Shearwall OT, does the moment arm start at top of wall or top of diaphragm? 1

[reverbz]

Hey Guys,

Basically what's in the title but, when you're evaluating shearwalls for OT. Do you consider the height of the shearwall the moment arm or the distance from bottom of wall to top of diaphragm the moment arm? Especially in truss diaphragms where the wall can start 2' below the top of diaphragm. Would we consider the entire truss and wall to be overturning together, therefore combining the height? It seems like you would if you have constant connection between the truss and the wall. Let me know what you guys think.

Thank you!

 

[Nick6781]

The truss has to dump the load into the shear wall at some point. What is the elevation of that point? If both the top & bottom chords are connected, I would say the middle of the truss.

 

[driftLimiter]

I guess it depends what elevation the shear transfer is happening at. Usually this is right at the diaphragm sheathing.

If the calc is for SW chord forces, I would measure from the base of the wall to the sheathing of the diaphragm.

The fact that your 'wall' starts 2ft lower does not reduce the overturning demand on the wall.

 

[azcats]

I'd generally put the wall height at the elevation of the drag strut or collector. And if that's the top plate at the top of wall, so be it. In that case you've probably (hopefully) got shear transfer from diaphragm to top plate the length of the wall. If not, I could be talked out of my answer. Like if there's only blocking above your wall and the collector is at the diaphragm, I'd use the diaphragm elevation.

 

[StrEng007]

I think the OP is referring to a 2ft deep truss sitting over the top plate of a shear wall line.

As others have mentioned, shear transfer begins at the location where your diaphragm is connected to the vertical part of the lateral system. If you look at this as 2 mechanisms working together, you have a discrete system that prevents the truss from rolling over and the shear wall system below that. Overall, the shear wall system has to be able to support the total lateral load.

I've provided a very simple example below to show that the analysis of two systems should equal a single overall system.

•Single System:
10 kip at top of a 10 ft x 20ft long wall:
Wall shear = 500 lb/ft
Chord force = (10 kip x 10ft) / 20 ft = 5 kip (tension/compression)

•Multiple Systems
10 kip at top of 2 ft truss, sitting on a 8 ft x 20 ft long wall

Truss discrete system #1:
Shear = 500 lb/ft
Chord force = (10 kip x 2ft) / 20 ft = 1 kip (tension/compression)

Wall system:
Shear = 500 lb/ft
Chord force = 1k + (10k x 8ft)/20ft = 5 kip (tension/compression)

An alternate way to look at this: If you can run sheathing continuous along your wall studs and the heel of your truss then you have a 10ft overall shear wall height. This is probably best with floor framing that aligns with wall framing.

I'm neutral on the subject because I don't use a whole lot of wood framing but this would fall into the category of "Advanced Wood Framing" (i.e., truss and wall studs at 24" O.C.). The consensus is, as far as I can tell, a majority of people here don't like it.

 

[Eng16080]

The moment arm would be the distance from bottom of wall to the height of the resultant force of all applied loads acting on the shear wall. Simple FBD.