Error: "small pivot at node XX ROTY"

[rdcowan]

I am new to this group and am sure my problem is very basic. For the purpose of learning, I am trying to duplicate the results of a lumped mass model of a vertical turbine pump (I have a copy of the input command file and have typed it into a file). The column pipes and shaft are modeled separately as BEAM4 elements with X & Z dir coupled nodes located at the same coordinates. (at bearing locations) There is one node that is anchored (D,20,ALL command) which is part of the column. There are only 62 nodes and 60 elements. I am getting 55 'small pivot at node NN ROTY' errors and don't know how to track down the problem. The message says 'Check for unconstrained model' I have checked my input file and it doesn't seem to have any data entry errors. Also, I don't understand what a 'Master DOF' is. Again, sorry for asking such basic questions, but I'm sure that if I can get this analysis working, I'll understand far more than I do now!

THANKS!