Modelling concrete slabs in SAP2000 and ETABS

[slowshow]

Hi all,
When is it most appropriate to model a concrete slab with a membrane or a shell(thin) element. For example, do you model a 150mm thick concrete slab with a membrane or shell element.

 

[slickdeals]

Depends.
If it is a 150mm thick slab supported by beams on all sides or two sides, model it as a membrane so as to accurately capture the design forces in the beams
If it is a two way slab, model it as a shell.

 

[slowshow]

@slickdeals
when you model as a membrane element, you do not capture the true displacements and moments acting on the slab. So how do one go about analyzing and designing the slab?

 

[johnbridge231]

It depends. Membrane and plate functions are both introduced in a shell element. I alsways use shell elements and (if needed) modify the property modifiers accordingly.

Regards.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[slickdeals]

You can't design a slab in ETABS anyways, and it doesn't give you a wonderful display of moments/shears.
I mainly use membrane when I am dealing with one way slabs.

Also, like John says, you can do with property modifiers in shell elements.

 

[pkjr618]

Good day all,

One benefit of using shells for concrete slabs in ETABS is that the program automatically considers T-beam action for all beams with shell elements properly placed on them. I use membranes mainly because it distributes the surface load assigned to it either one way or two way as you please and evenly at that compared to shells which you are required to mesh in some acceptable size in order to distribute the loads properly.

Also, my mentor uses membranes for slab in order to make the analysis faster (faster computer clock time) specially when modelling very large structures. Just imagine processing a 24x24 matrix for each shell compared to just 12x12 for eachy plane/membrane.

pkjr618

 

[slickdeals]

T-beam action is considered when shells are modeled, but you can't design based on that information, unless you do section cuts. The beam reinforcing reported is much less that what's truly required.