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?
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?