[WOOD] Shear transfer using bottom plate nailing only

[masterdesign]

I’d like to get opinions on the shear transfer of an upper wall into the floor system at the exterior of a multi-story building (2-5 floors), using bottom plate nailing only.

The nail is in single shear with the bottom plate as the side member. In order to design the nail per NDS Table 12N, assume floor sheathing acts compositely with rimboard as main member.

Nail capacity = 141 # (NDS Table 12N)
Connection capacity = 141*1.6*12/8 = 338 plf

The connection of the floor sheathing must account for the composite action.

Assume
[ul]
[li]Floor sheathing nailing = 10d common (0.148”x3”) @ 6”[/li]
[li]Demand = bottom plate contribution + diaphragm contribution[/li]
[li]Diaphragm construction factor (1.1) applies to fastener capacity[/li]
[/ul]

Nail capacity = 95 # (NDS Table 12Q)
Connection capacity = 95*1.6*1.1*12/6 = 334 plf
v bottom plate,fs = 0.375*338 = 127 plf (bottom plate contribution)
max v diaphragm = 334-127 = 207 plf

Therefore, you would be limited to 207 plf diaphragm shear when transferring 338 plf bottom plate shear using the bottom plate nail alone. If my goal is to have a single design that can be used throughout the structure, I don’t see this working without additional specification (wall sheathing lap, hardware or drag), since my diaphragm shears are quite frequently over this limit.

I know additional specification (wall sheathing lap, hardware or drag) can make this work. I am specifically looking at using the bottom plate nailing alone to contrast my experience. I have seen this detail used regularly by Engineers, including at firms I have worked for, across multiple floors. I just don’t see how the numbers work without additional specification.

Lastly, an alternate option which avoids putting increased demand on the floor sheathing is to neglect the floor sheathing in the design of the bottom plate nail and assume there is a gap between the side member (bottom plate) and main member (rimboard). This fastener design condition is covered in AWC TR12.

Nail capacity = 48 # (calculation omitted for brevity)
Connection capacity = 48*1.6*12/8 = 72 plf

This method is quickly ruled out since most shear demand will be much higher than this.

 

[driftLimiter]

Just sidestepping the question of whether the sheathing negatively effects nail strength....

You could just a fastener tested for the specific condition. Simpson SDS screws have load values through a piece of plywood specifically for this application.

I'm not sure how much shear transfer you are accumulating going down, but I have never tried to use a single sole plate connection spacing for a building like this.

Generally I have a specific sole plate fastener and spacing for each type of shear wall being used, and as I go down the building it I need more shear transfer capacity.

 

[masterdesign]

Drift: thanks for your response.

Haven't heard of sheathing negatively affecting nail strength, can you link me to some references on that. My assumption is that if the rimboard and sheathing both have G = 0.50, and they are adequately fastened together, then one could assume they are monolithic for the fastener calculation. I'm only having to use this logic for the purpose of my larger question below.

I understand the wall sheathing can be lapped or that Simpson screws or lag bolts can be used to transfer the forces. And that anything over a 2 story structure will require this regardless based on the magnitudes. What I'm trying to drill into is that historically I've seen designers use the bottom plate nail alone. Look at this detail, which is similar to standard details I was trained on in my early career.

Details like this reduce the bottom plate nail spacing to match the shear wall capacities. But I believe there is a problem with this. That is, part of the nail shear is being transferred through the floor sheathing and it's connection to the rimboard, which already has to contend with diaphragm shear. I have never seen any discussion on this.

In my designs, if I want to use the bottom plate nailing to transfer shear, I will permit it on the uppermost floor only. All other floors have wall sheathing lap requirements or screws/lags. But even with this approach, when the bottom plate nailing is used, the reduction in available capacity of the floor sheathing nailing should be considered.

I don't see any way around this. So I'm hoping to hear that others agree, or be proven wrong.

 

[masterdesign]

This thread touches on my concern

A couple notes on that:

1. OP says "The sill nailing in our schedule is 16d, which would appear to not be long enough for adequate penetration into the rim joist." This implies the rimboard alone is being treated as the main member. Now, if minimum penetration is taken as 6*d, then 16d box (3-1/4" long) and 16d common (3-1/2" long) nails do meet the minimum. But I presume he means minimum as 10*d corresponding to full design capacity, and this statement would be true, meaning you get quite the reduction on the nail capacity due to geometry factor. But! This logic has a problem in that it ignores the 3/4" distance between side and main members. I contend that in this condition, not only do you get hit by the penetration reduction, but you also have to design the fastener with a gap between members as detailed in AWC TR12. This renders the fastener essentially useless for common shear demands. So this whole line of thought should be abandoned.

2. Then a commenter speaks of using 20d. Certainly this is an option, but again should be designed with the gap. But specifying this is no better (and presumably worse) than specifying wall sheathing lap or proprietary screws or lag bolts.


In the end, I just don't think the bottom plate nail can be used for anything other than transferring shear from a single level. And when it is used, the diaphragm shear limit must be considered.

 

[driftLimiter]

Maybe I've missed your point here. Are you suggesting that the shear from the wall transfer needs to be added to the diaphragm shear when you check the diaphragm itself?

I don't really see that, the shear transfer is flowing from the sole plate to the nail and from the nail to the G=0.5 sheathing + rim. Shear stress in the plywood needs shear deformation, along the wall line I'm just not seeing that.

My point about the nail was so called gapped dowel connections. This could arise in your condition depending on if you think the plywood is 'composite' with the rim or not.
[URL unfurl="true"]https://awc.org/wp-content/uploads/2021/12/AWC-TR12-1510.pdf[/url]

 

[masterdesign]

Drift:

Either the floor sheathing is involved in resisting the bottom plate nailing, or it is not.

Case 1 - floor sheathing does resist bottom plate nailing
There must be a load path. Bottom plate nail > floor sheathing > floor sheathing nailing > rimboard. Demand on floor sheathing nailing must be combined with any demand coming from diaphragm. The capacity of this nailing needs to be checked.

No, I am not suggesting that the shear from the wall transfer needs to be added to the diaphragm shear when you check the diaphragm itself.

Case 2 - floor sheathing does not resist bottom plate nailing
This would be the gapped dowel connection. See the bottom of my original post (starting at "Lastly,") and #1 in the post above your last post. The fastener capacity is so small as to be useless.

Thoughts?

 

[driftLimiter]

I see better now what your doing here. TBH I've never seen anyone add load to the diaphragm nails based on contribution from the shear walls above. I believe the general approach is provide shear transfer fasteners from the wall and the diaphragm and treat them separately. Without getting into the weeds on stiffness compatibility of the fasteners I am not really sure how one would properly do this anyway.

That being said it doesn't mean that this is the correct approach. Its been a while since I have given this document a good read but it may contain some insight for you. Cant find much else available.

 

[masterdesign]

Drift:

Thanks again for your responses and the links.

To your point - I have never seen anyone add load to the diaphragm nails either. As well, I have always seen the shear transfer fasteners from the wall and the diaphragm treated separately.

This is precisely my point. In order to do the above and rely on the bottom plate nailing for shear transfer, you would need to treat the fastener as a gapped dowel condition, which will never meet the demand. Therefore, you have to use specific shear transfer fasteners or lap the wall sheathing - you cannot use the bottom plate nailing alone.

I would love to see how I'm wrong on this. As it's much easier to use the bottom plate nailing, and that is how framers prefer it and have done it forever.

 

[driftLimiter]

Yea I am with you and your conclusion on this. It seems this may be another area in wood where there is not consensus. The more experienced I get the harder wood structures become lol.