Steel Portal Frame

[MIStructE_IRE]

Hi,

In a typical portal frame we have horizontal cladding rails fixed to the outside face of the column and torsional V-restraints only in very specific locations.
Myself and a colleague disagree on the degree of restraint which is provided to the column in terms of its resistance to buckling under axial load.

He is of the opinion that the column's weak axis is fully restrained by the horizontal cladding rails on the outside face only. My view is that the column's weak axis is only restrained where both flanges are restrained - so only at the points where we have torsional V-restraints. As such, I believe the effective length to be the distance between V restraints - not the distance between cladding rails..

(The above relates to checks for axial loads only - obviously the compression flange is only restrained against buckling and lateral torsional buckling at V-locations only.)

Any thoughts?

 

[KootK]

If the rails themselves are preventented from moving axially then I would say that you have weak axis column bracing. What you won't have is comum torsional bracing. Depend how fancy you wanna get with your checks.

 

[civeng80]

If you have doubts about the rails providing restraint to the inner flange put in a fly brace.

 

[Retrograde]

I believe that the "torsional V-restraint" is the same thing as a fly brace.