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How to perform craig-bampton method

Altair Forum User

I have been working on Craig-Bampton method for normal mode analysis in which I aim to get a faster computational time result for 16 modes. Until now I unable to do so. I tried to use all the information obtained from the internet, but I failed miserably since I have no idea how to do it. I'm working with superelement . Just a simple structure. One superelement and one residual. With this, I attached my explanation of the structure I'm working with.

<?xml version="1.0" encoding="UTF-8"?>

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Rahul Rajan_21763

I am sending a sample model with the steps for creating the CMS deck for that model.

Unable to find an attachment - read this blog

Altair Forum User

For the grid data section in superelement, did I need to include the sub B grid also?

Rahul Rajan_21763

I am sending a simple file; cantilever_cms.fem. It writes out the pch file attached (it shows the MAAX) which we include in the final residual run – reduced_cms.fem.

Unable to find an attachment - read this blog

Altair Forum User

I keep on getting this error

. I used EIGRL and CMSMETH for load collector and Normal modes for load step

case control.JPG

Im not sure what type of load step use in the centilever example. Is it normal modes? for both EIGRL and CMSMETH load collector?

Altair Forum User

Then I only used EIGRL load collector for normal modes load step, but I dont know what type of load step for CMSMETH load collector

This is the error appear:

Control cards 'METHOD=n' are not allowed when CMSMETH card is present.

Rahul Rajan_21763

Could you share us file in which you are working?

Altair Forum User

i've sent the file to your dropbox

Altair Forum User

Hi Ahmad,

Can you share the .out file here in forum?

Altair Forum User

Ahmad,

Can you remove the normal modes subcase and run the analysis?

Have only a generic subcase with CMS active.

Altair Forum User

@Prakash Pagadalaplease check your dropbox. I have sucessfully generate the .pch file. But there is no mass matrix (MAAX) in it. Only stiffness matrix (KAAX). Plus at the .out file, there is warning and problem on the CMSMETH. Can you please check the .fem file if there is anything wrong I made.

Altair Forum User

Ahmad,

I am sorry, I don't have access to dropbox here.

Can you share the .out file?

Altair Forum User

FYI,

check in your out file whether mass matrix is written or not in .pch file.

<?xml version="1.0" encoding="UTF-8"?>

Altair Forum User

@Prakash Pagadala This are the files. I have checked the .out file. There is no mass matrix written. Only stiffness.Can you please check the .fem file if there is anything wrong I made.

One more, I supposed to get this 2 output from the optistruct but I only get the DMIG (.pch) only. Not the H3D file.:

1.The successful run will have DMIG matrix (Super Element).

2.The successful run will also have H3D file (binary Super Element)

Unable to find an attachment - read this blog

Altair Forum User

Ahmad,

provide number of modes or upper frequency in CMSMETH card and try again.