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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 Updated: 8/3/2018 9:00 AM

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KXR Gantry 249 - Existing Gantry.mcb

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