Harmonic Analysis for Dummies

[sybie99]

Hi guys,

I have a structure, basically a mixture of concrete and steel support frame, with a motor on top. I want to determine whether the structure is safe and has the capacity to carry the motor and the vibrations it will create. Now, I am a bit confused on the basic procedure and what exactly I am trying to determine. Firstly, I know I must calculate the natural frequency of the structure for a number of modal shapes. This I do by using the selfweight of the structure and motor only.

Now once I have determined this, I apply the dynamic load, in this case it is point loads (forces) at the motor bearings, and I input into the software programme the rpm or frequency of the load.

Now, this is where I get lost, having done the above, what do I consider next. I understand the natural frequency of the structure must be out of range of the operating frequency of the motor to prevent resonance. For the case where I apply the dynamic load, must I look at induced stresses, what about the frame's response to the harmonic loading? How do I check the response, what is meant by response and what must I look out for in the results.


I know that for some this is very basic engineering, but I have never really looked at this type of case and am not in an office where anyone can assist.

Hope someone hcan guide me through the procedure.

Thanks

 

[structSU10]

You are on the right track. you want to understand the transmissibility of the machine on the foundation. Here is a link that describes this further:

http://www.engineering.uiowa.edu/~sxiao/class/058-153/lecture-9.pdf

or you can try your own google search for transmissibility - you basically get a force from the resonance effect that you can design your structure for.

 

[GregLocock]

Transmissibility is not directly important in this case, as it is the ratio of acceleration or whatever across an isolator typically. The Op needs to add damping to his modal model and then examine the response to the excitation, so for example, if his machine creates a vibrating force of 25 N at 50 hz, he needs to build that laodcase into his model and examine the dynamic stresses created.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[sybie99]

Greg, could you, if possible, summarize what is meant by response? Is it basically the reaction of the structure to the applied dynamic load? SO if I check the resulting stresses and they are within limits, and the natural frequency of the structure is in a different range to that of the motor, all is okay? I suppose defelctions are also and issue and should also be within the criteria required.

 

[GregLocock]

Um. Well presumably you had some goal in mind when you started this analysis. Where you have got to might lead to a noise analysis, or stress, or vibration. So it really depends on why you did it in the first place. I'd be a bit surprised if running a motor made a building fall down, so I'm guessing noise or vibration.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?