Design of Steel Structures Questions and Answers – Design Strength of Laterally Unsupported Beams – I

This set of Design of Steel Structures Multiple Choice Questions & Answers (MCQs) focuses on “Design Strength of Laterally Unsupported Beams – I”.

1. The design bending strength of laterally unsupported beams is governed by
a) torsion
b) bending
c) lateral torsional buckling
d) yield stress

Explanation: Beams with major axis bending and compression flange not restrained against lateral bending (or inadequate lateral support) fail by lateral torsional buckling before attaining their bending strength.

2. The effect of lateral-torsional buckling need not be considered when
a) λLT ≤ 0.4
b) λLT ≥0.4
c) λLT > 0.8
d) λLT = 0.8

Explanation: The effect of lateral-torsional buckling need not be considered when λLT ≤ 0.4, where λLT is the non dimensional slenderness ratio for lateral torsional buckling.

3. The bending strength of laterally unsupported beams is given by
a) Md = βbZp /fbd
b) Md = βb /Zpfbd
c) Md = βbZp
d) Md = βbZpfbd

Explanation: The bending strength of laterally unsupported beams is given by Md = βbZpfbd, where βb is a constant, Zp is plastic section modulus, fbd is design bending compressive stress.

4. The value of βb in the equation of design bending strength of laterally unsupported beams for plastic sections is
a) 0.5
b) 2.5
c) 1.0
d) 1.5

Explanation: The value of βb in the equation of design bending strength of laterally unsupported beams for plastic and compact sections is 1.0. This constant depends on elastic and plastic section modulus for semi-compact sections.

5. The value of βb in the equation of design bending strength of laterally unsupported beams for semi-compact sections is
a) Ze/Zp
b) ZeZp
c) Zp/Ze
d) Zp

Explanation: The value of βb in the equation of design bending strength of laterally unsupported beams for semi-compact sections is Ze/Zp, where Ze is elastic section modulus, Zp is plastic section modulus.
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6. The value of design bending compressive stress fbd is
a) XLT fy
b) XLT fy /fy
c) XLT fy fy
d) XLT /fy

Explanation: The value of design bending compressive stress fbd is XLT fy /fy, where XLT is bending stress reduction factor to account for lateral torsional buckling, fy is yield stress, fy is partial safety factor for material (=1.10).

7. The bending stress reduction factor to account for lateral buckling is given by
a) XLT = 1/{φLT + (φ2LT – λ2LT)}
b) XLT = 1/{φLT – (φ2LT + λ2LT)}
c) XLT = 1/{φLT – (φ2LT + λ2LT)0.5}
d) XLT = 1/{φLT + (φ2LT – λ2LT)0.5}

Explanation: The bending stress reduction factor to account for lateral buckling is given by XLT = 1/{φLT + (φ2LT – λ2LT)0.5}, where φLT depends upon imperfection factor and non dimensional slenderness ratio, λLT is non dimensional slenderness ratio.

8. The value of φLT in bending stress reduction factor is given by
a) φLT = [ 1 – αLTLT + 0.2) + λ2LT].
b) φLT = [ 1 + αLTLT – 0.2) + λ2LT].
c) φLT = 0.5 [ 1 – αLTLT + 0.2) + λ2LT].
d) φLT = 0.5 [ 1 + αLTLT – 0.2) + λ2LT].

Explanation: The value of φLT in bending stress reduction factor is given by φLT = 0.5 [ 1 + αLTLT – 0.2) + λ2LT], where αLT is imperfection factor, λLT is non dimensional slenderness ratio.

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