Risa 3D Foundation Question

[Rabbit12]

Hello. I'm modeling a mat slab in Risa Foundation that is supporting some process equipment.

I have three different cases I need to examine; empty, operating, and hydrotest. Obviously, all of these have separate dead loads associated with them. The basic load cases are pre-defined in the Risa Foundation so I can't defined different basic dead loads. As a work around I defined the DL's for the operating and hydrotest loads as other loads (OL1, OL2, etc).

When I run the analysis I get an error that it's unstable for seismic load cases. It works for the empty load and the moments aren't that much different. Looking into it a little more if I change the load type from OL1 to DL it solves it and my mat is adequate.

It appears it isn't taking into account the OL1 (operating DL) load into account for stability checks because it isn't a DL. Can someone verify this is what is actually occurring? Any work around to this other than creating separate models?

 

[Guastavino]

Rabbit,

I think you are correct in how RISA is working. JOSHPLUM should be able to confirm.

Now, with that said, why don't you model the MAT slab and the empty equipment as dead load since I'm assuming that it won't change for any case. Then model the contents (Operating and hydrotest components) as other types of loads? That way, the actual dead load will be accounted for appropriately. But maybe I'm not understanding exactly what you're doing.

Or, copy the same thing over some distance to the right or left in the same model and change the parameters that way. At least prevents (3) model files.

 

[JoshPlumSE]

No, that's not the way it should behave in the matrix solution. The self weight of the foundation itself is only included in the DL category. So, your load combinations should be DL + OL1, or DL + OL2 instead of just OL1 or OL2 otherwise you're neglecting a lot of stabilizing self weight.