Similar eigenvectors with different load combination

[zsherliza]

Hi,

I have a few questions:

I have a bridge model, my column section is 1500 x 3500 mm (in local y and z axes). I have followed the script for obtaining eigenvectors and period. I applied horizontal components of ground motions simultaneously and run the analysis. Thankfully, the analysis run successfully. However there are a few observations which puzzled me:
1. I have 4 different load combinations, each one is DL + LL + EQ (earthquake load is following the 30% rule) applied differently. Each time I run with different combination, I observed that the periods and
modeshapes (eigenvectors) are the same. Shouldn't I get different modeshapes and period due to the different load combinations?.

Another question is: is this the correct way to apply the horizontal components i.e. NS and EW simultaneously in the analysis:
#----------------------------------------------------------------------------------------------------------------------------------------------------
# analyze using both horizontal ground motion components (EW, NS)
set outFile kobeEW.acc
set outFile1 kobeNS.acc
set accelSeries "Series -dt $dt -filePath $outFile -factor 10"; # 100% EW
set accelSeries1 "Series -dt $dt -filePath $outFile1 -factor 3" ; # 30% NS

# Set the direction of excitation
#-----------------------------------
set GMdirection1 1;
set GMdirection3 3;

# create UniformExcitation load pattern
#---------------------------------------
# ID loadtag grd motion dir
pattern UniformExcitation 100 $GMdirection1 -accel $accelSeries;
pattern UniformExcitation 200 $GMdirection3 -accel $accelSeries1

Thank you for your time.

Regards,
zsherliza

[vesna]

Mode shapes and eigen vectors are function of mass and stiffness of your structure. They do not depend on the external load applied on the structure.

Yes, the way you define your ground motions is correct. They will be applied simultaneously on your bridge during the analysis.

[zsherliza]

Dear Ves,

Thank you for the answers. I understand that modeshapes are function of mass and stiffness. This is why I think if section dimensions are different, and if I subject EW component longitudinally, I would expect the period and mode shape would be different, and vice versa if I subject it in transverse direction....this is because stiffnesses are different on each face.

Thank you Vesna, I only hope I can learn more from the discussion.

[vesna]

Mode shapes of of the structure in the initial condition is not function of the load applied to it. If you perform the eigen analysis after you have applied the load you will see the change in the mode shapes.

[zsherliza]

Dear Vesna,

Thank you for the reply. I got the point now.