Design of wood frame shear walls with openings- perforated vs force transfer method

[jsharkbait]

Hi everyone,

We are designing a 3 story wood portion that sits on one story of concrete; the lateral system is concrete shear walls for the concrete and light frame wood shear walls for the wood portion. Due to the building geometry I need to use the exterior walls as part of the lateral system on the to have at least 3 sides of the diaphragm supported.

Using the NDS Seismic & Wind Provisions 2015, the perforated shear wall method seems pretty simple but it comes with some limitations, one being that the amount of wall above and below the opening is the same. For me, it's 6" off- I am weighing up whether or not to ignore this, but in the meantime I wanted to explore the force transfer around openings method.

I have looked at a number of design examples including a nice spreadsheet produced by the APA, but both the design examples and the spreadsheet only look at one story at a time. The force transfer method derives of all the other forces in the wall including the straps around the windows from the hold down force at the end of the wall. For my lowest story, the cumulative shear in the wall and the hold down force at the end of the wall is obviously much higher. If I analyse this lowest wall I feel like I need to run two scenarios and then somehow join them together:

1) Applied cumulative shear force at the top of the wall to calculate sheathing requirements
2) Applied dummy force to replicate the hold down force at the end of the wall

Doing 2) gives me crazy forces around these openings and it doesn't feel correct. Am I doing this the right way?

As a side note, we had a previous design of this building from another engineer. I checked what they had used to see if I was heading in the right direction, but they had only a couple of typical notes about interior and exterior walls all being sheathed in gypsum with minimal connections and hold downs. I understand the code allows this to resist forces but their drawings just seemed remarkably light...

Thanks.

 

[JoshPlumSE]

Huh, I'm not certain I fully understand your question.

1) Applied cumulative shear force at the top of the wall to calculate sheathing requirements
2) Applied dummy force to replicate the hold down force at the end of the wall

I'm pretty sure your item 1 (apply cumulative shear force at the top of each story) is the correct way to calculate the sheathing requirements as well as any tie requirements around the openings.

My guess is that you're not quite understanding the method fully when you say that you're having trouble with the hold down forces. The moment from the floor above is resisted as just a tension / compression force couple at the chords of the wall. Add that to what you get in the chords from the shear (i.e. item 1) and you should be set with your design. This isn't "dummy forces", it's just the FTAO method.

 

[dold]

Ditto JoshPlum.

When using FTAO, you can design (with some caveats in certain situations) the hold downs at the ends of the wall the same as you would design a plain/segmented shearwall, independently of FTAO. But of course you still have to use the FTAO calcs to come up with forces needed to design your sheathing and straps.

We try not to use Perforated because (and I'm still kindof fuzzy on the reason for this) you have to provide hold downs along the entire length of the segments capable of resisting uplift that is equal to the applied horizontal shear in the wall. At least that is my understanding... [URL unfurl="true"]https://seblog.strongtie.com/2014/12/wood-shear-wall-design-example/[/url]

 

[JoshPlumSE]

I fully understand the concept of FTAO vs perforated wall design. I just didn't fully understand what was causing confusion to the OP.

I should point out that my response was only in response to the FTAO portion of the OP's post. While, I don't have my SDPWS in front of me, I believe the perforated method is not allowed for 3 story walls.

 

[dold]

Yea i'm still not entirely sure what he's asking. OP, you'll still have to have some sort of hold down between stories connecting the chords (posts) of your shearwalls together. Whether it be a coil strap, regular hold down (simpson HDU, etc.) or a continuous holddown system. Is that what you're asking about?

 

[jsharkbait]

My confusion was that when I applied the cumulative shear force at the top of my lowest wall the hold down force at the end of the wall was obviously less than the cumulative hold down force for the entire 3 story shear wall. Hence me assuming I needed a dummy force to replicate the hold down force as this is what is used to calculate the strap forces...

From your replies it seems that I would just apply the cumulative shear at the top of each wall, get the sheathing and strap requirements based on that story only and then sum the hold down/chord forces at the end of each wall and tie them together at each floor level. Am I understanding that correct? It makes more sense now I think about it.

Thanks for your replies.

 

[dold]

Yes so when youre looking at the lowest floor you will have the cumulative shear force applied at the top of the lowest wall PLUS the overturning moment associated with the shear applied at the top of the walls above. You'd get the same result if you draw a sketch of all three walls stacked and apply the individual story forces at the top of each level, then sum moments about the base Ma = [V1*h1 + V2*(h1+h2) + V3*(h1+h2+h3)]. Then of course your hold down reaction at the foundation would be (Ma-Mr)/length_of_wall, where Mr is the resisting moment due to gravity loads. If you're looking at seismic loads don't forget to modify your dead load by the vertical seismic effect (0.6 - 0.14Sds)D + 0.7QE (effectively reducing your resisting moment)