Flat Plate Modeling

[Stazz]

I'm modeling a 15 story post tensioned slab building and I just want to get some ideas on how to model the slab.

1) Can you model the slab as a true "Flat Plate" with just columns or do you need beams to define the slab edge and to collect the loads into the column.

2) I'm choosing 2 way for the plate but it won't do the live load reduction for 2-way. Is there a way around this?

3) If you do model the beams then do you fix them? such that the moments transfer into the columns? Or do you leave them pinned so that the columns only take axial loads without moments.

 

[Stazz]

Also, I can only model walls as Masonry walls in Risa Floor. Is there a way to model them as concrete so that I can export them to 3D and do a lateral analysis on them? Thanks.

 

[JoshPlumSE]

The RISAFloor program really doesn't do any analysis of the slab itself. The slab is really just viewed as a way to get the load into the supporting beams or walls. Therefore, it is required to model in beams and / or walls.

While the program isn't specifically geared towards flat plate design, I have seen people use it so that they can get a good LATERAL analysis done on their structure in RISA-3D.

In that case, you would model in a number of fictitious beams (gravity only) that frame into your columns. You'd use a two-way slab so that the load attribution is approximately what a flat plate should be. You might also make the fictitious beams out of a weightless material.

Then you'd get approximately correct column and reactions at each floor level. That will allow the program to give you good lateral loads when you bring it into RISA-3D. Once you are in RISA-3D then the lateral analysis results should be really good.... Because you're not generallyl relying on the flat plate to be part of the lateral force resisting system.

This process takes some getting used to. It's really just a way to simplify the creation of a lateral model. Therefore, it's debateable about whether or not it's worth the effort. Afterall, you could have just created the lateral model in 3D in the first place.

 

[Stazz]

Thanks Josh,

Doing the whole thing in 3D sounds like a good idea since I'm really only after column sizes and shear wall design. But what about the Live Load reduction? Can 3D do this? and why won't Risa Floor reduce the live loads for 2 way plates?