SAP vs ETABS 1

[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.

 

[ge0san]

CdeDo,
Have you used the "rigid diaphragm" feature in Etabs? This might explain it.

 

[CdeDo]

Nope.. I did not

 

[ge0san]

Well then, possible answers are a lot. Yet, it is impractical to continue guessing.

 

[slickdeals]

I would second geosan, check the rigid diaphragm assignment either to point objects or to the slab. I agree that there is no way a core can hold the wings.

 

[CdeDo]

I solved the problem reinstalling Etabs.

 

[stressed]

The meshing in your model does not look good. Meshing needs to be carried through to adjacent area elements. ETABS, by default, uses a line constraint which connects/zips together adjacent elements which do not share mesh points. That way, ETABS will give a result even with a poor mesh by attempting to interpolate between the closest mesh joints which may be relatively far apart.

With SAP2000, the line constraint (called "edge" constraint in SAP) is turned off by default.