Modal analysis - Dynamic global masses

[Anonymous]

Hello everyone,

I have done a modal analysis of a building and everything looks OK but when I checked the Modal Analysis Result, the dynamic global masses don’t match with the mass of the model or the mass that appears in the Modal Analysis Table.

I tried a small model because there are some warnings in the building calculation and I thought it could be the cause of this different values but I have the same problem with a model that is only one bar. It looks like the mass in this report is the double that the real mass when I have the “disregard density”... 

You can see below and attached the results of this simple model:

1- Model: Only one bar fixed, with the dead load as converted mass and disregarding density

2-The mass of the bar is 181kg and we can find this value in reactions and in the table Dynamic Analysis Result

3-However, when I look at the Analysis Report we find the double of mass 362kg (Analysis>Calculation Report>Simplified Note)

I have also tried two additional checks:

- If I do the same calculation with an additional load, the dynamic load is the total load and the double of the self-weight of the bar.

- The calculation when the “disregard density” box is not ticked is the same and the Analysis Report shows the correct masses.

Is there a mistake in the Analysis Report or this value has a different meaning?

Thanks in advance!

[Anonymous]

Hello

I have read a couple of times on other posts, that many often tend to add selfweight of the obejct twice in the load windows maybe it will help.

Can you post a list of your load.

 Edit: it has to be the table edition.

Best regards

Kim Christensen

[Rafal.Gaweda]

File please.

[Anonymous]

The model I have tried is so simple that I was ashamed to attach it. I checked some structures I have already calculated with seismic loads and it's always the same.

This model only a bar with the density disregarded and dead load converted to dynamic mass. You have to look at Analysis>Calculation report>Simplified note where you can find the dynamic global mass considered and it double the dead load.

All I can do is convert the dead load to the modal case and not to the dynamic mass but it should be OK also as dynamic load if we disregard density, isn't it?

Thanks,

[Rafal.Gaweda · Accepted solution]

Calculation note corrected in 2014 version.

Correct masses you can find in modal results table.

[Anonymous]

Thanks for your help

[structural.engineer79]

Greetings

1- Whats the difference between global mass and dynamic mass ? when we can use global and when we can use dynamic.

2- Mass direction will be in X,Y,Z OR either x,y or Z. according to what the selection will be ? this will effect on the seismic in vertical direction.

regards

[Artur.Kosakowski]

Hi @structural.engineer79

Greetings

1- Whats the difference between global mass and dynamic mass ? when we can use global and when we can use dynamic.

The prior applies also to forces of inertia (body forces) analysis.

2- Mass direction will be in X,Y,Z OR either x,y or Z. according to what the selection will be ? this will effect on the seismic in vertical direction.

These directions determine dynamic degrees of freedom. E.g. when you model a floor as a diaphragm you may want to exclude Z as no vibrations of the slab in this direction is going to happen anyway.

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.