Beam design with walls above

[Rayengr]

I have a 4 story concrete building, shear walls, flat slab floors and roof with beams at every column line. The Penthouse smaller than the overall footprint and is supported by beams at the main roof level. All beams are on the main column grid.

My problem is getting the roof beams to recognize the loads from the penthouse walls and roof the walls support in both Etabs and Safe. I would expect these beams to have a higher moment than adjacent beams that do not have the extra load but I'm seeing considerably less. Its almost as if the walls are being designed compositely with the beams or its thinking these walls are deep beams.

How do I set the model so that the beams supporting walls (and roof)recognize the added loads and are analyzed/designed properly? Is there a modifier or something that I need to assign to the walls?

 

[slickdeals]

In order to prevent composite action, you can release in-plane shear in the walls by changing the f11 factor in stiffness modifier. What this will is prevent shear transfer from the wall to the beam.

However, be very careful with the meshing of the wall. Point loads will get transferred properly but not not uniform loads.

Play around with it.

 

[Rayengr]

Thanks Slickdeals. I got a response from CSI Support saying to assign the walls a stiffness modifier of 0.01 for f22. Didn't say anything about meshing. Guess I'll try out both and see what happens.

 

[slickdeals]

@Rayengr:
f22? Interesting. I was thinking f11. Let me know how that works out.

 

[slickdeals]

Model the slab as a membrane element, and set f11 and f12 to zero on the walls. That will work out.
See uploaded model.