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#73252 - 05/07/19 09:40 AM Spectrum DLF vs. CII results
pooria1978 Offline
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Registered: 02/25/08
Posts: 234
Loc: Netherlands
Hi

my question is with regards a simple vertical cantilever model that I made in CII and the impact of a short term dynamic force on the bottom anchor of it.

I defined a "triangle" force-time spectrum and had CII convert it to a frequency-DLF spectrum and applied it in the middle of the cantilever. the largest contribution of the force is in the 4th mode of the system (4 X(1)) where the natural frequency is about 57HZ.
as I refer to the frequency-DLF spectrum, the DLF should be about 1.2 at about the same frequency.
to my surprise, the reaction force on the anchor is even smaller than one time the original applying force while I expected it to be 1.2 times of it according to the frequency-DLF spectrum.

can anyone explain what I am missing here please?

many thanks in advacne

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#73253 - 05/07/19 10:40 AM Re: Spectrum DLF vs. CII results [Re: pooria1978]
Dave Diehl Offline

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Registered: 12/14/99
Posts: 2376
Loc: Houston, TX, USA
You may expect a DLF of 1.2 for the 4th mode shape/frequency pair. But each mode holds only a portion of the total system mass. The 4th mode does not account for all the mass activated by your event. The modal response to the applied load is also affected by the location of this load. (The response to the same tap on a water glass changes based on where you tap the glass.)
Take a look at the participation factor for your 4th mode. If you change the DLF associated with that frequency, you will see a proportional change to the participation factor.
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#73255 - 05/08/19 01:37 AM Re: Spectrum DLF vs. CII results [Re: Dave Diehl]
pooria1978 Offline
Member

Registered: 02/25/08
Posts: 234
Loc: Netherlands
thanks Dave!

I am now even more puzzled! if I look at the participation factor of the relevant mode it is about 0.23 and when I multiply it to that DLF of 1.2, it gives me a factor of 0.276. considering that the peak force is 65000 N the maximum contribution of 4th mode should be about 0.276x65000= 17940 N. the results in the restraint summary of CII contradicts this value massively. please have a look at the attachments.

regards,


Attachments
3.JPG

4.JPG



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#73256 - 05/08/19 07:34 AM Re: Spectrum DLF vs. CII results [Re: pooria1978]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2376
Loc: Houston, TX, USA
I think you are pushing the definition of a DLF a little too far for this application. The DLF is the ratio of the maximum dynamic response to the the response to the same load, applied very slowly (a static load). Add to this that the system is assumed to respond as a single degree of freedom. Systems themselves are seldom have just a single degree of freedom but the way the math works, you can treat the natural modes shapes of vibration as those degrees of freedom - the total response is the sum of all those DOFs or mode shapes. In your model, you are activating three degrees of freedom (3 mode shapes). Each of these modes, alone, cannot account for all the response. The timing and location of the transient load modifies how mush mass is associated with each mode of vibration. If you had ALL the (X, Y, & Z) mass participate in the 4th mode, it wouldn't be the 4th mode, it would be the ONLY mode, a single DOF and your approach would apply.
Again, the 4th mode accounts for only a portion of the total system mass - note the higher participation factor for your 2nd mode.
Your participation factor for the 4th mode is 0.22181. This number includes your DLF of 1.2. If you change your DLF at 57.3 Hz to a different value, the listed participation factor will change to be (0.22181 * (new value/1.2)).
Aside: To simplify your test model, exclude all mass in the Z direction to eliminate your current modes 1, 4 & 6. If you add a guide EXACTLY where your current 4th mode does not move (a "nodal point" in this mode of vibration), you will eliminate your current 2nd mode (the first mode in the X direction). You can then use the control parameter switch to stop the eigensolution after the 1st mode. This will isolate the analysis to your single mode. Even so, all the mass will not participate.


Edited by Dave Diehl (05/08/19 07:35 AM)
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#73258 - 05/09/19 01:48 AM Re: Spectrum DLF vs. CII results [Re: Dave Diehl]
pooria1978 Offline
Member

Registered: 02/25/08
Posts: 234
Loc: Netherlands
Thanks Dave,

my main intention for making this model was, I wanted to see if I could do a sound engineering judgment on some surge situations simply by looking at the natural modes of the system and comparing them with the similar values in the frequency-DLF generated spectrum by CII.
I thought maybe I could avoid performing formal analysis for some situations if the corresponding DLF of the frequency close or evqual to natural frequency of the system is very small.


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#73259 - 05/09/19 08:00 AM Re: Spectrum DLF vs. CII results [Re: pooria1978]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2376
Loc: Houston, TX, USA
I agree that avoiding natural frequencies where the response spectrum shows a large DLF is a good design rule but does not fully develop the story. The response is also based on the amount of total mass that is participating in that mode. And, for force response spectrum analysis, the response is also related to the location and direction of the applied load.
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