Wood Stud Capacity - Weak Axis Lateral Support 1

[codySTR]

I'm creating a spreadsheet to compare wood stud capacities (based on length, width/breadth, bracing, wood species/grade, etc., etc.). I'm questioning my assumptions for the lateral bracing of the stud. Specifically, I'm questioning the correct application of the NDS 2015 provision A.11.3 (appendix), which states:

"When stud walls in light frame construction are adequately sheathed on at least one side, the depth, rather than the breadth of the stud, shall be permitted to be taken as the least dimension in calculating the le/d ratio. The sheathing shall be shown by experience to provide lateral support and shall be adequately fastened." (note: The main body NDS 2015 provision 3.6.7 pushed me to look at the Appendix provision A.11.3 quoted here)

Generally, if I'm considering a load bearing wood stud wall with at least one side sheathed, I use the full stud height (from top of sill plate to bottom of top plate) for the strong axis length and the center-to-center distance of blocking as the weak axis unbraced length. Based on the quoted NDS provision, would it be acceptable to use the sheathing nail spacing (I'd use the field-nailing nail spacing for general use) as the weak axis unbraced length and use the member depth (i.e. 5.5" for a 2x6 stud) as the "d" value in the le/d ratio?

Edit: I want to add a quick comparison of stud capacities. By my math, using my "general" bracing assumptions (full length for strong axis, 4'-0" unbraced length for weak axis due to blocking at 48" o.c.), a #1 2x6 Douglas-Fir Larch stud has an axial capacity of 3.8 kips. Applying the A.11.3 provision assuming the weak axis unbraced length is kept at 48" (i.e. I'm not assuming the field nailing spacing applies), the same stud now has 5.3 kips capacity. That's a nearly 40% increase in capacity.

 

[oldestguy]

How about what I see a lot these days? Drywall on inside and Styrofoam sheets plus plastic "clap board" siding outside? Usually on 2 x 6's. No blocking. At corners, maybe 1/2" plywood outside. Cavity with fiberglass batts.

 

[BridgeSmith]

I believe the quote from the NDS indicates that the attaching the sheathing fully braces the studs in the weak direction, so the critical buckling mode is about the strong axis. You can look at it in the weak direction for an unbraced length of the nail spacing, but it shouldn't be anywhere close to the critical buckling mode.

From a realistic standpoint, it seems the sheathing acting as a diaphragm should provide considerable vertical load capacity. I'm not sure what the code provisions have to say in that regard, though.

 

[codySTR]

HotRod10,

Thanks. That's how I'm interpreting it. Seems right to me.

 

[BridgeSmith]

Well oldestguy, the configuration you mention would probably not qualify as being braced, but if the vertical load is relatively small, which it typically is in residential construction, it may not be necessary for the studs to be braced for vertical load. The plywood at the corners is probably providing shear resistance to prevent racking of the wall.

 

[JoshPlumSE]

I believe all that code provision is trying to say is that sheathing is stiff enough to prevent weak axis buckling of the stud. Even if it's only sheathed on one side. Hence, your use of the blocking spacing would be considered a bit conservative.

Oldest guy brings up a good point about drywall on one side (instead of sheathing). Drywall is still pretty stiff, so I would consider it sufficient to brace the stud. But, I wouldn't generally consider the foam insulation.

Note: members that are continuously restrained on one side, but not on the other don't really fail by weak axis buckling. But, rather by some kind of partially constrained torsional - flexural buckling. For a 2x4, I wouldn't worry about the bracing / sheathing only being on one side. But, if you get much deeper, then I might not feel as comfortable with the assumption that the unsheathed side is braced.

 

[codySTR]

JoshPlum,

Good points. I agree.

 

[BridgeSmith]

Thanks JoshPlum. I know the drywall itself is plenty stiff enough; just wasn't sure about its local crushing strength around the nails or screws.

 

[Celt83]

CodySTR: give this thread a look also may prove useful for your spreadsheet.

https://www.eng-tips.com/viewthread.cfm?qid=435497

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[codySTR]

Celt83: Holy moly. That is one heck of a thread! I've got some serious reading/investigating to do. Thanks!

 

[Celt83]

quick heads up you got me curious so I'm adding in an option to completely turn off the sheathing in my program so the weak axis can either be considered unbraced or braced at the set blocking interval, will probably be uploaded in an hour or so.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[msquared48]

One point I would consider, alluded to in the '88 UBC, but not strongly emphasized since, is the tendency of the nails or screws in a GWB or GSB wall to carve out a gap around them during a seismic event, leading to the need to re-nail gyp shear walls after the event (which is seldom done).

This will lead to a reduced capacity of those SW types to provide the necessary lateral support in the weak direction if not maintained properly.

And since many buildings actually have as-built drawings denoting what walls are shear walls, and what walls are not, further confusion and more unknowns enter into the fray.

Hence the fact that I now only use ply or OSB shear walls...

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

Celt, since that older thread is closed; do you have any references on using python and getting your script running? I was looking at that thread a few weeks ago and annoyingly wasn't able to get your script to run. I'm fairly computer savvy but I wasn't able to figure out what I was missing as I don't use python. I get a flash of a command prompt window opening and closing and that's it. Ran out of spare time to try to solve it.

Got any references to figuring this out?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[Celt83]

TehMightyEngineer:
I use a standard install of Python-XY version 2.7.10.0:

https://python-xy.github.io/downloads.html

If you double clicking the script to run it and its flashing a command prompt and closing you might be missing one of the pre-reqs, try navigating to the directory with an command prompt/terminal and type python wood_stud_wall_gui.py that will keep the prompt open so you/I can maybe see whats crashing it.

Alternatively if your on windows I can get you a compiled version a bit later today if you'd like.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[sticksandtriangles]

I tried to account for this in the mathcad sheet that I had posted in the thread that celt83 linked.
We had one engineer in our office who was pretty adamant about this being an option.

S&T

 

[codySTR]

sticksandtriangles,

While your consideration of lateral restraint for the calculation of bending capacity is important, I was specifically referencing a NDS2015 provision that pertains to column-buckling and the determination of CP, the column stability factor. If you're already aware of this distinction, please ignore me.

 

[TehMightyEngineer]

Celt:
I downloaded Python 2.7.11 (64 bit); but don't believe I have the "xy" version you referenced. I'll uninstall and reinstall that version you linked.

After reinstalling and running it from a command prompt she's working. Thanks!

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[Celt83]

Sticks: If I've got it correct codySTR is looking at the weak axis bracing only being provided by the blocking in the wall in lieu of being considered fully braced with sheathing attached, which can significantly impact the column stability factor Cp.

TehMightyEngineer: Python-XY just comes with a bunch of bundled modules, the ones I take advantage of the most are SciPy, NumPy, itertools, and Matplotlib so in theory you could do a bare python 2.7.11 install and using the pip install tool install each of those other packages and be up and running, assuming non of the newer versions of those modules broke any functionality.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[TehMightyEngineer]

Celt82: Yeah, I knew you used Matplotlib so I had that installed; but was missing the other 3 so that's likely why it wasn't working.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[codySTR]

Celt82 (or anyone who can help):

Do you have a link to help me understand how to "open" your program? Or could you roughly explain how I can go about that? I've downloaded pythonxy, but I have almost no idea how to get from the little python(x,y) home window (the tiny window/box with tabs "shortcuts", "documentation", and "about"; options under the "shortcuts" tab such as "spyder:... options:..." and "applications:... IPython Qt console..." and so on) to using your program.

I've read some things about using command prompts and/or "live" interpreters... I'm lost

Any help would be very much appreciated!