Technical
AD 536: In-plane member buckling lengths for portal frames
The verification of members in portal frames leads to a common question about in-plane buckling lengths, especially when designers are using general software. General software requires an effective length factor or buckling length in both axes, leading designers to question what the in-plane buckling length is. The subject was covered in New Steel Construction of June 2020[1] so this AD serves as a summary reminder and a commentary on other potentially misleading guidance.
The in-plane verification of members in a portal frame is completed by verifying the in-plane stability of the entire frame. BS 5950 provided clear advice in 5.2.3.1 (2nd para) and 5.5.2. Once frame stability is verified and second-order effects are allowed for if necessary – by calculating αcr,est, the only in-plane verification is the resistance of the cross-section.
Out-of-plane verifications are of course necessary, using expression 6.62 of BS EN 1993-1-1. Comprehensive guidance on stability verifications is given in P399.
P397, also covering portal frame design, was written before Eurocode guidance had been fully developed. P397 includes guidance and an example covering the in-plane verification of portal frame members, which clearly conflicts with the above advice and should be ignored. A warning in the foreword to P397 points out that the guidance relating to in-plane buckling was likely to change – it did, and is clarified in P399.
SCI members have pointed out that SN031a (available on Steelbiz) and Table 6.2 of P360, both refer to buckling lengths in portal buildings. It’s likely that the original guidance in SN031a referred to the widespread continental practice of “portalised” frames with columns and flat roof beams (perhaps with pinned ends), rather than the pitched roof portals that dominate UK practice. The guidance in SN031a and Table 6.2 of P360 is not appropriate for pitched roof portals.
Sometimes, buildings with roof trusses are “portalised” by connecting both top and bottom chords to the column, thus providing in-plane stability. Designers should note that the buckling lengths of the columns in these frames may be well over twice the system length, depending on the depth of the truss and clear height of the column. The Steel Designers’ Manual contains helpful charts to determine the appropriate factor.
Contact: David Brown
Telephone: 01344 636555
Email: advisory@steel-sci.com
[1] D. G. Brown: In-plane stability of portal frames, New Steel Construction, June 2020