I should have listened to you in the first place, but I was trying to follow the method in the Caesar manual. I put in the displacements on all the C-nodes and ran the analysis. The motion looks right - the system looks just like a piping system swaying on a floating platform. The stresses are enormous, however, and I am not convinced that they are correctly calculated. I don't think Caesar's displacement function is really intended for movements like this, but rather for slow movements (maybe quasi-static is the word) that are similar to thermal expansion or static deck deflection from the weight of equipment. I'm not sure putting g's in as uniform loads is quite right either, but it seems more realistic, in the vein of doing static seismic analysis. Is the real answer to do some sort of dynamic analysis? I don't have time to do any more experimenting at this time, so right now I'll throw some accelerations in. Oh well, I learned something by trying it.