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Thread Subject:
Nat freq and mode shapes of MDOF

Subject: Nat freq and mode shapes of MDOF

From: Patrick Roberts

Date: 3 Sep, 1999 17:27:12

Message: 1 of 3

I am using Matlab 5.2. I have the mass and stiffness matrix of a
multi-degree of freedom system. How do I calculate all the natural
frequncies and mode shapes for a multi-degree of freedom system using
Matlab? I was using MathCAD but it takes a while and it's limited in
the size of matrices. I thought it should be pretty straight forward in
Matlab but I'm having more trouble than I thought. I've searched the
web to see if anyone has posted a Matlab solution and I've found
nothing. I would appreciate any help. Thanks.

Subject: Nat freq and mode shapes of MDOF

From: izhak

Date: 4 Sep, 1999 20:13:56

Message: 2 of 3



Patrick Roberts wrote:

> I am using Matlab 5.2. I have the mass and stiffness matrix of a
> multi-degree of freedom system. How do I calculate all the natural
> frequncies and mode shapes for a multi-degree of freedom system using
> Matlab? I was using MathCAD but it takes a while and it's limited in
> the size of matrices. I thought it should be pretty straight forward in
> Matlab but I'm having more trouble than I thought. I've searched the
> web to see if anyone has posted a Matlab solution and I've found
> nothing. I would appreciate any help. Thanks.

 if M is your mass matrix and K is the stiffness matrix, use
[phi w2]=eig(K,M);
  phi contains the eigenvectors (each column is a modeshape)
 w2 is a diagonal matrix, each diagonal entry contains wn(i)^2
 (i.e. the i'th natural frequency squared).
 HTH
 izhak

Subject: Nat freq and mode shapes of MDOF

From: Patrick Roberts

Date: 4 Sep, 1999 16:05:32

Message: 3 of 3

Great. I'll try that.

izhak wrote:

> Patrick Roberts wrote:
>
> > I am using Matlab 5.2. I have the mass and stiffness matrix of a
> > multi-degree of freedom system. How do I calculate all the natural
> > frequncies and mode shapes for a multi-degree of freedom system using
> > Matlab? I was using MathCAD but it takes a while and it's limited in
> > the size of matrices. I thought it should be pretty straight forward in
> > Matlab but I'm having more trouble than I thought. I've searched the
> > web to see if anyone has posted a Matlab solution and I've found
> > nothing. I would appreciate any help. Thanks.
>
> if M is your mass matrix and K is the stiffness matrix, use
> [phi w2]=eig(K,M);
> phi contains the eigenvectors (each column is a modeshape)
> w2 is a diagonal matrix, each diagonal entry contains wn(i)^2
> (i.e. the i'th natural frequency squared).
> HTH
> izhak

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