Prestressed Concrete Structures Multiple Choice Questions on “Design for Shear and Torsion”.
1. Which type of cables are advantages in reducing the effective shear?
Clarification: Curved cables are advantageous in reducing the effective shear and together with the horizontal compressive prestress, reduce the magnitude of the principal tension and the effect of shear is to induce tensile stresses on diagonal planes and prestressing is beneficial since it reduces the magnitude of the principal tensile stress in concrete.
2. The various codes recommend empirical relations to estimate ____________
a) Ultimate shear resistance
b) Ultimate torsional resistance
c) Ultimate bending resistance
d) Ultimate load
Clarification: The various codes recommend empirical relations to estimate the ultimate shear resistance of the section by considering the flexure shear and web shear cracking modes and the design shear resistance should exceeded the ultimate shear due to the transverse loads and if not, suitable transverse reinforcements are designed to resist the balance shear force.
3. Which type of shear reinforcement should be provided for members with thin webs?
a) Maximum shear reinforcement
b) Minimum shear reinforcement
c) Nominal shear reinforcement
d) Tensile reinforcement
Clarification: In members with thin webs such as I and T sections nominal shear reinforcements have to be provided to prevent cracking due to variations in temperature the provisions for design of shear reinforcements prescribed in British, American and Indian standard have been dealt with in the case off structural concrete members subjected to torsion, shear stresses develop depending upon the type of cross section and magnitude of torque and the shear stresses in association with the flexural stresses may give rise to principal tensile stress the value of which when it exceeds tensile strength of the concrete results in the development of cracks on the surface of the member.
4. The pre and post tensioned members with bonded tendons bond stress between ____________
a) Steel and concrete
b) Steel and water
c) Steel and aggregates
d) Steel and plastic
Clarification: Pre tensioned or post tensioned members with bonded tendons develop bond stresses between steel and concrete when the sections are subjected to transverse shear forces due to the rate of change of moment along length of the beam and in the case of type 1 and 2 members, which are uncracked at service loads, the flexural bond stresses developed are computed by considering the complete section.
5. In case of pre tensioned member, the computations of transmission length are influenced by?
Clarification: In the case of pre tensioned members, the computations of the transmission length at the ends is of practical significance since the support positions are influenced by bond and transmission length.
6. When prestress is transferred to concrete by means of external anchorages which pressure is developed?
Clarification: In the case of post tensioned members where prestress is transferred to concrete by means of external anchorages, the bearing pressures develop behind the anchorages have to be investigated and suitably controlled to prevent crushing failure of the end block zone.
7. The bearing pressure on the concrete is given as ____________
Clarification: According to the Indian standard code IS: 1343-1980, the permissible unit bearing pressure on the concrete after allowing for all losses is limited to: 0.48fci (Abr/Apan) or 0.8 fci, fci = cube strength of concrete at transfer, Abr= bearing area, Apan = punching area.
8. The effective punching area Apun is generally the contact area of ____________
a) Tendon device
b) Anchorage device
c) Stress device
d) Strain device
Clarification: The effective punching area is generally the contact area of the anchoring device and the bearing area is taken as the maximum area of that portion of the member which is geometrically similar and concentric to the effective punching area.
9. The end block of a beam has a rectangular section 100mm wide by 200mm deep, the force of cable is 200kn. Find the actual bearing pressure?
Clarification: force of cable = 200kn, b = 100mm, d = 200mm
Bearing area Abr = (100×100) = 10000mm2,
Actual bearing pressure = (200×103) / 10000 = 20n/mm2.
10. The end block of a prestressed concrete beam has a rectangular section; a cable carrying a force of 200kn is to be anchored against the end block at the centre if the cube strength of concrete at transfer is 30n/mm2. Design the maximum permissible bearing pressure?(Abr = 10000mm2)
Clarification: Assuming an anchor plate of size 50mm by 50mm, punching area Apun = (50×50) = 2500mm2, Abr = 10000mm2, maximum permissible bearing pressure = 0.48fci(Abr/Apun) or 0.8fci whichever is smaller, fb = 0.48x30x(10000/2500)1/2 = 28.8n/mm2 or (0.8×30) = 24n/mm22.