Bracing 18' Light-Frame Wood Shearwall At Mid-height to Reduce Aspect Ratio? 1

[swooneyWOODSTRUCT]

I have a 3.5'x18' shearwall I am trying to get to work (No diaphragm at level 2 on a custom home with daylight basement. If I brace the wall out of plane at mid height with a very stiff member (where a floor diaphragm SHOULD have been (thanks architect)) can I justify a smaller aspect ratio? Basically my question is are the aspect ratio's given in the NDS just to stiffen the wall out of plane or are they to force a shear action rather than flexure in a shear wall?

I have also been talking to Simpson Strongtie about using steel Strongwalls but it is not looking good on that route because they don't have testing for three story Strongwalls. I would basically be putting a balloon framing Strongwall over a single story Strongwall in one area and a balloon framing strongwall over a GLB in another area where the upper 2 levels are supported on beams.

HELP

 

[jayrod12]

I feel that a horizontal support at mid height, a girt as I would call it, would break it up into essentially 2 walls for that check. But that horizontal support would have to span to something competent for the load it would receive from the studs for out of plane wind.

Dont we all do that at outside walls of stairwells in wood framed buildings?

 

[r13]

jayrod's response make good sense, but I would call the girt a beam, at least strengthened to resist the out plane force.

 

[phamENG]

I think we're talking less about a girt (assuming the studs can handle the out of plane without intermediate support), and more about a collector spanning between two diaphragms. (This could be what jayrod means and we're confusing regional nomenclature.)

Assuming there is a diaphragm at that level somewhere and at a reasonable distance form this wall, you may be able to run a collector through the wall to dump the diaphragm load into it at that level.

Can you post an elevation view of the wall (not just the shear wall panel - the whole wall) and the plan view at the second level where you'd like to have the diaphragm? FTAO could be an option, but without knowing more it's hard to say.

 

[KootK]

1) This is bloody fascinating as I've always wondered about the reason for the aspect ratio limitation but have never seen anything proofy to back up any of the possible theories including:

a) Limits of applicable testing.

b) The need for a sloppy, racked set of sheathing panels to be kind of "tuned up" intermittently by the introduction of a a collector element of some kind. This has been my assumption to date and, I believe, what phamENG's comments have been speaking to. In my mind, all that would have to happen is for blocking & strapping to be installed across the panel horizontally to break it up.

c) This out of plane bracing business is new to me but I see where it's coming from. That said, I feel that this is the least likely candidate. I'd think that out of planes issues, if they exist at all, could easily be rectified by using deeper studs etc.

2) Obviously, one can indeed make a shear wall of any aspect ratio by simply breaking it up with floor levels. So the question is, what is so special about braking it up with floor levels?? Both & [c] above speak to this in different ways. In the absence of something definitive, I'd be tempted to apply both approaches simultaneously: [blocking + strapping + diaphragm].

3) Disclaimer: I'm 100% making this stuff up based on my own logical inferences alone.

 

[KootK]

Actually, upon further inspection, I see that phamENG and I are proposing different things, mine being the most permissive of the bunch so far. I stand by my proposal just the same.

 

[swooneyWOODSTRUCT]

I'll attach the wall elevation here as well as the plan sheets because it is a confusing building. NOTE that the center of the building above basement is bumped out on columns away from the rest of the exterior of the building. The shear walls are outlined in red.

 

[swooneyWOODSTRUCT]

Plan sheets:

 

[swooneyWOODSTRUCT]

phamENG,

There is no diaphragm to run a collector to, unfortunately.

jayrod12,

The studs here should be able to take the out of plane wind load, I am only worried about breaking this wall into two pieces somehow to get around aspect ratio limitations.

 

[r13]

To be fore warned, I am no wood designer.

I think by utilizing the girt/beam to reduce the aspect ratio, you are running into two levels of shear wall situation. You might need to provide extra details to make sure the walls act as an unit, or check each level individually.

 

[phamENG]

Alright. This architect has given you a fantastic opportunity to flex your engineering muscles.

I don't think trying to force the little slivers to work out on the gable end wall is a good idea. The resulting force couples at the hold downs, which land on either side of the columns, will probably make the beam to column connection an absolute nightmare.

I'll bring up FTAO again - force transfer around openings. I'm a recent convert to this method, and admit I haven't attempted it on anything quite so ambitious as this. I think it could work, though. I also think it forms the fundamental basis for KootK's strap suggestion.

I notice the big timbers are called out as a truss at the roof level, and you've got some more timbers in there. Any chance you could play with that a bit and make a trussed moment frame? Kind of out of the box, but may be doable. A lot of people seem to forget that structures don't have to be confined inside walls - a lot of the pretty architectural features of old buildings with large, open spaces were actually structural elements.

 

[swooneyWOODSTRUCT]

phamENG,

I don't know that much about FTAO but I unfortunately don't have the budget to learn it for this job. We thought about using the timber truss as a last resort but it would certainly complicate the job by mixing two different LFRS (the timber frame doesn't cover the whole building). I am aware that the beam to column connection is going to be a nightmare, this job has probably 30 nightmare connection details so what's one more? lol

I spoke with a Branch Engineer from Simpson today and he was able to give me a solution. It's not pretty but it should work. We have to put strongwalls on top of steal beams in the walls.

Thanks for all your help everybody!

 

[emmgjld]

Please, Don't stop the discussion! This is getting interesting.

 

[r13]

I start to wonder how this "shear wall" thing to work out.

 

[phamENG]

Chances are, you'll never have a budget that supports learning it. But if you do a lot of wood, I certainly encourage you to read up on it some evening while your spouse/significant other/dog/gold fish watches TV. First time I used it, I took an "impossible" wall of windows and made a very reasonable shear wall with manageable hold down forces. I also didn't have the budget for it, but it was the difference between the client having to pay an arm and a leg for some custom fabrication that I'd waste 4 times as many hours on trying to make the contractor understand during construction, or give them something that's easy to construct and not make quite as much money as I'd hoped on it.

Be sure to post the details once you have them worked out, and then pictures when they build it. Sounds like it could get interesting.

 

[WeezerMike1]

Hi all,

Before you guys were engineers, were some of you Jurassic Park scientists?

A wall that's 18ft tall and 3.5' long is not even a shear wall. It's a cantilevered column.

I know it's hard to meet the architect's expectations, but sometimes you need to sit them down and explain to them the corner that they've painted you into. A residential architect should be aware of shear wall aspect ratio requirements, so you'd be doing them a favor to educate them. They need to know that they need to be okay with longer walls, or with a more expensive lateral system like a moment frame.

 

[r13]

Weezer,

In a sense you are correct, individually, it is more a pier than wall. However, the OP denotes the entire south wall as a shear wall, in which, after discounting the openings, only 4 piers are effective in resisting the shear.

 

[dhengr]

HunterWOODSTRUCT:
I think you should follow the lead of your neighbors to the north and the south. In CA and WA, you see a lot of bolted or field welded moment frames in these kinds of situations. The Arch. just can’t remove all of the structure because he thinks it looks sexier, and then expect you to make the bldg. stand up under today’s wind and EQ codes/stds. It finally has to cost him something, someplace, to make an ‘all windows’ upper levels, and soft-story ground floor level to stay standing, as if by magic. Hell, you can’t make (maybe can barely make) those kinds of shear walls work at the O.H. door opening of a two car garage, one or two stories high, let alone to support the whole house at one elev., with two or three story high, 3’ wide walls, which don’t line up in a single plane. Of course, almost anything is possible, but some things cost a lot more than others, and the Arch. must realize this, instead of expecting you to fix it and cut cost too.

 

[North2South]

I’d be tempted to use an interior wall(s) as my shear wall and cantilever the diaphragm to support that side of the house.

 

[molibden]

I practice in Europe but logic is the same. In this case we go steel moment frames or CLT walls.