This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Cable Profile and Cable Layout”.
1. The concept of load balancing is useful in selecting:
a) Anchorage profile
b) Shaft profile
c) Tendon profile
d) Span profile
Explanation: The concept of load balancing is useful in selecting a tendon profile and they provide suitable force system in concrete member, consider a prestressed concrete beam which is provided with a tendon at an eccentricity and is subjected to a hogging moment such that the beam deflects, the slope gets modified as the beam is subjected to a downward external load, if the beam is subjected to a UDL of W per unit run for the complete span, then the net slope at each end can be calculated as:
Δ = Wl3/24EcI – Pel/2EcI.
2. In a prestressed concrete member, cable profile is suitable for balancing loads of:
a) External type
b) Internal type
c) Bent type
d) Curved type
Explanation: In a prestressed concrete member, external type of loads is balanced by transverse component of suitable cable profile, on effect of loading the net deformation increases the stress, strain and length of the tendon, extension of tendon = 2eδ, increase in strain 2eδ/l, increase in stress = 2eδ/l Ec.
3. The reactions of cable are obtained by replacing the forces acting on:
Explanation: The shape of profile depends on the reaction of cable and these are obtained by replacing the forces acting on concrete with tendons, a prestressed concrete beam with bent tendons has extreme fiber stress, f = P/A+M/z and P/A-M/Z, prestressed concrete beam with parabolic tendon has a net downward force acting on the beam as F = W-We.
4. In a concrete member, trapezoidal cable profile is adopted when the beam is subjected to:
c) Point loads
d) Concentrated loads
Explanation: In a concrete member, trapezoidal profile is adopted when the beam is subjected to two concentrated loads and parabolic profile is used when it is subjected to UDL, the curve and sharp angles of cable develop uniformly distributed and concentrated loads respectively and the straight portion of cable does not produce any reactions at the end.
5. The net downward force of prestressed concrete beam with bent tendon is given as:
Explanation: Bent tendons are used in prestressed concrete beams as they tends to provide an upward pressure in the beam and hence reduces the effect of external loading to a great extent, consider a prestressed concrete beam AB of length l it is subjected to a point load W at centre and a prestressing force of P at the ends, the tendon is bent at an angle of θ at the ends, neglecting the frictional losses, the tendon will develop an upward force of magnitude 2Psinθ at the bent at the centre of span, the upward force reduces the effect of the externally applied force in the beam considering equilibrium in vertical direction, the net downward force F = W-2psinθ.
6. The pressure line is also known as:
a) C line
b) E line
c) G line
d) I line
Explanation: Consider a beam which is lying over the ground provided with a tendon and is free from all external loads such that the beam remains unaffected by any external bending moments, the tension force and compression force act at the same level when no external bending moments are acting over the beam and the tendon line (p line) or the line in which compressive force is acting is known as pressure line and it is also known as P line or C line.
7. In the concept of pressure line the C line moves over a P line due to the effect of:
Explanation: If the beam is effected by a moment M, in pressure line then the C line moves over the P line at a distance x and the distance is known as lever arm,
x = M/P = external moment/p, x = shift of C line from P line.
8. If the section of a rectangular beam is subjected to the prestressing force and dead load then the stress distribution is given by:
a) Pressure line
b) Kern distance
c) Permissible pressure
d) Fracture of steel
Explanation: Consider a rectangular beam section subjected to a prestressing force of magnitude p and if the section is subjected to the prestressing force and live and dead load only then the stress distribution across the section is given by kern distance in the form of upper and lower kern distance.
9. If the force acts at the lower kern point then the bottom fibers of the beam are subjected to:
a) Maximum permissible stress
b) Minimum permissible stress
c) Tensile stress
d) Principle stress
Explanation: The points a, b, represents the points of application of forces P and C respectively, now if the beam is subjected to live load in addition to dead load and prestress then the point of application of force c reach point a from point a, the point of application of force p and the magnitude of force C remain unchanged, if 0 represents the centroid of the beam section, then the distance ob is kb and oa’ is kt, if the force C acts at the lower kern point the bottom fibers of the beam are subjected to maximum permissible stresses and the top fibers are subjected to minimum permissible stress.
10. The strength concept can also be used to determine the position of:
a) Kern line
b) C line
c) S line
d) E line
Explanation: The strength concept can also be used to determine the position of c line, eccentricity and distribution of stress in concrete and it is expressed as:
Extreme stress in concrete = C/A+ or – Eccentricity of c/Z.
Sanfoundry Global Education & Learning Series – Prestressed Concrete Structures.
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