Prestressed Concrete Structures Multiple Choice Questions on “Torsion Members”.

1. In case of structural concrete members subjected to torsion, shear stress develops depending upon the __________

a) Type of bending

b) Type of tendon

c) Type of anchorage

d) Type of cross section

Answer: d

Clarification: In the case of structural concrete members subjected to torsion, shear stresses develop depending upon the type of cross section and magnitude of torque, the shear stresses in association with the flexural stresses may give rise to principal tensile stresses, the value of which when it exceeds tensile strength of the concrete results in the development of cracks on the surface of the member.

2. The distribution of torsion shear stress is uniform in __________

a) Parallel sections

b) Rectangular sections

c) Trapezoidal sections

d) Circular sections

Answer: c

Clarification: The distribution of torsion shear stress is uniform in circular sections where the magnitude of the shear stress is proportional to the distance from the centre and in case of non circular sections involving warping of the cross section, approximate formula have been proposed based on elastic analysis.

3. The maximum shear stress of circular section is given as __________

a) 16T/πD^{3}

b) 20T/πD^{3}

c) 40T/πD^{3}

d) 100T/πD^{3}

Answer: a

Clarification: An analysis of principal stresses in prestressed concrete members should include the combined effect of shear stresses due to transverse loads and torsion, together with direct stresses due to flexure and prestress and the maximum stress of circular section is given as:

16T/πD^{3}, D = diameter, T = torsion.

4. The failure of prestressed concrete member without additional un tensioned reinforcement, to that of plain concrete is?

a) Zero

b) Less

c) 1

d) 4

Answer: b

Clarification: The failure of a prestressed concrete member without additional un tensioned reinforcement, under pure torsion, is more or less similar to that of plain concrete where sudden failure is imminent almost simultaneously with the formation of first crack.

5. Which type of prestressing is advantageous for the members subjected to pure tension?

a) Concentric prestressing

b) Tangential prestressing

c) Circular prestressing

d) Overloaded prestressing

Answer: a

Clarification: The research by Humphery and Zia has shown that by suitably adjusting the value of the prestressing force the torsional resistance can be increased by as much as 2.5 times that for the corresponding plain concrete member and for members subjected to pure torsion; concentric prestress is more advantageous than eccentric prestress.

6. When both longitudinal steel and spirals are provided in prestressed members, the ultimate torsional resistance is?

a) T_{wp}+T_{ws}

b) T_{tp}+T_{ts}

c) T_{vp}+T_{vs}

d) T_{ep}+T_{es}

Answer: b

Clarification: The use of longitudinal steel or spirals independent of each other does not increase the ultimate torsional resistance but when both longitudinal steel and spirals are provided in prestressed members, the ultimate torsional resistance is enhanced and according can be expressed as T_{tp} + T_{ts}, where T_{tp} is the torsional resistance moment of the prestressed concrete section and T_{ts} is the additional torsional resistance moment of the non-prestressed reinforcement, which must consist of spirals and longitudinal steel.

7. The behavior of a prestressed concrete member is affected by the relative magnitude of __________

a) Internal actions

b) External actions

c) Zero

d) Constant actions

Answer: a

Clarification: The behavior of a prestressed concrete member is affected by the relative magnitude of the internal actions, such as torque, bending moment and shear force, in circular sections and if torsion is small, it has little effect on the overall behavior and the failure are controlled by either flexure or shear.

8. The members subjected to torque, bending and shear are generally reinforced with __________

a) Parallel and perpendicular reinforcements

b) Longitudinal and transverse reinforcements

c) Rectangular and trapezoidal reinforcements

d) Circular and square reinforcements

Answer: b

Clarification: Members subjected to torque, bending and shear are generally reinforced with longitudinal and transverse reinforcements in order to study the contribution of the longitudinal and transverse reinforcement in resisting flexure, torsion and shear forces, it becomes necessary to analyze the system of forces acting on the warped cross sections of the structural element at the limit state of failure.

9. The skew bending theory is based on the __________

a) Plane Elasticity

b) Plane Deformation

c) Plane Torque

d) Plane Strain

Answer: b

Clarification: The skew bending theory is based on the plane deformation approach to plane sections subjected to bending and torsion, the skew bending theory was initially proposed by Lessing with subsequent contributions from Collins, Hsu, Zia, GEsund, Mattock and Elfgreen, of the several researches in this field, Hsu has made the most significant contribution based on the experimental investigations, his work forms the basis of the American, Australian(AS 1481) and Indian code (IS: 1343) provisions.

10. The space truss, which is composed of longitudinal bars and diagonal concrete truss subjected to twist is known as __________

a) Skew bending theory

b) Space truss analogy theory

c) Space truss theory

d) Compression failed theory

Answer: b

Clarification: The space truss analogy theory, which is a modification of the planar truss analogy for shear and according to this theory the space truss, which is composed of longitudinal bars and diagonal concrete struts is subjected to twist in which the stirrups and longitudinal bars are considered the tension members and the diagonal concrete struts at an angle θ between the cracks and considered the compression members θ is idealized to 45 degrees.