Prestressed Concrete Structures Questions and Answers for Entrance exams on “Design of Partially Prestressed Members”.
1. In partially prestressed members, to which extent tensile stresses are permitted?
Clarification: In partially prestressed members limited tensile stresses are permitted in concrete under service wads with controls on the maximum width of cracks and depending upon the type of prestressing and environmental conditions and the use of partial prestressing was first proposed by Emperger in 1939 and further progress in this field was mainly due to the sustained work of abeles, Goschy and others and the west German code DIN 42227 has provided for partial prestressing even before the C.E.B-FIP provisions were introduced in 1970.
2. Which type of reinforcement is required for the advantage of partial prestressing?
a) Un tensioned reinforcement
b) Tensioned reinforcement
c) Forced reinforcement
d) Embedded reinforcement
Clarification: The main point in favour of partial or limited prestressing is that required in the cross section of a prestressed member for various reactions such as to resist the differential shrinkage temperature effects and handling stresses.
3. The reinforcement can cater for which requirements?
a) Limited requirement
b) Serviceability requirements
c) Range requirements
d) Termed requirements
Clarification: The reinforcement can cater for the serviceability requirements such as control of cracking and partially for the ultimate limit state of collapse which can result in considerable reductions in the costlier high tensile steel and the saving in prestressing steel contributes to an overall saving in the cost of the structure.
4. The fully prestressed members are prone to excessive ___________
a) Downward deflections
b) Upward deflections
c) Side deflections
d) Transverse deflections
Clarification: Fully prestressed members are prone to excessive upward deflections especially in bridge structures where dead loads form a major portion of the total service loads and these deflections may increase with time due to the effect of creep.
5. The energy capacity is comparatively low in case of ___________
Clarification: It is well established that fully prestressed members due to their higher rigidity have a lower energy absorption capacity in comparison with partially prestressed members, which exhibit a ductile behavior.
6. Which are used as untensioned reinforcement?
a) Yield steel and mild steel
b) High tensile steel and mild steel
c) Tensile steel and mid steel
d) Principle steel and mild steel
Clarification: High tensile steel and mild steel have been used as un tensioned reinforcement and the present practice is to use high yield strength deformed bars which are considerably cheaper than prestressing steel and at the same time have higher yield strength and better crack control characteristics by virtue of their surface configuration as compared to mild steel bars with plain surface.
7. The width of crack is influenced by ___________
b) Limit states
d) Factor of safety
Clarification: The method of calculating the crack width is of considerable importance in checking the limit state of cracking at service loads and it is well established that the width of crack primarily depending upon the stress in the reinforcement is also influenced by the cover and the type of reinforcement.
8. The calculation based on the stress in the reinforcement is obtained by ___________
a) Limit state theory
b) Convential theory
c) Retardation theory
d) Principle theory
Clarification: Several empirical relations have been developed to estimate the width of cracks but it is considered that a calculation based on the stress in the reinforcement obtained by the convential theory of cracked reinforcement concrete section is inherently more accurate than a computation based on the fictitious tensile stress in an uncracked section.
9. The tensile strength of the concrete below the neutral axis is neglected in ___________
a) Cracked section analysis
b) Uncracked section analysis
c) Beam section analysis
d) Strain analysis
Clarification: The cracked section analysis of a partially prestressed flange section with tensioned high tensile steel and un tensioned reinforcement is carried out under the following assumptions: The strain distribution across the section is linear, the tensile strength of the concrete below the neutral axis.
10. The stresses and strains developed and the forces acting on cracked prestressed concrete sections which are subjected to a moment Mcr is given as?
a) εpe = P/ApEp
b) εpe = P/AeEp
c) εpe = P/AsEp
d) εpe = P/AlEp
Clarification: In excess of the cracking moment Mcr just prior to the application of the moment the tensile strain in the prestressing steel is εpe and the compressive strain in the concrete at the tendon level is εce and these strains can be evaluated from the prestressing force acting at this stage εpe = p/ApEp , εce = P/Ec(e2/Ic+1/Ac), P = effective prestressing force, e = eccentricity of tendons, Ac = cross sectional area of the concrete section, Ic = second moment of area of the concrete section.
Prestressed Concrete Structures for Entrance exams,