This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Ultimate Load Analysis”.

1. The reduction in the magnitude of prestressing force occurs due to:

a) Re block

b) Restraint

c) Relock

d) Re tension

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Explanation: According to Bennet the main effects due to axial deformation and tertiary moments are: Reduction in the magnitude of prestressing force in a particular member due to the restraint of the adjacent members, the development of tertiary moments due to bending deformation of the indeterminate structure due to axial contraction under the action of the prestressing force.

2. The ultimate load carrying capacity is controlled by:

a) Tensile strength

b) Compressive strength

c) Flexural strength

d) Axial strength

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Explanation: The ultimate load carrying capacity of a statically determinate prestressed concrete structure is mainly controlled by the flexural strength of a critical section in the member and in contrast to this, the ultimate load carrying capacity of an indeterminate prestressed concrete structure depends upon the flexural strength of several critical sections and their rotation capacity.

3. A conservative estimate of the ultimate resistance is obtained by assuming:

a) Extra reinforcement

b) Tensile reinforcement

c) Factor of safety

d) Collapse

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Explanation: However a highly conservative estimate of the ultimate resistance of the structure is obtained by assuming the collapse to occur when the moment, calculated by the linear elastic theory first reaches a value equal to the flexural strength at any section.

4. Due to large deformations at the critical sections which concentration develops?

a) Equilibrium

b) Curvature

c) Resultant

d) Loads

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Explanation: Due to large deformations at the critical sections in a statically indeterminate structure a concentration of curvature builds up over a few clearly defined short length of the member, such as the interior supports and midspan points of a continuous beam and these plastic zones are idealized as hinges at which a rotation is considered to take place.

5. If the partial redistribution takes place by the limited hinge rotations, the ultimate strength of the structure lies between:

a) Upper limit

b) Lower limit

c) Upper limit above and lower limit

d) Zero

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Explanation: If the partial redistribution takes place due to the constraints imposed by limited hinge rotations, the ultimate strength of the structure lies in between the upper limit above and the lower limit corresponding to the computations based on the linear elastic theory.

6. The equilibrium between the internal forces and the external loads is maintained for:

a) Stress combinations

b) Load combinations

c) Deformation combinations

d) Elastic combinations

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Explanation: The redistribution of moments is permitted under the following conditions: equilibrium between the internal forces and the external loads is maintained for approximate load combinations, at sections where the moment capacity after redistribution is less than that from the elastic maximum moment diagram the following relationship should be satisfied (x

_{u}/d) +δm/100)) < 0.8, x

_{u}= depth of neutral axis, d = effective depth, δ

_{m}= percentage reduction in moment.

7. The ultimate moment of resistance provided at any section of a member is not less than:

a) 80%

b) 40%

c) 25%

d) 95%

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Explanation: The ultimate moment of resistance provided at any section of a member is not less than:

80% of the moment at that section obtained from an elastic maximum moment diagram covering all appropriate combination of loads, the elastic moment at any section in a member due to partial combination of loads shall not be reduced by more than 20 percent of the numerically largest moment covering all appropriate combination of loads.

8. In the design of continuous prestressed concrete members, it is often necessary to determine a cable profile lying within:

a) Distribution zone

b) Limiting zone

c) Edge zone

d) Span zone

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Explanation: In the design of continuous prestressed concrete members, it is often necessary to determine a cable profile lying within the limiting zone of thrust and also satisfying the conditions of concordancy and a method based on the principle of virtual work is outlined for determining the concordant cable profile.

9. The difference of the maximum and minimum moments at a cross section in design of continuous prestressed beam is given as:

a) M_{t} = M_{max}-M_{min}

b) M_{f} = M_{max}+M_{min}

c) M_{t} = M_{max}

d) M_{t} = M_{min}

View Answer

Explanation: The maximum positive and negative moments due to the live and dead load moments at various sections of the member are calculated and hence computing the range of moment which is obtained as the difference of the maximum and minimum moments at a cross section, M

_{t}= M

_{max}-M

_{min}In the design of statically indeterminate prestressed concrete structures involves computation of maximum and minimum moments at various cross sections of the members.

10. The overall cross sectional dimensions are fixed using the:

a) Permissible tensile stresses

b) Permissible compressive stresses

c) Permissible principle stresses

d) Permissible strains

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Explanation: The overall cross sectional dimensions are fixed using the permissible compressive stresses in concrete, fc by equation

Z = bh

^{2}/6 = My/f

_{c}assuming a suitable values of b = 0.4-0.5h, the overall depth this obtained so as obtain the range of moments which generally determines the cross sectional dimensions of the member.

11. In the design of unidirectional members like continuous beams, the axial contraction due to effect of prestressing is:

a) Constant

b) Zero

c) Neglected

d) 1

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Explanation: In the design of unidirectional members like continuous beams, the axial contraction due to effect of prestressing is: Neglected as it has very little influence on the force and moments in the structure but in the case of two dimensional structures, such as portal frames and bents, the axial contraction of the members significantly influences the primary and secondary moments in the structure.

12. The data that should be available for purpose of design is:

a) Overall shape

b) Overall area

c) Overall height

d) Overall depth

View Answer

Explanation: The data available for purpose of design is the overall shape of the structure and the loading conditions and the computations of maximum and minimum live load bending moments require a knowledge of the stiffness of the members of the frame and the moments and thrusts determined by assuming a suitable stiffness ratio determined by assuming a suitable stiffness ratio by the normal methods of structural analysis.

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