Modal Analysis Results

[Anonymous]

Hi,

I'm running a modal analysis with an excitation force to simulate the 1st and 2nd harmonics on a steel staircase. I am obtaining the excitation location from the original modal analysis by observing where the node with the maximum modal mass occurs.

I am then applying an excitation force obtained from some documentation and then hoping to observe the amplitudes at various node locations. I cannon however see any output options to observe the amplitudes at various nodes?

These amplitudes also need to be based upon normalized modal masses, though again in Robot 2014 I cannot see this option without doing this myself in a spreadsheet?

If there is no way of obtaining the amplitudes, do you have any recomendations?

Regards,

Ben. 

[Pawel.Pulak]

Hi Ben,

are you running modal analysis or harmonic analysis?

In case of modal analysis, if you need the components of mode shapes (eigenvectors) in various locations, they can be displayed not only grapically but also in the table of results of Modal Analysis, after changing the displayed table columns. It is also possible to filter the output or to change the format of display.

See this video:

In case of harmonic analysis standard displacements or deformed shape correspond to amplitude during forced vibrations.

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Regards,

[Anonymous]

Hi Pawel,

I'm just running a modl analysis at present. I'm not sure if you got to look at the attachment I sent which basically explains the three items I'm trying to find in the output:

1. Amplitude at the location of the application of force

2. Amplitude at the node I want to read the results from

3. The modal mass at the location of applied force (normalised to 1)

I guess the amplitude is simply the deflection under a harmonic analysis then?

But by obtaining 1 and 2, the results need to come from an analysis where the modal mass is normalised to zero. Can you do this for a harmnic analysis? Can you talk me through the settings as I've never done one?

My process would go something like this:

1. Run Modal analysis

2. Locate the node with the maximum modal mass

3. Apply a force at this node (alpha x mass of person - taken from literature)

4. Somehow run a harmonic analysis based on these results with the modal mass normalised?

5. Observe the amplitudes and determine where the maximums are?

6. plug the amplitudes into the equation on the first attachment I sent

Would this sound correct? If so can you help me out with some of the analysis settings?

Thanks.

Ben.

[Pawel.Pulak]

Hi Ben,

yes, I have seen the screen capture attached to the first post.

What is it taken from? Is it from some footfall analysis regulations?

If yes - than may be it would be easier to use directly footfall analysis in Robot.

Regards,

Pawel

[Anonymous]

Hi Pawel,

No this is from a SCI guide for the vibration limits for floors and stairs. SCI P354. It sets out how to manually calculate a response factor based on some FEA results. In order to do so I need to be able to normalise the modal mass in RObot and run a harmonic analysis as I've described.

Can you help me?

I have Robot 2014.

[Pawel.Pulak]

If you try to use SCI P354 then why not the direct implementation of it inside Robot?

See the screen capture from parameters of footfall analysis in Robot:

Regards,

[Anonymous]

Wow, I had no idea this option was available! Great!

So there are two options here:

1. Self Excitation

2. Full Exitation

There are then two sets of results with regard to accelerations:

1. Overall

2. aRMS

I assume that the aRMS is the Root Mean Square of the 1st and 2nd harmonics which is the value that you then divide by 0.005 to get your response factor?

Regards,

Ben.

[Pawel.Pulak · Accepted solution]

Self excitation is the response analysed in the same location as the excitation -  so probably you are interested in full excitation, where the response is analysed in different locations than the excitation.

In the footfall analysis you can output not only accelerations but also directly the response factors.

Regards,

[Anonymous]

Ok great,

Can you please screen shot the option to have the Response factor shown?

Thanks.

[Pawel.Pulak]

Here is the video showing the various ways of output of results for footfall analysis:

Regards,

[Anonymous]

Hi,

You don't need for the modal analysis to add the mass contribution ? (by direction X, Y or Z to the global, dynamic mass)

thnks