Learning Forum

[javat33489]

Harmonic analysis and resonance

Hi all.
I'm doing a modal + harmonic calculation for a lamp post.

The column consists of two parts, a vertical 6 meters long from a 500x500 mm pipe, and a horizontal top 2 meters long from a 250x500 pipe, the thickness of the pipe sheet is 6 mm.

In modal analysis, I see that the first modes are below 20Hz, which prompts me to conduct a harmonic analysis with prestress from the modal to check if there are responses at natural frequencies to exclude resonance.

After conducting a harmonic analysis, even at the smallest loads, I always see responses at at least the first two natural frequencies.

I believe that the resonance is confirmed. Is my reasoning correct? Moreover, if we carry out a dynamic analysis of the structure, it will withstand with a large margin.

[dlooman]

Unless the load doesn't excite the mode at all, there will be a resonant response at each natural frequency.  You should turn on clustering in the harmonic analysis settings to get better resolution of the peaks.  Be sure to have some damping specified.

[javat33489]

Unless the load doesn't excite the mode at all, there will be a resonant response at each natural frequency.

Yes sir, this is exactly what I encountered. That's what I wanted to hear. If the response does not change at its own frequencies, this is definitely a resonance.

Sir I was unable to enable cluster result:

[dlooman]

Turning on clustering shouldn't invalidate a frequency response result.  If you try to evaluate the results do you get a message that says why they aren't available?

[javat33489]

Yes

[dlooman]

What are the details for the Frequency Response objects?  It's usually the Analysis Settings Output requests that make a result unavailable.  Have you specified damping?  The clustering option will try to solve for the resonant response which is infinite if no damping has been specified.

[javat33489]

Dumping enabled sir

Results settings are standard

Today I received this message. By the way, I considered modal analysis without dumping.

[dlooman]

I wonder if it's a bug with the comma character.  If you look at the Harmonic ds.dat file do you see a command starting with dmps?  (It wasn't necessary to specify damping in the modal.)

[javat33489]

 

Sir, due to an error, ds.dat is not generated, even if you save the project.

 

[javat33489]

Sir. I figured it out with the message above.
There will always be responses at natural frequencies. You need to watch the amplitude. The amplitude strength varies depending on the load.
Questions that I haven't answered:
1. How to understand which amplitude is sufficient and which is too large?
2. Clustering does not work (we discussed this above)

[javat33489]

 

Sir, I would like to clarify, initially I had a 160×160 mm aluminum profile pipe with a height of 6 meters. And I first did a modal analysis and then a harmonic analysis from it. The first two modes always show 4-5, and in harmonic analysis at these natural frequencies it always shows strong responses, even at the slightest load. Sir, I reduced the length of the pipe to 2 meters, I made a calculation for each meter, but it always shows peaks at natural frequencies, even on a 2 meter pipe. There can be no resonance there. Why does it show like this?

Sir. I figured it out with the message above.
There will always be responses at natural frequencies. You need to watch the amplitude. The amplitude strength varies depending on the load.
Questions that I haven't answered:
1. How to understand which amplitude is sufficient and which is too large?
2. Clustering does not work (we discussed this above)

[dlooman]

1. The usual way is to compare the stress of the resonant response to yield or some allowable stress per code.
2. Clustering does work if damping has been specified.  Clustering should be used so that you get a solution at each natural frequency.  The resonant response will be inversely proportional to the damping value so it's a critical input quantity.  2% is a typical value for welded structures.

[javat33489]

Yes sir, I did indicate 0.02, but it still didn’t help

[dlooman]

If you right click on Solution the fly-out menu has an option to Open Solver Files Directory.  After even a failed solution you should find the ds.dat file in that directory.

[dlooman]

There should be a command, dmps,0.02 in the ds.dat file.

[dlooman]

Try using Damping Ratio to define damping as shown below.  Also, set User Defined Frequencies to Off.