CFS Flat Strap Wall Design

[hdp321]

I know when designing wood panel shear walls it’s common to utilize the floor framing DL and self weight of wall to counteract overturning, at least a portion of that load is used.

Is using the DL common practice for Flat Strap shear walls in metal stud building as well? Most of the software on the market does not account for this unless you enter chord compression loads manually, so it got me thinking that perhaps the loading is not commonly considered in CF buildings.

 

[DaveAtkins]

You certainly can use dead load to counteract overturning. Of course, you must use the proper load combination, which means 60% of dead load for ASD, or 90% for LRFD.

In order for the wall to overturn and begin to put tension in the holddown, it must lift the dead load.

DaveAtkins

 

[phamENG]

You need to take a closer look at the load path here than you might on a WSP shear wall. A shear wall using wood structural panels has lots of individual, rigid shear panels that work together to create a shear wall. The dead load spread out over the wall is applied to the panel below it and contributes to the overturning resistance of the individual panel. Taken all together, the spread out dead load contributes to the overall stability and overturning resistance of the wall.

A flat strap light gauge shear wall is a little different. You have a very discrete load path for your lateral and over turning loads. Also, if I remember correctly, AISI recommends NOT fastening the flat strap to each stud, so those connections really only occur at the ends of the shear wall. So you need to be able to justify that the dead load resting on the middle of the wall will resist the uplift caused by a lateral force at the end of the wall. It can be done, but it's not as cut and dry as a WSP shear wall.

 

[ChorasDen]

pham, not to get too off topic, but I'd like to discuss this a bit more. In the past, I've always considered a WSP wall as a discrete, monolithic structure when it comes to resolving overturning forces for the hold down at end and the dead load overtop. The more I think about it, the less I like this approach for anything other than segmented wall. I just don't like the idea of considering the rigidity of a perforated wall sufficient to make the section act monolithically. What are your thoughts?

 

[phamENG]

My thought process goes another way. It's not that I view the segmented wall as rigid monolithic, it's that the load path is sufficiently diffused through the wall that the distributed dead load can have a meaningful impact on overturning throughout the length of the wall. Take the crude sketch below as an example. If we have a super simple segmented shear wall L=12' H=8', you can build it with three sheets of WSP. If we look at the middle panel, it has to be in equilibrium. That occurs from shear transfer between the panels to resolve the overturning couple, and shear at the base. If there's dead load on top, the dead load directly over this panel contributes to the overturning resistance of the panel. The result is less uplift being passed to the next, and less being passed to the hold down. Of course the sum of these results is equal to assuming the whole thing acts monolithically. So any disagreement I might have with you here is academic, not practical.

For perforated I think I agree with you - the dead load over headers should not be counted in full. From the sketch below, you'll see that the benefit of half of the dead load over the opening is lost. The opposite pier, however, would benefit from half of the dead load above the header as the dead load reaction from the header and the tension reaction vectors are coincident. So the summation of those vectors will be a net reduction in tension at the edge of that panel.

For an FTAO wall, though, I think I'd still count all of it. I haven't gone too far down this rabbit hole, but I rely on the Diekmann method and I understand it to require fixity of the segment above the opening to the piers next to it. If we're relying on enough rigidity to transfer moments for the lateral load through that joint, it seems reasonable to assume the dead load reaction will be resisted by the same mechanism.

 

[hdp321]

I appreciate everyone's feedback. I'm just looking for insight and theory with flat strapped walls. I believe they would perform as phamENG described.

 

[phamENG]

hdp321 - sorry. That last post was a response to ChorasDen and was about WSP.

Flat strap needs a similarly stiff distribution element across the top of the wall to what I was describing for an FTAO wall for it to really contribute much dead load to the mix.

 

[DaveAtkins]

Do you have GWB sheathing in addition to the flat strap bracing? I can't imagine a wall with bracing but without sheathing. If you have sheathing, it will hold the wall together so you can count on the full DL against overturning.

You can go through the mental gymnastics of contemplating how the wall can support the dead load as it is overturning, as the bottom of the wall lifts upward. Unless you have a lot of overturning counteracted by a lot of dead load, I think the wall can do it.

DaveAtkins

 

[KootK]

We discussed this in some detail here: Link.

There, I summed up my views on this as repeated below. I don't believe that using internal dead loads for OT resistance is appropriate for either:

1) Wood panel shear walls, including segmented shear walls.

2) Cold formed strap shear walls.

I used to espouse similar practices. Then, someone here at ET produced some research from the University of Washington pertaining to the exact case that I was interested in at the time: a shear wall with a heavy post load in the middle of it. I can no longer find the research paper but the result stuck with me: the wall doesn't really feel the post load with respect to it's shear wall behavior. I've sort of reconciled myself with that as follows:

1) The axial stiffness of the post sort of shields the sheathing from "feeling" the concentrated load under what is primarily a racking deformation mechanism.

2) We tend to always think in terms of things that are flexurally flexible rather than shear flexible. Shear flexible stuff like wood shear walls do this business where everything racks to the side by way of rotation of the individual panels. This, mechanically, is quite different from flexural "flexing".

3) I think that we only need boundary members at the ends of shear walls because, there, there's no other way to achieve equilibrium of those last shear panels. We need the boundary members for overturning too, of course, but that's just a different side of the same coin.

I know, I really wish that I could find that UW paper...

By a similar logic to that above, I'm not convinced that a shear wall feels an intermediate hold down in the way that we'd like it to.

 

[KootK]

When I consider the strapped CFM situation, this is how it looks to me: