Long Metal Stud Shear Wall with X-bracing

[Ben29]

I am designing a metal stud shear wall with X-bracing. My shear wall is 30ft long and 11ft tall. I was taught to take the wall and segment it into (3) parts, then the shear load in each segment would be V/3. I feel like I am missing something. Is this the correct way to go about designing a long shear wall like this? See attached detail for more info. This is a typical detail that my company would use for this type of wall. I modeled this in multiframe and the shear force (at the top track) was not distributed equally among the (3) wall segments. I cannot find any design examples online for this scenario.

 

[Ben29]

Here is the typical detail...

 

[XR250]

Yes, technically it would be V/3.
Coupla comments about you detail....
You are not showing any shearwall hold-downs - I imagine you need them at each strap intersection.
If the shear is anything substantial, getting your load from the top plate into the straps and back out into the bottom plate is going to be an issue
I would call out the PAF's at 6" O.C. instead of (2) @ 12" O.C. due to c/c spacing limitations Seems like a lot of them. If you are using them for uplift, check the stiffness of you bottom plate
Might want to talk to the installer about his ability to get 1 1/2" embedment out of a PAF. That seemed to be an issue for me in the past. Probably depends on the age of the concrete.

 

[phamENG]

I can't comment on the program directly as I've never used it, but my first suspicion would be that you applied a point load to the end and they're not properly connected in the model. Check you collector and make sure it's continuous, and then distribute your collector load along its length.

The strap to track connections are critical. You didn't include them, so maybe you've got this already - you should be using a gusset plate to connect the strap to the track (top and bottom of wall). Otherwise you're trying to go through a couple little screws in the ends of the chord stud(s). Not likely to check out. You also need shear wall hold downs at each point. These braces are tension only, so you can't try to rely on the opposing brace some how holding the connection down.

Also be mindful of the strap tension in your specifications. I've seen contractors install these with just a couple helpers holding it up and a screw gun. It's important that the strap be pulled tight before fastening. Clark Dietrich and a couple other manufacturers sell/lease specialized tools to apply an appropriate level of pretension to the straps. If they don't, and the building is loaded, the wall will rack until it comes taught. It can be loud, and it can cause damage (depending on how sloppy the installation is).

Another thing to consider. Architects usually hate having the straps on the interior because they can easily telegraph through GWB - especially if they're going with 1/2". That leads a lot of structural engineers to put them on the exterior only. Be careful doing this. It creates an eccentricity at the base connection that can significantly reduce the capacity of the wall.

 

[Ben29]

@XR250 @phamENG - Thank you so much. This is very helpful info. I appreciate it!

 

[DaveAtkins]

For a wall that long, do you even need straps? Can the sheathing do the job?

DaveAtkins

 

[XR250]

you should be using a gusset plate to connect the strap to the track (top and bottom of wall)

I wonder has anyone done an FEA on a gusset plate to see how the load actually gets resisted. Take a strap running upwards to the top right of a wall. Ideally, it is forcing the top track and jamb stud in compression (pulling left on the top track and down on the jamb stud). How do we know it is not doing the opposite and pulling the top track down and the jamb to the left? I believe that is actually what it is happening as the gusset has about zero buckling resistance (unlike a red iron gusset). If that is the case, the weak axis strength of those two components would have to be evaluated.

FWIW, when I was doing light gage design, I would use a red-iron angle or channel in the base and a deep leg, 14 or 12a track at the top. The strap was then welded to those members. Seemed to take a lot of doubt out of the equation.

I would be interested in other's thought on this.

 

[phamENG]

I've used a deep track as well - it works really well with lighter loads since there's room to get the connection in. If the connection needs more room, I've spec'd a 12ga "plate." Gets a bit tricky though - starts to really take up some space if you need it on the interior.

I think that, at least initially, it would behave in a similar fashion to a structural steel gusset. You could apply a similar approach to the Uniform Force Method. That is just statics, after all. The result would be a combination of the two. You'd get horizontal and vertical reactions on both the jamb and the track. I think that's where it breaks down. Since these members are significantly more flexible than structural steel, I think the deflections will happen so fast that the relative stiffness of each member in each direction will reveal itself a lot faster. So, as long as your gusset is sufficiently stiff, I think you'll be able to see the compression loads that we're looking for.

I guess that's your point though, isn't it? A FEA to determine how stiff is stiff enough.