Prestressed Concrete Structures Multiple Choice Questions on “Post Tensioning Systems”.
1. In post tensioning, the concrete units are cast by ___________
a) Ducts
b) Jacks
c) Anchorages
d) Wedges
Answer: a
Clarification: In post tensioning, the concrete units are first cast by incorporating ducts or grooves to house the tendons and when the concrete attains sufficient strength, the high tensile wires are tensioned by means of jack bearing on the end face of the member and anchored by wedges or nuts.
2. After the tensioning operation, the space between the tendons and the ducts are ___________
a) Layered
b) Grouted
c) Cemented
d) Drilled
Answer: b
Clarification: The focus are transmitted to the concrete by means of the end anchorages and when the cable is curved through the radial pressure between the cable and the ducts and the space between the tendons and the duct is generally grouted after the tensioning operation.
3. The Freyssinet system of post tensioning anchorages was developed in ___________
a) 1934
b) 1935
c) 1939
d) 1936
Answer: c
Clarification: Freyssinet system of post tensioning anchorages which was developed in 1939, gave impetus to the various new systems devised over the years and at present, according to abeles there are over 64 patent post prominent systems used worldwide.
4. The Freyssinet system of prestressing involves the arrangement of wires of number?
a) 10
b) 14
c) 12
d) 8
Answer: c
Clarification: Freyssinet system of prestressing involves the arrangement of high tensile steel wires of about 12 in number with diameter of 5mm to 8mm forming a group into a cable, cylinder is provided as an anchorage and it has a conical hole at its centre provided with heavy hoop reinforcement these cylinders are is provided as an anchorage and it has a conical hole at its centre provided with heavy hoop reinforcement.
5. In Freyssinet system which element behaves as a guide?
a) Spiral spring
b) Hallow spring
c) Trapezoidal spring
d) Cable spring
Answer: a
Clarification: In Freyssinet system of prestressing spiral spring is provided inside the cable for sufficient clearance between the wires and it even provides a cement grouted channel and the spiral spring behaves as a guide in transferring the reaction to concrete.
6. Which of the following is a disadvantage of Freyssinet system?
a) Stresses in the wires are not similar
b) Rapid attainment of stretching force
c) Safeguarding of wires is economical
d) Projection of plug left in concrete
Answer: a
Clarification: The stresses in the wires are not similar, even though all the wires are been stretched together, Freyssinet system involves the application of heavy jacks which are economical force applied for stretching is about 250kn to 500kn which is maximum and is still inadequate.
7. In Gifford-Udall system, the number of methods for prestressing members is?
a) 1
b) 2
c) 3
d) 4
Answer: c
Clarification: 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.
8. 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
Answer: c
Clarification: 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.
9. In Lee-McCall system, the prestressing tendons are in the form of ___________
a) Steel bars
b) Tension bars
c) Compression bars
d) Rings
Answer: a
Clarification: 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.
10. The drawback of Lee-McCall system is it cannot use ___________
a) Straight tendons
b) Elliptical tendons
c) Curved tendons
d) Trapezoidal tendons
Answer: c
Clarification: 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.
11. 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
Answer: a
Clarification: 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.
12. Which system of prestressing is widely used in India?
a) Gifford-Udall
b) Lee-McCall
c) Magnel-Balton
d) Freyssinet
Answer: d
Clarification: 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.