Stiffness of Element

[JaCl]

Hi.

I modeled a plane moment-resisting frame structure with concentrated plasticity at the girders and columns ends.
Therefore I modeled two types:
1) Type A: the element is elastic (and stays elastic) and the inelastic properties are assigned to rotational springs at the beam/column ends (with modified stiffness because of the elements in series)
2) Type B: the element is rigid and the inelastic properties are assigned to rotational springs at the beam/column ends
The (rotational) stiffness of the springs and the elements of TYPE A are chosen to match the stiffness of the Element in model Type B.

But if I do a modal analysis (eigen command), then i get a different period for the two cases. (T = 3.82s for Model type A and T=3.79s for Model type B)
Can you tell me any explanation, why this is the case? Is there a difference in the element-stiffness-formulation?

The problem seems to be in the column. When I changed the properties from model Type A to model Type B only for the beams, than the period is the same.
Just when I change the properties from the columns (from modeling type A to type B) than a difference in the period occurs (independent of the modeling of the beams).

First I thought that P-delta could be an explanation, but when I changed the tranfsform command to Linear (from PDelta), the same effect appears.

Do you have any further explanation, why I don't get the same periods? And why is it working for beams only, but not for columns?

Thanks a lot, Clemens

[vesna]

The reason for this lays in the way you define the stiffness of your elements. If you use approximate method you can not get the exact solution. How did you define stiffness of your elements?