> For the complete documentation index, see [llms.txt](https://alpaca4d.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alpaca4d.gitbook.io/docs/basics/sections.md).

# Sections

A **section** pairs a geometry with a [material](/docs/basics/materials.md) and hands the resulting stiffness properties to an element.

Beam sections are **uniaxial**: they take a [Uniaxial](/docs/basics/materials/uniaxial.md) material and reduce to an area, two second moments, a torsion constant and two shear factors. Shell sections are **multi-dimensional**: they take an [nD](/docs/basics/materials/nd.md) material and a thickness.

## Components in this tab

| Component                                                            | Nickname            | For    | Defined by                                      |
| -------------------------------------------------------------------- | ------------------- | ------ | ----------------------------------------------- |
| [Rectangular](/docs/basics/sections/rectangular.md)                  | `RectangleCS`       | Beams  | Width, Height                                   |
| [Circular](/docs/basics/sections/circular.md)                        | `CircleCS`          | Beams  | Diameter, Thickness (0 = solid)                 |
| [Rectangular Hollow](/docs/basics/sections/rectangular-hollow.md)    | `RectangleHollowCS` | Beams  | Width, Height, Web, two flange thicknesses      |
| [I Section](/docs/basics/sections/h-section.md)                      | `I Section`         | Beams  | Height, two flange widths and thicknesses, Web  |
| [Double L-Angle](/docs/basics/sections/double-angle.md)              | `2L`                | Beams  | Height, Width, Thickness, Gap                   |
| [Generic Section](/docs/basics/sections/parametric-cross-section.md) | `Generic Section`   | Beams  | A, Izz, Iyy, J, AlphaY, AlphaZ entered directly |
| [Steel Section Library](/docs/basics/sections/library.md)            | `Section Library`   | Beams  | A catalogue name — IPE200, HEA300, and 600 more |
| [Plate Fiber Section](/docs/basics/sections/plate-fiber.md)          | `PFS`               | Shells | Thickness                                       |

## Choosing a section

| You know                                                                         | Use                               |
| -------------------------------------------------------------------------------- | --------------------------------- |
| A catalogue steel profile by name                                                | **Steel Section Library**         |
| The dimensions of a parametric profile                                           | the matching parametric component |
| Only the stiffness properties — a homogenised deck, a section computed elsewhere | **Generic Section**               |
| A shell thickness                                                                | **Plate Fiber Section**           |

{% hint style="info" %}
All parametric beam sections reduce to the same OpenSees command — `section Elastic` — with `A`, `Izz`, `Iyy`, `J`, `AlphaY` and `AlphaZ` computed from the dimensions. They differ in how you describe the geometry and in what [Model View](/docs/basics/visualisation/model-view.md) draws when extruded preview is on.

For a genuinely non-linear section, build a fibre section instead: [Fiber Section](/docs/basics/moment-curvature/fiber-section.md).
{% endhint %}

## Local axes

`Izz` is bending about the local *z* axis; `Iyy` is bending about the local *y* axis. The orientation of those axes on a beam comes from the **ZAxis** input of the [ForceBeamColumn](/docs/basics/elements/force-beam-column.md) component, not from the section.