Risa 3D to Floor; diaphragm and seismic loads

[BoHboH]

Good afternoon, new to posting so hope I do this correctly!

I have a building modeled in Risa 3D and am wanting to use Risa Floor to generate my seismic loads. I know I can't just import my Risa 3D model to Risa Floor, but is there a way to draw up the floor plans in Risa Floor with semi-flexible diaphragms, generate the loads, then just import those into Risa 3d model? Or import the semi-flexible diaphragms into Risa 3D then generate loads there? I'm new to Risa Floor so not sure what's possible.

I'm trying to avoid re-drawing this building in Risa Floor but really need to get the seismic loads applied to my Risa 3D model and I know Risa 3D only generates seismic when using rigid diaphragms (which this building does qualify for). I've read you can simulate semi-flexible diaphragms in Risa 3D with mesh plates but heard its difficult or very time consuming?

Any suggestions would be greatly appreciated!

 

[jayrod12]

Have you reached out to the people at Risa?

The cross platform design is one of their selling points. So yes there must be a way to use floor, and a semi-rigid diaphragm, to generate your seismic loads and import them back in. I haven't done it personally, so I'm not much help in terms of the process, but I do know that it's possible.

Risa.com has many tutorials for how to do things, and in my experience their technical help people have generally been responsive and knowledgable.

 

[BoHboH]

@jayrod12 yes I spoke to Risa about model transfer, they specifically said you cannot go from Risa 3D to Risa Floor, only from Floor to 3D. But, good point, I didn't ask them about drawing a diaphragm in Floor and bringing it into 3D, I will reach out to them thank you. They also said I could possibly go from 3D to Revit then to Floor so I might try that too.

Do you have experience with seismic load generation? I live in an area where its not required so I've never had to do it but fell into a situation where I need to provide the seismic calcs and apply the loads to the structure and was hoping Risa 3D could help do that quickly but so far it wont cause it will only generate seismic loads when using rigid diaphragms. I really just need a seismic load generator if I can't get Floor to work, any suggestions for a good one? I know SkyCiv had a decent one?

 

[jayrod12]

Unfortunately I don't have much experience there. Our seismic loads are so low where I am that often we're able to prove that it doesn't govern the design of the main structure, just of minor component anchorage.

Is there a reason you can't then start your model in floor and then import it? Why must you start in 3d and then go to floor? I was under the impression that it could go back and forth once the link was made.

 

[dylansdad]

I don't believe Floor will generate any lateral (seismic, wind) loading, only vertical. I normally start with a Revit model, transfer to Floor (which in the latest versions of Revit have been a pain),clean it up, input the floor loading, then "direct" to 3D to generate seismic.

 

[BoHboH]

@jayrod12 the only reason I'm avoiding starting in Floor is because its kind of a complicated structure and I already have it drawn in Risa 3D and was hoping to avoid a re-draw in an unfamiliar software.

@dylansdad from what I've researched Floor can definitely generate lateral loads using its diaphragm system. But, I didn't think of going from Floor to Revit to 3D, maybe I can try that since I already have the Revit model drawn. Do you use semi-flexible diaphragms in Floor in order for 3D to generate seismic loads? I'm a noob with diaphragms and Floor sorry for the 20 questions.

 

[dylansdad]

I stand corrected on the "I don't believe floor will generate....." I have never used Floor to generate lateral loading, I generate lateral and wind loads after I direct to 3D from Floor. I have only used rigid diaphragms. In newer versions of Revit, an analytical model is no longer automatically generated. You have to go to Analyze, Analytical Automation, Physical to Analytical for Buildings to create an analytical model. Then invariably you have to clean up the analytical model in Revit or use the add in tools in Revit to export to RISA (3D or Floor).

 

[bones206]

is there a way to draw up the floor plans in Risa Floor with semi-flexible diaphragms, generate the loads, then just import those into Risa 3d model?

I would try this:

1) Create a diaphragm model in RISA Floor. Include some nominal gravity members to make sure the solver can run. Make sure to save a backup of this RISA Floor model before the next steps, just in case.

2) Export to RISA-3D. This step will include the seismic load generation.

3) While in RISA-3D, use the "detach model from RISA Floor" tool. This will create a standalone RISA-3D model.

4) Append your original RISA-3D model to the new diaphragm model. This may or may not work well, but it's worth a shot. Having a precise shared coordinate system between the two models will be key. Might also have to run a model merge after the append operation.

If it was me, I would just apply hand-calc'd seismic manually in RISA-3D. Seems like a lot less hassle. It's pretty easy to model plates manually in RISA-3D as well.

 

[JoshPlumSE]

Bones206's idea is pretty good.

Unfortunately, if I remember correctly there are internal flags on some of the seismic loads that came over from RISAFloor. It depends on how those come over. I believe you might be able to "capture" transient loads that get created by RISA-3D when the model comes over from RISAFloor. But, don't have access to any RISA-3D of RISAFloor programs anymore, so I can't test this out for you.

I used to be able to get Demo versions of their programs to test out things like this... But, I don't think I can do that anymore.

 

[bones206]

Might be able to append the original model in without first detaching the new model from RISA Floor. But I imagine the software would get very confused if you ever tried to bring the whole thing back over to RISA Floor later.

 

[dold]

OP, are you talking about semi-rigid diaphragms when you say semi-flexible? RAM Structural system has 'pseudo-flexible' diaphragms so I just want to clarify that, and they are an entirely different animal.

But if you are talking about semi-rigid, then yeah I like bones's idea too. Trying to draw in plates (then sub-mesh them) would be a huge headache for you, and it requires a ton of extra nodes and plates that bog everything down. The semi-rigid diaphragm feature is specifically designed to get rid of those issues (i would assume...). Although it's not impossible, and fairly simple to assign lateral loads to the plates prior to sub-meshing, it's still a headache.

There is no way to create a true semi-rigid diaphragm in R3D and have it generate lateral loads as it does for a rigid diaphragm. You must first go through Risa floor, then transfer to R3D. That is also when the lateral load generation occurs.

And yeah, as Josh pointed out, the in-plane lateral loads applied to walls/beams/etc from FLEXIBLE diaphragms are flagged as transient line loads that behave differently than other load types.

For semi-rigid I think the lateral loads are actually story forces that get smeared over the semi-rigid diaphragm either before or after the meshing takes place. That would make sense because force distribution to LFRS can't be computed before the diaphragm is meshed and solved. I believe forces from external masses (i.e., everything other than the mass dead load of the diaphragm itself) is actually applied to the diaphragm as transient line loads in the plane of the diaphragm (e.g., a brick wall leaning on the diaphragm perpendicular to the load case). And I *think* you can only view those loads after solving the model. That's definitely true for flexible diaphragms, i think it's also true for semi-rigid...?

Long story short, if you want to use semi-rigid diaphragm then you'd be doing yourself a favor by starting over in risa floor. Hopefully your model isn't enormous.

 

[JoshPlumSE]

I used to be able to get Demo versions of their programs to test out things like this... But, I don't think I can do that anymore. sad

FWIW, I have figured out how to access the demo versions of RISA on my new computer. A friend asked about their newer implementation of P-Delta (which involves geometric stiffness adjustment). Since this was one of the features I had been pushing for when I was there (and for which I wrote a pretty detailed specification), I had to look into it. Not surprisingly, they ignored my specification and went with a relatively "half-ass" implementation of the feature. It doesn't even work with Dynamic results! And, dynamic results is the MAIN reason why I considered implementing this method to be so important.