# Design of Steel Structures Questions and Answers – General Requirements for Plastic Design and Plastic Hinge

This set of Design of Steel Structures MCQs focuses on “General Requirements for Plastic Design and Plastic Hinge”.

1. Which of the following assumptions is correct for plastic design?
a) material obeys Hooke’s law before the stress reaches fy
b) yield stress and modulus of elasticity does not have same value in compression and tension
c) material is homogenous and isotropic in both elastic and plastic states.
d) material is not sufficiently ductile to permit large rotations

Explanation: The material obeys Hooke’s law till the stress reaches fy. The yield stress and modulus of elasticity have the same value in compression and tension. The material is homogeneous and isotropic in both elastic and plastic states. The material is assumed to be sufficiently ductile to permit large rotations of section to take place.

2. Which of the following assumptions is not correct for plastic design?
a) plastic hinge rotations are small compared with elastic deformations so all the rotations are concentrated at plastic hinges
b) segments between plastic hinges are rigid
c) influence of normal and shear forces on plastic moments is not considered
d) plane section remains plane after bending and the effect of shear is neglected

Explanation: The plastic hinge rotations are large compared with elastic deformations so all the rotations are concentrated at plastic hinges. The segments between plastic hinges are rigid. The influence of normal and shear forces on plastic moments is not considered. The plane section remains plane even after bending and the effect of shear is neglected.

3. Which of the following are the conditions that stress strain characteristics of steel should obey to ensure plastic moment redistribution?
a) yield plateau should be less than 6 times the yield strain
b) ratio of ultimate tensile stress to yield stress should be less than 1.2
c) steel should not exhibit strain-hardening capacity
d) elongation on standard gauge length should be more than 15%

Explanation: The conditions that stress strain characteristics of steel should obey to ensure plastic moment redistribution are: (i) yield plateau(horizontal portion of stress strain curve) should be greater than 6 times the yield strain, (ii) ratio of ultimate tensile stress to yield stress should be more than 1.2, (iii) elongation on standard gauge length should be more than 15%, (iv) steel should exhibit strain-hardening capacity.

4. Which of the following conditions are true for using plastic method of analysis as per IS 800?
a) members shall not be hot-rolled or fabricated using hot-plates
b) yield stress of steel should not be greater than 450MPa
c) cross section should be unsymmetrical about its axis perpendicular to axis of plastic hinge rotation
d) cross section of members not containing plastic hinges should be ‘plastic’ and those members containing plastic hinges should be ‘compact’

Explanation: The following are conditions for using plastic method of analysis as per IS 800: (i) yield stress of steel should not be greater than 450MPa, (ii) members shall be hot-rolled or fabricated using hot-plates, (iii) cross section should be symmetrical about its axis perpendicular to the axis of plastic hinge rotation, (iv) cross section of members not containing plastic hinges should be ‘compact’ and those members containing plastic hinges should be ‘plastic’.

5. Which of the following is true regarding plastic design methods?
a) design needs to satisfy elastic strain compatibility conditions
b) different factor of safety for all parts of the structure
c) saving of material over elastic methods resulting in lighter structures
d) design is effected by temperature changes, settlement of support, etc

Explanation: The following are the advantages of plastic design methods: (i) realisation of uniform and realistic factor of safety for all parts of the structure , (ii)simplified analytical procedure and rapidity of obtaining moments since no need to satisfy elastic strain compatibility conditions, (iii) no effect due to temperature changes, settlement of support, etc, (iv) saving of material over elastic methods resulting in lighter structures.
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6. Which of the following is true regarding plastic design methods?
a) difficult to design for fatigue
b) more saving in column design
c) lateral bracing requirements are less stringent than for elastic design

Explanation: (i) There is little saving in column design, (ii) lateral bracing requirements are more stringent than for elastic design, (iii) difficult to design for fatigue,(iv) moments produced by different loading conditions needs to be calculated separately(cannot be added together) and the largest plastic moment is selected.

7. What is plastic hinge?
a) zone of bending due to flexure in a structural member
b) zone of yielding due to flexure in a structural member
c) zone of non-yielding due to flexure in a structural member
d) zone of yielding due to twisting in a structural member

Explanation: Plastic hinge is a zone of yielding due to flexure in a structural member. It is used to describe a deformation of a section when plastic bending occurs.

8. Plastic hinge behaves like a ______
a) friction mechanical hinge except that there is always a fixed moment constraint
b) frictionless mechanical hinge except that there is no fixed moment constraint
c) friction mechanical hinge except that there is no fixed moment constraint
d) frictionless mechanical hinge except that there is always a fixed moment constraint

Explanation: Plastic hinge behaves like a frictionless mechanical hinge except that there is always a fixed moment constraint which is equal to plastic moment capacity of the section.

9. Which of the following is true about hinged length?
a) value of moment adjacent to yield zone is more than yield moment up to hinged length of structural member
b) value of moment adjacent to yield zone is less than yield moment up to hinged length of structural member
c) value of moment adjacent to yield zone is half the yield moment up to hinged length of structural member
d) value of moment adjacent to yield zone is equal to yield moment up to hinged length of structural member

Explanation: The value of moment adjacent to yield zone is more than yield moment up to a certain length of structural member. This length is called hinged length.

10. Hinged length depends upon
a) weight of member
b) type of connection
d) number of bolts used in connection

Explanation: The hinged length depends upon the type of loading and the geometry of cross section of structural member.

11. What is the hinged length for simply supported rectangular beam of span L with central concentrated load?
a) L/√2
b) 2L
c) L/2
d) L/3

Explanation: For simply supported rectangular beam with central concentrated load, the hinged length is equal to one-third of the span.

12. What is the hinged length for simply supported rectangular beam of span L with uniformly distributed load?
a) L/√3
b) L/√2
c) L/2
d) L/5

Explanation: For simply supported rectangular beam with uniformly distributed load, the hinged length is equal to span/√3.

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