eigen value analysis

[baozai]

Hello, All:

I have a quick question on modal analysis of a column in 3D domain.

I built an elastic column in 3D domain. I fixed the column base and assign the mass to every node along the column in x direction.
When I tried an eigen value analysis, it is very interesting that the first 6 modes are all with the same period 6.3s.
And from the 7th period, it seems the same as the 1st expected period of the colunm, which is 0.07s. I put the period of the first 10 modes in the following. Can anyone help me or give me some hints to correct the model analysis I did? Thank you for the help.
The command I used to build elastic column is
element elasticBeamColumn 1 1 2 1 4930 1496 1.e10 1.e10 1.e10 $IDNorthWallTransf;

T1=6.28319
T2=6.28319
T3=6.28319
T4=6.28319
T5=6.28319
T6=6.28319
T7=0.07117
T8=0.01159
T9=0.00421
T10=0.00230