Screw Capacities: Wood Side Member/CFS Main Member 1

[P1ENG]

I'm trying to determine how to calculate the shear and tension capacities of a wood side member with a cold formed steel (18 GA) main member. My only thought this far is combine AISI screw capacities with the NDS screw capacities. The problem with that is figuring out if the failure mode equations change or even become negligible. The NDS addresses steel side member with wood main members, but not the other way around. ITW and other manufacturers make screws for attaching wood to steel (self-drilling screws with the wings), but I haven't seen strength tables. Finally, I'm assuming the screw would have to be a metal screw so will that change my NDS equations? I'm thinking yes. Perhaps some other jurisdictions (Canada, Australia) has such a condition documented and can provide some help?

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[KootK]

Interesting. My thoughts:

1) No better info in Canada to my knowledge.

2) if you look closely at the NDS failure mode equations, I think that you'll find that the main member/side member distinctions are somewhat arbitrary. I think that you can apply the same theory with the roles reversed.

3) I would argue that the CFM is still the side member in your scenario. The connection is really the story of thin sheet steel attached to thicker wood. The fact that your sheet steel has returns and lips etc doesn't materially change that in my opinion.

4) Combining NDS and AISI provisions sounds good to me. You really just need to capture the few failure modes that would apply for a CFM side plate that wouldn't normally apply for hot rolled plate.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[P1ENG]

I would think side/main are not arbitrary as the screw head keeps the fastener from rotating at the side member in the failure modes. I would think the free end of the screw protruding out of the steel member would allow more rotation during a shear application than if the members were switched. However, after saying that, I was also contemplating doing just that: assume the nomenclature arbitrary and using the steel side member / wood main member of the NDS and calling it a day (provided thick steel attached to 18 GA per the AISI did not control).

I like your signature!

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[KootK]

My read of the NDS provisions is that no account is given to screw/nail heads at all. It's just straight dowel considerations. The heads may indeed restrict rotation but that's not part of the method.

And surely, when you look closely at the main and side member NDS equations, you'll agree that we're running the same checks on opposite sides of the main/side member interface in many instances?

Of the arguments that I made previously, I feel that the strongest is that your light gauge member really is still the side member anyhow. It's thickness is 18 ga in this context, not the structural member dimension.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RFreund]

I had posted a similar question to AWC some time ago. Here are the questions and responses:

My Questions:

1. Usually I think of the side member as the member in contact with the head of the screw. Is this correct?
2. If #1 is correct, is there a difference between steel side member vs steel main member. Meaning if the head is in contact with the wood member and fastened to the steel member (nic) is this the same as the other way around?

Responses:

Yes, the side member is the member in contact with the head of the screw. The definition usually used is that the main member receives the tip, so the other is the side, but it means the same thing.

Yes, there is a difference between having a steel side member and having a steel main member. Also, when you used “(nic),” are you meaning “not in contact?” If so, this makes the connection even more difficult as per 10.1.5 of the 2012 NDS, Chapters 11, 12, and 13 are based upon “the assumption that the faces of the members are brought into contact when the fasteners are installed…”

The attached technical report deals with hollow sections and gaps and may be useful.

I had replied to this with:

When I referenced “nic” I did mean “not in contact” but I meant the head of the fastener was not in contact with steel. Rather the head of the fastener is in contact with the wood. Therefore the steel member is the main member and the wood member is the side member. It appears that the AWC wood calculator does not address this case. Is that correct? Are the results for this case higher or lower than steel side member? Or does it depend? Is this situation solvable via the TR-12 report?

I don't think he responded. I will follow up.

Report 1 Link

Report 2 - 97 edition

Let me know if you make any ground on this.



EIT

http://www.HowToEngineer.com

 

[KootK]

Upon further consideration:

1) Side member = member that will be fully penetrated by dowel.

2) Main member = member that may be partially penetrated by dowel.

In cases, like OP's, where both members will be fully penetrated, I maintain that the distinction is arbitrary. This is also probably why steel plates are never presented as main members. There are no practical, partial penetration scenarios for steel plate.

Great doc post Rfreund. The HSS stuff actually answers a debate that I had with one of our new EIT's a couple of weeks back.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

P1ENG:
I think that Koot has pretty much hit the dowel (nail?) on the head here. Just as the head of the screw might have some affect on the exact curvature of the dowel and dowel action, it is probably true that the length of penetration and the hardness (resistance to bearing crushing) has some affect on the fixed end moment reaction curvature on the dowel into the penetrated member. That is to say... the screw, bolt, dowel will have a slightly different reaction/curvature/shape/strength when it is penetrating a 2 or 3" wood member (pretty soft and crushing) or a .5" steel plate (very strong, moment resistant) or an 18ga. CFS member (very much non moment curvature holding member, in fact a knife edge). I don’t know that we can really refine our every day calcs. and analysis to this fine a detail. The important thing is that we give some thought to how these things affect or joint in our analysis and design, so that we account for them in some way. But, be careful that just because our computers calc. things to 8 signif. places does not mean that we understand/know how that joint works to that same degree of assumed accuracy.

 

[XR250]

I imagine your loads are pretty small in this scenario. Using 100lbs per fastener would be likely conservative and would not likely result in more than would be needed anyway for standard practice.