Sign conventions for Node and Element

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Lei Lei
Posts: 82
Joined: Mon Jul 10, 2023 1:28 pm

Sign conventions for Node and Element

Post by Lei Lei » Thu May 16, 2024 1:13 pm

Dear STKOteam,

I have some questions regarding the sign conventions for Node and Element used in STKO.

1、For nodes, how are the positive and negative signs defined for the output items? Are these outputs based on the global coordinate system or influenced by local axes settings?

2、For elements, how are the positive and negative signs defined for the internal forces (Fx, Fy, Fz, Mx, My, Mz) in each direction? Additionally, for each degree of freedom, there are two equal values with opposite signs. How should these be distinguished?

Looking forward to your reply!

STKO Team
Posts: 2297
Joined: Tue Oct 29, 2019 8:45 am

Re: Sign conventions for Node and Element

Post by STKO Team » Fri May 17, 2024 8:53 am

Results are same as those in OpenSees
1、For nodes, how are the positive and negative signs defined for the output items? Are these outputs based on the global coordinate system or influenced by local axes settings?
Nodal results are always in global coordinates
2、For elements, how are the positive and negative signs defined for the internal forces (Fx, Fy, Fz, Mx, My, Mz) in each direction? Additionally, for each degree of freedom, there are two equal values with opposite signs. How should these be distinguished?
It depends on 1) the result type and 2) the element type.
For example:
  • In beams: "force" is in global coordinates "localForce" in local coordinates
  • "stress" in beams are in local coordinates, in solids are in global coordinates
There are two equal values with opposite signs
That makes perfect sense:
"force" are the beam's end-nodes forces: imagine a column with a vertical force F. The reaction will be -F. The axial force N will be equal to F, and the beam end-forces will be -F on top and +F on bottom

Lei Lei
Posts: 82
Joined: Mon Jul 10, 2023 1:28 pm

Re: Sign conventions for Node and Element

Post by Lei Lei » Fri May 17, 2024 11:00 am

STKO Team wrote:
Fri May 17, 2024 8:53 am
"force" are the beam's end-nodes forces: imagine a column with a vertical force F. The reaction will be -F. The axial force N will be equal to F, and the beam end-forces will be -F on top and +F on bottom
So, the two equal values with opposite signs are outputs for two ends of the element, and the order of the ends is arranged by local coordinates, right?

STKO Team
Posts: 2297
Joined: Tue Oct 29, 2019 8:45 am

Re: Sign conventions for Node and Element

Post by STKO Team » Fri May 17, 2024 1:59 pm

and the order of the ends is arranged by local coordinates, right?
The local X axis on a beam goes from first-node to last-node

Lei Lei
Posts: 82
Joined: Mon Jul 10, 2023 1:28 pm

Re: Sign conventions for Node and Element

Post by Lei Lei » Fri May 17, 2024 2:03 pm

Thanks for your reply!

STKO Team
Posts: 2297
Joined: Tue Oct 29, 2019 8:45 am

Re: Sign conventions for Node and Element

Post by STKO Team » Fri May 17, 2024 2:06 pm

You're welcome

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