Lip Buckling of Cold Formed Steel Purlin
Lip Buckling of Cold Formed Steel Purlin
(OP)
Looking for some opinions regarding local buckling of lips on cold formed steel C-purlins.
The subject building is comprised of 10 inch deep C-purlins spanning (continuously via bolted splices) over girts spaced at 20' o.c. No bridging is installed in the building. Sprinkler pipes are suspended from the purlins via steel rods which penetrate the bottom flange of the purlins attached to nuts with washers (no double nuts - just a nut and washer to provide bearing). During heavy snow, twisting of the purlins was observed, which subsided when the snow was removed.
The lips (not the flanges themselves) of the purlins exhibit deformations in the immediate vicinity of the steel rod penetrations. I assume that the deformations were caused lateral torsional movement of the purlins which compressed the lips, causing a localized failure.
What stumps me is why the lips have buckled not only at midspan locations, but also at locations near the girts. The purlins are continuous, so these regions are likely under negative bending, or at least not regions of maximum positive bending. I guess the heart of my question relates to the effects of lateral torsional buckling induced lip crippling, and the effects of point loads induced on the flanges of nonsymmetric cold formed sections.
The subject building is comprised of 10 inch deep C-purlins spanning (continuously via bolted splices) over girts spaced at 20' o.c. No bridging is installed in the building. Sprinkler pipes are suspended from the purlins via steel rods which penetrate the bottom flange of the purlins attached to nuts with washers (no double nuts - just a nut and washer to provide bearing). During heavy snow, twisting of the purlins was observed, which subsided when the snow was removed.
The lips (not the flanges themselves) of the purlins exhibit deformations in the immediate vicinity of the steel rod penetrations. I assume that the deformations were caused lateral torsional movement of the purlins which compressed the lips, causing a localized failure.
What stumps me is why the lips have buckled not only at midspan locations, but also at locations near the girts. The purlins are continuous, so these regions are likely under negative bending, or at least not regions of maximum positive bending. I guess the heart of my question relates to the effects of lateral torsional buckling induced lip crippling, and the effects of point loads induced on the flanges of nonsymmetric cold formed sections.






RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
Almost looks someone hung something from the edge - what or why - I cannot fathom.
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
On the other hand, I can't see any witness of contact on the extended sprinkler hanger itself, so maybe there was no contact, but the lip just buckled there because of the reduced section in the weak direction due to the flange hole.
.. Which might not have been an issue if the purlins were restrained from rotation, e.g. by bridging.
Or maybe there were temporary decorations or something hung from the purlins, at least that one. Just to the right of the extended sprinkler hanger in the image is what appears to be a circular witness mark on the lip's outer surface, as might be left by the clamping screw of some kinds of beam clamp. The paint appears to be spalled off inside the witness mark, which approximates a circle. There are also indistinct tooth-type marks below and to the right of the circular mark.
Mike Halloran
Pembroke Pines, FL, USA
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
Is this possible?
Jim
RE: Lip Buckling of Cold Formed Steel Purlin
No, to me, this deformation was not caused by rotation, but by installation. Looks the end of a pipe wrench or vice grip was used to get the rod assembly in place, and it was never bent back.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Lip Buckling of Cold Formed Steel Purlin
Another possibility is that they forced upwards the bottom free flange whilst pre-punching whitout upper restraint, or percuting to make a driving hole for the hangers; the C seems thick enough to have responded plastically locally.
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
RE: Lip Buckling of Cold Formed Steel Purlin
Regards,
Lutfi
RE: Lip Buckling of Cold Formed Steel Purlin