This set of Prestressed Concrete Structures Interview Questions and Answers for freshers focuses on “Prestressing of Shell and Folded Structures”.
1. The concrete shell roof has been widely used to cover __________
a) Small floor spaces
b) Large floor spaces
c) Middle span spaces
d) Edge floor spaces
View Answer
Explanation: The concrete shell roofs have been widely used to cover large floor spaces of industrial structures and they are generally preferred to other structural forms as they use a minimum amount of materials yielding maximum structural advantage, the cross section being optimally used to resist the forces.
2. Reinforced concrete shells are ideally suited to cover floor spaces of up to __________
a) 15m
b) 25m
c) 30m
d) 40m
View Answer
Explanation: Reinforced concrete shells are ideally suited to cover floor spaces over medium to long range spans of up to 30m and in the case of longer spans, the tension develops in the edge beams of cylindrical shells is very high and results in congestion of reinforcement and improper compaction of concrete in these zones.
3. Long spans exceeding 30m necessitate is?
a) Welding
b) Blocking
c) Anchoring
d) Spinning
View Answer
Explanation: Longer spans exceeding 30m necessitate the lapping or wielding of reinforcement in addition, the structure may be rendered unfit for its intended use at the serviceability limit states due to excessive deflections and objectionable cracking and most of these problems are eliminated by prestressing the edge beams of long span shell structures.
4. The parabolic profile of the cables in the edge beam counteracts the __________
a) Compression
b) Tension
c) Deflection
d) Deformation
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Explanation: The parabolic profile of the cables in the edge beam counteracts the deflections due to the dead and live loads, so that the resulting deflection is well within the safe permissible limits and prestressing the shell considerably reduced the quantity of reinforcement in the structure as demonstrated by Goldstein.
5. The investigations by Marshall have shown that prestressing considerably reduces __________
a) Sagging moment
b) Transverse moment
c) Hogging moment
d) Longitudinal moment
View Answer
Explanation: The investigations by Marshall have shown that prestressing considerably reduces transverse moments and the development of cracking due to high tensile stresses at the soffits of edge beams can be completely eliminated by eccentric prestressing and in addition precompression in concrete inhibits the formation of temperature and shrinkage cracks.
6. The compressive state of stress in the shell membrane results in which construction?
a) Airtight construction
b) Water tight construction
c) Lever construction
d) Soffit construction
View Answer
Explanation: The compressive state of stress in the shell membrane results in which construction water tight construction and according to Haas prestressing has a favorable influence in providing adequate safety against the limit state of collapse by inelastic bulking India in 1941 over spans of 36m, with a chord width of 10.5m, the thickness of the shell being 63mm and the prestressed shell roof of the aircraft hangar at Karachi was built in 1942 over spans of 40m and as these shells gave satisfactory performance and were also economical in France, U.K, Germany and other countries.
7. In general, concrete shells have thin cross sections, which prectude the use of large diameter?
a) Cables
b) Wires
c) Bars
d) Lens
View Answer
Explanation: In general, concrete shells have thin cross sections, which prectude the use of large diameter Cables and the tendons consisting of 5 to 8mm diameter wires accommodated in narrow sheaths are conveniently used in post tensioning the lower parts of the shell membrane to overcome the problem of correctly positioning the curved cables along the space curves at the junction of the shell and edge beam, it is general practice to impart prestress by post tensioning the curved cables that housed entirely in the deep edge beams and the edge beams being invariably deep, it is convenient to arrange the cables one over the other so that maximum eccentricity is available at the centre of span.
8. The analysis of circular cylindrical shells with prestressed edge beam is more or less similar to __________
a) Prestressed concrete beam
b) Reinforced concrete beam
c) Chemically prestressed beam
d) Partially prestressed beam
View Answer
Explanation: The analysis of circular cylindrical shells with prestressed edge beam is more or less similar to Reinforced concrete beam however, the effect of prestressing is to be considered in formulating the boundary conditions at the junction of the shell and the edge beam and in addition to the normal boundary conditions such as zero horizontal displacement and rotation of the shell edge, which are also applicable for reinforced concrete shells.
9. The rigorous methods generally involve computations to estimate the __________
a) Transverse reactions
b) Longitudinal reactions
c) Redundant reactions
d) Span reactions
View Answer
Explanation: The rigorous methods generally involve lengthy computations to estimate the redundant reactions between the shell and the edge beam and in the case of long shells with span/radius ratio exceeding 3, the beam theory developed can be conveniently used for the preliminary analysis of prestressed shells without the loss of much accuracy and in this method the shell is considered or a beam of curved cross section and the flexural and shear stresses are computed using the well known beam formulas.
10. The general bending theory of thin curved shells is governed by a __________
a) Differential equation
b) Parabola equation
c) Ellipse equation
d) Moment equations
View Answer
Explanation: The general bending theory by thin curved shells is governed by a differential equation of eighth order involving the main shell parameters and deformations and solution based on simplifying approximations have been developed by several investigators and a comparative analysis of various analytical approximations.
11. The hyperbolic parabolic shells, grouped under the category of __________
a) Singly curved shells
b) Doubly curved shells
c) Three curved shells
d) Four curved shells
View Answer
Explanation: Hyperbolic paraboloid shells, grouped under the category of doubly curved antielastic sells were first successfully used as roofing units by silberkuhl in germany and parts of one shear hyperboloid units, with a geometric from closely following a circular curve in the length(span) direction and hyperbolic curve in the direction of width are well suited for mass production since they are ruled surfaces.
12. Fold plates are widely used for __________
a) Roofs
b) Beams
c) Stress
d) Foundation
View Answer
Explanation: Folded or hipped plates are widely used for roofs of industrial structures, coal bunkers and cooling towers and the simplicity of the form used for casting folded plates makes them competitive shell construction for covering large floor space and the plates have a triangular or trapezoidal zig-zag cross sectional shape and prestressing is generally done by curved cables or straight tendons lying within the plate in the longitudinal direction to counteract the beam action.
13. The folded plate is also analyzed for __________
a) Longitudinal
b) Transverse
c) Straight
d) Reverse
View Answer
Explanation: The folded plate is also analyzed for transverse moment by considering the plate as a continuous slab with imaginary supports at the junctions and the transverse reinforcement are designed to resist these moments and according to IS:224 nominal reinforcements consisting of 10mm bars are to be provided in the compression zones at 200mm centers and the maximum spacing of reinforcements in any direction is limited to five times the thickness of the member and minimum reinforcement in the section should conform to the provisions in various national codes.
14. The spherical domes are supported by a ring beam at the __________
a) Base
b) Middle
c) Start
d) Edge
View Answer
Explanation: Concrete domes are generally preferred for covering circular tanks and for roofs of large span circular structures, such as sports arenas and churchus where an uninterrupted floor space is desirable and a prestressed concrete hemispherical dome of 40m diameter has been used for the roof of the atomic reactor at Kota, Rajasthan, India and the spherical domes are supported by a ring beam at the base, which can be conveniently prestressed by winding tensioned wires or by cables to counteract the hoop tension developed in the ring beam and the main disadvantage of the reinforcement becoming congested in large diameter reinforced concrete ring beams is overcome by prestressing the ring beams in addition there are significant savings in cost, when compared with other equivalent roofs of conventional design.
15. The peripheral ring beam is prestressed by which type of wire winding?
a) Circular
b) Trapezoidal
c) Elliptical
d) Curved
View Answer
Explanation: The peripheral ring beam is prestressed by which type of wire winding Trapezoidal similar to that of tank walls, or by cables housed in the ring beams with anchorage points at 90 degrees spacing, opposed and phased at 45 degrees and in the case of cables, due provisions should be made be about 10 to 15percent due to the large curvature of the cables.
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