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Cold Formed Strapped Shear Wall Mechanics and Design

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.


EIT
www.HowToEngineer.com

RE: Cold Formed Strapped Shear Wall Mechanics and Design

Your mechanics look pretty tight. What type of compression failure do you get? With the tracks braced continually about both weak and strong axes, I'd think you'd be able to compress them right up to the point of yield / local buckling.

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

The lateral force needs to go from the diaphragm into the lower track and then into the bracing and vertical element. So, the bottom track will see all of the lateral load as shown in your axial load diagram. Like KootK wrote, the slab will brace the track. I would use a 12 gauge deep-leg track before using concrete embeds at the braces. What type of fastener is used between the tracks and slab in the first sketch?

RE: Cold Formed Strapped Shear Wall Mechanics and Design

(OP)

Quote (Kootk)

What type of compression failure do you get?
You are right it is mostly a yielding failure as the track is braced about its strong axis at each fastener and about the weak axis at each stud.

Quote (WannabeSE)

I would use a 12 gauge deep-leg track before using concrete embeds at the braces. What type of fastener is used between the tracks and slab in the first sketch?
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

I believe your limiting factor here is weak axis bending of the track and jamb post due to the strap tension. If the load is significant, I generally emebed an MC6x15.3 in the track that is fastened to the slab. This eliminates the problem all together. This issue still occurs at the top connection however.

RE: Cold Formed Strapped Shear Wall Mechanics and Design

I like XR's idea for it's simplicity.

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

Note that my last idea probably wouldn't suit prefabrication and is therefore probably moot.

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

(OP)

Quote (XR250)

I believe your limiting factor here is weak axis bending of the track and jamb post due to the strap tension
Can you elaborate on this. The only time I would have weak axis bending in the End Post is in the "extended end post" configuration. Unless you are referring to eccentricities related to the strap location, but I'm unclear on what that would look like. Also on clear on the track weak axis.
Here is a sketch of the extended end post:

Quote (XR250)

I generally emebed an MC6x15.3 in the track that is fastened to the slab
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:

Quote (Kootk)

If there's space available, you could run the tracks out beyond the extents of the braced panel.
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

Quote (RF)

This is possible it would just decrease your overturning moment arm and increase your vertical forces.

It shouldn't alter your vertical forces or overturning arm at all.

Quote (RF)

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?

- 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

(OP)

Quote (Kootk)

It shouldn't alter your vertical forces or overturning arm at all.
Well, right. But I'm assuming that there is a limited length of shear wall. I don't actually extend the tracks I move the end posts 'inward'.

Quote (Kootk)

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.
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

Quote (RF)

Well, right. But I'm assuming that there is a limited length of shear wall. I don't actually extend the tracks I move the end posts 'inward'.

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

(OP)

Quote (Kootk)

And really, what benefit is composite action here anyhow? The only perk beyond load sharing is higher buckling capacity.
I suppose I really don't need composite action. I really am just interested to see if I can use load sharing. Which I guess I suppose I could. See sketch below. Does this seem right? Basically If I can transfer the force to the different elements within the length of the connection (gusset / strap) then I can distribute the load to all members.


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? bigsmile

EIT
www.HowToEngineer.com

RE: Cold Formed Strapped Shear Wall Mechanics and Design

My version of the statics is shown below. You may be getting at the same thing, I'm not sure.

Quote (RF)

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".

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.

Quote (RF)

What type of diaphragms could you use as a strut and how to evaluate them as a compression member.

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 might be an interesting thread for you to review RFreund. It includes more excellent advice from XR250 as well as a good sketch by MacGruber of XR's channel detail.

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

(OP)
Thanks Kootk I'll take a look.

Quote (Kootk)

Go Pack!
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

Quote (RFreund)

Can you elaborate on this. The only time I would have weak axis bending in the End Post is in the "extended end post" configuration. Unless you are referring to eccentricities related to the strap location, but I'm unclear on what that would look like. Also on clear on the track weak axis.
I did not realize you were using an HSS post, but the mechanics are still the same. The strap is tugging upwards on the track and sideways on the jamb post. The location of this force depends on the required width of the strap and length of welds. Unless your strap is really narrow, your resultant on the track will be away from the jamb post (say 1"). If your strap load is 3000 lbs, you might have 2000 in-lbs or so of weak axis bending on the track. This gets even worse when designers resort to light gage gusset plates yet I have rarely seen anyone consider it. As long as they can get enough screws in the gusset plate, they think they are good.

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