Slab Analysis and design

[Anonymous]

Hi all,

I am modeling an RC podium slab which is the lower ground floor of a building. The podium slab is supported RC columns, shear walls at some locations and transfer beams at some others. The purpose of my model in to take the slab reactions on the columns, transfer beams and shear walls (those elements will be designed independently) and analyse and design the slab. Some modeling comments so that you are aware of what I've done so far:

- The slab is 300 deep and I've offset the top of the columns by 150mm downwards.

- I've also offset the transfer beam so that the top of the beam is at the same level as the top of the slab.

To start with, I am getting some strange results. Transfer beam 20 shear force diagram for comb1 is as bellow:

That means that the intermediate column axial force at the top should be 895.94+752.75=1648.69 kN. In contrast it is 1961.06 kN. To sum up, can I trust those results? Do you any any mistake in my model? I have attached my model as well.

Thanks!

[Anonymous]

Does anyone have any ideas?

[saclovitzky]

What I normally do is to get the reactions from columns/walls or beams etc. and transfer it to the slab(separate model) as a point/line loads.

This will make the design of slab much simpler. There's no problem directly designed it from the 3D model and hoping that everything that attached to the slab will be accounted for.

But as a checked, I would normally model it separately, specially when it is a transfer slab.

[Anonymous]

Hi saclovitzky and thanks for the response. Maybe I was not clear. In this case the slab does not take any load from the superstructure. Only the transfer beams do. But I haven't put those reactions in as the transfer beams have been designed elsewhere. I have model the transfer beams, walls and the columns for two reasons:a) to reflect the slab supports (which is indeed supported by the beams, walls and columns) and b) to determine if possible the vertical reactions to those members.

Another issue I've got is that i don't want a fixed connection to the top of the columns so in the releases I made the top column connection as pinned. I checked and the columns do not have any moment at the top but i would expect that the slab would also have a moment near zero at the edge column position. This is not happening in my model (see attached picture).

Any ideas?