note that “transverse stiffeners are simply along for the ride”
3. Introducing stiffeners on their own, even when aligned with a side rail,
does not constitute a torsional restraint, as the connection to the side rail
or purlin is in no way equivalent to the “clamp” shown in Figure 3c. Bolts
in clearance holes in very thin material cannot be considered to provide
a rigid joint.
In the UK, the common way to restrain the inside flange is to provide
small diagonal links from the inside flange to the side rail or purlin, as shown
in Figure 4.
Conceptually, this triangulated system is equivalent to the web stiffeners
shown in Figure 3c and the secondary steelwork provides lateral restraint.
The necessary torsional restraint, equivalent to the “clamping” described
above, is delivered by the stiffness of the secondary members acting as
“U-frames” as shown in Figure 5.
U-frame action and its application to portal frames was discussed at
length in New Steel Construction in June 2018.1 This article included advice
on when and how the stiffness of the secondary steelwork forming the
U-frames should be assessed.
The small diagonal ties shown in Figure 4 are normally designed for a
lateral force equal to 2.5% of the compression force in the flange, but their
stiffness is equally important. If out-of-plane buckling is prevented in the
first place, there is no lateral force. Since in the UK we believe that U-frame
action is the underlying structural mechanics, we do not believe that the
restraint forces translate into tension and compression in the side rails or
purlins, nor do we insist that to be effective as part of a restraint system, the
side rails and purlins must intersect with the nodes of bracing. Some other
European countries make this a requirement. The secondary members must
be continuous, otherwise there is no U-frame. Side rails interrupted by roller
shutter doors, for example, are clearly not forming a “U” with the adjacent
It is self-evident that a purlin or side rail must be located at the position
where a restraint is needed, which means that judicious positioning of
secondary steelwork is required, to suit both the cladding and the out-ofplane
restraint to the members.
Figure 6 shows a frame during construction. The judicious spacing of
purlins is evident – closer spacing in zones of high bending moment and
more widely spaced purlins elsewhere.
An alternative approach often used at the most heavily loaded location
– where the underside of the haunch meets the column flange – is to
position a member at this level, immediately adjacent to the inner flange. It
is not adequate to simply tie all frames together at this point, as all the
frames could buckle in the same out-of-plane direction. The members at this
point must be triangulated back to the outside flange at some point, or
connected to the foundation.
Figure 4: Stays from secondary steelwork to inner flange
Figure 5: U-frame behaviour with secondary steelwork
Figure 6: Portal frame with thoughtfully spaced purlins