Why is the maximum sagging moment being used in the LTB checks when I have restrained the top flange?

Environment

British Standard

Eurocode

Question

Why is the maximum sagging moment being used in the lateral torsional buckling (LTB) checks when I have restrained the top flange?

Answer

For lateral torsional buckling checks we would normally expect the top flange to be checked against the sagging moment and the bottom flange to be checked against any hogging moments that we may have in the beam. The logic behind this is that the sagging moment will cause compression in the top flange and visa-versa which would cause that flange to buckle.

However, where a one flange is restrained and the other is not, and the unrestrained flange is in compression for at least part of its length, different design rules apply. These are detailed in Annex G for BS 5950 and Section 6.3.2.4 of EN 1993.

Annex G

Section 6.3.2.4

In effect, what this means is that if a steel member undergoes moment reversal between restraint positions, it is the maximum moment in that span that is used, regardless of whether this is a hogging (compression side) moment or a sagging (tension side) moment.

Example

For example, if we consider this backspan cantilever with a moment profile in which the sagging moment is greater than the hogging moment.

If we look at the design results for the backspan, we can see that the maximum absolute moment is taken, despite this being the sagging moment, which we would expect to cause buckling on the top flange.