1. In Gifford-Udall system, the number of methods for prestressing members is?
a) 1
b) 2
c) 3
d) 4
Explanation: Gifford-Udall system gave three methods for prestressing members by small wedging grips, anchoring of wires by wedging, application of tendons of H.T type with a diameter of about 28mm and this method of prestressing involves the anchoring of wires by wedges, due to which the wires directly fit into the tapered recesses present over the bearing plate.
2. In Gifford-Udall system, which method involves prestressing by H.T type tendon?
a) Method 1
b) Method 2
c) Method 3
d) Method 4
Explanation: Gifford-Udall system consists of 3 methods in that third method of prestressing involves the application of a tendon of H.T type with a diameter of about 28mm and it is quite stronger than 6mm diameter wires.
3. In Lee-McCall system, the prestressing tendons are in the form of __________
a) Steel bars
b) Tension bars
c) Compression bars
d) Rings
Explanation: In Lee-McCall system, the prestressing tendons are in the form of high tensile alloy steel bars and the bar size ranges from 12-40mm diameter and after tensioning, the bars are anchored by special threaded nuts which bear against a distribution plate.
4. The drawback of Lee-McCall system is it cannot use ___________
a) Straight tendons
b) Elliptical tendons
c) Curved tendons
d) Trapezoidal tendons
Explanation: In this system, the forces are transmitted by the bearing at the end blocks while the system eliminates the loss of stress due to anchorage slip by screwing a nut and washer against end blocks and the drawback of this system is that it cannot use curved tendons.
5. The method of tensioning Magnel-Balton system involves ___________
a) Hydraulic jack and wires
b) Hydraulic jack and bars
c) Multi strand hydraulic jack
d) Jack inserted at centre of beam
Explanation: The method of tensioning Manel-Balton system involves hydraulic jack tensioning two wires at a time, the arrangement of Magnel-Balton system constitutes of a rectangular cable section provided with layers of wires of diameter about 5mm to 8mm such that each layer is supplied with four wires and the arrangement of wires in the geometric pattern is to be considered by supplying spacers/grills at uniform intervals throughout the length of the cable, an adequate resistance against friction for the wires in the of sandwich plates.
6. Which system of prestressing is widely used in India?
a) Gifford-Udall
b) Lee-McCall
c) Magnel-Balton
d) Freyssinet
Explanation: The Freyssinet anchorage system is widely used in Europe and India consists of a cylinder with a conical interior through which the high tensile wires pass and against the walls of which the wires are wedged by a conical plug lined longitudinally with grooves to house the wires.
7. The method of pre stressing by heated tendons, achieved by passing an electric current is known as ___________
a) Chemical pre stressing
b) Thermo electric pre stressing
c) Dynamic pre stressing
d) Thermo blasting pre stressing
Explanation: In this method the tendons are heated through the high tensile wires and by allowing electric current to flow through it, this process is referred as thermo electric pre stressing, in the erstwhile U.S.S.R, the electro thermal method has been widely used since 1958 for pretensioning bar reinforcements of precast units.
8. In thermo electric pre stressing, heating the bars is done to a temperature of ___________
a) 300-400degrees
b) 500-600degrees
c) 200-300degrees
d) 150-450degrees
Explanation: The temperature in the bars is raised from 300 to 400degrees for a time duration of 3to 5 minutes, with this the bar elongates up to 0.4 to 0.5%, empirical relations for the estimation of the current, voltage and power requirements of the transformer are reported by Graduck
9. The period of cooling in thermo electric pre stressing is observed as ___________
a) 20-30 minutes
b) 12-15 minutes
c) 5-10 minutes
d) 15-20 minutes
Explanation: After the process of cooling, the bars get shortened but which can be known with the fixed arrangement at both the ends and the period of cooling is observed as 12 to 15minutes, a steel metal sheath of enlarged diameter and sufficient length is generally used to cover the splice, screw threaded connectors are not recommended for splicing heat treated prestressing steels, which are highly suspectable for stress corrosion.
10. The thermo electric pre stressing, initial stress in tendons ranges between __________?
a) 500-600n/mm2
b) 200-300n/mm2
c) 100-150n/mm2
d) 600-800n/mm2
Explanation: Thermo electric pre stressing helps to cause on initial stress in tendons of magnitude from 500 to 600n/mm2 and concrete is placed in moulds only after the temperature of the wires falls below 90degrees.