Modal Analysis Outputs

[Anonymous]

I have been requested to provide dynamic properties for one of our structures modelled in Robot.

I have undertaken the modal analysis but after looking through the help file and the community boards I am still struggling to locate all the data required.

Could you please advise if/where I can locate the information detailed in the attached PDF.

I have attached the modal analysis results in Excel format for reference and the analysis options to ensure there is not a required box that I have not checked.

Thanks in advance.

[Anonymous]

Check out both of these tables.  If you don't see the information you want, try right-clicking and selecting "table columns" and see if the information is in there.  Let us know if there is still information you need that you can't find.

[Anonymous]

Ken,

Thank you for your response.

The stories table has provided further useful information for me, so thank you. I had seen the dynamic analysis results table before which thought I had attached in my first message but turns out it wasn't.

I am still in need of mode shapes for each floor (as shown in page 6 of the previously attached document), is this possible?

Thanks.

[Rafal.Gaweda]

I am still in need of mode shapes for each floor (as shown in page 6 of the previously attached document), is this possible?

Seelct all elements of a floor, open them in New Window then:

[Anonymous]

You can get numeric values for the eigen vectors (at nodes) on the modal analysis results table by using table columns and selecting the eigen vectors tab as shown below.  I do not believe that there is a way to get reduced eigen vectors for a story as you have shown in your PDF file.  Maybe Rafal can help us with that.

[Tuctas]

I will just add that you can also view the values of eigen vectors in parallel with the displayed deformed shape (i.e the values of displacements of nodes corresponding to modes) through the window of results-diagrams for bars-deformation (see the attached file). These displayed values are the un-normalized total - resultant values (consisting of all components ux, uy, uz). However, sometimes the values cannot be viewed... (in case of more complicated models like shell structures e.t.c)

[Rafal.Gaweda]

 I do not believe that there is a way to get reduced eigen vectors for a story as you have shown in your PDF file.  Maybe Rafal can help us with that.

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Yue are right Ken, no way.

[Anonymous]

Rafal,

As a work around, I have created a node at the centre of mass and used that for the 'per story' data, which seems to have given reasonable results.

The only problem is that the torsional rotations (RZ eigenvectors) appear to be of the wrong magnitude.

I have attached photoshots of the modal shapes and the eigenvectors.

Is there something I have missed with setting up the geometry or the modal analysis?

Kind Regards,

[Anonymous]

Please help me 

could you please simply explain to me the difference between the analysis mode (model , seismic and psedo mode ) and when i have to use each one ?

also if you help me to let me know the same things about the method of solution for the eign-problem , which and when i use each one ?

 thanks 

[Pawel.Pulak]

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@Anonymous wrote:

 As a work around, I have created a node at the centre of mass and used that for the 'per story' data, which seems to have given reasonable results.

The only problem is that the torsional rotations (RZ eigenvectors) appear to be of the wrong magnitude.

I have attached photoshots of the modal shapes and the eigenvectors.

Is there something I have missed with setting up the geometry or the modal analysis? 

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Please see the description in message 2 of this forum post:

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Modal-Analysis-Outputs/m-p/4877418#M21562

or in message 14 of this forum post:

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Mode-shape-vectors/m-p/3948314#M14271

It describes relation between translation and rotation components of eigenvectors.

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If this post answers your question please click the "Accept as Solution" button. It will help everyone to find answer more quickly!

Regards,

[Anonymous]

Pawel,

Thank you for your response.

I am still unsure of the magnitude of the rotations that Robot is producing. Given the modal shapes uploaded previously, the rotation RZ of modes 2, 4, 7, 9 and 12 has a greater impact on the final modal shape than lateral displacements. The magnitude of UX and UY reflect this but not RZ, this is what the cause for concern is.

On another note, I am also looking into the frequencies of individual slabs within a structure, do I have to remove the rest of the structure to get the dynamic analysis results for each individual panel?

Regards,

Adrian

[Pawel.Pulak]

As concerns the abnormally small values of rotations in relation to displacements in vibrations shapes it is difficult to tell anything without the model.

Such effects can be obtained for the model without any shear walls or beams (only slabs and columns) and it is related to "drilling rotation" degree of freedom in surface finite elements used by Robot (see the screen capture made for top view of some simple test model - the sinusoidal shape of deformation). But after adding any beams in plane of the slab or connecting any shear walls to the slab this effect  is almost not visible.

... and in the screen captures of your model I have seen some cores and shear walls - so the above explanation does not fit and your model would be useful.

As concerns the frequencies of individual slabs it will be very difficult (if at all possible) for the complete structure. You can try to obtain it increasing the number of modes and /or considering only the vertical mass. But possibly there will be interactions with the adjacent walls etc, if fixed connections are defined between them.

Best regards,

[Anonymous]

Pawel,

Thank you for your response. I have attached a link to our model for your reference.

https://www.dropbox.com/s/q36kqyj8n7eoer9/T101VH%20-%208729%20Building%201%20%20-%2027.02.14.rtd

Regards,

Adrian

[Anonymous]

Thank you for teh advice ,

strangely I found a diference of 0.01 between displayed value and calculated value

I have  attached the screenshot

I tried to calculate participation factors but it doesnt seem to be coherent by hand and by RSA values .

[Tuctas]

Keep in mind that the displayed values are the un-normalized and total - resultant ones (consisting of all components ux, uy, uz).

[Anonymous]

Hi,

Could you please explain how did you create the node int the centre of mass of the level?

Thank you

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@Anonymous wrote:

Rafal,

As a work around, I have created a node at the centre of mass and used that for the 'per story' data, which seems to have given reasonable results.

The only problem is that the torsional rotations (RZ eigenvectors) appear to be of the wrong magnitude.

I have attached photoshots of the modal shapes and the eigenvectors.

Is there something I have missed with setting up the geometry or the modal analysis?

Kind Regards,

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