Modeling of composite floor in SAP2000

[AleksaBgd94]

Hi everyone,

I am trying to model composite floor in SAP2000. What is the best way to do that? Which area section should I assign? And what about Modulus of Elasticity? The thing I am the most concerned about is its behaviour due to horizontal loads.
Please write some notes and tips if you have experience in this field.
I appreciate your help

 

[271828]

There are a couple of options.

If you're OK with fully composite behavior, then do the following. Use an orthotropic shell to represent the slab. Use the stiffness property modifiers to result in the slab being stiffer parallel to the ribs. Use the regular concrete material properties for this. Use a regular steel frame element for each beam and girder. Set the insertion points so that the beam is offset downward from the shells.

If you need to consider partially composite behavior, then compute the effective moment of inertia, such as with Iequiv in the AISC Commentary to Chapter I. Assign this moment of inertia to the frame element.

 

[271828]

Here's the most important part:

You should know how the composite elements should perform. Run manual calculations on some simple cases. Compare those results with results from SAP2000.

 

[AleksaBgd94]

Thank you very much 271828!

I have some more precise questions, so I would be really satisfied if you could consult me.

My current task is to make a 3D-model of steel building with composite slabs. I've already made one model, so here are some notes.

Since I have no questions about frame elements, I'll just focus on this shell element.

I've chosen shell element - plate(thin). Its total height is 140 mm, where height of steel deck is 60 mm and height of concrete above slabs is 80 mm, so I've put that both membrane and bending thicknesses are 140mm = 0,14m.

As a material I've chosen concrete C30/37 defined in Materials (I'll attach my model in SAP2000 at the end of the post). This concrete's Weight per Unit Volume is defined as 0, because I've assigned all gravity loads, including dead load as Uniform Load(Shell to Frames)to my Area Section (slab).

I've already checked results in frames due to vertical loads and everything seems to be ok. Gravity loads which act on slabs are transported in the right way.

I am really confused about my Modal Analysis results. All modes and shapes look ok to me and my
colleague got almost the same periods in Etabs, but I've figured out later that the concrete's Modulus of Elasticity in my model is 199 GPa, which is a huge mistake. When I change to 33 GPa, I get torsion as a second mode...

I also think that some axial forces should appear in my beams due to horizontal loads - WIND (I haven't run quake yet), but that doesn't happen. I guess it is for the same reason - my shell element is not defined correctly.
What do you think? Do beams and slabs have to be conneceted by constraint elements - diaphragms or it is something that we don't need because our shell elements are rigid enough to transport all horizontal loads? I've read some things about generating mesh in slabs... Should I do that? I am correcting my latest model right now in order to get better results, and I've noticed that some axial forces appear in beams due to wind, when I define insertion points for beams(Top-center).

Here is my model, so take a look if you have an idea how to help me. If you have any questions about model, I'll answer as soon as possible.

Thanks again.

 

[AleksaBgd94]

And here is the photo of the slab.