Shear Transfer Frame 2nd Floor Shear Wall to 1st Floor Shear Wall

[00Z]

I have a hypothetical question. When designing a 2 story wood structure with stacked shear walls, should the connection of the rim joist to the shear wall below be designed for the floor diaphragm shear or the shear wall load from the wall above?

For example lets assume we have two shear walls of the same length stacked one directly above the other. The second floor shear wall has a load of 350 plf. The second floor diaphragm has a load of 100 plf along the line. When designing the connection of the rim joist/blocking between the two walls, should the nailing be designed for 350 plf or 100 plf?

In other words is it assumed that the load from the shear wall is transferred into the diaphram and then into the shear wall(s) below, or is the load transferred directly from the 2nd story shear wall through the rim joist (over the length of the shear wall) directly into the 1st floor shear wall below?

I think the connection should be designed for the diaphragm load (100 plf). I am having a disagreement with a colleague over this and have a lunch (and my pride ) riding on this.

It has been surprisingly difficult to find a example/source where this is specifically addressed. Sorry for the length of the post and thanks in advance to anyone who chooses to reply.

 

[msquared48]

It's carrying the shear of the wall above plus the diaphragm shear of the floor, so 350.

Mike McCann
MMC Engineering

 

[00Z]

Thanks for the response. Hearing that from you is really shaking my resolve. I have learned a lot from your responses in the past. It looks like I may have gotten this one wrong.
Just for clarification I probably shouldn’t have said diaphragm shear but the shear along that line. For instance let’s say I have a 28 foot long wall with a lateral load along that line at the second of 1400 lb and I have one 4 foot long second story shear wall with a load of 350 plf (1400/4=350 plf). The shear along the same line at the first floor is 2800 lb giving us shear along that line of 100 plf (2800lb/28’=100plf). Lets say I have two shear walls along that line at the first floor. One 4 foot shear wall directly below the 4 foot wall above and a 6 foot long shear wall. Wouldn’t the shear in those walls be 280 plf (2800lb/10’=280plf)?
Or lets assume that we have one second story shear wall and one first story shear but they are not stacked directly on top of each other. Do you still need to clip that rim joist to double top plate below the wall so that it can take 350 plf. Essentially bypassing the diaphragm and passing the shear directly from the shear wall to the rim joist and down to the double top plate over the 4 foot length of the shear wall in my example or can you assume the shear is transferred into the diaphragm and then down to the double tops over the entire 28 foot length of the wall. The double top plate splice is designed to drag/collect the load and transfer it to the shear wall.

 

[OldPaperMaker]

OOZ,
If you have concerns about the loads & load paths I suggest that you pick up a copy of the book:The Analysis of Irregular Shaped Structures, Diaphragms and Shearwalls by R. Terry Malone and Robert W. Rice.

 

[00Z]

OldPaperMaker,

Thanks the book recommendation.

 

[RFreund]

Well I think it depends on how it is detailed. If the sheathing were continuous from the 2nd floor wall down to the top plates of the wall below, then I would say the shear from the wall above is transferred to the wall below via the connection of the shearing to the top plates. The diaphragm shear is transferred to the rim joist then to the top plates (or alternatively from diaphragm to rim joist to wall sheathing of the shearwall below).
As long as there is a path you should have an answer. Although some paths are better than others.

EIT

http://www.HowToEngineer.com

 

[00Z]

Thanks for your input. It is my opinion that the shear from the second floor shear walls is transferred to the blocking/rIm joist and that the shear from the plywood diaphragm is also transferred to the blocking/rIm joist. The shear is then transferred from the blocking/rim joist to the double to plates. The double to plates then drag/collect the shear into the first floor shear walls which then transfer the shear to the foundation providing a complete load path.

It is also my opinion the the shear from the second floor shear wall that is transferred to the blocking/rim joist can then be transferred to the double top plates below over the entire length of the line (wall) and not just over the length of the shear wall above. However, I am apparently in the minority in this opinion and perhaps wrong.

 

[OldPaperMaker]

One thing to keep in mind when talking about shearwalls is that there are at least four methods for analysis of shearwalls with openings:
1. Segmented shearwalls, which uses only the segments that are sheathed for the full height
2. Perforated shearwalls which is an empirical method
3. Force Transfer arouind Openings (FTO)
4. Cantilevered wall segments

The assumed load path is different for those different methods

 

[00Z]

I am familiar with three types: Individual Full Height Segments, FTO, Perforated Shear walls.

In my question I was assuming individual full height segments, but I think the question works for FTO too. Perforated shearwalls have some different detailing requirements at the sill plate.

 

[masterthenight]

Agree with Refund. It depends how it's detailed and the assumptions which are made, therefore I believe both answers could be right (referencing original question).

Option 1: If the sheathing is detailed to be lapped across the floor without horizontal joints (aka horizontal joints above/below the floor framing), then I would assume the shear transfers from the upper wall to the lower wall without entering the rimboard. In this case the rimboard connection would only carry the 100 plf from the floor diaphragm.

Option 2: If no accommodation is made to prevent a horizontal joint at the floor framing, then you have to assume a horizontal joint may occur anywhere and have to design for the worst case. If there is a horizontal joint in the wall sheathing at the bottom of the upper wall, then you get the combined shear forces (wall + floor) in the rimboard and it's connections. (side note: I believe it would be 350 wall + 100 floor = 450 total design shear, no?)

Lastly, the lengths use for shear transfer along the floor diaphragm and rimboard can be longer than the wall below, but you need to check T/C forces in the double top plates to make sure that your double top plate lap splice connection can transfer the correct potion of this (all of this is dependent on geometry). Basically you need to make sure your drag load path is solid.

 

[MileyDiley]

Your posts are a bit confusing, but judging only by the picture attached, it looks like your detail shows sheathing extending to the rim from both shear walls. That's the way the top wall transfers loads to the bottom wall.

The 2nd story sole plate can just be nailed nominally to the rim.

As long as you edge nail the bottom shear wall to the rim and to the top plates, you don't even the 2x backing/blocking connection. The floor diaphragm is connected to the rim which is already connected to the bottom shear wall. The remaining floor diaphragm beyond the shear wall will be edge nailed to rim which will be connected down to the top plates for at least 100plf, and the top plate splice needs to be designed for any drag that could develop due to the 100plf 1st story load.

 

[00Z]

Thanks for the replies masterthenight and MileyDiley. I suppose the answer could be it depends. In other words I guess you could detail the transfer so that the shear force goes directly from the wall above to the wall below, but I still don't think you have to. My posts are a bit wordy and hard to follow so I have added another sketch that may help clarify things for those interested.

And I'll take one more try at verbally describing my understanding of the load path:

Roof Diaphragm to Double Top Plate (Connection of the blocking/rafters/trusses to the double top plate)
Double Top Plate to Shear Panel (The double top plate transfers (drags) the load to the shear wall. The edge nailing at the shear wall transfers the load into the shear panel)
Shear Panel to Sole Plate (The edge nailing at the shear wall transfers the load into the sole plate)
Sole Plate to Rim Joist (The nailing of the sole plate to the rim joist/blocking)

At this point the second story shear forces have been transferred into the floor diaphragm. This 2nd story load + the lateral load from the floor then needs to be transferred to the 1st story shear walls:

Floor Diaphragm to Double Top Plate (Connection of the blocking/rim joist to the double top plate)
Double Top Plate to Shear Panel (The double top plate transfers (drags) the load to the shear walls. The edge nailing at the shear wall transfers the load into the shear panel)
Shear Panel to Sole Plate (The edge nailing at the shear wall transfers the load into the sole plate)
Sole Plate to Foundation (The bolting of the sole plate to the foundation)

That probably has not helped at all...

 

[MileyDiley]

At this point the second story shear forces have been transferred into the floor diaphragm.

I believe you're assuming the rim is continuous for the full length of the floor diaphragm or that it is strapped together at splices. But you are not saying that as far as I can tell. This isn't standard practice around me, either. I usually see rim in ~20ft lengths or so.

Also, in general, the more direct you can make the shear connection the better. Instead of spreading the concentrated shear out (possibly through straps), then collecting it and dragging it back to the same location below, just make the direct connection from wall to wall.