Modeling WSP with Plates
Modeling WSP with Plates
(OP)
I've been playing around with RISA3D a bit more lately and I think it might be useful in doing some in depth analysis of portal frames. However, after reading through some posts by others on the board I'm trying to figure out whether or not one can get reasonable data with this sort of thing. I want to model all of the studs, beams and plates and then use plate elements between them to represent the sheathing. A previous thread had some mention about "E" and "G" not being compatible for this type of modeling and using a fictitious "E" value based on a poisson's value of 0.2
I am wondering what others experience with this sort of thing has been and what suggestions they might have in this regard. I did notice that if I input the typical "E" and "G" values that I found listed for plywood my model was too "stiff" and I didn't get any realistic deflections.
I am really curious how the wall panel element within RISA works and what type of "E" and "G" values are used to get deflections that compare with the calculated methods of the NDS SDPWS.
I am wondering what others experience with this sort of thing has been and what suggestions they might have in this regard. I did notice that if I input the typical "E" and "G" values that I found listed for plywood my model was too "stiff" and I didn't get any realistic deflections.
I am really curious how the wall panel element within RISA works and what type of "E" and "G" values are used to get deflections that compare with the calculated methods of the NDS SDPWS.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com





RE: Modeling WSP with Plates
That previous post was likely from a number of years ago (prior to 2008-ish). Back then, the plate element formulation was based on E and poisson's ratio and ignored the G value. That's no longer the case and hasn't been the case since the version 8.0 release.
The wall panel element in RISA-3D uses true ortho-tropic plate elements where different stiffness properties are used for axial vs shear deformation. We do this so that the axial stiffness represents the stiffness of the studs, but the shear stiffness represents the stiffness of the sheathing (based on the "apparent shear stiffness" designated in the NDS SDPWS spec.)
As such, the program doesn't attempt to model individual studs or posts or such. Rather, the desire is to get a representative (but uniform) axial stiffness based on the total number of studs. And, a representative shear based on sheathing and nailing.
There is a "Shear Stiffness Adjustment Factor" that can be applied to the walls. This is used to soften the walls in the case that the wall deflection doesn't match up with the NDS estimated deflections. Probably more useful for walls which are very narrow and hold down deformation becomes a major consideration.
RE: Modeling WSP with Plates
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Modeling WSP with Plates
I've seen post beam construction with kicker braces. But, to me, that's more of a braced frame than a moment frame. At least in terms of how it gets analyzed.
RE: Modeling WSP with Plates
http://publicecodes.cyberregs.com/icod/irc/2009/ic...
My method of manual analysis is based on a paper by Zeno Marti "Principles of Mechanics Models for Wood Structural Panel Portal Frames". However, I would like to validate this manual analysis further with an appropriate FEA model.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Modeling WSP with Plates
Since JoshPlum is now involved. I do wish he could talk to his programming buddies next door and have them add in Wood panels in their plates so i could model a sloped wooden diaphragm straight in RISA-3D! :) Or maybe i need to make a new post if this is possible!
RE: Modeling WSP with Plates
My thinking is if I use larger holdowns, triple chords, larger and strong straps, beefed up stemwalls, and both sides sheathed I can get quite a bit more out of one of these portal frames. The trick is to quantify this amount which is what I have tried to do using the methods of Zeno Martin's paper and also referencing Terry Malone's book on the subject. Malone uses FEA along with his manual calculations throughout his book and I would like to follow his lead and see if I can get comparable numbers using RISA3D.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com