# Design of Steel Structures Questions and Answers – Lateral Stability of Beams

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This set of Design of Steel Structures Multiple Choice Questions & Answers (MCQs) focuses on “Lateral Stability of Beams”.

1. Which of the following assumptions is not an ideal beam behaviour?
a) local and lateral instabilities of beam are prevented
b) any form of local buckling is prevented
c) compression flange of beam is restrained from moving laterally
d) compression flange of beam is not restrained from moving laterally

Explanation: Two important assumptions are made to achieve ideal beam behaviour: (i) compression flange of beam is restrained from moving laterally, (ii) any form of local buckling is prevented. A beam loaded predominantly in flexure would attain its full moment capacity if local and lateral instabilities of beam are prevented.

2. In beam design, sections are proportioned as such that _____ to achieve economy.
a) moment of inertia about principal axis parallel to the web is equal to moment of inertia about principal axis normal to the web
b) moment of inertia about principal axis parallel to the web is considerable larger than moment of inertia about principal axis normal to the web
c) moment of inertia about principal axis normal to the web is considerable larger than moment of inertia about principal axis parallel to the web
d) moment of inertia about principal axis normal to the web is considerable lesser than moment of inertia about principal axis parallel to the web

Explanation: In beam design, sections are proportioned as such that moment of inertia about principal axis normal to the web is considerable larger than moment of inertia about principal axis parallel to the web to achieve economy. Such sections are relatively weak in bending resistance.

3. To ensure that compression flange of beam is restrained from moving laterally, the cross section must be
a) plastic
b) semi-compact
c) slender
d) thin

Explanation: To ensure that compression flange of beam is restrained from moving laterally, the cross section must be plastic or compact. if significant ductility is required, section must invariably be plastic.
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4. What are laterally restrained beams?
a) adequate restraints are provided to beam
b) adequate restraints are not provided to beam
c) economically not viable
d) unstable beams

Explanation: In laterally restrained beams, adequate restraints are provided to beam in plane of compression flange.

5. Characteristic feature if lateral buckling is ___________
a) entire cross section do not rotate as rigid disc without any cross sectional distortion
b) entire cross section rotates as rigid disc without any cross sectional distortion
c) entire cross section rotates as rigid disc with cross sectional distortion
d) entire cross section do not rotate as rigid disc

Explanation: The characteristic feature if lateral buckling is entire cross section rotates as rigid disc without any cross sectional distortion. This behaviour is similar to axially compresses long column which after initial shortening in axial direction, deflects laterally when it buckles.

6. Lateral buckling in beam is _________
a) does not occur in beam
b) one dimensional
c) two dimensional
d) three dimensional

Explanation: Lateral buckling in beam is three dimensional in nature. It involves coupled lateral deflection and twists that is when beam deflects laterally, the applied moment exerts a torque about the deflected longitudinal axis, which causes the beam to twist.

7. What is elastic critical moment?
a) bending moment at which beam do not fail by lateral buckling
b) bending moment at which beam fails by lateral buckling
c) shear force at which beam do not fail by lateral buckling
d) shear force at which beam fails by lateral buckling

Explanation: Bending moment at which beam fails by lateral buckling when subjected to a uniform end moment is called elastic critical moment.

8. Which of the following condition causes lateral instabilities?
a) section possesses different stiffness in two principal planes
b) section possesses same stiffness in two principal planes

Explanation: Lateral instabilities occurs only if following conditions are satisfied : (i) section possesses different stiffness in two principal planes, (ii) applied loading induces bending in stiffer plane (about major axis).

9. Which of the following is not a method for providing effective lateral restraints?
(i) by embedding compression flange inside slab concrete
(ii) by providing shear connectors in compression flange
(iii) by bracing compression flanges of adjacent beams
a) i only
b) i, iii
c) ii, iii
d) i, ii, iii

Explanation: Effective lateral restraints can be provided by embedding compression flange inside slab concrete, by providing shear connectors in compression flange and embedding in concrete slab, by providing torsional bracings in the compression flanges of adjacent beams preventing twists directly.