topic Re: Mass participation in Robot Structural Analysis Forum

Mass participation

Anonymous — Mon, 02 Sep 2013 16:40:00 GMT

I have only 18% mass participating in 40 modes. Could you give me your advice

Thanks

Re: Mass participation

Anonymous — Mon, 02 Sep 2013 23:23:47 GMT

very broad question?

Re: Mass participation

Anonymous — Tue, 03 Sep 2013 00:05:22 GMT

Thanks a lot for your interest, if you see my file in the attachment at the first message, I have claddings with wind loads and vertical loads to simulate the weight of exterior walls (self weight), this wall not contribute to stiffness but are mass to calculate the seismic force. Maybe is it necessary replace the loads by nodal mass at the join points of columns?
I can’t find what is wrong but I can’t increase the seismic mass participation.

Re: Mass participation

rsousa_ — Tue, 03 Sep 2013 10:19:49 GMT

With your model, i'm getting this results. My version of robot is lower than yours, so I had to make some corrections in Analysis-->Analysis Types--> Load to mass conversion and Combination sign. When you are dealing with modal analysis these tabs are very important.

I think with 40 modes i'm getting 99.9% of mass participation.

Re: Mass participation

Anonymous — Tue, 03 Sep 2013 10:54:24 GMT

Thanks, could you explain me what changes are you made?

Re: Mass participation

rsousa_ — Tue, 03 Sep 2013 11:20:49 GMT

in load to mass conversion table, the load cases were missing. there were values of "0" and I modified to "1", "2" and "3". then hit calculation button and get that results.

I don't know if the third line was load case "3" snow with coeficcient 0.25.

if it ins't put the right loadcase and I guess you will get similar results in terms of mass participation.

Re: Mass participation

Anonymous — Tue, 03 Sep 2013 11:53:42 GMT

Not, in my model (first message this post) I have :

load to conversion table Cas 1, 2 and 3
Snow load coefficient =0.25 is good
Total mass of building are 2 008 924 kg

In your post you have only 101 586 kg

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 12:06:41 GMT

David,

Have you managed, checke, corrected the instability in model?

Re: Mass participation

Anonymous — Tue, 03 Sep 2013 12:24:42 GMT

Yes,

If I check DSC algorithm I have instability type 3 only (I guess because I have a diaphragm)

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 13:20:39 GMT

Apply "Shell" and use 80 modes.

Re: Mass participation

t.sautierr — Tue, 03 Sep 2013 13:25:45 GMT

I took a look to your model :

-> see the modal shapes : acutally the modes are only local modes, I think this is due to the fact that your roof is light and your bracinf is quite stiff (St andrew crosses), in the other hand, your columns are quite high.

that's why the local modes appears before the global ones. To illustrate this you can try two things : just to see, try to change the thickness of your roof into something heavy (for example EP30 which is concrete 30cm), or remove the crosses and put fixed support at the bottom; In both case, you will find that 5 modes will give you more than 80% of the mass.

I the model I also remove the rigid links because I think it is double role with the fact you define a soft diaphragm.

Either there is no diapgram and you define Rlinks manually - Either diaphram but no Rlinks ? Am I right Rafal?

Rafal, this lead to a question about the local modes :

Is it possible toinactivate some element only for the modal/seismic analysis? I don't think so but ....

If not, the typical solution to solve the issue is :

- use concentrated matrix without rotation

- use fake material (very low density (0 is not possible but maybe 0.01 kN/m3) )

- Other tricks?

Re: Mass participation

t.sautierr — Tue, 03 Sep 2013 13:27:07 GMT

That's what I used to test the model, but why is it better working with a shell than a diaphragm ?

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 13:44:14 GMT

This is small model - use 80 modes

Of course to erduce number of modes you have to get rid of some local modes giving small % of masses.

You can disable bars ("Inactive") but for all calculations.

You can exclude them for SW generation (sw on Part of Structure) and instaed of their sw apply nodal loads or masses directly

You can make fake material

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 14:11:23 GMT

That's what I used to test the model, but why is it better working with a shell than a diaphragm ?

To calculate 80 modes 🙂

Re: Mass participation

t.sautierr — Tue, 03 Sep 2013 14:19:38 GMT

🙂
didn't understand? why for 40 modes it should be diaphragm and for 80 shell?

Re: Mass participation

t.sautierr — Tue, 03 Sep 2013 14:21:35 GMT

"You can exclude them for SW generation (sw on Part of Structure)"

How : create SW load case, suppress SW and reapply SW only for element I want to have taken into account?

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 14:25:52 GMT

didn't understand? why for 40 modes it should be diaphragm and for 80 shell?

Guess.

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 14:36:17 GMT

How : create SW load case, suppress SW

http://screencast.com/t/oPwCEmwun

Re: Mass participation

t.sautierr — Tue, 03 Sep 2013 15:08:00 GMT

Diaphargm reduces the number of degreee of freedom but ..... in this structure we have alread plenty .... grrr I don't find 🙂

Re: Mass participation

Rafal.Gaweda — Tue, 03 Sep 2013 15:13:09 GMT

Diaphargm reduces the number of degreee of freedom but ..... in this structure we have alread plenty .... grrr I don't find 🙂

Because he used "meshed" diaphragm = meshed panel with rigid link.