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# What is the Difference Between CQC and ROSE Method?

05/18/2017 8:52 AM

Hi!
For response spectrum analysis I use the SRSS, CQC and ROSE method to combine the result value (eg acceleration) of the modes. If I have closely spaced eigenfrequencies, I use CQC or ROSE. But what is the difference and when should I use which? Thanks!

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Location: Lubbock, Texas
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#1

### Re: What is the difference between CQC and ROSE method?

05/18/2017 10:03 AM

Does this relate to FFT analysis of vibration wave-forms? I am on the fringe here, not really involved, just asking for my own edification.

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#2
In reply to #1

### Re: What is the difference between CQC and ROSE method?

05/18/2017 10:10 AM

Hi. Its about FEM analysis, there are different equations for superposition of modal solutions. It depends on whether the eigenfrequencies are closely together (than they correlate) or not which equation to choose.

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#3
In reply to #2

### Re: What is the difference between CQC and ROSE method?

05/18/2017 10:32 AM

I have seen advanced applications of combustion dynamics on gas turbine burner cans, where FEM is applied in a sort of skewed grid. Is that related?

In the analysis I witnessed as a presentation, the algorithm could pick and choose the size of the volume elements, and the area of the surface elements for the model to compute on.

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#4
In reply to #1

### Re: What is the difference between CQC and ROSE method?

05/18/2017 6:23 PM

I think it has to do with analyzing the effect of earthquakes on buildings.

https://en.wikipedia.org/wiki/Seismic_analysis

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#5

### Re: What is the Difference Between CQC and ROSE Method?

05/19/2017 4:16 AM

I found this:

"Rosenblueth and CQC are similar - they both utilize random vibration theory to represent the seismic loading as white noise, although Rosenblueth assumes finite duration whereas CQC assumes infinite duration. If you plug in some numbers, you can see how the coefficients may compare between the two, although I think that they are generally similar.
The choice tends to be driven by the specification."

on http://www.eureka.im/5132.html

but I don't really know what that means "finite duration" and "infinite duration". Is that with regard to the excitation? If so in regard to earthquakes does the infinite duration make even sense?

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#6
In reply to #5

### Re: What is the Difference Between CQC and ROSE Method?

05/19/2017 10:29 AM

Is it that finite or infinite describe the application of driving oscillation in each mode?

Consider a strictly slip fault motion, would that not represent a finite case?

A long duration tremble might be considered to be infinite duration if the duration is long compared to the failure time of the structure...just throwing that out there.

The coupling of closely spaced vibration modes compared to non-closely spaced one is interesting, even in the study of intramolecular energy transfer of quantized vibration from one mode (such as C=C stretching to C=O stretching) to another in such small molecules as biacetyl.

or propynal

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#8
In reply to #5

### Re: What is the Difference Between CQC and ROSE Method?

05/19/2017 4:42 PM

The theoretical calculation for the combined effect of two modes assumes an infinite duration of the excitation. This obviously is worst case and overly conservative for design. A correction by Rosenblueth adjusts the result for a short (finite) duration.

It's explained in "Fundamental Concepts of Earthquake Engineering", p203

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#9
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### Re: What is the Difference Between CQC and ROSE Method?

05/19/2017 10:24 PM

Page number in "Fundamental Concepts of Earthquake Engineering" should be 394, paragraph 10.3.2.3 Rosenblueth's Rule.

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#10
In reply to #9

### Re: What is the Difference Between CQC and ROSE Method?

05/22/2017 4:06 AM

So what I got is
- CQC and ROSE differ when the ground motion duration is short, similar if long

-ROSE includes a correction for nonstationary effects

- both can be used with or without closely spaced eigenfrequencies, only if damping ratio is close to zero I should use SRSS (I use a damping of 2%, is that close to zero?)

- ROSE gives the best results
So.. does that mean that the computing times goes up with SRSS->CQC->ROSE

but the accuracy goes up also?

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#11
In reply to #10

### Re: What is the Difference Between CQC and ROSE Method?

05/22/2017 4:27 AM

Edit: sorry I just read: CQC is bad for damping ratio <2%

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#7

### Re: What is the Difference Between CQC and ROSE Method?

05/19/2017 3:50 PM

This document might help (or might raise more questions )