> 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/sel-faq/using-the-software/calculation-modules/columns/column-slenderness-definition.md).

# Column Slenderness Definition in ENERCALC

How ENERCALC defines column slenderness and why K and L subscripts now refer to buckling about each axis.

## How does ENERCALC define column slenderness?

Column slenderness quantifies a column’s tendency to resist buckling. The industry has settled on the use of the radius of gyration, r, about each axis as the section property of interest. The other critical parameters are the unbraced length, L, and the restraint conditions described by the factor commonly called K.

These parameters come together in the familiar form of KL/r.

If you look up the r value for a steel section, or if you calculate it for a concrete section, the value is described as the radius of gyration ABOUT the named axis. So, in most sections, there is a value for rx and for ry. And it makes sense that the slenderness should be evaluated about BOTH potential axes to find the critical condition.

Naturally, when evaluating buckling about the X axis, the slenderness needs to be calculated with rx, along with the K and L values that describe the column’s tendency to buckle **about the X axis**.

When ENERCALC was initially developed, the Slenderness tabs referred to buckling as an effect that happens “in the direction of” an axis. So ENERCALC historically collected values of K and L from the perspective of “the K factor (or L value) that applies to buckling **in the direction of** the X (or Y) axis”. When subscripted, these were referred to as “Kx and Lx for buckling **in the direction of** the X axis”, for example.

This method creates an expression like Kx Lx / ry, which has an obvious inconsistency in the subscripts.

The majority of the engineering community defines K and L as values that affect the buckling resistance of a column “about” a particular axis (just like the subscripting on r values). In this way, the common term KL/r appears with **consistent** subscripts, such as Kx Lx / rx.

ENERCALC has recently been modified to bring the input variable naming and the output labeling in concert with the method commonly used by the engineering community, which is to use consistent subscripts, all referring to effects “about” the named axis. For existing calculations, previous settings have been interpreted and converted to this industry standard system. You can review these settings on the Slenderness tabs.

![](https://1076748670-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FtBL2fEQRwrlKSM2irFL0%2Fuploads%2Fgit-blob-64131d099f77cccf4c63512e1629d2d9ae7d2862%2Fslenderness_definition.jpg?alt=media)


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