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Subject: Re: Modal Participation Factor Calculation
Author: Chris Andersen
Date: 2001-01-29 15:38:00

Hello Carlos:

I have some old notes that say the PF is calcuated by multiplying
{u}T{M}{V), where {u}T is the transpose of an eigenvector and [M] is the
mass matrix. {V} is a vector of 0's and 1's indicating direction. Like in a
3D model {V} is 0,0,1,0,0,1 for excitation in the Z direction. So you can
calculate a PF for each direction of excitation for each model. By comparing
the PF's in all the directions for a particular mode you can see which
directions of excitation will excite that mode.

If PF is 0 for a particular direction of excitation, then you will not
excite that mode with excitation in that direction. The biggest PF for a
particular mode indicates that direction that will most excite that model.
So PF's can be pretty useful.

PF's are also used to scale modes in a response spectum analysis along with
the spectral value from the response spectrum curve.

I may not have explained this completly correctly, but I think this is the
general idea.

Chris

> -----Original Message-----
> From: Cosme, Carlos [SMTP:ccosme@w...]
> Sent: Monday, January 29, 2001 12:36 PM
> To: 'xansys '
> Subject: [xansys] Modal Participation Factor Calculation

> Hello All,
> I've been running a modal analysis (Block Lanczos). After Ansys computes
> the
> natural frequencies, it does a participation factor calculation. It
> computes
> these in the X, Y, Z and ROTX, ROTY, ROTZ directions. For each of these it
> prints six columns:

> Mode Cumulative_Freq_Mass Fraction Period Participation_Factor
> Ratio Effective Mass

> Can someone please help me interpret this information? Is it something I
> should be interested in?
> Also, some of the participation factors are negative while some are
> postive.
> What is the significance of this?
> Should rigid body modes have a special interpretation?

> I've always thought participation factors were something you computed
> after
> a dynamic simulation, i.e., here is the participation of each mode in the
> final response of the structure. How does this Ansys information fit in
> with
> such an interpretation?

> Any help would be greatly appreciated.

> Thanks,
> Carlos

> Carlos Cosme
> Western Star Trucks, Inc.
> Kelowna, British Columbia
> Canada V1Y 6H8
> tel. (250)-868-6250 ext 5234
> fax. (250)-868-6210
> email: ccosme@w...

Posts possibly associated with message #20600AuthorDateScore
20585Modal Participation Factor CalculationCosme, Carlos ccosme@2001/01/2910
20596Re: Modal Participation Factor CalculationPrapoj Phataraphruk2001/01/29 
20600Re: Modal Participation Factor CalculationChris Andersen2001/01/29 
20603Re: Modal Participation Factor CalculationChristopher Wright1991/01/29