Modal analysis problem

[Anonymous]

Hello,

I'm doing modal analysis of concrete bedplate supported on steel spring isolators. Stiffness of the isolators is same in x and y direction. My question is: How is it possible that the natural frequency of the structure is smaller in y direction than x? 

[Krzysztof_Wasik]

Hi @Anonymous 

Could you please share your model for verification.

[Anonymous]

Hi

For some reason I cant share the file. Here is some info:

b=7m, h=2,5m l=9,5m 

spring stiffness: kx=ky=9500kN/m kz=12100kN/m

damping:              cx=cy=160kN*s/m kz=240kN*s/m

[okapawal]

@Anonymous what do you mean exactly by "the natural frequency of the structure is smaller in y direction than x"?

Do you mean mass participation in X, Y direction or displacement shape in a mode?

[Anonymous]

Rigid link master node is elevated 3m from the top of concrete bedplate and nodal force at master node is 1500kN

[Anonymous]

Here is my modal analysis results

[Krzysztof_Wasik]

Hi @Anonymous 

It seems that it is caused by mass eccentricity. 

Normal mode2 ( in X direction) has probably symmetrical rotational component around Y (mass in z =0). While mode 1 has non zero mass in z which suggest non symmetrical rotation around X.

Look at normal modes shapes you should see the difference 

[Anonymous]

Hi

When I move the rigid connections to the centre of the structure I have following results

[Krzysztof_Wasik]

Hi @Anonymous 

Could you check normal frequencies for UZ fixed in supports?

[Anonymous]

fixed the model a bit. Also made support fixed in UZ direction

results:

[Krzysztof_Wasik]

Hi @Anonymous 

Now frequencies are similar. In my opinion effect is caused by different stiffness for rotational vibrations coming from different diameters in Y and X directions when elastic supports are defined in UZ.