Prestressed Concrete Structures Multiple Choice Questions on “Cracking Moments”.
1. The soffit of the beam after the transfer of prestress to concrete will be under ____________
a) Tension
b) Compression
c) Breakage
d) Bondage
Answer: b
Clarification: The bending moment at which visible cracks developed in prestressed concrete members is generally referred to as the cracking moment after the transfer of prestress to concrete, the soffit of the beam will be under compression.
2. The compressive and tensile stresses developed in cracking moments are due to ____________
a) Bending loads
b) Transverse loads
c) Tensile loads
d) Compressive loads
Answer: b
Clarification: These compressive stresses are balanced by the tensile stresses developed due to the transverse loads on the beam, so that the resultant stress at the bottom fiber is zero, a further increase in loading results in the development of tensile stresses at the soffit of beam.
3. The micro cracks develop as soon as the tensile strain in concrete exceeds about ____________
a) 80-100×10-6 units
b) 100-150×10-6 units
c) 150-300×10-6 units
d) 300-500×10-6 units
Answer: a
Clarification: As concrete is weak in tension, micro cracks develop as soon as the tensile strain in concrete exceeds about 80-100×10-6 units and the parameters in concrete are influenced by various materials and their permissible values according to Indian standard codes.
4. When the loads in a concrete member are further increased than permitted, the crack widths are of an order of limit?
a) 0.01-0.02mm5
b) 0.05-0.10mm5
c) 0.03-0.05mm5
d) 0.07-0.08mm5
Answer: a
Clarification: If the loads are further increased, than 80-100×10-6 units visible cracks appear in the tension zone and at this stage, it is estimated that the crack widths are of an order of 0.01-0.02mm5 and these width of cracks are considered from a value of minimum to maximum not excess till permitted.
5. A rectangular concrete beam of cross section 120mm wide and 300mm deep is prestressed by a straight cable, effective force of 180kn at an eccentricity 50, area of 36×103mm2(z=18×105mm3). Find the total stress due to prestress?
a) 10
b) 25
c) 35
d) 45
Answer: a
Clarification: P = 180kn, A = 36×103mm2, e = 50mm, b = 120mm, d = 300mm, z = 18×105mm3
Stresses due to prestress = (p/a) = (180×103/36×103) = 5n/mm2, (pe/z) = (180×103×50/18×105) = 5n/mm2
Total stress = ((p/a)+(pe/z))=(5+5) = 10n/mm2.