1. 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.
2. The loss due to relaxation of stress in steel is reduced temporarily by overstressing at a period of ____________
a) 20 minutes
b) 5 minutes
c) 15 minutes
d) 2 minutes
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.
3. 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.
4. The cold drawn steel wires have a modulus of elasticity of ____________
a) 200
b) 300
c) 400
d) 500
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.
5. The modulus of elasticity of cold worked high tensile alloy steel according to British code is?
a) 177
b) 175
c) 176
d) 170
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.
6. The Indian standard code provides relaxation loss for prestressing steels at a temperature of ____________?
a) 27˚c
b) 30˚c
c) 37˚c
d) 57˚c
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.
7. The relaxation loss given for initial stress of 0.7fpu is?
a) 70
b) 80
c) 90
d) 100
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.
8. The loss of prestress due to friction generally occurs in case of ____________
a) Post tensioned members
b) Pre tensioned members
c) Chemical members
d) Electrical members
Explanation: Loss of prestress due to friction occurs in the case of post tensioned members, the tendons are housed in the ducts performed in concrete and the ducts are either straight or follow a curved profile depending upon the design requirements.
9. The total loss of prestress due to friction is of ____________
a) 4 types
b) 2 types
c) 8 types
d) 3 types
Explanation: The total loss due to friction is divided into two types:
Loss of prestress due to effect of curvature, Loss of prestress due to wobble effect and frictional losses can be reduced by over tensioning the tendons by an amount equal to the maximum frictional loss and jacking the tendons from both ends of the beam adopted generally, when the tendons are long or when the angles of bearing are large.
10. The loss of stress due to curvature effect depends upon ____________
a) Alignment
b) Midpoint
c) Centerline
d) Exterior point
Explanation: The loss of stress due to the curvature effect, which depends upon the tendon form or alignment which generally follows a curved profile along the length of the beam, curvature coefficient is expressed as μ and wobble coefficient is expressed as k/m.