Minimum Screw Engagement

[glorfindel87]

I know this general topic has been dealt with elsewhere on this forum but the threads I visited never addressed the question I have. If it has been dealt with elsewhere feel free to provide me a link.

But basically I have some corrugated steel panels that are attached to the girts of a metal building with some 1/4" diameter x 3/4" long sheet metal screws (14 threads per inch). The problem is that with most of these screws, the point barely sticks out past the back of the girt and I have heard that with sheet metal screws it's either engaged or not, depending on whether or not the tapered point extends beyond the edge of the metal being attached to. If this is the case, is there documentation for this? And if not, what is the requirement and is there any reduction for thread engagement for a screw? I know there is now for anchors but I haven't seen anything to indicate it is the same for screws.

And if you care for some background to the question, I ask because I am writing an RFP for the refurbishment of a metal building and am concerned with the pressure washing knocking the panels off.

Thanks in advance for your replies.

 

[ishvaaag]

From
Residential Steel Framing Handbook
Scharff
McGraw Hill 1996
p. 70

Selecting a Screw Length

"should be approximately 3/8 to 1/2 inch longer thant the thickness of the connected materials. At least three exposed threads should extend through the steel to assure an adequate connection. These are minimum guidelines."

 

[glorfindel87]

So am I to assume then that in the design I should take it as a black and white issue, and if it does not meet these minimum guidelines I should treat it as not engaged at all?
Thanks for your reply.

 

[ishvaaag]

The text writes that. I think that the precision on that this is a minimum requirement should be taken by the author more as a warning against misinterpretation on some readers thinking that screws should protrude as much (that to some might think was too much). That is, this is the minimum protrusion, says, not the maximum the screw can protrude.

However, since all the screws portraited in the book are mostly cylindrical (unlike some for wood) one must think that the intent of the author, fom his sources is that, yes, these should be the appropriate dimensions of the screws. The two prescriptions, dimensions and threads must not be much different, given the conical point, for I wouldn't call thread but those on the conical shaft. A triple backup against tensile disengagement.

At the end of the book when talking on how to give specifications for such steel framing buildings, when talking about screws says:
"Contractor shall refer to installation instructions by the screw manufacturer and ASTM C-954 for minimum spacing and edge distance and torque requirements"

So in the end it seems there was not an accepted industry-wide standard by 1996 (for otherwise would have quoted it in the book) and discharged the load of proper placement of screws on the recommendations of manufacturers of screws (or maybe metal framing systems) for installation. Can't say more.

 

[dinjin]

If the metal screws pass thru the panels and into the metal building at least equal to 3 times the thickness of the panels, I would think it would be sufficient and not have to
protude thru the building walls. Could you be more specific on how thick the panels are and also how thick the building wall are? It seems to me that would be a consideration.

 

[ishvaaag]

Of course when I wrote threads "those on the conical shaft" it should read on the cylindrical shaft.
And as dinjin points, I understand these are provisions for just the steel framing substructures, the steel parts, not at all for any finishing gypsum works and of course less towards a seen face! The drywalls and floor ceilings must be built according to their specifications, industry or fabricator.

 

[glorfindel87]

Thank you for the replies.

dinjin, the thickness of the girt is 7/16" and the panel is ~0.08in, it was hard to get too accurate of a measurement since the panels were on the building and the owner is unable to locate the drawings which would show this information. Keep in mind though that the building is ~40 years old and whether it's from the thickness of the girts or possibly the thickness of the coating varying the protruding length of the screw varies from 1/4" to none at all. As to your comment, I would think it sufficient as well going off instincts, but is there any documentation that you are aware of that states something to that effect?

 

[dinjin]

I was thinking of the general rule that 3 threads are needed to develop the full strength of the screw so
3 x (1/14) gives you .214 length required. This general rule assumes the screw and material it is going into is the same strength. They used to use the rule that metal screws were of strength equal to Grade 2 bolts. I have no idea how strong the metal girts are.