Self-Drilling Screws 4

[IreneAdler]

Buenas Dias! Could anyone provide me information on the pros & cons of Self-Drilling Screws? We have this case wherein a frame is supported at two points. At these two points, the fixing is proposed to be fastened on a 6mm thk steel RHS using Self-Drilling Screws, for installation convenience. From my understanding, frames or trusses are fixed though a welded or bolted connection. This is the first time I have encountered such case. Help!

 

[Ron]

As long as the fastener is sized to carry the appropriate loading (shear, tension or both simultaneously), self-drilling screws are no different than other fasteners. Self-drilling fasteners are generally used for light loading, but are commonly used for light-gauge steel framing and other building applications.

 

[bridgebuster]

Professor Moriarty likes them too.

 

[TehMightyEngineer]

You could almost say that the design of a self-tapping screw is "elementary".

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[SteelPE]

This might not pertain to you as you are not in the US, but in the attached link you will see the code allowable values for self drilling TEK screws so you can get a feel for what they might be capable of supporting. Your code should have a similar document listing the values of the screws.

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

You will see in the literature that the FOS for the screws is pretty high for obvious reasons.

 

[BUGGAR]

Remember you will need an allowable combined shear and tensile load. Individual loads only may be listed in the literature.

 

[XR250]

Make sure you pre drill the outer ply if it is thicker than 16ga +/- Sounds like it may be structural steel.
Tek screws are not good about pulling plys together.

 

[rb1957]

the advantage of self-drilling screws is that they reduce installation costs, doing two operations at once. From the sketch, there don't look to be a lot of screws involved, hence not much savings. Why not use a "proper" fastener (a bolt or a rivet) into a drilled hole ?

another day in paradise, or is paradise one day closer ?

 

[IreneAdler]

Nice try Sherlock.

I have no issues in terms of the allowable values. It's just that every time I analyse these fasteners, it's for light loading only. I'm just worried that I may have overlooked a few points when considering self-drilling screws. Won't there be any problem in installation? Is it really okay to fix it at a 6mm thick steel? What if for some reason, the frame should be removed, and it is needed to unscrew the fasteners, and put back again, would there be any complications?

 

[nonplussed]

Buildex Series 500 Teks can go through steel up to 12mm. As for removing and reinstalling a frame, you would generally want to go up 1 size of screw for the re-installation so that it grips properly. That is why for screws used in roof sheeting, you never want to use the largest available size screw, because when you eventually have to replace the sheeting due to corrosion, you always want larger screw sizes available for the re-installation.

 

[samdamon]

Irene as an alternative you could also use a "self tapping" screw. These require the extra step of predrilling but can be used with thicker pieces and may still can be less costly than field welding. The contractor needs to use the correct, exact predrill bit diameter, as required by the mfgr, though. If he doesnt and the holes are even slightly too small the screws break off before being fully driven and you have a mess, or if the predrill holes are slightly too big the screw threads dont bite properly and you will not have the capacity you designed for. The work should be inspected as it happens.