Multi-story analysis of Cantilevering Concrete Slabs

[CJoeC]

I am doing a very unique kind of structural analysis with concrete slabs cantilevering out from a giant concrete column. We have already done the hand calcs for rebar and now just need to analysis the entire structure as a whole.

My experience with SAP2000 is limited as most of my work is comprised of steel designed by center lines. What would be the best approach modeling these slabs with the rebar design that we have? I am lost in terms of even drawing the slabs around a concrete column.


See attached for a rough sketch-up of what we're trying to model.

http://tinyurl.com/pk6mokl

Thank you in advance,
Joe

 

[stressed]

If modeling with shell elements, use cylindrical grid coordinates and specify each radius for drawing. For inside cylinder draw "dummy" lines at the top in plan view and Edit>Extrude lines to areas downward (-Z) to create inside cylinder, then draw outer slab rings in plan view. I created the attached shell element model in about 4 minutes using this method. You can modify the radius to help subdivide the ring slabs when drawing. Use tz view to "unwrap" inner cylinder if you want to select and edit to refine mesh.

Another modeling approach could be to model the vertical cylinder using pipe line/frame element and use SD section designer for ring slab drawing a circular concrete section and then draw another circular section inside it with "opening" as the material. Right-click SD section to modify dimensions and material.

 

[CJoeC]

Stressed,

Thank you so much for your helpful response.

I apologize for a couple follow-up questions.

1.) Is it OK that my extruded edges don't come in contact? (see attached) Or am I drawing something wrong? I set the degrees to increments of 10.

2.) I assigned the section area type as "shell-thick" since my concrete thicknesses range between 8" - 18". I read that a thick shell assignment accommodates for traverse shear deformation. Am I correct with this area assignment? or should I use another section type?

3.) Since my concrete column is hollow and has an inner and outer diameter, I drew my inner column and assigned it a material with a thickness of the outer diameter. How do I draw my circular, cantilevering slabs to insure it is connected to the outer diameter of the column if I only have the inner radius drawn?


Thank you again for all of your help. I truly appreciate your expertise.

Regards,
Joe

 

[stressed]

Joe, regarding your questions:

1)Graphically your cylinder looks fine. You need to extrude them every 2 - 4 feet or so in the Z direction, taking care to have a Z elevation mesh at your ring slab locations for connectivity. For example, if your column was 40ft. tall and you had ring slabs at Z elevations 10ft, 20ft, and 30ft, you would need to be careful to have Z gridlines and divided mesh points in your cylinder column at each of those elevations.

2)Shell thick is fine. You can easily copy the same slab section and make one section with thin shell and the other thick shell and use each in separate models to compare results. I suspect that with your slab, the differences will be minimal.

3) Thickness should equal wall thickness of your cylinder column. Not sure what you mean by outer diameter thickness. To draw the circular ring slabs, modify your cylindrical grid to add additional radius rings with the final ring matching to the outer radius of the ring slab and also add Z grids as mentioned in #1 above. In plan view, use up/down arrow buttons to navigate to each ring slab elevation then use 'quick draw areas' to window around the radius grids to add slab elements. If your column radius was, for example, 4ft and your ring slab radius 8ft, you might want to add intermediate radius grid lines every 2ft or so to help you mesh.

The ring slab drawing approach is a little different in that you do not extrude like you did in drawing your cylinder column. Quick draw is usually easiest when you have your grid lines in place.