SS Self Tapping Screw Capacity 5

[ScottVH]

I frequently work on curtain wall designs for one of my clients, and they recently came back on a project that their client wants to use self-tapping stainless steel screws. Typically they use TEK screws, and I follow the ICC reports for the limit states of the screws. I figured that the limit states controlling tilting, pullout, and other failure modes of the connected metals would be similar enough since they are still connecting steel members (I also verify with the equations from AISI S100). My issue is that the manufacturer does not have, or just won't supply the shear and tension values for the screws beyond that they are SS304. I can't find anywhere in AISI that has a way to calculate the shear and tension capacities of the screw itself. I'm tempted to follow the equations from ASCE for bolt tension and shear and use an increased phi factor, but am hesitant to do so. Does anyone have any guidance on where I can find equations for calculating SS screw strengths?

 

[EdStainless]

Unless they are being made to a spec and have a minimum strength that is verified, no.

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P.E. Metallurgy, consulting work welcomed

 

[ScottVH]

They do have a minimum strength. The manufacturer did tell me that they are made to SS304 specs with a minimum yield strength of 30 ksi, and a minimum tensile strength of 70 ksi.

 

[BAretired]

There is some strength information in the link below:

https://www.hilti.ca/medias/sys_mas...chnical_information_ASSET_DOC_LOC_1577675.pdf

BA

 

[phamENG]

The key to EdStainless' post is the word "verified." Just because you know the base metal that went into the manufacturing process doesn't mean you understand how it will behave now that it has come out in screw form. If that manufacturer hasn't invested in the testing of their product to verify its capabilities, they obviously aren't interested in profiting from true engineered applications for their products.

Find another manufacturer that certifies their product.

 

[brut3]

ScottVH,

You need to purchase a copy of AAMA TIR 9-14 which is a design guide by the American Architectural Manufacturers Association. The design guide gives the ultimate, yield, and shear strength of different SS alloys depending on the diameter of the fastener that is used. Please see the below link to the design guide and should give you exactly what you're looking for.

https://pubstore.aamanet.org/pubstore/ProductResults.asp?cat=0&src=tir-a9

 

[jjl317]

We have had success using the Elco (now Dewalt) Bi-Flex stainless steel self drilling / self tapping screws in the exterior cladding industry. Many self drilling / self tapping screws have issues with Hydrogen Assisted Stress Corrosion Cracking, which can be created during the hardening process required to create the cutting threads. The Bi-Flex are made by connected a hardened tip to a more ductile screw body, eliminating that concern. I believe that Elco originally developed the Dril-Flex, which is a similar carbon steel version, and there are now similar carbon steel versions on the market (i.e. ITW Teks Select), though I don't know about any other stainless screws with similar technology. And they have pretty complete technical data online. The only complaint we have gotten is that they are not inexpensive, especially compared to the random screws that can be found online.

And I agree completely that AAMA TIR 9-14 is a very good document with exterior cladding.

 

[XR250]

Try this...

Link

 

[Ron247]

Seems like you have 2 questions/issues here. The first is where to get information on calculating the screw capacity when it is not provided by the supplier and the second issue is "Once calculated, how reliable is the value?"

I would address the 2nd issue first and if I felt the reliability is questionable, I would not worry about the 1st one. That depends on the price of screw failure. If it fails, what is the downside?

I have been involved in several instances like this where I specified a bolt (Hilti, TEK or Redhead for example) and AFTER awarding the project, the Contractor or Owner wanted to substitute a much lower cost product. In about 90% of the cases, the proposed substitution was much cheaper but had ZERO design information. They would not even tell you Fy or Fu in some cases. My take was that if the manufacturer was not willing to spend any money on testing, research, verification, documentation and quality control, their price should be a lot less than someone who does. When questioned, many of them had no quality control at all. Just make a million screws and distribute them. Going back to what has already been stated, "Verified".

When I refused to allow the change here are the most common "types of response" I got:
[ul]
[li]The supplier's legal won't let them supply that information due to increased liability.[/li]
[li]Your costing me $4,000. I bid the job with these screws, not TEK.[/li]
[li]My supplier said to just use Hilti's design information since that is what they are substituting for. (I refer to that as a "Chameleon Bolt". Its properties match what you need.)[/li]
[li]I don't see the difference, they are both 3/4" diameter and made of steel. (I call those "Carnival Bolts". Give the Barker a $1 to guess your age or weight and if they get it wrong you get 1 of those bolts as a prize but you will never get a Hilti. You get some glass that looks like a diamond but not a diamond)[/li]
[li]I guarantee you they are just as good as Hilti. (I call these the "Grand Prize bolts". Grand Prize is 1 bolt, 2nd place is 2 bolts, 3rd place is 3 bolts etc.)[/li]
[/ul]

Final point, when I told them there would be a fee to even research their proposed change that has no real information, they felt I should do it for free.

 

[dik]

so true...

Dik

 

[sticksandtriangles]

Ron, what a lovely post, thank you for outlining all the fun excuses you get.

I am going to have to steal the differing bolt names if I may. Many a times I get the "Chameleon Bolt" substitution. "What strength to you need? Oh yeah, this one has that"

S&T

 

[Ron247]

Sad thing is, these were real statements that I have gotten more than once. Especially those with the word "guarantee". My best magic trick is making the word "guarantee" disappear. "OK, I'll give you a guarantee to write me out on your insurance company's letterhead and have them sign it". Poof, guarantee is gone. Their next statement is that they can't give a real guarantee because they are not a structural engineer. My answer, you don't have to be an engineer to give a guarantee, just have the money to back it up.

 

[ScottVH]

Thanks everyone. I was mostly wondering if there was some sort of way that you could calculate the screw strength when you did know the material it was made from. It sounds like there really isn't a good way to do so. I will have to tell them that they are going to have to stick with a manufacturer that does provide testing results. Here come the complaints that Ron247 was talking about. I think we all love when our clients don't like our answer that they can't do something they want....and that we don't work for free.

My favorite is when I specify a Hilti, or similar, connector and 2 months later they say "We used [off brand] anchor, can you update your calcs?" Then I have to explain that they can't do that because said off brand anchor doesn't work. The ensuing tantrums are oh so fun. So many companies don't seem to understand that when we specify a specific anchor, material, or even member size, we do it for a reason.

 

[EdStainless]

The properties that they gave you are the starting raw material, not the fasteners unless they have been re-annealed after forming. Just to give you an idea, when heavily cold worked 304 has no problem reaching over 150ksi.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[JoshPlumSE]

There is an OPD (OSHPD pre-approved detail) or two that cover the capacity of sheet metal screws. This might help. It's based on the diameter of the screw and the material it's going into. It's not exactly what you're looking for, but it's the closest that I know of:

https://oshpd.ca.gov/construction-finance/preapproval-programs/oshpd-preapproved-details-opd/

https://oshpd.ca.gov/ml/v1/resource...ction-And-Finance/PreApproval/OPD-0003-13.pdf

 

[brut3]

Below is a snapshot from the AAMA TIR 9-14 document I reference earlier. The strength values that you're looking for should be in the document. Their are many different tables depending upon what type SS or carbon steel you're using. The different "condition types" in the tables give design values ranging from raw material to materials that have been cold-worked or annealed.