Lip Buckling of Cold Formed Steel Purlin

[AZengineer]

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.

 

[AZengineer]

Here's a photo of the condition.

 

[MiketheEngineer]

Could it be that the reaction loads at the frames were exceeded and you are seeing some effects of overload at these points. The reason we spec web stiffeners at bearings??

 

[chicopee]

looks like the sprinkler installer caused the damage and not the snow load.

 

[MiketheEngineer]

While I can't disagree w/ chicopee - that is some "REAL" damage from a simple hanger?? - unless that purlin really rolled a LOT.

Almost looks someone hung something from the edge - what or why - I cannot fathom.

 

[AZengineer]

Mike, but these deformations were observed at midspan too, not just near the supports! Chicopee - are you aware of other cases where this has happened due to installation? How are the holes in the purlins made (or, how WERE they made 30-40 years ago?)

 

[MikeHalloran]

If the extended sprinkler hanger was able to touch that lip, it suggests the web must have rotated ~45 degrees, maybe more.

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

 

[wannabeSE]

Have you checked the moment capacity of the purlins? I hope the design did not assume a fully braced purlin if the compression flange is not braced for the negative moment. But, I don't think this explains the bucking of a flange in tension at mid span.

 

[hokie66]

Couple of comments. Just to clarify, the purlins are supported by girders or rafters, not girts. Lack of bridging would lead to twisting under load. The deck above appears to be permanently deflected...or is that just my eyes? The local flange distortion may be due to suspension of the sprinkler mains from the flange edges at an earlier time, and they rehung the pipes, but left the flanges in their damaged state. Sprinkler mains full of water are heavy.

 

[jimstructures]

The buckling looks like what would happen if they used a clamp with a throat depth that was too shallow and they attached the hanger only to the flange lip which would mean the hanger would hang at an angle and then they bent the hanger straight down to attach the sprinkler or load. I realize that would take two operations. Then they had to re-hang the load/sprinkler with the hanger shown.

Is this possible?

Jim

 

[msquared48]

If torsional rotation caused the rod to impinge on the flange of the joist, I would expect any deformation seen in the lip to be semi-circular and gradual, not straight and kinked as in the picture.

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

 

[ishvaaag]

I agree with msquared48 it looks deformation of the lip made whilst putting the hanger outfit; maybe by whatever the reason they found they needed more clarance for the vice thing suggested in place during perforation. A discerning issue would be if the imperfection appears anywhere where the lip is above some depth; for it would be rare from structural work the failure be overspread except, again, some general kind of local failure, like forcing at an angle the hangers to meet a grid below, was imposed; but the hanger seems not bent.

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.

 

[chicopee]

The holes were probably drilled by the sprinkler installer unless the purlin manufacturer had prior knowleged on the hole spacing before the purlins were formed. Afterward during the installation of the sprinkler hangers, the installer just had a plain wrench or a socket not deep enough to tightened the nuts so the installer had to deform the lips for the wrench to work.

 

[chicopee]

The purlin manufacture would have punched the holes

 

[AZengineer]

Thanks for all of the input. I'm convinced that the deformations were not due to snow loading or lateral torsional effects after all!

 

[Lutfi]

I think the conclusion is clear. Buckling of the bottom flange will not occur unless the bottom flange sees compression under an uplift load. The damage appears to have been caused during erection or during installation.

Regards,
Lutfi

 

[hokie66]

Lutfi, the purlin is continuous, and would likely be in compression under gravity loading at that point, although it is hard to tell the distance from the support. But I agree, that local distortion is caused by something other than uniform loading.