Cold Formed Strapped Shear Wall Mechanics and Design
Cold Formed Strapped Shear Wall Mechanics and Design
(OP)
Attached is a diagram of a CFS strapped shear wall. My general question or intent is to understand how I might be able to increase the compression capacity of the top/bottom track in a cold formed steel strapped shear wall. It seems like the tracks are typically the limiting factor when designing these shear walls. I am wondering if the top/bottom track could be considered a built-up member or used as some sort of composite section with the diaphragm.
Any comments would be appreciated.
Any comments would be appreciated.






RE: Cold Formed Strapped Shear Wall Mechanics and Design
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
RE: Cold Formed Strapped Shear Wall Mechanics and Design
Yeah, I have tried this but the track is still the limiting factor. I can have large straps on each side and HSS end posts without creating much of an issue for the wall manufacturer. As for the fasteners, I am just showing a generic fastener but in reality it might be a, PAF or tapcon, or small Titen HD fastener.
There are several means for beefing up the track - nested track, inserting a steel plate in the track, using a tube or channel. You can even by-pass one of the tracks by extending your posts up to the base of the posts above and welding them together. However before encroaching on some of these tactics I just want to make sure I'm taking advantage of what is already there.
In the second sketch with the embed plates, how would you check the slab as a strut there? Use some sort of effective width (say for concrete use your effective slab width rules for 2-way strips)? There is probably a lot of capacity considering it is braced out of plane by the wall.
Now what happens if you change the floor material to wood sheathing or metal deck?
EIT
www.HowToEngineer.com
RE: Cold Formed Strapped Shear Wall Mechanics and Design
RE: Cold Formed Strapped Shear Wall Mechanics and Design
If there's space available, you could run the tracks out beyond the extents of the braced panel. That way, you could split the force between a compression on one side of the post and a tension on the other, effectively halving the force being dealt with in the track. You would need to adjust the tie down detail to suit and be cognizant of the maximum length track available in your market.
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
Here is a sketch of the extended end post:
How do you handle this when it is a bearing wall. Meaning that the infill studs (between end posts) would only be attached at the flanges. Section below:
That's an interesting idea. This is possible it would just decrease your overturning moment arm and increase your vertical forces. So basically you would assign however many fasteners you outside the end post to tension on the track and however many fasteners on the inside the track to tension. So now you effectively have 2*Atrack*F.y depending on unbraced length, etc.
Still no takers on the whole composite section (say we take out the diaphragm)? Maybe it's just not a good idea. Checking the diaphragm as a strut?
EIT
www.HowToEngineer.com
RE: Cold Formed Strapped Shear Wall Mechanics and Design
It shouldn't alter your vertical forces or overturning arm at all.
- For many situations, other than perhaps balloon framing, the stuff sandwiched between the tracks IS the strut in the vertical trussing system. I really just view the tracks as rather long connections between the bracing and the struts. Just like your axial force diagram.
- To the extent that one may rely on the diaphragm to brace the tracks about their weak axis, we're already utilizing some composite action implicitly.
- So long as all of the load is initially delivered to the track, the track would need to deal with 100% of that load for the first few fasteners regardless of composite action.
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
Right, I suppose that is true for any built up section with a side connection. But lets say that I put a gusset in the corner or a really wide strap and a bunch of fasteners directly adjacent to the end post so that I get all the load into the diaphragm with the length of the strap to track connection. Like wise I can pick up all that load in the top track below within the length of the strap/gusset connection. So all is in all three elements before the stap (or gusset) connection ends. Now remove the diaphragm. Do both tracks act together? Need a sketch?
EIT
www.HowToEngineer.com
RE: Cold Formed Strapped Shear Wall Mechanics and Design
Got it. I was imagining the other where you might extend the track under a window or something.
As you've descscribed it, you could definitely make a case for non-composite load sharing between tracks. You've probably got composite action too but that's difficult to demonstrate quantitatively. And really, what benefit is composite action here anyhow? The only perk beyond load sharing is higher buckling capacity. And you're already braced in the plane of the wall by the studs.
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
This seems kinda odd but I'm not sure of a counter argument. The odd part is that If I don't transfer enough load by the time I get to the end of the strap/gusset then bottom track would yield just after the gusset. Maybe the part that seems odd is that I am transferring load a member which is really now part of a system that can't "handle the load".
Next:
What type of diaphragms could you use as a strut and how to evaluate them as a compression member.
XR's to responses to previous questions above?
EIT
www.HowToEngineer.com
RE: Cold Formed Strapped Shear Wall Mechanics and Design
Yup. I'd be hesitant to go down this road simply due to uncertainty regarding the load sharing and the length over which transfer occurs. I'd either introduce more capable tracks and space out the fasteners or or switch to making a more convincing local connections to a diaphragm strut designed to deal with 100% of the load.
As I see it, it's nearly impossible to avoid having something akin to a diaphragm strut. That could be:
- concrete slab or beam.
- steel beam or strut.
- wood or CFM rim.
- track group in balloon frame.
Obviously not plywood nor untopped metal deck.
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
This thread has me thinking a bit more deeply about the mechanics of these details than I have in the past. XR's detail is pretty much a slam dunk at a foundation or slab constituting the base of the braced frame. At upper levels, however, it would get a bit more interesting. The vertical component of the strap force would need to be transferred from the channel connection back to the stud pack via localized one way shear or punching shear. It's totally doable but worthy of some explicit attention to be sure. Go Pack!
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.
RE: Cold Formed Strapped Shear Wall Mechanics and Design
I only cheer for two teams on Sunday. The Chicago Bears and whoever is playing the Green Bay Packers...
Although after this season, why not,
go pack.Edit: Sorry that just didn't feel right.
EIT
www.HowToEngineer.com
RE: Cold Formed Strapped Shear Wall Mechanics and Design