Stud axial force from gravity load in wood frame shear walls? 1

[drile007]

Hello,

I’m wondering how to correctly model wood frame shear walls (sheeted from both sides) to get best estimation of axial forces in (edge) studs.
I’m working on 4 storey building where gravity load will govern the design since there are low wind and earthquake forces. I’ve decided to model it with shell elements, since floor plans are not regular through elevation (wall don’t stand directly on each other).
To design edge studs on axial forces I’ve tried two models:

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1)Walls are modelled just with shell elements (with E and G of sheeting boards).
2)Walls are modeled with shell elements (with E and G of sheeting boards) and vertical line elements to model studs.
Results are quite different? In second model I’ve got high axial loads in studs and much lower in sheeting elements. The difference is not negligible.
Do you thing that if I model studs with line elements, their axial stiffness is too high?

Thank you for all your comments and suggestions.
Also, any direction for further readings is most appreciated.

 

[KootK]

Ideally, I think that you'd want no axial load in the sheathing. We typically assume that the studs take all axial load and the sheathing only shear. In your modelling effort, you may want to give some account of a) nail slip and b) that sheathing is not generally axially continuous. It's installed with small gaps between panels.

I'd recommend analyzing your walls either by hand/spreadsheet or with a software package purpose built for wood like Woodworks or Risa.

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.

 

[XR250]

I wish I has enough fee in my wood jobs to do this :>

 

[RFreund]

I don't want to get off topic but I've really wanted to design wood shear walls with some sort of elastic method P/A +/- M/S, even it it means multiple hold-downs in a wall. I mean lumping all the load on the end posts just doesn't seem right. But then again, maybe that is closer to what the behavior actually is.

Also Ditto what XR said.

EIT

http://www.HowToEngineer.com

 

[mike20793]

My thoughts:

1. The 2015 SDPWS has provisions for using the sheathing for uplift on studs. There is some pretty strict nailing requirements and you have to deal with some eccentricities, but I assume the same logic could be applied to gravity loading.

2. Wood is cheap and often times contractors would prefer to add studs than have complicated details. Contractors dealing with wood often deal with only wood and like to keep it as simple as possible.

3. Ditto what Koot said about gravity loading on your sheathing.

4. The rigid body assumption for overturning is more than adequate to estimate the stud loads. An FEA model would have to include load distribution through the plates, nail slip through the sheathing, etc to be truly accurate and worth it.

5. If you want to use multiple studs to resist the compressive force from lateral loading, you need the same amount to resist the uplift force. This means more straps or holddowns and more installation time and more detailing time and more cost.

6. The sheathing doesn't line up with the sill plate, or, sometimes the edge of slab, so you will need to transfer the load the foundation/slab somehow.

7. How does nail slip under gravity loading affect the cladding?

There's more that will come to mind. Wood is so cheap and relatively easy to analyze in a spreadsheet, so modeling it in anything other than RISA or Woodworks is unnecessary in my opinion. That being said, I'd love to get a peak at the results if someone did model it.