RISA 3D v14 Lateral Load Distribution

[dcStrucEng]

I am using RISA 3D v14 and I do not have RISA Floor. I am modeling a somewhat complicated two-story structure (high-end residence) composed of steel posts, steel beams, CFS floor joists, and CFS shear walls. I can design the CFS floor joists and CFS shear walls easily enough using catalogs. Therefore, I am not planning on modeling the CFS floor joists, but I do want to model the walls so I can have the model determine the distribution of lateral forces to the shear walls. However, if I don't model a diaphragm (using plates), it seems like I can't use RISA's auto-generated wind load and seismic load generator since how will the program know how to accurately distribute the lateral loads to the walls without a diaphragm?

I may be answering my own question, but if I don't model a diaphragm (since RISA 3D can't model flexible diaphragms, unless I define a plate properly), will I need to manually input all of the lateral loads (and load combinations) versus having the program auto-generate them?

Thanks!

 

[JoshPlumSE]

When the program talks about "diaphragms" it is usually talking about "rigid diaphragms" per the diaphragms spreadsheet. Either that or a diaphragm (rigid, semi-rigid, or flexible) that came over from RISAFloor. Therefore, if you are using RISA-3D by itself the only way to get the auto-generated loads (equivalent lateral force) is by using rigid diaphragms at each floor level.

Therefore, you will likely need to manually enter in your wind and seismic loads. Though you could apply these to a semi-rigid diaphragm made out of plate elements and let the program distribute them to the appropriate shear walls based on relative rigidities.

For seismic, you might have some luck with a response spectra analysis. But, generally my experience is that most engineers will avoided this unless it is required by code or they have a particular interest or career focus on structural dynamics.

 

[dcStrucEng]

Thanks for your reply. I would really like to use the auto generated wind loading in RISA-3D to avoid creating the numerous cases and combos. So I will model my diaphragm with plates and modify its properties appropriately. But please confirm that this procedure will work, because looking at the bottom of page 356 of the RISA-3D v14 General Reference Manual it states: "For a RISA-3D only model you will need to manually apply your loads and build your diaphragm with plates. See the Semi-Rigid Diaphragms section for more information on this."

So just to confirm: with a RISA-3D only model, if I model my diaphragm using plates to approximate the semi-rigid/flexible behavior, can I still utilize the auto generated wind loading to distribute the wind loads to the shear walls in my model?

 

[JoshPlumSE]

No, you cannot use the Auto-generated wind loads. Unless your diaphragms are defined with the diaphragms spreadsheet.