This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Long Term Deflections”.
1. The deformation of prestressed members change with time as a result of:
a) Creep and shrinkage
b) Friction and torsion
c) Deformation and flexibility
d) Cracking moment
Explanation: The deformation of prestressed members change with time as a result of creep and shrinkage of concrete and relaxation of stress in steel, practically the change in stress obtained is relatively very small and hence it may be assumed that at constant stress the formation of creep occurs in concrete.
2. The deflection of prestressed members can be computed relative to given:
a) Bending moment
b) Strain diagram
Explanation: The deflection of prestressed members can be computed relative to a given datum, if the magnitude and longitudinal distribution of curvatures for the beam span are known based on load history including prestressing forces and live loads.
3. The prestressed concrete member develops deformation under the influence of:
a) Flexural moments
b) Stress strain diagram
c) Prestress and transverse loads
d) Self weight
Explanation: The prestressed concrete members develop deformation under the influence of two usually opposing effects, which are the prestress effects and transverse loads and the deflections caused are to be changed first because the loss incurred due to prestress which in turn decreases the deflection and effects of creep which increases the deflection are suggested in the method for long term deflections.
4. The net curvature ϕtat a section at any given stage is given as:
a) ϕt = ϕw + ϕe
b) ϕt = ϕm + ϕn
c) ϕt = ϕp + ϕs
d) ϕt = ϕmt + ϕpt
Explanation: The net curvature ϕt at a section at any given stage is obtained
ϕt = ϕmt + ϕpt, ϕmt = change of curvature caused by transverse loads,
ϕpt = change of curvature caused by prestress, as the time changes the compressive stress distribution in the concrete also changes under sustained transverse loads.
5. The section of sustained transverse loads under compressive stress distribution in the concrete changes with:
c) Bending moment
Explanation: Under the section of sustained transverse loads, the compressive stress distribution in the concrete changes with time, axial force is a measurement of the forces required to pull something such as rope, wire or structural beam to the point where it breaks compression force is the application of power, pressure and erection against an object.
6. The creep strain due to the transverse loads is directly computed as a function of:
a) Strain coefficient
b) Creep coefficient
c) Stress coefficient
Explanation: The creep strain due to transverse loads is directly computed as a function of the creep coefficient so that the change of curvature can be estimated by the expression,
Φmt = (1+ϕ)ϕi,
ϕ = creep coefficient, ϕi = initial curvature immediately after the application of transverse loads.
7. Which of the following person made attributions to evaluate the curvature under simplified assumptions?
Explanation: Several methods have been proposed to evaluate the curvature under simplified assumptions and important ones are attributed by Buseman, Mchenry, Douglas, Corley, Sozen and Siessand and the numerical solutions developed ignore the influence of the tensile concrete zone on the strain distribution in the section, which considerably effect deflection the equation for long term deflection of cracked members.
8. The creep curvature due to prestress is obtained on the simplified assumption that creep is induced by the average prestress acting over the given time is according to:
Explanation: According to Neville and the ACI committee report, the creep curvature is obtained due to prestress based on a very simplified assumption that the creep is induced into the concrete by the average prestress acting over with respect to the given time.
9. A simplified but an approximate procedure for computing long time deflections is given by:
Explanation: “Lin” suggested a procedure which is not exactly accurate but it helps in calculating long term deflections in a very simplified manner, this procedure helped in calculating the long term deflections.
10. The long time deflections are expressed as:
a) af = (ail-aipxpt/pi) (1+ϕ)
b) af = (ail-aipxpt/pi)
c) af = (ail-aipxpt/pi)
d) af = (ail-aipxpt/pi)
Explanation: The principle of reduced modulus involving the creep coefficient is used to amplify the initial deflections and according to this method, the final long time deflections are expressed as:
af = (ail – aipxpt / pi) (1+ϕ).
Sanfoundry Global Education & Learning Series – Prestressed Concrete Structures.
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