topic Re: Seismic Vs Spectral in Robot Structural Analysis Forum

Seismic Vs Spectral

Anonymous — Wed, 07 Mar 2012 11:11:02 GMT

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

Re: Seismic Vs Spectral

Artur.Kosakowski — Wed, 07 Mar 2012 12:48:33 GMT

@Anonymous 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.

Re: Seismic Vs Spectral

Anonymous — Wed, 07 Mar 2012 14:03:47 GMT

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

Artur.Kosakowski — Wed, 07 Mar 2012 15:26:36 GMT

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.

Re: Seismic Vs Spectral

Anonymous — Thu, 08 Mar 2012 15:42:11 GMT

Thank you Artur,

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

Re: Seismic Vs Spectral

Artur.Kosakowski — Thu, 08 Mar 2012 15:44:19 GMT

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

Re: Seismic Vs Spectral

Anonymous — Thu, 08 Mar 2012 15:59:51 GMT

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

Artur.Kosakowski — Thu, 08 Mar 2012 16:03:15 GMT

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?

Re: Seismic Vs Spectral

Anonymous — Thu, 08 Mar 2012 16:15:42 GMT

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

Artur.Kosakowski — Fri, 09 Mar 2012 13:50:39 GMT

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/GUID-9A9795B9-E2B1-4F70-BC47-FF61A312699-621.htm

Re: Seismic Vs Spectral

Anonymous — Sat, 10 Mar 2012 08:16:51 GMT

You are right about the CQC rule, i missed the fact that it's damping dependant.

Thank you.

Re: Seismic Vs Spectral

Anonymous — Fri, 08 Jun 2012 07:27:21 GMT

Hi,

can you please explain ( maybe in a simple example) the last question

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.

I am new to spectral analysis, and I dont understand in the second interation how can I apply the scaling factors and how to get this factors?

Thank you

Re: Seismic Vs Spectral

Artur.Kosakowski — Mon, 11 Jun 2012 07:10:24 GMT

You can check the sum of reactions and compare it to the targeted value. This will give you the required scaling factor. Then you can e.g. use the corresponding load factor for given ~~seismic~~ spectral case.

Re: Seismic Vs Spectral

Anonymous — Sun, 10 Jun 2012 09:48:23 GMT

That is, check the sum of reactions for the dominant mode in each direction. (not the CQC combo)...

Tony

Re: Seismic Vs Spectral

Anonymous — Wed, 09 Mar 2016 22:36:04 GMT

I have a question over the scaling factor for the modal masses that are added in a modal / spectrum analysys ,

With a given spectrum (acceleration spectrum)

Costa Rica Code stablishi a Seismic Base Shear by

V= W x C,

where C= FED x R, W the seismic mases

Since the Dunamic spectral factor "FED" comes from the spectrum is correct to suposse that

the scaling factor for modal masses in the RSA Analysys tool is is R?

Best Regards ,

Rodolfo R

Re: Seismic Vs Spectral

Anonymous — Mon, 25 Feb 2019 04:35:54 GMT

Hola Rodolfo, dijiste que sos de Costa Rica? Sera que nos podemos poner en contacto para hacerte algunas preguntas de Robot, mi numero es el 8742179.

Re: Seismic Vs Spectral

Anonymous — Mon, 25 Feb 2019 20:45:44 GMT

Hola mucho Gusto Rodolfo Rodriguez ,

Si tiene alguna consulta , con mucho gusto puede contactarme al correo
rrodriguez@ijlcr.com