Upper Floor Perforated Shear Wall Sill Plate Anchorage

[jeffhed]

I frequently use perforated shear walls in residential engineering. However, I only use it in applications where the shear wall sill plates will be anchored to concrete so I can get the anchor bolts to resist shear and the uniform sill plate uplift force that is required by section 4.3.6.4.2.1 of the 2008 NDS Special Design Provisions for Wind and Seismic. Where our shear walls are on a wood floor of an upper story, we specify nailing for shear transfer from the sill plate into the rim joist. Just looking at the uplift values for nails you probably never get much to work, not to mention, the code typically prohibits using nails in withdrawl, especially for seismic loads. So I came across this document below that specifically states that nails can be used for shear and uplift. Is anyone else using nails for withdrawl on upper floor perforated shear walls? The link for the document is below.

http://www.awc.org/publications/Papers/WDFPLine.pdf

 

[MiketheEngineer]

I generally use a Simpson or equal PDH5 or TD5 or whatever works. Then you can follow a complete load path directly to foundation.

 

[jeffhed]

Miketheengineer,
Are you talking about hold downs at the ends of the shear wall? I am talking about sill plate anchorage between the ends of the shear wall that the code requires for perforated shear walls.

 

[RFreund]

I have used combination tension and uplift. There is a detail that seems to work pretty well where you put layer of plywood on the outside of the rim joist and nail the ends of the sheathing to this. The 'sistered' piece of plywood on the rim joist transfers the uplift forces as opposed to perpendicular to the grain loading on the rim joist. See figure 5 attached.

EIT
www.HowToEngineer.com

 

[MiketheEngineer]

Depending on the loads I have used them and also what RF says

 

[jeffhed]

RFreund,
I have looked at the tables in the document you have provided, something similar is also in the 2008 NDS Special Design Provisions for Wind and Seismic. I didn't say before, but I am in a higher seismic area where seismic sometimes governs. the provided into is for wind loads only. Maybe when seismic governs there are no perforated shear walls on the upper floor because you can't provide the continous uplift anchorage?

MiketheEngineer,
So if your loads are low, you specify nails for the sill plate uplift anchorage? Do you specify the sill plate nailing for perforated walls differently than for regular shear walls?

 

[RFreund]

Hmmmm, interesting.

You are correct in that they do specifically say "Combined shear and uplift for resisting Wind loads"
The 2008 does provide an example in the commentary for determining the nailing pattern (C4.4.1-1) for shear and uplift.
I would think the previous detail (Fig 5) would be 'better' than nailing into the sill plate but I'm not sure if these details are acceptable for EQ loads.
They don't specifically say that perforated shear walls are not allowed on upper floors in high seismic areas. Maybe this means that they want strapping at X' intervals. T'=V/(Co*Li's)*X'
So in the end I'm not sure what is correct.


EIT

http://www.HowToEngineer.com

 

[MiketheEngineer]

Yes they can be different.

 

[jeffhed]

RFreund,
I have thought of the strapping before too. I don't know of any other way to do it where seismic loads are involved. But at that point, I might as well design a shear wall with reinforced openings. It would probably require the same amount of strapping.

MiketheEngineer,
You must have pretty low loads on the upper story and in a low seismic area? Let me give an example of what I run into that forces me into using a shear wall with openings designed for force transfer instead of a perforated shear wall. If you try to get a perforated wall to work on the upper floor with a uniform shear load of 140 plf (adjusted with the Co factor). Using 16d common nails (not gun nails) according to the NDS I can get 32 lbs/in. The amount of penetration into the rim joist and floor sheathing after the sill plate would be 2" which would only give me 64 lbs per nail. (64 lbs/nail)/140 plf*(12) = 5.5 in. o.c. nail spacing. Does that look like the numbers you see? I would love to used perforated walls on the upper floors, but when I run the numbers I am having trouble justifying their use. Am I missing something?

 

[RFreund]

Jeff - good point on the strapping.

In your example - I'm confused with your detail "the amount of penetration into the rim joist and floor sheathing after the sill plate" this is a withdrawal loading condition? If so 5.5" on center does not sound unreasonable. Out of normal framing practices we would usually nail walls down with at least (2) 16D nails every stud space (8" oc). I don't think going to 4" oc would be extraordinary. The only question is whether you are ok with the withdrawal loading condition.

I was thinking the sheathing was continuous from sill to top plate or sill above. In which case you would be nailing into the edge of the sill.

EIT

http://www.HowToEngineer.com

 

[jeffhed]

RFreund,
Typically here if we are framing 9'-0" walls the sheathing is only as tall as the wall. So the sill plate is nailed down to transfer the shear from the sill plate to the wood floor. Typically we specify that the sill plate nailing occur over the rim joist or blocking below. With 2 rows of sill plate nailing some nails (or maybe all) would be into just the floor sheathing. Which is fine for shear transfer, but I am more trying to figure out how (if?) it is possible to use sill plate nailing to satisfy the minimum sill plate uplift anchorage requirement for perforated shear walls. If you try to get uplift to work with nails just in the floor sheathing, the numbers I have previously given would be significantly lower.