This set of Prestressed Concrete Structures Problems focuses on “Loss Due to Relaxation of Stress in Steel”.
1. The phenomena of reduction of stress in steel at a constant strain are known as:
a) Relaxation of stress
b) Shrinkage of concrete
c) Creep of concrete
d) Anchorage slip
Explanation: In high tension strength steel, it is noticed that at a particular time the stress in steel reduced at a constant value of strain and this phenomena of reduction of stress in steel at a constant strain is known as relaxation of stress.
2. The relaxation of stress in steel changes according to the variation in:
Explanation: The phenomena of relaxation of stress occur as a result of creep in steel and the relaxation of stress in steel changes according to the variation of percentage of creep, the phenomenon of creep is influenced by the chemical composition, micro structures, grain size and variables in the manufacturing process, which results in changes in the internal crystal structure.
3. The codes provided for the loss of stress due to relaxation of steel are expressed as a percentage of:
a) Initial stress
b) Final stress
c) Major stress
d) Minor stress
Explanation: Most of the codes provided for the loss of stress due to relaxation of steel are expressed as a percentage of initial stresses in steel and the high tensile steel tendon in a prestressed concrete member does not remain strictly under a condition of either stress or strain, the most severe condition occurs generally at the stage of initial stressing subsequently the strain in the steel reduces as the concrete under the prestressing force.
4. The Indian standard code recommends a value for stress in wires varying from:
a) 0.5fpu to 0.8fpu
b) 0.2fpu to 0.6fpu
c) 0.4fpu to 0.10fpu
d) 0.6fpu to 0.8fpu
Explanation: The Indian standard code recommends a value varying from 0 to 90n/mm2 for stress in wires varying from 0.5fpu to 0.8fpu and the Indian standard codes for wires and bars prescribe the 1000 hour relaxation test with no relaxation exceeding 5 percent of the initial stress alternatively one can resort to the 100 hour relaxation test with no relaxation exceeding 3.5 percent of the initial stress similar provisions have also been made in the British and American codes.
5. The loss due to relaxation of stress in steel is reduced temporarily by overstressing at a period of:
Explanation: Experiments have shown that a reduction in relaxation stress is possible by preliminary overstressing by 5-10percent for a period of 2 to 3minutes considerably reduces the magnitude of relaxation and some codes permit temporary overstressing with corresponding lower magnitudes of relaxation stress.
6. The reduction of loss due to overstressing does not appear to be beneficial for:
a) Cold drawn wires
b) Hot drawn wires
c) Stabilized wires
d) Tensile wires
Explanation: Overstressing does not appear to be beneficial for stabilized wires which as a result of heat treatment, have 0.1 percent proof stress in excess of 85 percent of the tensile strength, since such wires suffer very little permanent deformation when overstressed.
7. The cold drawn steel wires have a modulus of elasticity of:
Explanation: The type of tendon cold drawn steel wire to BS: 2691 are of two types: Pre-straightened (normal relaxation), Pre-straightened (low relaxation) both have a modulus of elasticity of 200kn/mm2 having a percentage loss of prestress value of fpi 5, 2 and fpu 8.5, 3.
8. The modulus of elasticity of cold worked high tensile alloy steel according to British code is:
Explanation: The cold worked high tensile alloy steel bars to BS: 4486 has a modulus of elasticity of 175kn/mm2 and the initial prestress should not normally exceed 70% of the characteristic tensile strength and in no case should it exceed 75% at the time of initial tensioning, at the time of final stress after allowing the losses of prestress not greater than 60% of the characteristic tensile strength of tendons.
9. The Indian standard code provides relaxation loss for prestressing steels at a temperature of:
Explanation: The recommendations of Indian standard code (IS:1343) relaxation loss for prestressing steels at 1000 hours at 27˚c and at the time of initial prestress not to exceed 80% of the characteristic tensile strength of tendons final stress not less than 45% of the characteristic tensile strength of tendons.
10. The relaxation loss given for initial stress of 0.7fpu is:
Explanation: The initial stress of 0.7fpu has a relaxation loss of 70n/mm2, 0.5fpu has a relaxation loss of 0, 0.6fpu has a relaxation loss of 35n/mm2, 0.8fpu has a relaxation loss of 90n/mm2 fp is the characteristic strength of steel used for prestress.
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