of portal frames
David Brown of the Steel Construction Institute offers a reminder of the
guidance covering in-plane stability of this very common form of construction.
Judging by recent questions received by the SCI, the topic is not as clearly
understood as it should be!
The problem(s) identified
In March and April of 2020, to offer some light relief during
COVID-19 lockdown, the SCI ran a number of free webinars
covering the analysis, member verification and detailing of portal
frames. The webinars were significantly over-subscribed, and
were repeated four times. Two topics gave rise to the most
questions – in-plane stability and restraints to the inside flange.
Many delegates wanted to know what in-plane effective length
should be used when verifying members – particularly the
column. Others wanted to apply Annex E of BS 5950 to determine
an in-plane effective length. There was no problem with out-ofplane
lengths – no-one questioned that out-of-plane, members
should be verified between restrained positions.
Perhaps the problem is highlighted if designers are using
general elastic analysis software to determine the design forces
and moments around the frame and then to verify the members
within it. Such software expects to complete both in-plane
checks and out-of-plane checks, which naturally demands an inplane
buckling length. Portal frames are a special case, with
particular rules discussed in this article.
What does BS 5950 say?
Before opening the Eurocode, it is valuable to look at the
particular rules for portal frames given in BS 5950. The UK would
claim to have developed most of the rules for portal frame
design, backed up by many decades of successful application, so
one might expect definitive guidance in our previous standard.
Portal frames are one example of a continuous frame, and
may be designed elastically or plastically, so we need to look
carefully at the relevant clauses.
Within the “Continuous structures” section, clause 188.8.131.52
discusses plastic analysis. The second paragraph should be
sufficient to clarify the in-plane verifications needed:
The in-plane stability of the members in a continuous frame
designed using plastic analysis should be established by
checking the in-plane stability of the frame itself, see 5.5.4.
Designers should note that according to this clause in-plane
checks of individual members are not required.
Portal frames are addressed in section 5.5. Clause 5.5.2 covers
If elastic global analysis is used for a portal frame, the crosssection
capacity should be checked… and the out-of-plane
buckling resistance should be checked….
For portal frames with no in-plane bracing…the in-plane
stability of the frame should be verified by checking the crosssection
capacity and the out-of-plane buckling resistance of the
members (amplified if necessary)