transverse loads on the beam.
9. The development of crack widths is comparatively larger in ____________
a) Bonded beams
b) Un bonded beams
c) Localized beams
d) Strengthened beams
View Answer
Explanation: In post cracking stage, while the bonded beams are characterized by small cracks, which are well distributed in the zone of the larger moments, unbounded beams develop only a few cracks, which are localized at weaker sections and the crack widths are correspondingly larger in comparison with the bonded beams.
10. Which beams are preferred mostly due to their higher flexural strength?
a) Bonded beams
b) Un bonded beams
c) Exhaustive beams
d) Extended beams
View Answer
Explanation: In general bonded beams are preferable due to their higher flexural strength and predictable deformation characteristics, after the onset of cracking the stress in steel increases at a faster rate in both types of beams since steel does not reach its ultimate strength in case of unbounded beams the ultimate load supported by the beam is smaller than that of bonded beam in which the steel attains its ultimate strength at the failure stage of member.
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