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Queensferry

Module 08

Beams and lateral-torsional buckling

A beam loaded downwards can fail sideways. Understanding why turns a mysterious code check into something you can reason about — and shows that where you put the load matters as much as how big it is.

What this module covers

  • Explain LTB physically, in terms of the two flanges behaving differently
  • Read the two terms in Mcr and say which dominates at a given span
  • Explain why uniform moment is the worst case
  • Quantify the effect of applying load above or below the shear centre
  • Calculate what a lateral restraint buys, and when further restraint stops helping
  • Say what a restraint must actually do to count as one

Lessons

  1. 8.1

    Why a beam fails sideways

    Load it downwards and it moves sideways and twists. The reason is that the two flanges are doing opposite things.

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  2. 8.2

    Restraint, moment shape, and where the load sits

    Three things change the answer. One is free, one is decided by the structure, and one is a detailing choice most people never think about.

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Module checkpoint

Check what you have taken in

6 questions

  1. Question 1

    A beam has Mc,Rd = 826 kNm and Mcr = 450 kNm. What is λ̄LT?

  2. Question 2

    A beam has λ̄LT = 1.20. What is the elastic cap on χLT?

  3. Question 3

    A beam has χLT = 0.727 and Mc,Rd = 826 kNm. What percentage of the cross-section resistance is lost to LTB?

  4. Question 4

    A 12 m beam has two intermediate lateral restraints, equally spaced. What length governs the LTB check, in m?

  5. Question 5

    The warping term Iw/Iz is 66 950 and the St Venant term is 73 530. What percentage of the resistance to twist comes from warping?

  6. Question 6

    For double curvature C₁ = 2.60. If Mcr under uniform moment is 290 kNm, what is it in double curvature?