Modal Analysis Mass Participation

[AJA14]

Hi. 

No matter what I do, tha mass participation ratio reaches a certain percentage (56%) and stops increasing. 

Before doing some modifications, the mass participation would reach 90 % from the 2nd mode. The changes are:

   1- (Foundations were not included with elastic supports, the columns had pinned supports

   2- Changing loads and addition of new loads. 

   3- minor modifications to upper floors (2 floors only)

 Thanks. 

[teixeiranh]

Well, I guess we would need the model.

Have you tried to make RSAP find out how many modes are necessary to mobilize 90% of the mass?

I guess the problem would be a blunder in the model...

[AJA14]

Hi. I couldn't attach the model. I removed the foundaiton, applied pinned supports for the columns, and now the participation ratios are above 90%. This is strange.

I do have one question. When you include the raft foundation, the mass of the raft foundation is included in the mass participation. With pinned connections, it is not. Could that be the reason. 

I will retry to upload the file. 

Thanks. 

[Artur.Kosakowski]

Try to exclude the mass of  the raft foundation from the analysis. It may be difficult to get the vibrations which correspond to its movements in the horizontal directions 

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.

[patkson]

Let me add a few points to what has been raised,

Sometimes due to skipped vibrations and the likes your total mass of the system nevers converges to 100%, 

This is well explained as "missing mass". It happens all the time especially for quite irregular structures with relatively stiff base.

And it is not bad to model your foundation to capture Soil Structure Interaction.

Robot has an option called residual mode under siesmic analysis parameters,

Once you have quite significant mode and still couldin't reach your target of 90% I will advice you to use the residual mode option to round up your mass to 100%.

It's very cool. check it out!! 

http://knowledge.autodesk.com/support/robot-structural-analysis-products/learn-explore/caas/CloudHel...

[AJA14]

Thanks Guys for your help. Much appreciated. I will take a look into utilizing residual mode.