Converting Load Cases to Modal Mass

[Anonymous]

All,

I have been using Robot to try and analyse frequencies for a steel balcony. However, there are a few different loading conditions (i.e. mass distributions) that I want frequencies for.

I have tried to separate out the different load cases by only assigning the relevant cases in the convert to mass option (in the analysis type menu) but noticed that the total mass is in excess of the actual mass applied. This excess is the smae in each case (in my model approx 403kg) and applies even if I don't convert any load cases to mass.

Is there a better way of going about this task or have I just made a mistake somewhere?

Link to model: https://www.dropbox.com/s/pf4wdtr23mf04sb/Balcony%20Frequencies.rtd

[Artur.Kosakowski]

403 kg is less than selfweight of the structure (will be the same if you use the consistent mass matrix type). I can't see any issue here (unless you compared kg with kN)

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]

Artur,

I was not using the consistent mass matrix type and it corrected itself when I changed this option, thank you.

Please could you clarify the differences between the consistent, lumped with rotations and lumped without rotations mass matricies and why this effected the total mass taking into the system?

Out of interest, why does the programme automatically convert the self-weight to mass for modal analysis but for all other functions a load case for self-weight needs to be created?

Also, further to my initial inquery, is there an efficient way of obtaining frequencies to multiple loading conditions?

Thanks.

[Artur.Kosakowski]

Please could you clarify the differences between the consistent, lumped with rotations and lumped without rotations mass matricies 

The last two are masses that are generated in nodes (transitional d.o.fs and translational and rotational d.o.fs respectively). The first one is distributed along elements.

and why this effected the total mass taking into the system?

For the lumped one some part of weight is 'assigned' to supported nodes so it is not 'active' for vibrations.

Out of interest, why does the programme automatically convert the self-weight to mass for modal analysis but for all other functions a load case for self-weight needs to be created?

I'm not sure what you mean. Would you like to have all other load cases also automatically converted?

Also, further to my initial inquery, is there an efficient way of obtaining frequencies to multiple loading conditions?

You can create several modal analysis cases and convert different set of loads to masses for each of them. 

[Anonymous]

Also, further to my initial inquery, is there an efficient way of obtaining frequencies to multiple loading conditions?

You can create several modal analysis cases and convert different set of loads to masses for each of them. 

Please could you provide a quick tutorial on how to do this, the 'load to mass conversion' option in the 'analysis type' menu appears (to me) to be independent of analysis type. i.e when I convert a load type to mass and have multiple modal analysis types, all the modal analysis types give the same answer.

[Artur.Kosakowski · Accepted solution]

Here you are

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]

That's great, thanks.

[Anonymous]

can I also do this to determine reactions per mode shape?

[Artur.Kosakowski]

I'm not sure if I understand the question correctly. The modal analysis does not calculate reactions.