self tapping screws to HSS

[JStructsteel]

I have a lightly loaded connection (250# Shear) that was originally designed as a plate weled to a HSS section. The contractor wants to use a scew, #8 self tapping. I am doubtfull it will cut through the 1/4" plate and the 1/4" HSS, so I am considering pre drilling. Does anyone know of any trouble pre-drilling for self tapping screws?
Thanks

 

[SteelPE]

When ever I have self taping screws this is what I reference:

http://www.icc-es.org/reports/pdf_files//ESR-1976.pdf

The only time I have had the clients pre-drill holes is when I am attaching through something that is thicker to something that is thinner.

 

[jayrod12]

I'm pretty sure teks has screws that will self tap through 1/4" thick plate.

 

[JStructsteel]

Thanks,
I am just worried that they will not get properly installed if the screw will not drill thru the two thicknesses.

 

[jayrod12]

I would pre-drill the plate. not the hss.

 

[racookpe1978]

For two plates, drill through the FIRST plate for this small diameter of a self-tapping screw.

Drill size could be full nominal thread diameter (0.188 inch) for fastest assembly. Could be root diameter, but screws forcing their way through base plate 1 into baseplate 2 often "push" the two plates apart when the threads try to begin tapping/drilling/machining their way into the second plate unless a LOT of force is used on the screwdriver.

 

[steuey]

As a rule of thumb, I limit the thickness of the base member to 3/16" for any self-tapping screws to avoid deformation or capacity reduction of the screw. 1/4" plate seems a little thick to screw through. You might also consider a Lindapter hollo-bolt that is a nice replacement for a welded connection.

http://www.lindapter.com/american/Products/Hollow_Section_HSS_Connections/2/Type_HB_Hollo-Bolt

 

[spats]

The problem is what is the proper drill point for going through 1/2" of material. According to my ITW Buildex Teks catalog, it takes a TEKS/5 point to drill and tap 1/2" of material. The smallest screw they have with a TEKS/5 point is a #12 screw. Also, I hope it's a typo when you say "a" screw, as in single? No connection should ever have a single fastener.

 

[XR250]

A #8 will likely shear off trying to make threads in the HSS. Use a #12 minimum.

 

[JStructsteel]

Thanks. The connections are part of multiple ties for small precast panels. Why it maybe only "a" screw holding this particular plate, the panels have multiple connections.

Where do we stop engineering, and just start using prescriptive design?

 

[zcp]

Check the tech sheets for the particular fastener they want to use. Then check the drill points. Then predrill the top/outer plate. Then build in redundancy. #8 will be too small to get through 1/4" plate.

If they are pushing, you can always choose a couple of predrill sizes and mockup a couple of test articles. Load to failure and go from there with a healthy FS.

ZCP

http://www.phoenix-engineer.com

 

[XR250]

If you pre-dril the plate to larger than the diameter of the screw, a #12 tek5 will easily go into the HSS and pull them tight. I have installed alot of them.

 

[mrsutton]

I believe some of the new Simpson screws for attaching their new line of light gauge clips to structural steel will drive through 1/4" material. Teks is certainly the most commonly specified hardware though.

 

[Ron]

Agree with jayrod12.

 

[XR250]

I agree with XR250 and Jayrod12 but not Ron

 

[WiSEiwish]

I used to design components for facades that connected plates to structural steel and our primary method was to pre-drill or punch the plate and then use a self drilling screw (TEKS or Elco Drilflex) to fasten to the steel. I think I remember that the contractors having difficulty self drilling at the limiting thickness of the screw point, so if the screw "can" drill through up to 1/2" it may be a good idea to pre-drill that hole too if it is 1/2" thk. material.

 

[Ron]

XR250...don't do that while I'm drinking coffee!

 

[moments]

Definitely pre-drill the clearance hole. The base metal is not supposed to need predrilling, however I've gotten feedback about various circumstances where screws don't work as advertised without it. It is critical to drill the base metal at a smaller diameter. The idea is to still let the screw ream and tap the hole. The predrilling just eases the installation. You could ease your worries and help your installer by specifying a predrilled clearance hole in the connected part of a standard hole size for your product and an optional predrilled guide hole in the base metal not to exceed a reduced diameter you deem fit. The TEKS are supposed to be great, as are the Elco Drillflex AKA Hilti Kwik Flex, and all of these have ICC reports for all the nitty gritty details you'll want to look at: capacities, steel drilling thickness limits, bearing lengths for different screw lengths, clearance hole diameters, etc. There is also information for the contractor to keep in mind such as drill rpms and which type of tool to use that will affect the quality of the installation and which you may want to have a conversation about with your installer.