Inconsistency of periods of Example 5

[igidaris]

Hi,

I have noticed something really weird in the "Example 5. 2D Frame, 3-story 3-bay, Reinforced-Concrete Section".

The first three period of this model are :
T1 = 0.43 sec
T2 = 0.11 sec
T3 = 0.06 sec.

I wanted to check the effect of the gravity loads in the stiffness of the RC elements; thus I removed the gravity loads and I performed eigenvalue analysis. In this case the periods are:

T1 = 0.53 sec
T2 = 0.14 sec
T3 = 0.08 sec

So, the structure is more flexible without the gravity loads, which is anticipated since axial load in general increases the stiffness of RC columns.

Then, I perform time history analysis using as an earthquake input the excitation that you provide in the examples page
(H-E12140.AT2). I changed the scale factor (GMfact) of the excitation to a small value (e.g. 0.01) in order to avoid inelastic response of the structure. Finally, after the time history analysis I performed again eigenvalue analysis and the periods are:

T1 = 0.43 sec
T2 = 0.14 sec
T3 = 0.08 sec

Hence, it seems that the structure has "stiffen" after the end of time history analysis and its first period is the same with the one that I obtained when the gravity loads were taken into account!!!

Isn't it weird? I can't think any physical meaning for this trend!!
Do you have any ideas?

Thank you very much in advance!