## Robot Structural Analysis

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# Seismic Vs Spectral

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Rafal/Artur,

It seems to me the difference between seismic and spectral analysis in Robot is that;

In SEISMIC analysis;

- Based on the chosen code, seismic analysis works out the response spectrum etc automatically based on the input parameters (region etc).
- The user can set an expected base shear that Robot will limit seismic base shear to.
- Eccentricity option is available within the seismic options (can leave eccentricity zero in the preceeding modal case)

But using SPECTRAL method;

- I need to define the correct response spectrum;
- I need to apply any eccentricity in the preceeding modal analysis;
- I need to do an initial run, then go back and do a second iteration to apply scaling factors to limit my base shear to the expected value.

Please confirm if my assumptions are correct, feel free to make corrections!

Tony

Solved! Go to Solution.

# Re: Seismic Vs Spectral

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tony.ridley wrote:

Rafal/Artur,

It seems to me the difference between seismic and spectral analysis in Robot is that;

In SEISMIC analysis;

- Based on the chosen code, seismic analysis works out the response spectrum etc automatically based on the input parameters (region etc).
Correct.

- The user can set an expected base shear that Robot will limit seismic base shear to.
For selected codes it is possible. For the ones this option is not available you can 'scale' it defining a load combination with the appropriate load factor.

- Eccentricity option is available within the seismic options (can leave eccentricity zero in the preceeding modal case)
Is available in both modal analysis (accurate solution) and seismic case (simplified approach). It is the either / or choice.

But using SPECTRAL method;

- I need to define the correct response spectrum;
Correct

- I need to apply any eccentricity in the preceeding modal analysis;
Correct

- I need to do an initial run, then go back and do a second iteration to apply scaling factors to limit my base shear to the expected value.
Correct

*If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.*

**Artur Kosakowski**

# Re: Seismic Vs Spectral

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Hi,

i' ve noticed that in spectral analysis the results are influenced by the the damping ratio (controled in the modal analysis menu).

How is this possible?

# Re: Seismic Vs Spectral

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Intended for situation when you have two spectra with e.g. 1% and 5 % damping and you want to average them. Then what you type in the modal analysis e.g. 3% influences the way these spectra are averaged.

**Artur Kosakowski**

# Re: Seismic Vs Spectral

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Thank you Artur,

5% damping will have no influence on the spectrum, is that right?

# Re: Seismic Vs Spectral

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According to my knowledge if you do not average spectra the value of damping should be irrelevant.

**Artur Kosakowski**

# Re: Seismic Vs Spectral

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Hi,

i have no averaged spectrum, just a single custom spectrum defined from a text file.

Damping (the one controled by the modal analysis menu) seems to influence the results in an inverse maner, high damping values give high response values.

This behaviour is more clear in systems with many degrees of freedom.

# Re: Seismic Vs Spectral

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In such case I need to have a closer look at this situation. Could you send me the rtd file and indicate the differences in results you noticed as well as the corresponding values of damping you set in the parameters of the modal case please?

**Artur Kosakowski**

# Re: Seismic Vs Spectral

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Here it is.

You can try some values of damping (e.g 1%, 50%) in the modal analysis menu and check the the columns Fx reaction.

# Re: Seismic Vs Spectral

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As far as I can see the results for single modes and SRSS combinations are the same. The difference is for CQC combinations but this is justified as damping is included in the formula.

http://docs.autodesk.com/RSA/2012/ENU/filesROBOT/G

**Artur Kosakowski**