DYNAMIC ANALYSIS 3

[sposhni]

Hello!

I am designing Table Top Structure supporting centrifugal compressor.I need to carry out the dynamic analysis on Staad Pro.In my case,I need to opt for Time History analysis.I have got the unbalanced weight of rotating part of my compressor.Now,I need to establish relationship between Time and force function to get the maximum amplitudes of my system which should be less than the amplitudes suggested by the compressor vendor.My problem is that how can I model the compressor on top of table top structure to get the desired results.This is the first time,I am carring out vibrational analysis on staad pro.I want to seek your guidance and help.

Regards
sposhni

 

[Qshake]

Typically when using a time history analysis in staad, you assign the time history (displacement or acceleration vs time) to a joint. The joint is displaced or the mass attributed to the joint is excited and the appropriate stiffness analysis can be carried out.

For the typical transmissability problem one assigns the time history to the support(s) assuming base excitation. I suspect that you can apply these histories to any joint that directly supports the machinery so long as it is physically possible to transfer that motion to those joints directly. If the load path isn't direct, you will lose energy from your original input and the input to the joint could be far less than what you planned.

I suggest that you look into a structural dynamic text for machine vibrations or the shock and vibration handbook for additional tips.

Regards,
Qshake

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[ThomasH]

First of all, if you work with vibrations it is usually more efficient to do the analysis in frequency-domain instead of time-domain. That gives you the opportunity to check how sensitive you design is.

But since your choice is STAAD I think you don't have frequency-domain available. There might be something in the latest version but I'm not sure.

Also, is STAAD mass is applied as load and it has direction, make sure that you know what you are doing in terms of +/- on the forces.

Good Luck

Thomas

 

[GWSystems]

I run a complete time history analysis in STAAD all the time. Usually on large steel structures though.

In your problem, you may need to actually model the piece of equipment using plates, beams, or solids. You can use beams with releases in appropriate places to simulate the behavior of the compressor. Include the elements in the model of the table top. The reason is so that when the mass matrix is built, the mass of the compressor will be included in the eigensolution. I have found it to be better to represent the vibrating equipment with actual elements because the proportion and location of the mass as well as the behavior of that mass is figured more exact and gives a better representation of the behavior.