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Error in Random Response Analysis Setup in FEMAP??


Greetings all,


I have been struggling to understand why I get vastly different stresses using the large mass method vs the direct method when I setup random response for some components.


The large mass method, I just attach a 1e8 mass element (CONM) to the RBE2 and constrain all 6DOFs except for the excitation direction.  I also apply a force of 1e8 to the same node (in the direction of excitation) and  I use units of mass and my PSD input is {g^2/hz x (386.4)^2}.


In the direct method, i will remove the large mass element and just apply a 386.4 in/s^2 to the same RBE2 master node.  I leave my PSD input in g^2/hz and I also use units units of mass.


What am I doing wrong?  Please if anybody can help.