Where are restraints needed?
The short answer is wherever the member verification demands. Member
verification demands a buckling length, which in the out-of-plane direction
depends on the position of the restraints. It is surprisingly difficult to find
this fundamental requirement in the Eurocode. Clause 6.3.3 which covers
combined bending and axial compression and is therefore applicable to
members in portal frames points out in Note 1 that “the interaction formulae
are based on the modelling of simply supported single span members with
end fork conditions….”. As shown in Figure 2, end forks provide a torsional
During the recent SCI webinars on the design of portal frames, most
discussion centred on the restraint where the underside of the haunch
meets the column flange, generally referred to as “Point A”. Horne and
Ajmani, who were responsible in the 1970s for much of the research relating
to portal frames which we see in BS 5950 and now repeated in the
Eurocodes, described this important location as “Point A” and the
description has remained ever since.
A number of designers were not convinced that a restraint was essential
at “Point A”. It can be inconvenient, because if the cladding is supported at
the top of the column, the next side rail down could usually be far below
“Point A” if support to the cladding was the only requirement. However,
there is nowhere around the frame where the compressive force in the
flange is higher, so nowhere more deserving of an effective restraint. A side
rail positioned for that purpose (if that system is adopted) is not an
expensive and unnecessary addition, but an essential contribution to
Some designers suggested that with a restraint to the inner flange some
way down the column, and another restraint some way along the haunch,
the situation would be adequate. The SCI response is to ask which clause is
being used to verify the member – which is partly tapered and includes a
change of direction of usually 84°. There are no clauses that cover a
member with a nearly right-angle kink within the length.
The second common question recognised that there is very often a
compression stiffener in the column at “Point A”, and suggested that this
combined with a side rail would restrain the inner flange. However, as
explained above, a connection in the very thin material of the side rail with
ordinary bolts in clearance holes is hardly the “clamp” necessary for this
system to be effective.
Designers using bespoke software for portal frame design should make
Figure 7: The result when Point A is not restrained
sure they are entirely clear what type of restraint (one flange only, or
torsional, demanding restraint to both flanges) they have modelled. “Point
A” will invariably be modelled in software with a torsional restraint, which
must be provided in the physical structure.
Figure 7 should serve as a dramatic warning. No restraint at “Point A” has
simply allowed the point to buckle laterally. This should not be allowed to
happen – and yet – it is sometimes possible to see buildings under
construction without this point restrained. It is also possible to see
structures where the restraints have been detailed and provided to the
bottom flange of the rafter, rather than the bottom flange of the haunch. At
the deep end of the haunch, we would expect the compression to be in the
bottom flange of the haunch and this location should be restrained. The
bottom flange of the rafter, being approximately on the neutral axis of the
compound section, should have hardly any force at all.
The importance of restraints to the compression flange (the location of
which will vary in different combinations of actions) cannot be overemphasised.
Such restraints are fundamental to the structural stability of
the frame, and omission could lead to collapse. Restraints to the inner flange
must be identified, specified and provided in the actual structure.
1 U-frames in bridges
New Steel Construction, June 2018