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self-tapping screw connections
when using a self-tapping screw for a connector, we are basically relying on the integrity of one thread, at least when connecting light gauge materials. if the installer doesn't have the clutch set correctly on his screw gun, he can easily strip the threads on the screw or the part being fastened (the screw is harder than the materials being fastened).
does anyone know of information on this, any test data that could give some idea how much residual strength might be left in the connection if the threads are stripped?
i am looking at a situation where i know the tested, average pullout strength of the self-tapper; say 500 lbs. the connection fails in a wind storm, but the wind speed is less than the rated pullout capacity of the screw. working backwards, if the screws are spaced at 10 sf/screw, the "rated" capacity is 500/10 = 50 psf. however, the windspeed associated with this load is much higher than that reported for the storm. any thoughts???
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how did the screws fail? by pullout or did they shear? for pullout there could have been a prying action on the connection. that could develop a greater force than just the wind pulling directly. just some thoughts.
what was the material guage? the pullout will differ from 16ga to 12ga.
if the screw was over-tightened and stripped the material being fastened, the screw has virtually no pullout capacity! you may have to determine what percentage of the fasteners installed have been stripped,on the average, and count on only those that were properly installed.
the screw is a #12x24 self-tapper and is being fastened through a predrilled, standing seam roof clip into an 18 guage hat channel. the failure is a "pull out" from the 18 guage hat channel, but in some instances there is pring action as seen in the shape of the hole. the entire 40'x120' section of roof blew off and not one clip was left on the roof!
anyone have information on something like this?
it could be difficult to determine the magnitude of the forces required. seems like a straight 'pull-out'... could have initiated at a weakened screw, or a series of them and just overloaded the adjacent ones as the first ones failed. if this is the case, it may not be possible to determine the strength of the fasteners as they failed; they could be overloaded beyond their design capacity.
i would suspect that the flexural stiffness of the clip is too thin for significant forces due to 'prying action', even though the deformed shape of the hole indicates that this has occurred.
were there similar buildings in the area that suffered similar windstorm damage?
no wind damage to other parts of the same building and no downed trees in the area. i'm coming to the conclusion that the wind "happened" to be in the right direction at the right speed to start the pull up of the roof. once it started there was no stopping it.
what got it started? well i think that there were most likly some screws that were torqued too high and stripped the threads, they would then have no pull out strength. i don't think it would take too many to get the roof started. i was just hoping that there might be some information out there of similar failures of standing seam roofs.
sometimes with over-tightening, the metal deck in the area of the fastener head is deformed or even cut... this may be visible at some of the clips by possible corrosion.

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