Parking Garage modeling in RAM SS

[Strucasgdf]

thread726-230354

RAM SS does model flat plate, two way slabs without spandrel beams.. the only problem is that to do it you must call it a semi-rigid diaphragm, which causes a problem with your lateral (rigid diaphragm) action.

My question- how did you model the ramp from floor to floor? I am having difficulty modeling the ramp, and getting accurate lateral load distribution into and outof these "dummy levels". More specifically, I am getting lateral load into and outof levels where there is no connection to the diaphragm.

 

[SteveGregory]

Could you design the ramp by hand and put the ramp reactions as point loads and line loads at the ramp connection points?

 

[Strucasgdf]

Sure. And I could detail the ramp-floor interface as a slip in all degrees but the two... but a detail FEM analysis would take too long, and would be too difficult.

Is that what you did?

 

[ishvaaag]

I don't know in RAM steel but in RISA you can simply model the ramp in autocad if complex and import or merge into your model. If simple, you can model it directly in the program. In every case is practical to use triangular plate or shell elements for ramps to care for adaptation to the form and not have element problems. If entirely flat, you need not. This way you can have a reasonalby accurate geometrical representation of any ramps you have. Then, elastic analysis with isotropic materials knows scarce thing about diaphragm classifications, you can do the same and use a more complex material to get a proper representation of what you want. But you can use rigid floors if you want and keep the ramps at their natural stiffness, or tweak them by hinging and sliding supports to be inactive from a lateral load resistance viewpoint. What I am saying doesn't require at all any definition of levels, just modeling the geometry; you can use levels and diaphragms etc but leave out the ramps etc. Simply, model the geometry, with the proper relative support and then decide if you make some things rigid for convenience of analysis.

 

[SteveGregory]

Another possibility, if you have RAM Elements (was RAM Advanse), you can import the model directly from RAM SS. Then generate the geometry for the ramp.

 

[Strucasgdf]

Its not that the modeling is hard to do. The model is already built. I have concerns with the lateral load distribution.
In RAM SS (RSS) you have to model the tiers in a parking garage with dummy levels (in RISA you do not but I am concerned with the time it will take)and unfortunately those dummy levels are modeled trapezoidal in shape due to the shearwall locations. I am concerned that the trapezoidal dummy walls are analyzed in the program with unrealistic rigidities. Also, I am having the much more serious problem of load distribution in and out of dummy levels that are not connected to the diaphragm.
Because of this I have modified the model to semi-rigid diaphragms (instead of rigid), and it looks like its solved that issue but now my semi-rigid diaphragm is having to resist horizontal loads at the ramp without the help of the beam elements and is undulating like the ocean.... it appears to me that no matter what I do with RSS there will be significant inaccuracies.
Any suggestions?