Wood Plywood Roof Diaphragm Modeling in Risa

[Everroux]

Anyone know the best way to model 1/2" plywood on new single level gable roof addition to house such that it can be shown to provide adequate lateral load resistance. I have created plates between each roof rafter and meshed them (this I believe I have done correctly to transfer load from plate points to members) but do not know what property values to give the plywood under "General Material" spreadsheet.
Should it be modeled as Isotropic or Orthotropic?, E and G values? Poison ratio?
Once modeled correctly, couldn't I check the maximum in-plane force in x and y direction and compare to unit force allowable values of plywood/fastening panel by referencing the NDS diaphragm tables?

Would the model be giving reasonable results for lateral deflections?

The diaphragm is actually transferring load as a cantilever reaction against the host structure.

 

[JoshPlumSE]

I didn't realize RISA allowed users to specify non-isotropic plates! That must be relatively new.

Technically plywood is a bit orthotropic, but it's not like ribbed steel deck which is crazy different in the two directions. I've usually modeled plywood as isotropic plates. APA used to have a document that helped with the properties. Something like the plywood design specification....? I'll see if I can dig it up for you. I think they re-named it though since it now covers OSB as well.

 

[Celt83]

Would the model be giving reasonable results for lateral deflections?

Assuming you get the properties correct the main component you'd be missing from the deflection is the chord splice slip. I'd also double check in the APA document that Josh may end up posting that they work out isotropic stiffness adjustments so that you get an effective Ga that accounts for fastener slip, being APA it likely does but the nail slip would be a second component of error.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[XR250]

Why not do this by hand?

 

[Celt83]

Here is a good AWC presentation with each of the deflection components explained: Link

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Come on, XR...why do something by hand when a computer can do it for you...geesh...

Josh is right about that document, though I can't find it either. Last time I did this I was designing an office building - it was mostly wood with a bunch of steel where it needed it (about 1/3 of the building) with some concrete, masonry, and even light gauge thrown in just for fun. So modeling it to take advantage of RISA doing code checks in 5 different codes made it worthwhile...I just had to take the diaphragm results out and check them against the requirements manually. I don't recall exactly what I did, I'm afraid.