SAP2000 Error when changing height of structure 1

[rllo]

I'm getting a pretty common SAP error, but I can't track down what the issue is. the model is very simple: I have a rigid member with a load on top connected to a HSS column. if I change the height of the column, the error goes away. can someone help me understand what is going on? below is a picture of my model and rigid member definitions.

Error message:
Load Case : MODAL
Element Type : N.A.
Element Name : N.A.
ZERO OR NEGATIVE MODAL-MASS VALUES WERE DETECTED DURING ITERATION, THE RESULTING EIGEN-MODES MAY NOT BE VALID - CHECK THEM CAREFULLY !! THE STRUCTURE IS UNSTABLE OR ILL-CONDITIONED !! CHECK THE STRUCTURE CAREFULLY FOR: - INADEQUATE SUPPORT CONDITIONS, OR - ONE OR MORE INTERNAL MECHANISMS, OR - ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR - EXTREMELY LARGE STIFFNESS PROPERTIES, OR - BUCKLING DUE TO P-DELTA OR GEOMETRIC NONLINEARITY, OR - A FREQUENCY SHIFT (IF ANY) ONTO A NATURAL FREQUENCY

thank you!

 

[JoshPlumSE]

I'll suggest that when you lower the height to that specific value, you have two nodes that are essentially overlapping and that you need to Edit > Merge Duplicates command to get rid of that error.

 

[PMR06]

This sounds like a runaway pdelta problem.

What is the top node deflection in those various models?

Is there a gravity load at the uppermost node?

Have you tried stiffening the lower post section in 16.75ft error model to say HSS8x8x3/8 and see if it run stable?

 

[rllo]

I'll suggest that when you lower the height to that specific value, you have two nodes that are essentially overlapping and that you need to Edit > Merge Duplicates command to get rid of that error.

Thanks Josh, no duplicates in the model

This sounds like a runaway pdelta problem.

What is the top node deflection in those various models?

Is there a gravity load at the uppermost node?

Have you tried stiffening the lower post section in 16.75ft error model to say HSS8x8x3/8 and see if it run stable?

Thanks PMR06! You're right. I do have a vertical gravity load on top of the rigid member that is not shown in the screenshots. Checking modal deflections, I'm seeing 40 inches horizontal.