This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Nature of Losses”.
1. The initial prestress in concrete with ongoing time undergoes?
Explanation: The initial prestress in concrete undergoes a gradual reduction with time from the stage of transfer due to various causes and this is generally referred to as “loss of prestress” since there is no bending moment at the span the stress in the tendon changes only at the midspan but not at the end, since concrete and steel are considered as one section after bonding the change in stresses due to the bending of this section can be calculated by using transformed section method.
2. The loss in prestress is necessary to make an estimate of:
Explanation: A reasonably good estimate of the magnitude of lines of prestress is necessary from the point of view of design, if the prestress present in the steel is considered as the force applied on the concrete at the ends and the change is stress along with length of entire beam is not concluded as change in prestress.
3. How many types of losses in prestress are observed in pretensioned member?
Explanation: The losses in prestress in pretensioned member are four types: Losses due to elastic deformation of concrete, Loss due to relaxation of stress in steel, Loss due to creep of concrete, Loss due to shrinkage of concrete, in pretensioned members, due to prestress present at the ends of the members the tendon bends upwards due to bending action and the tendon becomes shortened, this is not considered as loss in prestress.
4. How many types of loss in prestress are observed in post tensioned members?
Explanation: The loss of prestress in post tensioned members is of 6 types: Elastic deformation of concrete, relaxation of stress in steel, creep of concrete, shrinkage of concrete, slip of anchorages, friction, if the tendon is not the particular line from centre of gravity of concrete section the calculation of the change in length of the beam due to bending moment will be complicated and this is considered as ordinary loss or gain 2 to 3%.
5. The frictional and anchorage slip losses are observed in:
a) Post tensioned members
b) Pretensioned members
c) Ruptured members
d) Tensile members
Explanation: The frictional losses and loss due to anchorage slip are observed in post tensioned members only because pretensioned members do not require anchorages for prestressing and in post tensioned beams which are unbounded the loss and gain of prestress depends upon the upward bending movement of the beam and the beam when loaded fully respectively.
6. The concrete members which are prestressed by providing the tensioned tendons are termed as:
a) Pre tensioning members
b) Internally prestressed members
c) Linear prestressed members
d) Circular prestressed members
Explanation: The concrete members which are prestressed by providing the tensioned tendons are termed as internally prestressed members due to bending moment in the beam is developed and the bending in the beam results in change of unit stresses, unit strains in the tendons.
7. Which of the following is a loss of prestress in post tensioned members?
a) Loss due to slip of anchorages
b) Loss due to deformations
c) Loss due to tensioning
d) Loss due to pumped concrete
Explanation: The loss of prestress in post tensioned members in the following is the loss of slip due to anchorages and friction, in post tensioned beams which are bounded, before grouting action the prestress in the steel is affected by the bending action of the member.
8. The sudden changes in loss of prestress may be due to:
Explanation: In addition to loss of prestress in post and pre tensioning members, there may be losses of prestress due to sudden changes in temperature, especially in steam curing of pretensioned units before the bonding of tendon to the concrete, the prestress in the tendon will be influenced by the bending of the member.
9. The rise in temperature in loss of prestress causes:
a) Partial transfer
b) Shifting of prestress
c) Full transfer of prestress
d) Prestress elongation
Explanation: The rise in temperature causes a partial transfer of prestress due to elongation of the tendons between adjacent units in the long line process which may cause a large amount of creep if the concrete is not properly cured.
10. The loss of change of temperature is between:
a) Tension and transfer
b) Compression and prestress
c) Bending and shear
d) Principle stress and loading
Explanation: If there is a possibility of a change of temperature between the times of tensioning and transfer the corresponding loss should be allowed for design, due to bending of a member the prestress may vary depending upon the line of bending, position of tendons(high strength steel cables which are flexible) number of tendons which are kept at different stages and also the type of prestressing whether it is a post tensioned or pretensioned the effect of bending may be either a positive or negative in prestress.
Sanfoundry Global Education & Learning Series – Prestressed Concrete Structures.
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