> 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/elements/support.md).

# Support

A **support** restrains degrees of freedom at a node. Alpaca4d supports can be axis-aligned, in which case they are exact point constraints, or **skewed** — turned to an arbitrary plane, for an inclined roller or a support on a sloping face.

## 🔧 Grasshopper component

`Support (Alpaca4d)` — **Alpaca4d ▸ 02\_Element**

### Inputs

| Name     | Nick       | Type    | Default | Description                                                                                                                                               |
| -------- | ---------- | ------- | ------- | --------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Position | `Position` | Plane   | —       | Where the support acts, in `m`. A **Point** restrains the global axes; a **Plane** restrains its own axes instead, which is how you get a skewed support. |
| Tx       | `Tx`       | Boolean | `true`  | Translation along the support plane's *X* axis. `true` = restrained.                                                                                      |
| Ty       | `Ty`       | Boolean | `true`  | Translation along the support plane's *Y* axis.                                                                                                           |
| Tz       | `Tz`       | Boolean | `true`  | Translation along the support plane's *Z* axis.                                                                                                           |
| Rx       | `Rx`       | Boolean | `true`  | Rotation about the support plane's *X* axis.                                                                                                              |
| Ry       | `Ry`       | Boolean | `true`  | Rotation about the support plane's *Y* axis.                                                                                                              |
| Rz       | `Rz`       | Boolean | `true`  | Rotation about the support plane's *Z* axis.                                                                                                              |

A point arriving on **Position** is cast to a world-aligned plane at that point, so a model built before planes were an option behaves exactly as it did.

### Outputs

| Name    | Nick      | Type    | Description                                                              |
| ------- | --------- | ------- | ------------------------------------------------------------------------ |
| Support | `Support` | Support | Support object, to be connected to [Assemble](/docs/basics/assemble.md). |

### The seven presets

Seven common DOF patterns are recognised by name and drawn with their own symbol in [Model View](/docs/basics/visualisation/model-view.md). Anything else is drawn as a short text tag listing the restrained DOFs.

| Preset                            | Tx | Ty | Tz | Rx | Ry | Rz |
| --------------------------------- | -- | -- | -- | -- | -- | -- |
| Rigid / Fixed                     | ✓  | ✓  | ✓  | ✓  | ✓  | ✓  |
| Hinged / Pinned                   | ✓  | ✓  | ✓  |    |    |    |
| Sliding in X and Y + Rz restraint |    |    | ✓  |    |    | ✓  |
| Sliding in X + Rz restraint       |    | ✓  | ✓  |    |    | ✓  |
| Sliding in Y + Rz restraint       | ✓  |    | ✓  |    |    | ✓  |
| Sliding in Z and Y + Rz restraint | ✓  |    |    |    |    | ✓  |
| Free                              |    |    |    |    |    |    |

## 📈 When to use it

**Use it when**

* Anywhere the structure meets the ground or a stiffer neighbouring system.
* You need a roller on an incline, or a bearing on a sloping face — feed a rotated **Plane** rather than trying to fake it with constraints.

**Do not use it when**

* You want two nodes of the model to move together rather than to be held still → use [Rigid Link](/docs/basics/constraints/equal-dof.md) or [Rigid Diaphragm](/docs/basics/constraints/diaphragm.md).

## 🔗 Relation to OpenSees

**Axis-aligned** supports are a single point constraint, exact and free of any penalty:

```tcl
fix $nodeTag  $tx $ty $tz $rx $ry $rz
```

**Skewed** supports are different. OpenSees has no notion of a nodal coordinate system — `fix` only ever speaks global DOFs — so the restraint is carried by a `zeroLength` element between the support node and a coincident node that is fixed outright:

```tcl
uniaxialMaterial Elastic $springTag $k $eta $kNeg
node $auxTag $x $y $z
fix  $auxTag 1 1 1 1 1 1
element zeroLength $eleTag $auxTag $nodeTag -mat $springTag ... \
        -dir $d1 $d2 ... -orient $xx $xy $xz  $yx $yy $yz
```

* `-orient` hands the element the support plane, after which the `-dir` indices count along the plane's own axes: **1–3 translation, 4–6 rotation**.
* Directions left out of `-dir` carry no stiffness at all, so a release along a local axis is exact. Only the restrained directions are a penalty.
* The penalty stiffness is **10⁶ × the stiffest term in the model** — recomputed per model from the element diagonals, so it is unit-independent and leaves the solver about ten digits of conditioning.

{% hint style="info" %}
For a skewed support the reaction lands on the auxiliary node, because that is where the point constraint lives. [Reaction Forces](/docs/basics/results/reaction-forces.md) reads it back from there and reports it in the support's own axes, so this is invisible from Grasshopper.
{% endhint %}