Participation masses are not satisfactory

[Anonymous]

Hi There,

I am modelling a Chimney, 85m high. I am using the Dynamic-seismic analysis to UBC2009. To start I have only activated masses on X-direction to achieve 90% mass participation. I am getting error message, "Participation masses are not satisfactory". I have run the model without any mass participation target and it gives me about 45% only. How can I increase the mass participation? Is there anything wrong in the model?

Also, when we perform a dynamic or a modal analysis with masses in all three directions (x, y, z); how do we know that the frequency we are getting is in horizontal X or Y direction. The thing I want to know is, if the robot is reporting the local frequency of the structure rather than the overall structural frequency. 

I have attached the model. You help is appreciated.

Raju

[Artur.Kosakowski]

I am modelling a Chimney, 85m high. I am using the Dynamic-seismic analysis to UBC2009. To start I have only activated masses on X-direction to achieve 90% mass participation. I am getting error message, "Participation masses are not satisfactory". I have run the model without any mass participation target and it gives me about 45% only. How can I increase the mass participation? 

You need to increase the number of modes to be calculated.

Also, when we perform a dynamic or a modal analysis with masses in all three directions (x, y, z); how do we know that the frequency we are getting is in horizontal X or Y direction. The thing I want to know is, if the robot is reporting the local frequency of the structure rather than the overall structural frequency. 

Both. You can check the shape of vibrations or look at the current mass in given direction.

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.

[Anonymous]

Hi Artur,

Thank you for your quick response.

I was under impression that if I chose for 90% participation, Robot will automaticaly work out the number of modes required. Looks like this is not the case.

I tried incresing the number of modes to 25 and I gives me 88%. I have checked all the modes and what I have noticed is that the deformation shapes appears to be local rather than global. Is there any way to get the overall global mode shapes only rather than the local vibrations? I was expecting more like a mode shapes in textbook where higher modes have multiple sine curves.

I am also interested in the first mode natural frequency which is 0.87 as per Robot. Looking at the deflection shape it looks that the first mode is correct but the mass participation is only 42%, do you think it may have been affected by local vibration?

Can you please confirm that my understanding regarding the Robot's dynamic analysis as follows is correct;

1. The analysis I am doing above ("Seismic") is a modal analysis.
2. "Spectral" and "Seismic" analysis are similar except in "Spectral" analysis we need to input site specific spectrum. If we input the Specturm as per the code (like based on ASCE 7-05 Sd1 and Sds) then Seismic and Spectral should give similar results.
3. When we prepare Spectra (acceralation Vs time) the Sd1 and Sds should be multiplied by g(gravity). Is there any sample Spectra? I could not find on the Robot setup folders.

Thank you,

Raju

[Anonymous]

Hi Artur,

Sorry, I missed the screen shots you had sent before replying. You have suggested to use 50 modes.

Raju

[Anonymous]

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@Anonymous wrote:

Hi Artur,

Sorry, I missed the screen shots you had sent before replying. You have suggested to use 50 modes.

Raju

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Try using pseudo seismic and set mass participation to 90%.. this option will make robot run modes further until it reaches mass participation of 90% on given directions.

[Anonymous]

Hi MIT-BsCE,

Thank you for your help. I will try your suggestion. Also can you please help me reagrding the other query that I have in my post above.

Raju