Learning Forum

[35020306]

I am trying to model a tuned mass damper (TMD) unit attached to the midpoint of a 12 m long simple supported beam as follows.

From what I read, you can model TMD as a combination of spring-damper element and point mass which are modeled using COMBIN14 element and MASS21 element respectively. I have tried entering the unit model parameters in Mechanical using APDL command script, but it encountered an error when I tried to mesh it. Here is the script as a reference:

 

/PREP7
! Definition of Material and Spring Element
MP, BETD, 21, 5513928
MP, DMPR, 21, 27420.78
ET, 21, COMBIN14, 0, 2

! Point mass
MP, ALPD, 22, 1000
ET, 22, MASS21, 0, 0, 2

! Definition of keypoints and spring
K, 1, 6,0,0
K, 2, 6, -1, 0
LSTR, 1, 2
LSEL, S, LINE,,1

! Definition of orientation keypoints
K, 3, 6, 0, 1
LSEL, S, LINE, , 1
LATT, 21,,,, 3

! Assign point mass
KSEL, S, KP,, 2
KATT, 22,,,0

!Meshing
!LSEL, S, LINE, , 1
KMESH, ALL
LESIZE, ALL, 0.1
LMESH, ALL

FINISH

/SOLU

 

Executing the command above resulted in the following error:

*** ERROR ***                           CP =       0.844   TIME= 11:12:38
 Element type 3 is CLOAD201, which cannot be used with the LMES         
 command.  The LMESH operation is aborted at line 1.

Is there a mistake in my script? For your information I inserted this command in the Transient Structural analysis block after I did Static Structural and Modal analyses.

Any help would be appreciated.

Thank you.

[Claudio Pedrazzi]

Hi, please don't misunderstand my suggestion, I like APDL very much, but since you are in Workbench already, why not define an additional small body (e.g. a small cube), add a point mass on it, and connect it with a spring (it is available and has even damping, if you need it).  Possibly you should constrain the body so that it can move only in the appropriate direction (Y, judging from your screenshot)

[Erik Kostson]

 

To add to what Claudio said you can do all what you want, natively inside WB Mechanical, using combin14 for spring and mass21 for mass.

1. You need a freestanding remotepoint at the mass location.

2. Define a point mass using remote points on the above defined point/location (this will be the mass21 element)

3. Finally define a spring connection using the remote point for the mobile (spring def.) the remote point and a node or if you have a vertex on your beam element for the reference side (spring def.). This is the combin14.

 

So we should have in our Mechanical tree, 1 remote point (free standing), 1 point mass (under the geometry tree), and one spring (under connections).

All the best

Erik

 

[35020306]

Thank you for the suggestions. It runs perfectly now with a lot less headache for me 😅.