RISA3D for Wood Framing/ Residential Design 3

[ctcray]

Has anyone used RISA3D to model the framing of a wood structure, ie, a house? I would love to use it to capture all the loads from the ridge beam down to the beam in the basement. I just wonder if that's a reasonable request of the program.

-chuck

 

[lsmfse]

You could do this in RISA, however, it seems to me that it would be a lot quicker to find the reactions by hand rather than build the RISA model.

 

[JoshPlumSE]

Ctcray -

Disclaimer: While I am a California registered SE, I actually work for RISA.... Just want to make that clear so that I can't be accused of mis-representing myself.

The RISA program can certainly be used for that purpose. However, there are some general issues with doing wood in FEM. Specifically related to stud walls and shear wall. Modelilng those is often more work than you'd like. For wood structures, it's much easier to limit your use of FEM to trusses, continuous beams, and such while avoiding the wood framed walls.

That being said, we'll see what we can do for shear walls and stud walls. Over the next six months or year, we'll be releasing a couple of versions specifically targeting commercial wood structures. The largest part of that development is related to modeling the walls.

 

[ctcray]

I guess I should have asked this question of Risa-Floor as I'm not really looking to do finite elements on a house. Obviously, for a 2-story Colonial with a middle bearing wall, modeling isn't worth the time. But for cut-up 5000-10000 sf houses, with 45 degree bearing walls here, and beams supported by beams on posts over window headers elsewhere, tracking the gravity loads down to the steel beam in the basement would be sped up using something like Risa-Floor, especially if it will transfer concentrated loads from one level to a level below.


Is this something that Risa-Floor does?

 

[JoshPlumSE]

Well, the gravity ANALYSIS of walls in RISAFloor is better than in RISA-3D.... At least right now. The layout for these types of systems is certainly easier in Floor than in 3D.

However, we do have some things in the works for this October's release that should really help with this type of structure (sloped floors in RISAFloor and wood bearing and shear walls). This forum is really NOT the best place to discuss these features.... It just sounds too much like a sales pitch. And, for the most part, we want to avoid that on these forums.

Therefore, you would probably be better served contacting the RISA support group directly (support@risatech.com) or the sales group (info@risatech.com) for a more detailed discussion of what those new features will do for you.

 

[ctseit]

ctcray,

It should also be noted, that RISA3D does a much better job with additive deflection calculations thant RISAFloor. Actually, if a beam in the roof is deflecting more due to it being supported on a beam in a lower level deflecting, RISAFloor only shows the deflection at the roof, due to the loads applied. The addition of the deflection of the support beam is not included, and therefore needs to be done by the user. RISA3D however, does provide the added deflection.

 

[Indianola]

A quick word of caution in case you were thinking about using plate elements for modeling wood shear walls (say, for force transfer). Without a lot of care, you can get grossly inaccurate results.

1) you MUST model for the shear stiffness "G", rather than "E". (You cannot model for both. The plate algorithm can't handle a poisson's ratio greater than 0.5 - choose "G" and backsolve for an "E" value based on a "normal" poisson's ratio of about 0.2) (Good "G" values can be found in section 8 of APA's Report T2003-24.)

2) you MUST model the boundary elements (end studs, top & bottom plates, and the full ht trimmer studs around openings.) Model studs for the actual "E" value, not the "G" of the plywood.

3) If you are taking any vertical load, add studs to the model to take the load.

4) be careful about how you model slab support, and uplift stiffness of plate anchors.

The above may be more than you want to do, but anything less is "garbage in - garbage out". (Hand modeling is often easier.) The reasons for the above can be found in Advanced Mechanics of Materials texts. I can expand further if you would like, but it should probably be as a separate thread.

Scott Beard, PE, SE

 

[ATR2]

I have used the 3d program to model complex roof framing systems....it works very well but you have to be careful about kinds of supports you use otherwise the stress output will not me accurate
atr2
structural eng.