Large Mass Method

Hi,

Could anyone please give me some idea about using the "large mass method" in FEA to calculate the structural dynamic response to random or sinusoidal excitations? Why this method is used and what are the advantages?

If I understand it correctly, a paper in my hand says that acceleration excitations are not applied to the corresponding DOFs of the structure directly, instead, a large mass (*10^6 of the mass of the structure) is rigidly connected to these DOFs and an equivalent force (force=largeMass*accerlaration)is applied to the large mass. Why is that?

It seems to me that this is quite a common technique in NASTRAN to calculate the steady state or random responses (to base excitation?) but in the ABAQUS it is not mentioned at all, I am not a Nastran user, but I would expect that implementing such a technique in abaqus should be straight forward, am I on the right track? I notice that in abaqus acceleration can be applied directly to the DOFs, then under what situation that the “large mass method” can be used?

Your any advice is much appreciated.

Thanks

John

 

[izax1]

The large mass method is used for base excitation, i.e. you want to analyze a structure mounted on a much larger structure and the vibration is transfered through the larger structure. Imaging shaking a tree. The branches on the tree will start shaking as well even if you have not touched them. The vibration input is not applied directly to the branches but to the tree.

The branch is your Fe model, the tree is the big mass.

Hope this helps

Bernt

 

[brad]

Good answer Bernt.

The same can be done in ABAQUS as in Nastran.

The major reason that this is not mentioned in ABAQUS is that ABAQUS has this capability directly (do a search on "base motion&quot, which is easier to implement than the large mass method.

One reason for which this may still be necessary is if there are two separate excited bases being excited at different levels (i.e. node 1 excited in dof 1, node 101 excited in dof 2). In this case, base motion in ABAQUS wouldn't work, and old-fashioned "large mass method" would work fine.

Hope this helps.
Brad