Seismosoft | Earthquake Engineering Solutions

Hi,
I need help with a problem present in my model.
I modeled a structure, similar to a table, in one case with a slab and in another case without a slab. The structure is only subjected to the self-weight of the elements, with the slab in the case with a slab being defined without self-weight and with a thickness equal to 0.
I performed a linear dynamic analysis with seismic acceleration close to zero to evaluate only the CQP combination. I found that the cases present very different moment diagrams, which is strange since the slab with zero thickness cannot provide stiffness to the structure.
Why does this hapenn?
[img][https://files.fm/u/tcnc8prksz]
Diagrams without slab
[img][https://files.fm/u/xavwed5gbx]
Diagrams with slab
[img][https://files.fm/u/fy8xn3wa66]

Best regards,
Nuno Ribeiro

Dear NunoRibeiro,

As it is written in SeismoBuild's Help System (https://help.seismosoft.com/seismobuild ... s/slab.htm): The slab modelling is carried out with rigid diaphragms; hence, a rigid slab is implicitly considered in the structural configuration, which is the case for the vast majority of RC buildings. The slab’s loads (self weight, additional gravity and live loads multiplied by the corresponding coefficients in the Static Actions module) are transformed to masses, based on the g value, and applied directly to the beams that support the slab.

This means that, if a slab is assigned in the model, the rigid diaphragm will be correctly modelled regardless of the specified slab height. The height value primarily serves purposes such as accounting for the slab's self-weight, applied loads, and inclusion in the exported CAD drawings.

However, I am curious to understand the rationale behind modelling slabs if there are, in fact, no slabs present in the structure.