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Prestressed Concrete Structures Multiple Choice Questions on “Resultant Stresses”.
1. The resultant stresses in concrete at any section are obtained by the effect of ___________
a) Prestress and flexural stresses
b) Prestress and bending stresses
c) Prestress and shear stresses
d) Prestress and torsion stresses
Answer: a
Clarification: Resultant stresses in concrete at any section are obtained by superimposing the effect of prestress and flexural stresses developed due to the loads, other common types of corrosion frequently encountered in prestressed concrete construction are pitting corrosion and chloride corrosion, a critical review of the different types of corrosion of high tensile steel in structural concrete is reported.
2. The resultant stress distribution due to eccentric prestressing, dead and live loads at any given section are obtained as ___________
a) Fsup = (p/a-pe/zt)+(mg/zt)+(mq/zt)
b) Fsup = (p/a-pe/zt)+(mg/zt)+(mq/zt)
c) Fsup = (p/a-pe/zt)+(mg/zt)+(mq/zt)
d) Fsup = (p/a-pe/zt)+(mg/zt)+(mq/zt)
Answer: a
Clarification: If Mq and Mg are live loads and dead load moments at the central span section;
Mq=ql2/8
Mg=gl2/8
3. A concrete beam of rectangular section, 250mm wide and 600mm deep. Calculate the bending moment that can be applied without applying tension at the soffit of the beam with given m/z value as 5.74?
a) 26.4
b) 54.8
c) 34.5
d) 86.1
Answer: d
Clarification: m/z = 5.74, b = 250mm, d = 600mm
Z = (250×6002/6) = 15×106mm3,
M = (5.74×15×106) = 86.1×106nmm = 86.1knm.
4. A prestressed concrete beam of section 200mm wide by 300mm deep of imposed load 4kn/m at a span of 6m, density of concrete is 24kn/m3. Find the concentric prestressing force necessary for zero fiber stress at the soffit?
a) 490
b) 560
c) 230
d) 310
Answer: a
Clarification: b = 200mm, d = 300mm, A = (200×300) = 6×104mm2, g = (0.2×0.3×24),
Mg = (0.125×1.44×62) = 6.48knm, Mq = (0.125×4×62) = 18knm, Zb=Zt = (200×3002/6) = 3×106mm3
P/A = 8.16, P = (8.16×6×104) = 489.6kn.
5. The locus of the points of application of resultant force in any structure is termed as ___________
a) Pressure line
b) Hollow line
c) Beam line
d) Tendon line
Answer: a
Clarification: The combined effect of prestressing force and the externally applied load will result in a distribution of concrete stresses that can be resolved into a single force and the locus of the points of application of this resultant force in any structure is termed as “pressure or thrust line”.
6. The concept of pressure line is very useful in understanding the concept of ___________
a) Load carrying mechanism
b) Bending mechanism
c) Shear mechanism
d) Torsion mechanism
Answer: a
Clarification: The concept of pressure line is very useful in understanding the load carrying mechanism of a prestressed concrete section, if a prestressed concrete member is cracked, it behaves similar to that of a reinforced concrete section, in contrast to the direct method of analysis of resultant stresses at a section of a prestressed concrete beam outlined the pressure or thrust line concept can also be used to evaluate stresses.
7. The location of the pressure line depends upon ___________
a) Breakage and Bondage
b) Magnitude and direction
c) Shear and Torsion
d) Pressure and equilibrium
Answer: b
Clarification: In the case of prestressed concrete members, the location of the pressure line depends upon the magnitude and direction of the moments applied at the cross section and the magnitude and distribution of stresses due to the prestressing force, the increase in resultant forces are due to a more or less constant lever arm between the forces, characterized by the properties of the composite section.
8. The eccentricity e in the pressure line diagram is?
a) h/16
b) h/12
c) h/15
d) h/8
Answer: a
Clarification: At the support section, since there are no flexural stresses resulting from the external loads, the pressure line coincides with that of the centroid of steel, located at an eccentricity of h/6, at the centre of the span section the external loading is such that the resultant stress developed in maximum at the top fiber and zero at the bottom fibers and it can be easily be seen that for this section the pressure line has shifted towards the top fiber by an amount equal to h/3 from its initial position.
9. The change in the external moments in the elastic range of prestressed concrete beam results in ___________
a) Bending moment in pressure line
b) Torsion in pressure line
c) Flexure in pressure line
d) Shift of the pressure line
Answer: d
Clarification: “A change in the external moments in the elastic range of a prestress concrete beam results in a shift of the pressure line rather than in an increase in the resultant force in the beam, this is in contrast to a reinforced concrete beam section where an increase in the external moment results in a corresponding increase in the tensile force and the compressive force.
10. The shift of pressure line measured from centroidal axis is obtained as ___________
a) (m/p)-e
b) (m/q)-e
c) (m/r)-e
d) (m/i)-e
Answer: a
Clarification: The shift of pressure line is measured from the centroidal axis is obtained as
E’= (a-e) = (m/p)-e, basically the load carrying mechanism is comprised of a constant force with a changing lever arm as in the case of prestressed concrete sections, and a changing force with a constant lever arm prevailing in reinforced concrete sections.