# Prestressed Concrete Structures Questions and Answers – Ultimate Shear Resistance

This set of Prestressed Concrete Structures Multiple Choice Questions & Answers (MCQs) focuses on “Ultimate Shear Resistance “.

1. How many types of modes of shear cracking are present in structural concrete beams?
a) 4
b) 2
c) 5
d) 8

Explanation: Research over the years have shown that there are two major modes of shear cracking in structural concrete beams, web shear cracks, flexure shear cracks and different modes of shear failure patterns are also considered like diagonal tension failure, shear compression failure, web compression failure.

2. The web shear cracks generally start from __________
a) Interior point
b) Exterior point
c) Edge
d) Mid span

Explanation: Web shear cracks generally start from an interior point, when the local principal tensile stress exceeds the tensile strength of concrete, the British code (BS:8110:-1985) and the Indian code (IS:1343-1980) specify a modified version of this relation given by: Vcw = 0.67bwh(f12+0.8fcpft)1/2 .

3. The web shear cracks are developed when the beam is subjected to __________

Explanation: Web shear cracks are likely to develop in highly prestressed beams with thin weds, particularly when the beam is subjected to large concentrated loads near a simple support, in which the value of 0.67 h is somewhat lower for flanges sections this together with reduced value of 0.8fcp results in conservation estimates of the shear resistance of flanges sections and if there are inclined cables, the sheaing force vcw is increased by an amount equal to the vertical component of the prestressing force, in the above expression for computing vcw the tensile strength of concrete may be assumed as ft = 0.24(fck).

4. In which direction the flexural cracks are first initiated?
a) Straight
b) Inclined
c) Bended
d) Dotted

Explanation: Flexure – shear cracks are first initiated by flexural cracks in the inclined direction and they are developed when the combined shear & flexural tensile stresses produce a principal tensile stress exceeding the tensile strength of concrete.

5. What is provided in order to protect a member from collapsing suddenly after the development of shear cracks?
a) Edge reinforcement
b) Principal reinforcement
c) Span reinforcement
d) Shear reinforcement

Explanation: In order to protect a member from collapsing suddenly after the development of shear crack, a minimum shear reinforcement is provided (Asv) is provided in the form of stirrups which is obtained by satisfying the following condition:
Asv/bsv = 0.4/0.87fy
In member without shear reinforcement, the inclined shear cracks extend to the compression face resulting in sudden explosive failure this is sometimes referred to as the diagonal tensile modes of failure.
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6. The ultimate shear resistance of prestressed concrete with web shear cracking but without flexural crakes & mainly governed by __________
a) Limiting value
b) Constant value
c) Zero
d) Infinity

Explanation: The ultimate shear resistance of prestressed concrete sections with web shear cracking but without flexural cracks, is mainly governed by the limiting value of the principal tensile stress developed in concrete, the failure is assumed to take place when the principal tension exceeds the tensile strength of concrete.

7. The relation for computing the ultimate shear force is given as __________
a) Vcw = bw dp((0.3λ)fcy1/2 – 0.3fcp) + vp
b) Vcw = bw dp((0.3λ)fcy1/2 + 0.3fcp) + vp
c) Vcw = bw dp(0.3λ + vp)
d) Vcw = bw dp

Explanation: The relation for computing the ultimate shear force, which includes a multiplying factor to multiplying factor to account for the type of concrete is given as Vcw = bw dp((0.3λ)fcy1/2 + 0.3fcp)+ vp
Bw = breadth of web, dp = effective depth to tender or 0.8h, λ = 1.0 for nominal weight concrete and less for light weight concrete, fcy = cylinder compressive strength of concrete(n/mm2), fcp = compressive prestress at centroid of a section, vp = the vertical component of the effective prestressing force at section.

8. The ultimate shear resistance vcf of section cracked in flexure is expressed as __________
a) vcf = (1-0.55fpc/fpcbwd+(m0/m)v
b) vcf = (1-0.55fpc/fpcbwd
c) vcf = (1-0.55fpc/fp)
d) vcf = (1-0.55fpc)

Explanation: vcf = (1-0.55fpc/fpcbwd+(m0/m)v less than or equal to 0.1bwdfck1/2, fpc = effective prestress, fp = characteristic strength of prestressing steel, τc = ultimate shear stress capacity of concrete, bw = breadth of the member, d = effective depth of tendons, m0 = moment necessary to produce zero, v and m = shear force and bending moment at section.

9. The flexure shear cracking load in a prestressed member is given by __________
a) ACI
b) Indian
c) British

Explanation: The American code (ACI: 318-1989) recommendations for the flexure shear cracking load in a prestressed member are based on experimental observations that flexure shear cracking initiates at the flexural cracking load plus an additional shear which is a flexural cracking load plus an additional shear which is a function of the strength and density of concrete & the dimensions of the section.

10. According to Mast, a complete shear analysis is necessary only in beams of __________
a) High span
b) Mid span
c) Low span
d) Edge span

Explanation: In general, the web shear and flexural shear resistance of the beam at important sections are compared with the ultimate shear requirements to identify zones where web reinforcement are required, According to Mast, a complete shear analysis is necessary only in beams of low span/depth ratio and in other cases the specified minimum proportions of web reinforcement is sufficient.

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