Prestressed Concrete Structures Multiple Choice Questions on “Types of Flexural Failure”.
1. The failure due to fracture of steel in tension in the beam is because of __________
a) Excess amount of steel reinforcement
b) Excess amount of prestressed concrete
c) Least amount of reinforcement
d) Least amount of prestressed concrete
Clarification: The percentage of steel provided, both tensioned and un tensioned taken together should be sufficient so that when the concrete in the pre compressed tensile zone cracks, the steel is in a position to bear the additional tensile stresses transferred to it by the cracking of the adjacent fibers of the concrete, thereby preventing a sudden failure of the beam due to fracture of steel in tension.
2. If the concrete in tension zone get cracks will lead to development of __________
a) Principal stresses
c) Tensile stresses
Clarification: The sudden failure of a prestressed member without any warning is generally due to fracture of steel in the tension zone, this type of failure is imminent when the percentage of steel provided in the section is low that when the concrete in the concrete in the tension zone cracks, the steel is not in a position to bear up the additional tensile stress transferred to it by the cracked concrete.
3. The failure due to fracture steel in tension can be prevented by providing a steel reinforcement is?
a) 0.15 – 0.2%
b) 0.20% – 0.35%
c) 0.2% – 0.6%
d) 0.6% – 0.10%
Clarification: In order to prevent this failure a minimum steel reinforcement is provided in the cross section of beam IS: 1343 recommended a minimum reinforcement of 0.15% – 0.2% of the cross sectional area in pretensioned units of small sections when a high yield strength deformed reinforcement is used, the minimum steel percentage is reduced to 0.15 percent.
4. In under reinforced section failure the beam observes __________
a) Excess elongation
b) Less elongation
c) Edge elongation
d) Mid span elongation
Clarification: In this mode of failure, the beam observes excess elongation of steel along with the crushing of concrete this is because large amount of steel is provided in the compression zone (greater than minimum steel reinforcement in the tension zone).
5. Due to excess elongation of steel, the neutral axis near the compression face gets __________
Clarification: If the cross section is provided with an amount of steel greater than the minimum, the failure is characterized by an excessive elongation of steel followed by the crushing of concrete and as the bending loads are increased excessive elongation of steel raises the neutral axis near the compression face at the critical section of beam.
6. Which type of deflections is observed in compression face during under reinforced section failure?
a) Small deflections
b) Large deflections
c) Bondage deflections
d) Shrinkage deflections
Clarification: Large deflections and wide cracks are observed in compression face and this is because the area of concrete provided is unable to resist the resultant internal compression forces and the member also fails in flexure due to crushing of concrete considerable warnings are observed before impending failure.
7. The over reinforced section fails due to sudden crushing of __________
Clarification: Over reinforced section fails due to sudden crushing of concrete and this section observes small deflections and narrow cracks the effective reinforcement due to which the compressive strength of concrete and tensile strength of steel are increased up to a certain range of values then the section is said to be over reinforced.
8. The amount of steel provided in this section should not be greater than __________
a) Steel required
b) Concrete required
c) Water required
d) Tension required
Clarification: In this section large amount of steel is provided which resists the stresses developed at the failure to reach the tensile strength of steel and the amount of steel provided in this section should not be greater than the steel required for balancing the section.
9. Which one of the following are other modes of failure?
a) Failure due to tension
b) Torsion failure
c) Reinforcement failure
d) Transverse shear failure
Clarification: Other modes of failure like transverse shear failure and web crippling are caused due to improper design of member in shear and by providing thin webs in the section, in case of pretensioned members, the failure of bond between the steel and the surrounding concrete is likely due to the inadequate transmission lengths at the ends of members, in post tensioned members anchorage failures may take place if the end block is not properly designed to resist the transverse tensile forces.
10. The failure due to other modes like transverse shear failure is mainly caused due to __________
a) Principal stresses
b) Tensile stresses
c) Compressive stresses
Clarification: These failures are mainly caused due to excessive principle stresses developed in the members when they are subjected to transverse loading, prestressed concrete members subjected to transverse loads may fail in shear before their full flexural strength is attained, if they are not adequately designed for shear, web shear cracks may develop if the principal stresses are excessive and if thin webs are used, the failure may occur due to web crushing.