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# Transient response model

Creator

Im running a direct transient model using the large mass method. I have used an RBE2 rigid element as the large mass, which is connected to the base of the model. I have run the model with the RBE2 element close to the base, then further away just as a test as I assumed it wouldnt change the results as this is infinitely rigid. However the results do change and I was wondering if anyone knows why?

Also I was wondering if anyone could help me with the frequency of the system damping value which is input in the transient analysis set up. I have used info from the Femap tutorials where the frequecy of the first mode of the structure is chosen but was wondering why this is and is this allways the case?

Many Thanks,

Dan

4 REPLIES

# Re: Transient response model

Phenom
Do you have all DoF ticked for the RBE2; what element type(s) is the RBE2 connected to; and (perhaps most importantly) is the node of the large mass element constrained in rotations? If the last condition is not satisfied, keep in mind that RBE2's are not large rotation elements.
Also your mass may be huge, but if there are no moments of inertia or rotation constraints it could rotate with ease, with only a small amount of rotation required to change results.

# Re: Transient response model

Creator

I have all DoF except the direction in which the large mass is loaded ticked for the RBE2, and the node is also constrained in rotation. The RBE2 is connected to the nodes at the bottom of the model (see attached picture). I havent included any moments of inertia as the tutorial I was working on did not specify to, do you think this is the problem and if so how could I get some reasonable moments of inertia values?

# Re: Transient response model

Phenom
Are you saying that the checkboxes for the RBE2 element does not include the translation DoF which is the same direction as the load direction? If so, that is not correct. You must have that DoF ticked for the element to properly transfer the load. Safest to have all six DoF checked for the RBE2 element itself.
But the constraint on the mass node should be all DoF except the translation DoF in the load direction.

Just in case you do have those set correctly, the next thing is... in what way do the results change? Stresses? Displacements? Velocities? If you have used the large mass method, you should always do a Chart of mass node velocity or acceleration vs time to make sure that you have correctly enforced the exact motion. If the problem appears to be displacement, do you have the deformation scale set to "Actual" (true scale), rather than the default exaggerated scale? If you are using the default exaggerated scale, this scale is likely to change if the mass is moved from one place to another, because it may well change the overall model size.

# Re: Transient response model

Creator

Solved! I was confusing nodal constraints with the RBE2 constraints and I didnt have the rotations ticked.Thank you very much for your help!