Eigen analysis before and after gravity analysis

[Alana]

Hi,

I am creating a 3D asymmetric model in opensees. And when i run the eigen analysis before gravity analysis, opensees gives one set of time periods while runnig the eigen analysis after gravity analysis results in another set of time period. What might be the possible error.
Please help.....

[2015James]

the difference of the periods should be very small, i think it is normal.

[fmk]

if this is a concrete building, the results are expected as the concrete stiffness properties can change a lot based on initial 0 state, slight compression or tension.

[Alana]

Thank you for the reply.
Yes,it is an 8-story RC building. And I am trying to do modal pushover analysis, so the accuracy of time period and mode shapes is very important.

Considering elastic section,
The time periods before gravity analysis are:
T1= 1.387s , T2=1.344s , T3=0.430s , T4=0.417s , T5=0.236s
After gravity analysis are:
T1= 1.409s , T2=1.365s , T3=0.434s , T4=0.420s , T5=0.238s

Considering fiber section,
The time periods before gravity analysis are:
T1= 1.200s , T2=1.162s , T3=0.372s , T4=0.360s , T5=0.204s
After gravity analysis are:
T1= 1.249s , T2=1.210s , T3=0.386s , T4=0.374s , T5=0.212s

I think the time period for fiber section should be greater than that of elastic section as there is softening in fiber section. But in my case, it is opposite. I have gone through my model several times, still cannot find out the error.

Please help.

[m12s12sa]

1) As said above, the fact the period can be reduced after the gravity analysis is to be expected, and indeed you have this result; in your case, anyway, the reduction is quite small (8 ms).
2) The comparison between the periods you obtain with the fiber and the elastic section is not straightfoward as you are putting it, because with the fiber model of the section you do not control the stiffness with just few geometric/inertial parameters (A,E,Ix,Iy, etc.) as you do with elastic section, but rather with the material relationships you define. For example, even if you use the same initial elastic module for the concrete, in the fiber discretization you are also modelling the steel bars whereas probably you are not accounting for them in the elastic definition of the same section. Hence, the fact the periods you get with the two type of modelling (elastic or fiber) is not an error, nor you can expect that the fiber section to give you only smaller periods: it depends on the parameters you use for the materials in the section!
3) What is difficult to understand for me is how you can get different periods before and after the gravity analysis on the ELASTIC section. This should not be the case, because both stiffness and mass should be constant in your elastic model. Do you have other non-linear entities in your model, besides the columns?

[Alana]

Thank you for the information.

When I use pattern command to give elemental loads then, the period before and after gravity analysis for elastic section varies.
For example:
pattern Plain 101 Linear {
#Column
eleLoad -ele 1 -type beamUniform 0 0 -$SelfUDLColB
eleLoad -ele 2 -type beamUniform 0 0 -$SelfUDLColB
eleLoad -ele 5 -type beamUniform 0 0 -$SelfUDLCol
eleLoad -ele 6 -type beamUniform 0 0 -$SelfUDLCol
...................
# Beam
eleLoad -ele 129 -type beamUniform -$UDLBeamBXS 0 0
eleLoad -ele 130 -type beamUniform -$UDLBeamBXS 0 0
........................

However, if I do not use this command, the time period is the same as expected.

[andreascj]

A couple of things:
1. Period before and after gravity should be unchanged if your model is completely elastic i.e., no nonlinear material and NO PDelta transformation. Make sure you are using linear geometric transformation for all elements.
2. Period using fiber sections will reduce because of the increased stiffness contribution from reinforcement. Now if your model is nonlinear then it will tend to increase depending on the gravity load that you have applied.

Hope this is helpful.

[Alana]

Thank you very much. Your comments are very helpful to a beginner like me.