> 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/h-section.md).

# I Section

An I-shaped (H-shaped) cross-section, allowed to be **asymmetric**: the top and bottom flanges have independent widths and thicknesses.

## 🔧 Grasshopper component

`I Section (Alpaca4d)` — **Alpaca4d ▸ 01\_Section**

<figure><img src="https://590400797-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F45NIwTYdUWtCNKbU9DWg%2Fuploads%2Fgit-blob-e06b4b7d2241c1c3619a9b36dc91646179c9ebfb%2Fi-section.png?alt=media" alt="I Section component" width="558"><figcaption></figcaption></figure>

### Inputs

| Name                  | Nick                    | Type     | Default   | Description                                                                                                                                                                                                                                   |
| --------------------- | ----------------------- | -------- | --------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| SectionName           | `SecName`               | Text     | *(empty)* | Label for the section.                                                                                                                                                                                                                        |
| Height                | `Height`                | Number   | `0.3`     | Overall height, in `m`.                                                                                                                                                                                                                       |
| TopFlangeWidth        | `TopFlangeWidth`        | Number   | `0.15`    | Width of the top flange, in `m`.                                                                                                                                                                                                              |
| TopFlangeThickness    | `TopFlangeThickness`    | Number   | `0.02`    | Thickness of the top flange, in `m`.                                                                                                                                                                                                          |
| BottomFlangeWidth     | `BottomFlangeWidth`     | Number   | `0.20`    | Width of the bottom flange, in `m`.                                                                                                                                                                                                           |
| BottomFlangeThickness | `BottomFlangeThickness` | Number   | `0.02`    | Thickness of the bottom flange, in `m`.                                                                                                                                                                                                       |
| Web                   | `Web`                   | Number   | `0.02`    | Web thickness, in `m`.                                                                                                                                                                                                                        |
| Material              | `Material`              | Material | —         | A [Uniaxial](/docs/basics/materials/uniaxial.md) material.                                                                                                                                                                                    |
| Colour                | `Colour`                | Colour   | *(empty)* | The colour the section is drawn in when [Model View](/docs/basics/visualisation/model-view.md) colours the elements **by section**. Left empty, Model View gives each section a colour of its own. Display only: the analysis never reads it. |

### Outputs

| Name    | Nick      | Type    | Description                                                                        |
| ------- | --------- | ------- | ---------------------------------------------------------------------------------- |
| Section | `Section` | Section | Beam section, for a [ForceBeamColumn](/docs/basics/elements/force-beam-column.md). |

## 📈 When to use it

**Use it when**

* The member is a welded plate girder, or any I-profile that is not in the catalogue.
* The section is **monosymmetric** — a composite beam approximated with a wide bottom flange, a crane girder.

**Do not use it when**

* The profile is a rolled IPE, HEA, HEB or similar → [Steel Section Library](/docs/basics/sections/library.md), family `I`, which has 90 of them and gets the root radii right.

## 🔗 Relation to OpenSees

```tcl
section Elastic $secTag $E $A $Izz $Iyy $G $J $alphaY $alphaZ
```