Provide a continuous load path for angled wood shearwall

[jeffhed]

I am working on a wood framed 2 story commercial building in a high seismic area. I need an interior shear wall line but only have one 10'-0" long shear wall. However, there is some additional wall I can use for shear but it is at a 45 degree angle. I can use just the component length in that direction for the wall and it will really help out, however, how do I detail the drag forces going through the rest of the building? If my wall angles do I provide collectors at both ends, in the middle only the far end? Maybe a partial collector with blocking and strap for the projected length and then a collector on the other end? The seismic loads are killing me where I have to have a shear wall in the middle of the building. I have attached a pdf of the roof framing plan to illustrate what I am talking about. I will have a similar condition in the longitudinal direction on the front of the building. I can't get the shear walls to work taking the shear out to each end because of lack of shear on the bottom floors.

 

[KootK]

I'm sure you could make something work at the floor level without too much trouble. diagonal blocking and strapping etc. The roof detailing seems as thought it would be brutal though.

The sketch below shows the best alternative that I could think up. Some issues:

1) Because the one girder would run cross-slope, sheathing nailing into that girder could be a bit sketchy. Maybe you could scab a beveled 2x to the side of the top chord to rectify that.

2) There's a danger that your truss supplier will put out a contract on your life. It might almost be better to build the wall up to the diaphragm and stick frame around it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jeffhed]

Kootk,
I think we could just frame some shear transfer blocking from the top of the shear wall to the underside of roof sheathing. After thinking about it I realized I need to resolve both components of the angled drag force and would need blocking and strapping or a truss to provide shear transfer for both components. I have attached what I came up with this morning after thinking about it last night.

 

[KootK]

I think we could just frame some shear transfer blocking from the top of the shear wall to the underside of roof sheathing.

I'd considered that but was concerned about constructability. Both the top plate and chords of each panel would be skewed. Probably doable but not much fun.

After thinking about it I realized I need to resolve both components of the angled drag force and would need blocking and strapping or a truss to provide shear transfer for both components.

Yeah, that's why I ran the diagonal girder out to the two other girders. I was hoping that you could dissipate shear within that space between the two girders and just call that area a sub-diaphragm.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jeffhed]

Kootk,
The diagonal girder truss does take care of both tension and compression. But I think changing the truss configuration to something so different would cause other problems we can't see right now. There just isn't a clean way to do it either way unless we go with a moment frame, but there isn't really any allowance in the floor plan for the columns, which is why I am where I am at. The shear walls actually work decent, there are some large uplift forces that will require cast in bolts, but it won't be anything compared to framing the transfer in the roof. The framers around here to shear transfer blocking on sloped roofs all the time, I think they are used to that sort of thing by now.

 

[KootK]

But I think changing the truss configuration to something so different would cause other problems we can't see right now.

Like what? Is this still in the design phase or are you committed to some things externally?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jeffhed]

Like you said previously, push back from the truss company. Usually anything atypical like that seems like we get pushback or even worst the truss company doesn't notice it and we are stuck doing blocking and strapping anyway. So I am trying to keep the truss stuff like I have it and working out the transfer with something the framers can do.

 

[KootK]

One of the reasons that I mentioned the truss guys is that, back in the 90's, I was the truss guys.

With your original layout, the full truss profile would be jigged and the the tails would be lopped of as required. With my layout, the same process would be used but, as tails were lopped off one side, additional truss tail would be added to the other side.

So my layout is less truss friendly than your but not by much. And, in exchange for that added complexity, you get:

1) No awkward shear wall to diaphragm blocking panels.
2) Possibly no roof diaphragm blocking at all in that area.
3) A system that seems much more robust structurally.

My suspicion is that the diagonal girder truss system may well be quite competitive. Remember, the whole reason that the truss industry exists is because it's almost always cheaper to do a thing in the plant than it is in the field.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.