Wood shear wall FTAO 1

[focuseng]

General talking point: Anyone know of limitations to the aspect ratio of the side segments on a wood shear wall when the Force Transfer Around Openings method is used? Published or otherwise? I know the limits for the perforated and segmented method are documented are are reasoned to limit wall flexibility. The FTAO method is not so codified though.

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MAP

 

[focuseng]

OK, so I found some reference to the aspect ratio as applying to the "wall pier". The wall pier is defined as the segment immediately adjacent to the opening and the height of the "wall pier" is the height of the opening. So using a 42" high window as an example and allowing for the 3.5:1 aspect ratio I could potentially have a 12" wide segememt of wall to use in my Shear wall including the window width even in a 10'. Looking for comments and references to this...

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MAP

 

[focuseng]

So maybe answering some of my own question. CBC 2305.3.8.1 (California Building Code) as far back as 2007.

"2305.3.8.1 Force transfer around openings.

Where shear walls with openings are designed for force transfer around the openings, the limitations of Table 2305.3.4 shall apply to the overall shear wall, including openings, and to each wall pier at the side of an opening. Design for force transfer shall be based on a rational analysis. Detailing of boundary elements around the opening shall be provided in accordance with the provisions of this section[see Figure 2305.3.5(b)]."

Interestingly this is not in the the IBC-2012 or as best I can tell NDS-2012 or SPDWS-08.

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MAP

 

[OldPaperMaker]

Check out SDPWS-2008, Section 4.3.5.2. This has the limitations for Force Transfer Around Openings (FTO)

 

[focuseng]

Excellent! Thanks OldPaperMaker.

I don't know why I couldn't find that before . SDPWS 4.3.5.2 gives 2'-0" as the minimum pier length regardless of aspect ratio -Which is an additional constraint from the CBC provision.
I see 4.3.4.2 applies the ratio table to the piers and defines them.

Anyone know a rational basis for the 2' limit?

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MAP

 

[RFreund]

I think the 2'-0" limit is new and just based on test results.

For these narrow situations I've always wondered how a built up plywood box beam (cantilevered up from the floor) with hold downs would work. The problem might be web buckling and am not sure there is a calc for that but it would be an interesting approach.

These shear walls keep getting narrower and narrower....

EIT

http://www.HowToEngineer.com

 

[focuseng]

Yes narrow is in demand. But also the issue is, can the shear wall length start at the corner or do I have skip around the windows 100%. Say I have 8' of windows starting 18" from the corner and I have a 14' long wall overall. My possible 14' shear wall just went to 4.5' because of 6" (18" is less than 24" -see the code). That is a big deal in hold down calculation and anchorage. I can still work with 4.5' yes, but now I maybe anchoring into footings (what crazy nonsense us engineers can think up!). Yes there are other options... always options. Getting architects, owners and contractors to understand anything, much less appreciate the difference between 18" and 24" because of a code provision, is all near impossible however.

So ranting done. 2'-0" limit seems somewhat arbitrary while being mostly reasonable. Anybody familiar with the source of this?

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MAP

 

[AELLC]

If the ratio of the 18" segment is less than 3.5:1 for the height of the window, then why don't you use the perforated method?

 

[focuseng]

Well I don't have an 18" segment or pier -it was only hypothetical for the discussion so it is not less than 1:3.5 but way more. For a typical 8' wall, 1.5'/8' = something like 5.33. I must have 28" for an 8' wall. The segment aspect ratio on a perforated shear wall is based on the overall height of the wall not the window opening - So I can't count the 18" segment at the end or the length of the windows using the perforated method or include it in my wall length because I must end with a legit 1:3.5 segment. Most walls are over 8' now-a-days in my experience. Additionally, the shear capacity of the wall could be degraded (or not) depending on my % of opening height. So a perforated wall for one reason or another isn't going to be an option and that is just the way it is to eliminate any confusion or red herrings. No portal frames or engineered shear panels etc either...

I just want to engineer the nuts off a wall because I can (it is a sick pleasure but a darn good one). Let's say for the sake of argument that I don't care about anything other than discussing FTAO and I am just curious about how the provisions of the limitations came to be for the pier size. Guess I got more digging to do to satisfy this itch. Most of the stuff out there seems to be preoccupied with modeling and testing comparisons. I don't have a good juicy report where they say: "2' is best and 18"... meh, not so good".

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MAP

 

[RFreund]

haha, nice.

Take a look at M410 report from the APA. Lots of testing in there.

http://www.apawood.org/level_c.cfm?content=pub_searchresults&pK=M410&pT=Yes&pD=Yes&pF=Yes&pubGroup=

Personally I'd like to learn more about their "advanced" modeling techniques, which they say are not "reasonable" for the typical practicing engineer... I'll be the judge of that
Woodworks has a presentation regarding the M410 report which is OK I suppose. I thought the 24" came from the M410 report but it does not appear so. They actual test a wall with an 18" segment (wall 12). Does the commentary in the provisions for wind and seismic say anything about it?

EIT

http://www.HowToEngineer.com

 

[focuseng]

Yes that is one of the reports I have looked at too. I think the software is referenced by name somewhere and I tried to search it. My guess: it is a very basic modeling software which is more code than show put together by some grad students. So when I say "basic" I mean "not user friendly". Probably a text only input and output kind of thing which doesn't represent the height of polish we are used to in our software, you know "not reasonable [to use]".

Nothing in the commentary or even a reference tag specifically. There is this document in the reference list which looks like it might be close to what I am looking for but not really helpful ultimately because it seems to support not using 24" as a limit:

http://www.apawood.org/pdfs/TSD/T-Reports/T2005-08.pdf‎

Hmmm.

My ultimate reason is to know if I should put a lot of emphasis on 24" being safe or if it was just some back room compromise and it is a nice round number and increment of blah blah blah. What if I have 23" because the R.O. of the window is just so. and the architect says "dammit you are screwing up the symmetry in this room and EVERYBODY can tell its wrong, it is just an inch. what's the big deal". to which my response would be: [dumb sheepish looks]. I love that feeling.


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MAP

 

[OldPaperMaker]

Focuseng,
If you haven't found an answer to your question about 24" minimum wall segments you might want to try the AWC Help Desk at
(202) 463-4713 or the apawood Product Support Help Desk at (253) 620-7400.