Singularity warning - cause?

Question:

Dear Sir / Madame,

When running the model attached, I get three warning that some nodes may be singular in the direction Rx.


ENTRY STATIC ANALYSIS
  MULTI-FRONTAL SOLUTION HAS BEEN PERFORMED 50 % .
WARNING : NODE NO.     99  RX  DOF MAY BE SINGULAR.
  MULTI-FRONTAL SOLUTION HAS BEEN PERFORMED 70 % .
WARNING : NODE NO.     56  RX  DOF MAY BE SINGULAR.
  MULTI-FRONTAL SOLUTION HAS BEEN PERFORMED 90 % .
WARNING : NODE NO.     36  RX  DOF MAY BE SINGULAR.
  MULTI-FRONTAL SOLUTION HAS BEEN COMPLETED.     
  DISPLACEMENT/FORCE/STRESS OUTPUT.
  ELEM. :    972 OF     972

-----------------------------------------S O L U T I O N   T E R M I N A T E D
YOUR MIDAS JOB IS SUCCESSFULLY COMPLETED.......V:\Projects\2018\L118008\ENG\04-Calcs\Gantry 249\Midas Model\KXR Gantry 249 - Existing Gantry
TOTAL SOLUTION TIME..:     2.84 [SEC]
------------------------------------------------------------------------------

      3 WARNING MESSAGES ARE FOUND.  
        PLEASE CHECK YOUR WARNING MESSAGE FILE  :...Gantry 249\Midas Model\KXR Gantry 249 - Existing Gantry.OUT


I reviewed the model and the results and I can't find anything wrong. 

Any chance you could shed some light on this?

Answer:

Hi,

As you see below, the top chord of the truss system is supported just by truss elements. Thus, the top chord has no resistance to the rotation about the X axis. If you assign point spring support with negligible rotational stiffness to any one node of the top chord, the warning will not appear. 

Regards,
DK Lee
Creation date: 8/2/2018 11:30 PM (dklee@midasit.com)      Updated: 8/3/2018 9:00 AM (dklee@midasit.com)
Files   
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KXR Gantry 249 - Existing Gantry.mcb