# Shell

**Shell elements** in Alpaca4d are used to model **thin and moderately thick surfaces** (plates, walls, slabs, shells)\
that carry both **in‑plane (membrane)** and **out‑of‑plane (bending, shear)** actions.

In Alpaca4d these are implemented mainly via:

* `ASDShellQ4`: 4‑node quadrilateral shell element.
* `ASDShellT3`: 3‑node triangular shell element in the core).

Both are exposed through a single Grasshopper component and share the same section definition.

## 🔧 Grasshopper component

The `ASD ShellQ4 (Alpaca4d)` component creates shell elements from a mesh and a shell section.

* **Inputs**
  * **Mesh**: Surface mesh representing the shell geometry.
    * Type: `Mesh` (quads and/or triangles).
    * Internally, the component **explodes** the mesh into individual faces:
      * **4 vertices** → `ASDShellQ4` (quadrilateral shell).
      * **3 vertices** → `ASDShellT3` (triangular shell).
  * **Section**: Shell/plate section assigned to the element(s).
    * Type: Alpaca4d multi‑dimensional section (thickness, material, reinforcement, etc.).
  * **Colour** (optional): Display colour for the generated shell elements.
* **Outputs**
  * **Element**: List of Alpaca4d shell elements (`ASDShellQ4` and/or `ASDShellT3`) ready to be connected to the assemble/model component.

## 📈 When to use shell elements

* **Use them when**
  * You are modelling **slabs, walls, plates, shells**, or other thin 3D structures.
  * Bending and membrane actions are both important (e.g. floor diaphragms, shear walls, tanks).
  * A full 3D solid (brick) model would be unnecessarily heavy, but a beam model is too simple.
* **Do not use them when**
  * The body is **volumetric / massive** and requires full 3D stress state → use **Brick** elements.
  * The behaviour is **1D member‑like** (beams, columns, braces) → use **Force Beam Column** or other frame elements.

## 🔗 Relation to OpenSees

Alpaca4d shells write OpenSees shell element commands, for example:

```tcl
element ASDShellQ4  $eleTag $n1 $n2 $n3 $n4 $secTag
element ASDShellT3   $eleTag $n1 $n2 $n3 $secTag
```

where:

* the mesh face vertex ordering defines `$n1 ...`,
* the **Section** input maps to `secTag`,
* the specific element type (`ASDShellQ4` vs `ASDShellT3`) is chosen internally based on face type and nonlinearity settings in Alpaca4d.


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