SAP vs ETABS

[CdeDo]

The structure that I am trying to model has a strange shape. The shape of a butterfly (two wings and a small "core" between this wings).It is a steel structure. It has 21 levels. In our country, because it is a seismic one, the building's elements are "sized by the Earthquake".
I have made a model in Etabs and everything seemed to be perfect .. too perfect. The building has in the first two modes "pure" translation and mass participations is over 65%. From my point of view it was something strange .. because I could not understand how can that small core hold the two big wings to work together.
So I modeled in SAP and the same structure had a total diferent behaviour. The way I expected. The wings are approacing or departing, they do not translate as a "whole" like in etabs. But the forces in sap are huge (It is imposible to make such enormous elements).The difference between SAP and etabs in terms of efforts is almost triple.
The concrete slab I modeled in both cases with shell (The shell was mesh) and I degraded the modulus of elasticity a thousand times.
The both models are identical. There are no warnings, the models have passed trough others persons to be verified and the conclusion was that they are identical and correct, but they did not have any explanation for the different outcomes (results).
My dilemma is why this difference? Which can I rely on?
The attached file is a plan view.. This a plan from other attempts, with the slab as a membrane, because of that the mesh isn't visible.

 

[WillisV]

First thought is to to make sure that the diaphragm constrains are the same between the two programs (make sure ETABS isn't asigned a Rigid diaphragm while SAP is not). Second thought is to review the mass that is participating in the modal analysis (the settings for what to use for Mass are slightly different in SAP vs ETABS).

 

[CdeDo]

Thanks for the suggestion..
There are the same diaphragm constraints. I did not assigned rigid diaphragm in Etabs.
Regarding the declaration of the participating mass, I am quite convinced that this is not the problem.
It is not the first project when I use both programs for the same structure. It is the first where the results are different.
I am convinced that I have made a mistake somewhere but I could not find where.

 

[CdeDo]

I solved the problem reinstalling Etabs.