> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/e3d-user-manual/design/steel-design/member-internal-forces-and-mom.md).

# Member Internal Forces and Moments

Understand the sign conventions for member axial force and moments and the critical ratio used in design.

1. Axial force P acts perpendicular to the section. Moments Mx and My act about section local X and Y axes respectively. They have the following sign conventions.

Axial Force P: positive for compression; negative for tension

Moment Mx: Positive when section top most fiber is under compression.

Moment My: Positive when section rightmost fiber is under compression.

![](https://560001759-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FWNe8U6EBBBdUJHdb2X9G%2Fuploads%2Fgit-blob-b90e2038c9154d2b762a688ddf52f1e2a5095d24%2Fclip0133.jpg?alt=media)

2. All moments are referenced about the geometric centroid of the gross section.
3. Loads are the required strength computed by the code-specified factored load combinations using either hands or analysis program. It is assumed that an overall 2nd order P-Delta (P-∆) analysis has been performed on a sway structure. If desired, the program uses moment magnification procedure to calculate the P-delta (P-δ) effect, which accounts for slenderness of columns in non-sway structure or for slenderness along the lengths of columns in sway structure.
4. Critical ratio (also called capacity ratio) is computed for each section based on the magnified factored loads and the capacity of the section. Critical ratio equal or less than 1.0 means the design strength is greater than the required strength and the section is adequate. Critical ratio greater than 1.0 means the design strength is less than the required strength and the section is inadequate.


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