Diaphragm Force Transfer At Roof Valley

[EngSD]

I have a question about diaphragm load transfer that I am hoping someone can help me with. I am working on a wood framed building that has a gable roof coming in perpendicular to another gable roof (see sketch). I don't have any lateral resisting system in the portion of the building that is below the perpendicular gable roof due to wall openings. I am wondering if it is possible to transfer the diaphragm forces through the valley of where the two roofs meet so that I can use the lateral force resisting system of that portion of the building. Was thinking something maybe like Simpson coil straps evenly spaced along the valley?

 

[KootK]

You probably could do what you've proposed but I wouldn't recommend it. The main roof sheathing should extend beneath the valley set. As such, you can transfer bump out shear to the main roof diaphragm using the last full height bump out truss as a drag strut connected to the main roof diaphragm via the adjacent sill plates. The main roof trusses either side of the bump out will also need to be drag struts to restrain rotation of the bump out. It's a mouthful, I know. Let me know if you need a sketch for clarification.

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.

 

[msquared48]

If you do what KootK suggests, you would have to specify the drag forces that each truss would have to be designed for to the truss manufacturer.

Mike McCann, PE, SE (WA)

 

[EngSD]

I follow what you are saying, thanks.

I am wondering what happens if the length of the bumpout decreases to a point where there isn't the minnimum 3:1 ratio needed for a roof diaphragm. In that case I assume you can't figure the load is going to get out to the sill plates to be transferred to the main roof can you?

 

[KootK]

The bumpout diaphragm has to meet the maximum aspect ratio limitation to be code compliant. That wouldn't really change the load path, however, unless new elements were introduced to improve diaphragm aspect ratio.

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.