This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Analysis of Achieving Continuity”.
1. To develop continuity the short and straight tendons may be used over the:
Explanation: Continuity in prestressed concrete construction is achieved by using curved or straight cables which are continuous over several spans and it is also possible to develop continuity between two precast beams by using cap cables alternatively short straight tendon may be used over the supports to develop continuity between two precast prestressed beams.
2. Based on method of construction, continuous beams may be classified into:
a) 3 types
b) 2 types
c) 5 types
d) 4 types
Explanation: Based on the method of construction continuous beams may be classified as fully continuous beams in which tendons are generally continuous from one end to the other and particularly continuous where each span is first precast as a simple bend and the elements are assembled to form a continuous member by using cap cables or short tendons over the supports.
3. The primary moment is the apparent bending moment at a:
c) Cross section
d) Equal distance
Explanation: The primary moment is the apparent bending moment at a section in a statically indeterminate structure due to the actual eccentricity of the tendons from the centroid axis and primary moment at every cross section of the two span continuous beams is Pe as it is a hogging moment.
4. The secondary moment is also known as:
a) Parallel bending moment
b) Eccentric bending moment
c) Parasitic bending moment
d) Elliptical bending moment
Explanation: The secondary moment also known as parasitic bending moment are additional moments induced at a section of a statically indeterminate structure due to the redundant reactions developed as a consequence of prestressing the structure and the variation of secondary moment is a two span continuous beam prestressed by a straight eccentric tendon.
5. The resultant moment is a section of:
a) Determinate prestressed structure
b) Indeterminate structure
c) Hollow structure
d) Transverse prestressed structure
Explanation: The resultant moment at a section of an indeterminate prestressed structure is the sum of primarily and secondary moments:
Resultant moment = (Determinate moment +Secondary moment).
6. The resultant thrust line in a two span continuous beam is prestressed by a:
a) Trapezoidal cable
b) Straight cable
c) Bent cable
d) Parabolic cable
Explanation: The resultant thrust line in a two span continuous beam prestressed by a parabolic cable with zero eccentricity at all the supports and if tendon profile is made to coincide with the resultant thrust line, the redundant reactions are completely eliminated and the cable may be considered as concordant.
7. A Transformation profile is any tendon profile consisting of:
a) Parallel lines
b) Straight lines
c) Bent lines
d) Transverse lines
Explanation: A transformation profile is any tendon profile consisting of straight lines between the rigid supports and having zero eccentricity at simple end supports and a tendon following such profile will produce support reactions and uniform longitudinal compression but bending moments.
8. How many common methods are there for analyzing statically indeterminate prestressed structures?
Explanation: There are several methods for analyzing statically indeterminate prestressed structures to compute the secondary moments that develop from prestressing the structure and the commonly used methods are based on the principles of three moment theorem, consistent deformation and tendon reaction and the methods in b and c are also generally referred to as the flexibility influence coefficient method and the method of equivalent loads respectively and the merits and demerits of these methods and their suitability for given cases are outlined with the help of examples.
9. Which diagram is considered in three moment method analysis of secondary moments?
a) Stress strain diagram
b) Bending moment diagram
c) Pressure line diagram
d) Tee beam diagram
Explanation: The classical method of linear structural analysis such as the three or four moment theorem can be conveniently used to analyze the second moments developed in a continuous prestressed concrete structure and in this method the free bending moment diagram to be considered is that due to the primary moment represented by the tendon profile with the longitudinal axis of the member as the horizontal axis.
10. The simplified form of moment equation is given by:
a) Mab+2Mba+2KMbc+KMcb = Kba+KKbc
b) Mab+2Mba+2KMbc = Kba+KKbc
c) Mab+2Mba = Kba+KKbc
d) Mab = Kba+KKbc
Explanation: The simplified form of moment equation is given by: Mab+2Mba+2KMbc+KMcb = Kba+KKbc depending upon the degree of indeterminacy of the structure a suitable number of equations are formulated and then solved to evaluate the secondary moments and the resultant moment at any section is computed as the sum of primary and secondary moments and this method can also handle members of variable cross section along the length of the structure.
11. The tendon reaction method is based on the principle of:
a) Equivalent tendon placing
b) Replacing the tendons
d) None of the mentioned
Explanation: The tendon reaction method, suggested by Guyon, for analyzing statically indeterminate prestressed structure is based on the principle of replacing the tendon by an equivalent system of loads acting on the concrete member which correspond to the reactions exerted by the tendon concrete.
12. The analysis of indeterminate structure supporting the equivalent loading directly yields the:
a) Span moments
b) End moments
c) Resultant moments
d) Strain moments
Explanation: It is important to note that the analysis of the indeterminate structures supporting the equivalent loads directly yields the resultant moments consequently, the secondary moments, if required are obtained as the difference of the resultant and primary moments at every cross section of the structure.
13. In statistically indeterminate prestressed concrete structures it is possible to make simple modification to:
a) Predetermined tendon profile
b) Elongated tendon profile
c) Collapse tendon profile
d) Ridge tendon profile
Explanation: In statistically indeterminate prestressed concrete structures it is possible to make simple modification to Predetermined tendon profile without altering the pressure line I the members this is an important property of continuous prestressed beams first enunciated by Guyon as follows: In a continuous prestressed beam, if the tendon profile is displaced vertically any of the intermediate supports by any amount but without altering its intrinsic shape between the supports, the resultant line of thrust is unchanged.
14. The axial contraction in case prestressed structures does not influence:
a) Force and Moment
b) Reactions and moment
c) Deflection and moments
d) Area and moments
Explanation: In case of prestresssed structures comparing unidirectional members, such as continuous beams, the axial contraction due to the effect of prestressing does not significantly influence the force and moment in the continuous structure.
15. In structure like portal frames prestressing of transform results in:
a) Tensile contraction
b) Axial contraction
d) Principle contraction
Explanation: In structures like portal frames with members in different directions, prestressing of transform results in an axial contraction, which in turn introduced tertiary moments in the frames due to the lateral displacement of the junction of the transform and column members.
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