Sign conventions for Node and Element

[Lei Lei]

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]

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]

"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]

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]

Thanks for your reply!

[STKO Team]

You're welcome