Restraints around portal frames
In this second technical article on portal frames, David Brown of the Steel Construction
Institute reviews the all-important correct positioning and arrangement of restraints to the
inside flanges of columns and rafters. Having considered in-plane buckling in the previous
article, the focus is now on controlling out-of-plane buckling.
The problem(s) identified
Charles King, well-known to many in the portal frame world and responsible
for much of the guidance on this popular form of construction, used to
comment when leading SCI courses that some errors in the analysis and
design of a portal frame may not lead to collapse, but incorrect detailing
almost certainly would. It is clear from inspecting some bare frames during
erection and from questions received at the SCI that some designers remain
uncertain about where restraints should be located, and what form an
effective restraint might take.
The bending moment diagram around a portal frame due to primarily
“gravity loads” is well known, shown in Figure 1. At various locations, notably
the column and around the haunch in the rafter, the inside flange is in
compression under this combination of actions. Elements in compression
wish to buckle, and eventually, if unrestrained, will buckle in the out-ofplane
direction. The moment is greatest at the eaves – consequently the
compression in the inner flange is at a maximum, resulting in great
enthusiasm to buckle out-of-plane – which must be restrained if the frame
is to remain stable.
The classic assumption about members is that they have “fork” supports,
as shown in Figure 2.
It should be noted that a “fork” support provides lateral positional
restraint to each flange, thus forming a torsional restraint. It should be
equally obvious that a restraint to one flange only, as shown in Figure 3a, is
not providing a torsional restraint at that location.
Figure 3a: Lateral restraint to one flange only
Figure 3b: Lateral and torsional restraint to one flange only
Figure 3c: Lateral and torsional restraint to one flange with web stiffeners
Some arrangement to “clamp” the one flange, as shown schematically in
Figure 3b, is still not a torsional restraint, as the unattached flange is free to
buckle. An arrangement with stiffeners to connect the flanges together, and
a “clamped” flange, as shown schematically in Figure 3c is the only way to
provide restraint to the “other” flange, but note the requirement for both
stiffeners and a “clamped” flange.
These schematic diagrams illustrate the sorts of questions – and answers
- which arise concerning restraints around portal frames. In summary:
1. A side rail or purlin connected to one flange only provides lateral
restraint to that flange only, but does nothing of value for the other
2. Introducing full depth stiffeners in isolation does nothing to prevent
lateral-torsional buckling – the whole cross section is still able to move
laterally and twist. In this situation the AISC (American equivalent of SCI)
Figure 1: Being moment diagram – “gravity” combination of actions
Figure 2: End fork supports – a torsional restraint