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Tuned Mass Damper (TMD) creation with RSA

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Message 1 of 11
aurelijus
6710 Views, 10 Replies

Tuned Mass Damper (TMD) creation with RSA

Hello engineers,

 

I am trying to model damping system for pedestrian cable - stayed bridge and reduce pedestrian induced vibrations.

Do you have any ideas how to model these dampers in RSA:

1. Tuned Mass Damper (TMD)?

TMD figure.gif

I tried to do it with compatible nodes: added one node where the TMD should be located. Applied weight to that node [kN] and converted to dynamic mass. Here is the print screen:

 

TMD creation.png

 

Problems:

- I did not suceed to notice any stabilization effect in THA although damper mass, stifness and damping are choosen properly. I compared THA results With TMD and Without it. Is this the right method that I am doing for TMD modeling? Is there any others?

- Due to nonlinearity I can analyze only THA. But I need Harmonic FRF analysis as well and diagrams that shows sensitivity in different range of frequencies. Besides I need diagrams that shows damper and point of anchorage acceleration due to pedestrian induced forces in different range of frequencies. And the efect of damper. for example like this:

like this.png
Should I make a linear model in order to analyze Hormonic FRF? if yes, how should I than create TMD? becuse nonlinearity appears due to compatible nodes, cables, even tention only bars...

 

 

2. Another dampers - Hydraulic or piston viscous dampers.

Have no clue how to model these in RSA. Is it possible?

 

I am struggling with damping task in RSA for already long time... I have tried Staad - there is no possibility to model dampers. Only possible to model pedestrian induced vibrations. I have also tried Cosmos. Looks like there is possibility to model dampers but there is no information how to do it... I didn't suceed to figure out it by myself.

I know that Midas has great options for this but I don't have it and have no time to learn another program. 

 

I would very much appretiate your support.

I have attached my file.

Thanks in advance.

 

 

10 REPLIES 10
Message 2 of 11
Pawel.Pulak
in reply to: aurelijus

Aurelijus,

your way of modeling tuned mass dampers in Robot (compatible nodes with appropriate stiffness, viscous damping and added mass) is generally correct.

I have some remarks related to modeling compatible nodes which can influence results (especially the first one):

  • compatible nodes should not be modeled with different coordinates (see message 4 in this post)
  • bars should be divided in compatible nodes - it concerns beam 436 in your model. In opposite case it is impossible to control which of the compatible nodes it is connected to when they have the same coordinates (as recommended above)

As concerns the results you obtain with TMD active and inactive (load to mass conversion for load case 7 active and inactive respectively) the amplitudes of displacement of the node 107 almost do not change. With TMD it is oscillating between +20.95 and +28.69cm and without TMD between +21.06 and +28.5cm. So with TMD it is even slightly worse than without it.

But are you sure that your tuned mass damper is really tuned and/or that you check the frequency of excitation where TMD is really effective?

 

Some time ago I have made some tests of the efficiency of tuned mass dampers in Robot but these tests were made for linear model. Because viscous dampers can be used in Robot only in time history analysis (THA) and not in harmonic or FRF analysis, it was necessary to run a series of THA with various frequencies to verify the efficiency of TMD. Here it the diagram of the amplitude of displacement in the function of the frequency of excitation I have received for my test models:

tmd.png 

As you can see the efficiency for a correctly tuned damper was quite good - but even in such case there are frequency zones (between 2.1 and 2.45 Hz for diagram above) where TMD is deteriorating and not improving the situation. Of course this deterioration can be ignored observing improvement in other frequency zones (between 2.5 and 2.9 Hz for diagram above).

 

If it can be helpful for you to verify your model I have attached the document with steps describing the creation of my test model and attached one of the files used (with some of frequencies of excitation used to create the diagram above).

 

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

 

Regards,

 


Pawel Pulak
Technical Account Specialist
Message 3 of 11
aurelijus
in reply to: Pawel.Pulak

Thank you very much Pavel for this valuable information! You are amazing!

TMD modeling algorythm and example that you provided are very deatail and just great!

You are right, my TMD isn't tuned well. My mistake. I'll correct my model and make a sequence of diferent THA as you suggested.

 

I have studied your exaple of TMD. want to clarify couple things:

1. In the description you wrote that "a. fundamental frequency of the structure f0 = 2.6986 Hz"

After analysis I found that its 2.34 Hz.

Besides, you wrote that "c. modal mass for mode 1 M=966 kg". How do you define value of this mass?

Are you taking persentage of Total dynamic mass like in the printscreen below?

Modal Mass.JPG

 

2. Second question is how do you describe TMD mass? Just simply adding mass to the compatable "node 22" when you want to see effect of TMD and removing this mass when you want to see exication of induced vibrations like in the below printscreen?

 TMD mass.JPG

 

Is it needed to describe this mass as dynamic? Like I was doing with loads in my previous message>> Defining a load of TMD and afterwards converting it to Dynamic Mass?

Besides, do you mark box "Apply to all cases"

 

Thank you for your great work!

Aurelijus

Spoiler
 
Message 4 of 11
Pawel.Pulak
in reply to: aurelijus

Some explanations to your questions

Fundamental frequency of the structure f0 = 2.6986 Hz was calculated without tuned mass damper.

Modal mass was taken directly from results of modal analysis (without tuned mass damper) activating its display.

modal_mass2.png

 

Yes, I have defined the mass of TMD defining added mass in node 22 (BTW: I have made here a mistake of unnecessary definition of mass in rotation CY but fortunately because of the symmetry of the model and symmetry of excitation it has not influenced the results 🙂 ).

Deactivation of TMD was done by deleting this mass in the table of masses.

Mass was defined as "Global" (with "Apply to all cases" active) but in this model it gives the same effect as "Dynamic" or assigned to specific load case (so called "local") - see also message 3 of this post.

added_mass.png

 

Regards,


Pawel Pulak
Technical Account Specialist
Message 5 of 11
aurelijus
in reply to: Pawel.Pulak

Thanks for explanation.

Appretiate this!

Message 6 of 11
ec11051
in reply to: aurelijus

Good Afternoon,

I have a question I hope you might answer, respecting to modeling TMD in RSA.

One way to model  the damping of the TMD, is  introducing the value C in the "compatible nodes" option.

My question is, if I create a equivalent two degrees freedom system and calculate  by modal analysis the natural frequecies and modal damping ratio, is it correct to introduce that values in time history analisys damping option? 

 

I did that and the results of structure response were rather similar respecting to "compatible nodes" option but less conservative.

 

Sorry if my english is not completely correct.

Best regards,

Message 7 of 11
ec11051
in reply to: Pawel.Pulak

Good Afternoon,

I have a question I hope you might answer, respecting to modeling TMD in RSA.

One way to model  the damping of the TMD, is  introducing the value C in the "compatible nodes" option.

My question is, if I create a equivalent two degrees freedom system and calculate  by modal analysis the natural frequecies and modal damping ratio, is it correct to introduce that values in time history analisys damping option, and in

"compatible nodes" option put just stifness value k, but disregard the value C, in other words, C=0?

 

 

I did that and the results of structure response were rather similar respecting to "compatible nodes" option but less conservative.

 

Sorry if my english is not completely correct.

Best regards,

Tags (1)
Message 8 of 11
luismrf
in reply to: Pawel.Pulak

Dear Pawel,

 

i post a doubt concerneing the study of a TLD (Tunned Mass Damper),instead of a TMD, and gave this link because the  the theoretical rational is similar to the TMD.

 

The diference is that instead of i mass i have 2 masses, one m0 (is a mass of water and can be considered death weight to the structure), m1 is the mass attached to a spring and as damping( similar to the TMD mass).

In the case of the TMD is attached toa a node of the structure, and to preform a THA is needed to make a compatible node.( i read the pdf)

 

In my case i need to place this masses in the topo of the tower (see the image it shows the mechanical priciple, and the dimension of the water tanks ( TLD), so i made 2 nodes in the top of the tower in four cases to mitigate the motion of a rotating radar of frequency o 1 hz).

 

to simulate each tank of water (TLD), i made 2 nodes link through "rigid links", in the node at 0.89 cm height corresponding to m0 placed a nodal force corresponding to the weight.

In the case of the m1 i placed a nodal force in the above node with the corresponding weight.

Then in the top nodes i placed a compatible node with the stifness K, and damping c, that i´ve calculated.

 

i´ve preformed the THA the way you  explain in the pdf, but some error ocurr, "type 1 at the compatible nods sayng there is a problem with the uz and rz,"

Why this error happens if i fixed the displacement on this direction, because in the compatible nodes i only free the directions x and y so the mass mittigate the oscilation  of the tower?

 

Could you give a look in my file and help me with it?

 

Thanks in advance!

 

 

Message 9 of 11

Hi @Pawel.Pulak 

You wrote:

Here it the diagram of the amplitude of displacement in the function of the frequency of excitation I have received for my test models:

HoshangMustafa_0-1631625056669.png

How can you access this diagram? Is it through Results > Advanced > Time History Analysis - Diagrams …?

 

Message 10 of 11

Hi @Pawel.Pulak 

Any comment?

Message 11 of 11

Hi @HoshangMustafa,

no, this diagram was not created in Robot.

It was created in MS Excel - but basing on results from "Results > Advanced > Time History Analysis - Diagrams" in Robot:

  • each point in this diagram corresponds to the stabilized (after dissipating transient oscillations) response from the time history diagram in Robot for specific frequency of excitation

Regards,


Pawel Pulak
Technical Account Specialist

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