Prestressed Concrete Structures Multiple Choice Questions on “Basic Assumptions”.

1. The analysis of stresses developed in prestressed concrete structural elements is based on how many assumptions?

a) 5

b) 4

c) 3

d) 2

Answer: c

Clarification: The analysis of stresses developed in a prestressed concrete structural elements is based on the following assumptions: Concrete is a homogeneous elastic material, Within the range of working stresses, both concrete and steel behave elastically, notwithstanding the small amount of creep which occurs in both the materials under sustained loading, a plan section remains plain even after bending.

2. The tensile stresses when do not exceed the limit of modulus then change in loading of member results in ___________

a) Change of bending moment

b) Change of stress

c) Change in shear

d) Change in rupture

Answer: b

Clarification: As long as tensile stresses do not exceed the limit of modulus of rupture of concrete (corresponding to the stage of visible cracking of concrete), any change in loading of the member results in a change of stress in the concrete only, the sole function of the prestressing tendon being to impart and maintain the prestress in the concrete.

3. Which conditions are negligible at the stage of visible cracking on concrete?

a) Bending conditions

b) Joint conditions

c) Zone conditions

d) Loading conditions

Answer: d

Clarification: Up to the stage of visible cracking on concrete, the changes in the stress of steel, the loading being negligibly small, are generally not considered in the computations, the phenomenon of stress corrosion in steel is particularly dangerous, as it results in sudden brittle fractures.

4. The stresses due to prestressing are referred as ___________

a) Combined stresses

b) Bending stresses

c) Anchoring stresses

d) Tensioning stresses

Answer: a

Clarification: The stresses due to prestressing alone are generally combined stresses due to the action of direct load and bending resulting from an eccentricity applied load, the stresses in concrete are evaluated by using the well known relationship for combined stresses, this type of attack in alloys is due to the internal metallurgical structure, which is influenced by composition, heat treatment and mechanical processing.

5. Which of the following notations are used for evaluating the relationship for combined stresses?

a) P, E, M, A, I

b) P, σ, M, g, I

c) P, W, M, π, L

d) P, θ, M, Z, L

Answer: a

Clarification: The notations and sign conservations used for the analysis of prestress are

P-Prestressing force, e-eccentricity of prestressing force, m – p × e – moment

A – Cross sectional area of the concrete member, I – second moment of area of section about its centroid.

6. The uniform prestress in a concrete member subjected to concentric prestressing is ___________

a) P/e

b) P/s

c) P/t

d) P/a

Answer: d

Clarification: Uniform prestress in concrete p/a, which is compressive across the depth of the beam and the applied loads, dead loads of the beam include tensile stress towards the soffit and are counterbalanced more effectively by eccentric tendons.

7. In case of eccentric prestressing force the support force F_{sup} is ___________

a) F_{sup} = p/a(1+ey_{b/i}^{2})

b) F_{sup} = p/a(1+ey_{b/i}^{2})

c) F_{sup} = p/a

d) F_{sup} = p/a-e_{b}

Answer: b

Clarification: The stresses developed at the top and bottom fibers of the beam are obtained by the two relations:

F_{inf} = (p/a+p_{e}/z_{b}) = p/a(1+ey_{b}/i^{2}), F_{sup} = (p/a-p_{e}/z_{b}) = p/a(1-y_{b}t/i^{2}).

8. The diagram indicating the direct stress of the beam is?

a)

b)

c)

d)

Answer: a

Clarification:

9. The diagram indicating the bending stress of the cross section the beam is?

a)

b)

c)

d)

Answer: a

Clarification: