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Narrow Shear Wall - Cantilever Diaphragm

Narrow Shear Wall - Cantilever Diaphragm

(OP)
I have a couple questions regarding situations where you have relatively small lateral loads but not much for shear walls. Attached is a project where they are adding a 3-Seasons room and a family room which will project out from the existing structure. There is not much for lateral bracing. Here where my thoughts on ways to provide LFRS -

Family Room:
  1. Although the aspect ratio of the pier does not meet the 3.5 requirement, detail the shear wall using FTAO including sheating on both sides and blocking / Strapping at the window header/sill.
  2. Check the wall as a cantilever box beam with plywood on both sides. Detail accordingly.
  3. Use the ceiling and/or roof diaphragm as a cantilever diaphragm similar to how you would for a deck/balcony. Provide 'hold downs' at the ends which would actually be connecting into the shear walls which have adequate aspect ratios and using a drag strut to tie into the adjacent wall at the structure.
3-Seasons Room
  1. Possibly try to cantilever the diaphragm but I'm just not too sure I can get that to work or that I want it to work.
  2. Tempting to tie into the masonry but I believe you must provide clearance around a fireplace. Also it is currently shown as 4" brick.
  3. Possibly use an embedded pole type foundation
Any ideas/suggestions?? Or do I need to tell the Arch he is pushing things a bit far.

http://files.engineering.com/getfile.aspx?folder=8...

http://files.engineering.com/getfile.aspx?folder=3...

Thanks in advance, I hope I'm not 'crutching' too hard.

EIT
www.HowToEngineer.com

RE: Narrow Shear Wall - Cantilever Diaphragm

Here are my thoughts. Since you didn't mention what kind of seismic zone you are located in I'll assume you are in a high one...

Family Room:

1) Personally, if you are not meeting the 3.5:1 requirement I would not try and use a conventional shearwall with Force Transfer Around Openings. I suppose you could do it, but you are on an island by yourself going outside of the recommended literature.

2) As far as I know a cantilevered box beam is not an accepted lateral element unless you can argue that it meets the requirements of a Timber Cantilevered Column System and you detail for an R= 1.5. I've never done this before, so I'm not sure what kind of pushback you would get from your building department or if it would even be able to meet those requirements.

3) A cantilevered column system here could work in my opinion. When I've done these in the past they almost always end up being 5x5 or larger HSS tubes so it can be expensive.

-- I think the most obvious solution along this line would be to use a Simpson Strong Wall or an iLevel Shear Brace (which was recently just bought by Simpson). We use the Shear Braces all the time and think its a great product. It's ICC tested and can be rake cut to match the slope. It's essentially just a big piece of LSL that has special holdown assembly at the bottom. I think they run for ~$1000 each.

3-Seasons Room(I'm assuming this is Screened Porch)

1) It doesn't look like you can get a cantilevered diaphragm to work as I don't think you meet the requirements of Section 4.2.5.2 of NDS SDPWS. Even if you could argue a rigid diaphragm I don't think you meet the 1:1 aspect ratios for an Open Front Structure either.

2) I would not try and tie into the masonry. You are correct that there are clearance requirements to combustibles. Additionally, if you are in a high-seismic area this wall would need to be a Special Reinforced Masonry Wall which its doubtful you could achieve.

3) A cantilevered column system here could work in my opinion. When I've done these in the past they almost always end up being 5x5 or larger HSS tubes so it can be expensive.

-- Similarly, to the Family Room, I think the most obvious solution along this line would be to use a Simpson Strong Wall or an iLevel Shear Brace (which was recently just bought by Simpson). They both come in 12" widths I believe. We use the Shear Braces all the time and think its a great product. It's ICC tested and can be rake cut to match the slope. It's essentially just a big piece of LSL that has special holdown assembly at the bottom. I think they run for ~$1000 each. I think since you are very tight to 12" you probably should be using the Shear Brace as you can hang the headers into it with a Header Hanger or Concealed Flange Hanger if you don't have space for a trimmer.

If you go the iLevel Shear Brace route just make sure to spec them long. You can trim them but you can't shim like you can with a Strong Wall.

RE: Narrow Shear Wall - Cantilever Diaphragm

(OP)
jd - Thanks for the response, it's greatly appreciated. I will look into those products. Actually now that you mention it we did use the Simpson Strong Wall on a project a while back but I didn't do much work on that project.

As for seismic - Low siesmic loads, sorry for not clarifying. This is in Midwest, USA, fyi.

EIT
www.HowToEngineer.com

RE: Narrow Shear Wall - Cantilever Diaphragm

RFreund,
I know the problem has already been taken care of, but I wanted to chime in on the FTAO option. When you use FTAO, your height of pier is just based on the height of the opening. But the SDPWS does specify that the wall piers of the FTAO shear wall cannot be less than 2'-0" even if it meets the 3.5:1 height to width ratio. Just FYI.

RE: Narrow Shear Wall - Cantilever Diaphragm

(OP)
Jeff thanks for the info. I wish they had more commentary on this. Or explain the failure modes. Especially for instances of low loads. I mean I'd like even justify the design as a cantilevered built up plywood box beam, use plywood on both sides.

For this instance we did use the shear brace but personally I think a 'plywood wood box beam' would have worked just the same.

EIT
www.HowToEngineer.com

RE: Narrow Shear Wall - Cantilever Diaphragm

RFreund,
I haven't done anything with plywood box beams so I don't have a feel on how well that may work or not. I probably would have gone the shear brace route as well. Around here they are about $300 - $400 depending on the height so they do make a large impact on cost. Well worth the cost especially wen the alternative is steel.

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