This set of Tricky Prestressed Concrete Structures Questions and Answers focuses on “Elastic Design of Sections”.
1. The code used for determining the ultimate flexural strength of rectangular section:
a) IS: 1444
b) IS: 1440
c) IS: 1343
d) IS: 1543
Explanation: IS: 1343 code method is used for determining the ultimate flexural strength of rectangular sections and T sections, all the bending stresses and direct force shall remain compressive in any direction, in structures subjected to dynamic loading, under the same circumstances on the other structures a tensile stresses Ft may be allowed which is less than one tenth of maximum permissible compressive stresses.
2. If the neutral axis of the section lies within the flange, the moment of resistance of the section is given by the equation:
a) Mu = fp ap (d-0.42Xu)
b) Mu = fp ap (d+0.42Xu)
c) Mu = fp ap (0.42Xu)
d) Mu = fp ap (0.52Xu)
Explanation: If the neutral axis of the section lies within the flange, then the moment of resistance of the section is given by the equation, Mu = fp ap (0.42Xu), Mu = ultimate moment of resistance, fp= tensile stress developed in tendon at failure, ap = area of prestressing, d= depth, Xu = depth of neutral axis.
3. In pre tension & post tension members, the value of tensile stress fp and depth of neutral axis Xu is obtained based on:
a) Shear reinforcement
b) Effective reinforcement
c) Edge reinforcement
d) Span reinforcement
Explanation: In pretension & post tensioned members, the value of tensile stress fp and depth of neutral axis xu is obtained based on effective reinforcement ratio (Apf/bdfera) and effective bond (or) un bond between concrete & tendon and these values are given in tables of IS: 1343 code.
4. If the neutral axis of the section lies outside the flange then the ultimate moment of resistance of flanged section is:
a) Mu = fp Apw (d-0.4 xu)+0.45fck (b-bw) Df (d-0.5Df)
b) Mu = fp Apw (d-0.4 xu)+0.45fck (b-bw)
c) Mu = fp Apw (d-0.4 xu)
d) Mu = fp Apw
Explanation: If the neutral axis of the section lies outside the flange, then the ultimate moment of resistance of flanged section is calculated by combining the moment of resistance of web & flange portion
Where Apw =area of prestressing steel for web (Ap-Apf ), Apf = area of prestressing steel for flange (0.45fck (b-bw) (df/f), f= characteristic tensile strength of the prestressing steel, df = flange thickness, b = width of beam, bw= width of web after considering are the cases in the tendon, the effective prestress fpc should be greater than 0.45f.
5. The number of steps involved in designing a rectangular prestressed concrete beam are:
Explanation: There are 6 steps involved in designing of a rectangular, prestressed concrete beam 6, moment (M1), section modulus z, width & depth, area amount of steel required As, self weight of beam Wd, Md, position of reinforcement.
6. The section modulus z, of designing a rectangular prestressed concrete beam is given as:
a) Z= ML/FC
b) Z= Me/Fc
C) Z= MC/FC
d) Z = Md/Fc
Explanation: The required section modulus z from the equation Z= ML/FC fc is the permissible stresses for concrete, but section modulus, Z=bd2/6 where d- depth of beam and the beam should be equal to L/20 or L/25, b-width of the beam, which is given as, b=6z/d2.
7. The position of reinforcement of a recta ngular prestressed concrete beam is evaluated from the relations:
a) e =2Md-Mc/2F
b) e =2Md-Me/2F
c) e =2Md+Mc/2F
d) e =2Md+Me/2F
Explanation: From the moment due to live loads & dead loads, the position of reinforcement is evaluated from the relation e = 2Md+ML/2F,e = eccentricity, inoder to protect a member from collapsing suddenly after the development of shear cracks a minimum shear reinforcement is provided this minimum shear reinforcement (As) is provided in the form of stirrups which is obtained by satisfying following condition:
Asv/bsv = 0.4/0.8fy.
8. The number of steps are involved in designing a prestressed concrete beam of I section is:
Explanation: Following are step by step procedure adopted for designing the prestressed beam of I section stress ft , permissible stress of concrete at transferred fr, fc, allowable tensile stress ft, permissible tensile stress in steel, loss of prestress b/w 15 to 20%, moment due to super imposed load ML, total bending Mt over all depth d, final prestressing force f, area required A, thickness of flange & web in b/w 120mm to 150mm, width of the flange bf, area of tendons At , no of cables required, after final dimensions check the beam for safety.
9. The overall depth in a beam of I section is given as:
a) d= k(me)1/2
b) d= k(mt)1/2
c) d= k(ml)1/2
d) d= k(ma)1/2
Explanation: The overall depth if beam d from the equation:
d= k(mt)1/2 k- coefficient,
The value ranges in between 30-45 adopt average value, this is the seventh step used in designing the I section.
10. The area of tendons` At` is given as:
a) At = F/Safe tensile stress in steel
b) At = L/Safe tensile stress in steel
c) At = D/Safe tensile stress in steel
d) At = C/Safe tensile stress in steel
Explanation: The area of tendons `At` is given as At = F/Safe tensile stress in steel by considering this area, provide suitable diameter of wires & determine the required number of wires, find the number of cables required and in each cable provide 8 to 12 wires the spacing for cables should be 120mm, after adopting the final dimensions check the beam for safety.
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