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

View Answer

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) λ_{LT} ≤ 0.4

b) λ_{LT} ≥0.4

c) λ_{LT} > 0.8

d) λ_{LT} = 0.8

View Answer

Explanation: The effect of lateral-torsional buckling need not be considered when λ

_{LT}≤ 0.4, where λ

_{LT}is the non dimensional slenderness ratio for lateral torsional buckling.

3. The bending strength of laterally unsupported beams is given by

a) M_{d} = β_{b}Z_{p} /f_{bd}

b) M_{d} = β_{b} /Z_{p}f_{bd}

c) M_{d} = β_{b}Z_{p}

d) M_{d} = β_{b}Z_{p}f_{bd}

View Answer

Explanation: The bending strength of laterally unsupported beams is given by M

_{d}= β

_{b}Z

_{p}f

_{bd}, where β

_{b}is a constant, Z

_{p}is plastic section modulus, f

_{bd}is design bending compressive stress.

4. The value of β_{b} 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

View Answer

Explanation: The value of β

_{b}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 β_{b} in the equation of design bending strength of laterally unsupported beams for semi-compact sections is

a) Z_{e}/Z_{p}

b) Z_{e}Z_{p}

c) Z_{p}/Z_{e}

d) Z_{p}

View Answer

Explanation: The value of β

_{b}in the equation of design bending strength of laterally unsupported beams for semi-compact sections is Z

_{e}/Z

_{p}, where Z

_{e}is elastic section modulus, Z

_{p}is plastic section modulus.

6. The value of design bending compressive stress f_{bd} is

a) X_{LT} f_{y}

b) X_{LT} f_{y} /f_{y}

c) X_{LT} f_{y} f_{y}

d) X_{LT} /f_{y}

View Answer

Explanation: The value of design bending compressive stress f

_{bd}is X

_{LT}f

_{y}/f

_{y}, where X

_{LT}is bending stress reduction factor to account for lateral torsional buckling, f

_{y}is yield stress, f

_{y}is partial safety factor for material (=1.10).

7. The bending stress reduction factor to account for lateral buckling is given by

a) X_{LT} = 1/{φ_{LT} + (φ^{2}_{LT} – λ^{2}_{LT})}

b) X_{LT} = 1/{φ_{LT} – (φ^{2}_{LT} + λ^{2}_{LT})}

c) X_{LT} = 1/{φ_{LT} – (φ^{2}_{LT} + λ^{2}_{LT})0.5}

d) X_{LT} = 1/{φ_{LT} + (φ^{2}_{LT} – λ^{2}_{LT})0.5}

View Answer

Explanation: The bending stress reduction factor to account for lateral buckling is given by X

_{LT}= 1/{φ

_{LT}+ (φ

^{2}

_{LT}– λ

^{2}

_{LT})0.5}, where φ

_{LT}depends upon imperfection factor and non dimensional slenderness ratio, λ

_{LT}is non dimensional slenderness ratio.

8. The value of φ_{LT} in bending stress reduction factor is given by

a) φ_{LT} = [ 1 – α_{LT} (λ_{LT} + 0.2) + λ^{2}_{LT}].

b) φ_{LT} = [ 1 + α_{LT} (λ_{LT} – 0.2) + λ^{2}_{LT}].

c) φ_{LT} = 0.5 [ 1 – α_{LT} (λ_{LT} + 0.2) + λ^{2}_{LT}].

d) φ_{LT} = 0.5 [ 1 + α_{LT} (λ_{LT} – 0.2) + λ^{2}_{LT}].

View Answer

Explanation: The value of φ

_{LT}in bending stress reduction factor is given by φ

_{LT}= 0.5 [ 1 + α

_{LT}(λ

_{LT}– 0.2) + λ

^{2}

_{LT}], where α

_{LT}is imperfection factor, λ

_{LT}is non dimensional slenderness ratio.

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