Plastic design is covered in clause 5.5.3:
Plastic global analysis may be used for a portal frame provided
that the conditions in 5.2.3 are satisfied (which is a reference
back to the clause previously quoted).
Checking the in-plane buckling of individual members in a
portal frame is inappropriate – the frame buckles as a single
entity, and therefore the standard demands that stability is
verified by checking “the in-plane stability of the frame itself”.
One potential exception to the preceding general rule is an
internal column in a multi-span frame (Figure 1). In the so-called
gravity combination, the bending moment in the internal column
may be very small. The in-plane buckling of this member should
be checked. P292 recommends an effective length factor of 1.0
for truly pinned bases, 0.85 for nominally pinned bases and 0.7
for nominally fixed bases.
Internal columns probably have no restraint at any level
below the haunch. If the internal column is orientated in the
orthodox direction (major axis in the plane of the frame) then the
minor axis resistance will of course be critical, not the in-plane
buckling. If the internal column was turned 90°, such that its
weak axis was in the plane of the frame (Figure 2), or if the
internal column was a fabricated section with a larger inertia out
of the plane of the frame, then in-plane buckling could be critical,
but it seems most unlikely.
In-plane buckling of the frame
According to BS 5950, in-plane stability of portal frames can be
verified by three methods:
1. The sway-check method – commonly known as the h/1000
check, with a limited scope (and a snap-through check for
2. The amplified moments method, requiring the
determination of λcr and an amplifier if necessary. No
amplifier is required if λcr > 10;
3. Second-order analysis.
In each method, the impact of second-order effects is
Figure 1; Multi-span portal with internal column
Figure 2; Orientation of internal column
considered. Satisfying the sway-check method means that
second-order effects are small enough to be ignored. The
amplified moments method allows for second-order effects with
an amplifier unless the effects are small enough to be ignored.
Second-order analysis will always allow for those effects.
Member checks in BS 5950
Having completed the in-plane buckling checks of the frame in
its entirety and allowing for second-order effects if necessary, the
cross section has to be checked and then out-of-plane checks
completed. BS 5950 has a range of clauses covering different
conditions – next to plastic hinges, with intermediate restraints
to the tension flange only, tapered sections etc.
Why not Annex E?
The introduction to the Annex seems to offer opportunities for
use, describing “the effective length LE for in-plane buckling of a
column or other compression member in a continuous structure
with moment resisting joints should be determined using the
methods given in this annex.” That sounds appropriate for
portals, but as one reads further, it becomes abundantly clear 28