Prestressed Concrete Structures Multiple Choice Questions on “Analysis of Stresses”.

1. The stresses developed in the prestressed structures are computed using __________

a) Simple bending equations

b) Stress strain theory

c) Strain analysis

d) Stress curves

Answer: a

Clarification: The stresses developed in prestressed and cast insitu concrete are computed using the simple bending equations until the stage of cracking and if the precast prestressed unit is unpropped during the placing of insitu concrete, the stresses that develop in the precast unit are sum of stresses due to self weight of members.

2. Once the insitu concrete hardens, we assume the section to be ___________

a) Dilithic

b) Monolithic

c) Transverse

d) Longitudinal

Answer: b

Clarification: After the insitu concrete has hardened the whole section is assumed to be monolithic and the stresses that develop due to subsequent live loads are computed using the properties of the composite sections.

3. If the precast unit is propped during the placing of insitu concrete, the stresses due to self weight are computed using ___________

a) Section modulus

b) Eccentricity

c) Tension

d) Principle stress

Answer: a

Clarification: The stresses developed due to self weight of the insitu concrete are computed using the section modulus of the composite section and in all cases the live loads stresses are based on the composite section.

4. In most composite constructions which involve prestressed units and insitu cast concrete the latter is of ___________

a) High strength concrete

b) Medium strength concrete

c) Colored concrete

d) Reinforced concrete

Answer: b

Clarification: In most composite constructions which involve precast prestressed units and insitu cast concrete the latter is invariably of low or medium strength concrete while the former are generally made of high strength concrete of grade exceeding M35.

5. For computing the live load stresses, the effect of different moduli between the cast in situ and precast unit is considerable by ___________

a) Eccentricity

b) Modular ratio

c) Tensile stresses

d) Deflections

Answer: b

Clarification: For computing the live load stresses, the effect of different moduli between the cast in situ and precast unit is considerable by using the modular ratio of precast insitu concrete for calculating the area, centroid, second moment of area and second modulus of the equivalent composite sections.

6. In modulus of elasticity of insitu concrete of grade M20 will be about ___________

a) 25kn/mm^{2}

b) 30kn/mm^{2}

c) 35kn/mm^{2}

d) 40kn/mm^{2}

Answer: a

Clarification: In most practical instances, the modulus of elasticity of insitu concrete of grade M 20 will be about 25kn/mm^{2}, while the modulus of concrete in precast prestressed units could vary from 28 to 36kn/mm^{2} for concrete grades of M30 toM60.

7. The modular ratio of precast prestressed unit is ___________

a) 2.0 to 1.6

b) 1.1 to 1.5

c) 2.4 to 1.4

d) 2.2 to 1.7

Answer: b

Clarification: The modular ratio of precast prestressed unit varies in the range of 1.1 to 1.5 however this value could be larger if light weight concrete with a modulus in the range of 5 to 12kn/mm^{2} is used in conjunction with precast units made of normal weight aggregates.

8. A precast pretensioned beam of rectangular section has a breadth of 100mm and depth of 200mm and the beam with an effective span of 5mm is prestressed by tendon with their centroidal coinciding with the bottom kern and the initial force in the tendons is 150kn. Find prestressing force?

a) 20n/mm^{2}

b) 34n/mm^{2}

c) 15n/mm^{2}

d) 12n/mm^{2}

Answer: c

Clarification: A = (100×200) = 20000mm^{2}, p = 150kn = 150×10^{3}

Stresses due to prestressing force = (2P/A) = (2x150x10^{3}/20000) = 15n/mm^{2}.

9. Calculate stresses due to slab weight in precast sections given moment due to slab weight is 1200nm of section modulus 667×10^{3}?

a) 1.8n/mm^{2}

b) 1.5n/mm^{2}

c) 2.5n/mm^{2}

d) 2.3n/mm^{2}

Answer: a

Clarification: Section modulus Z = 667×10^{3}, moment due to slab weight is 1200nm

Stresses due to slab weight in the precast sections = (1200000/667×10^{3}) = 1.8n/mm^{2}.

10. Calculate the stress of pretensioned beams assuming as propped during the casting of the slab if Z_{t} is 225×10, Z_{b} is 128x104mm^{3} and moment due to self weight is 1200nmm?

a) 12.3 and 4.5

b) 0.53 and 0.94

c) 0.23 and 0.45

d) 1.23 and 0.67

Answer: b

Clarification: Z_{t} = 225×10, Z_{b} = 128×10^{4}mm^{3}, moment due to self weight = 1200nmm

Stresses due to this moment in the composite section:

At top = (1200000/225×10^{4}) = 0.53n/mm^{2} (compression), At bottom = (1200000/128×10^{4}) = 0.94n/mm^{2} (tension).