## Large Mass Method

## Large Mass Method

**NPL101**(visitor)

(OP)

Hi,

Could anyone please give me some idea about using the "large mass method" 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 a 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

Could anyone please give me some idea about using the "large mass method" 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 a 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

## RE: Large Mass Method

The large mass method is easy to emplement but there

are a few cautions.

1. You must define a large mass. Large is a relative

term. This mass magnitude needs to produce an

inertia force which is significantly larger than

and load in the attached members.

2. If you make the mass too large it will create

numerical overflow problems which will produce

unexpected results.

3. To guarantee a good simulation. Compute the fixed

base frequency and compare it to the large mass

model frequency. The first mode of course is zero

but the second mode should be the equivalent

fixed base mode.

Have fun

Warren Hoskins

warren_hoskins@yahoo.com

Warren Hoskins

warren_hoskins@att.net

## RE: Large Mass Method

NPL101(visitor)A further question that is still puzzling me. If I know excitations in terms of accelerations, say, acceleration time histories, on one hand, I can applied such excitations directly onto the corresponding nodes(DOFs), and will get the calculated response, which I think this is what I can do in ABAQUS.

One the other hand, if the large mass method is to be used, what I need to do is that first attach a large mass to the node(DOF) on which the excitation is going to be applied, then convert the acceleration history to force history by F=LargeMass*acceleration, then applied the force history to the node(DOF) attached with the large mass, which seems to be a method recommended by NASTRAN.

Both ways can get structural response, but it seems to me that the first method is more straightforward and free of assumptions, but why Nastran uses the second method? Am I misunderstanding something here?

Thank you again for your time

regards

John

## RE: Large Mass Method

## RE: Large Mass Method

## RE: Large Mass Method

NPL101(visitor)In ABAQUS, when subspace projection method is to be used then again boundary conditions need to be approximated by the "Large Mass Method". As described in its users' manual, "in the subspace projection method it is not currently possible to specify nonzero boundary conditions directly".

Does it mean that it is "theoretically currently unavailable" or it is actually theoretically available but has not yet been implemented in abaqus.

It seems to me that when transforming the EOM from the physical space to the modal space will not affect excitation if the excitation is forces. But if the excitation is disp, velo or acc. then I have the problem. How to enforce the known acceleration from the physical space in the modal space, or do I need to? Is this the reason why the nonzero boundary conditions can not be directly applied when subspace technique is used?

Thank you again for your time and any of your advice will be much appreciated.

## RE: Large Mass Method

NPL101(visitor)My questions is:

1) Whether the assumptions made for the lumped mass selection are correct. In other word, do I need to change the masses to get better frequency results.

2) The next step for my analysis is to apply the excitations. These are in a format of acceleration. My second question is whether they are needed to be downscaled for removing the artificial effects of the large mass added to the model.

Any suggestions will be highly appreciated.

Thanks,

Moss