> 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/results/reaction-forces.md).

# Reaction Forces

The force and moment each [Support](/docs/basics/elements/support.md) applies to the structure, reported **in the support's own axes**.

## 🔧 Grasshopper component

`Reaction Forces (Alpaca4d)` — **Alpaca4d ▸ 08\_NumericalOutput**

### Inputs

| Name        | Nick          | Type    | Default | Description                                                                                                                                                          |
| ----------- | ------------- | ------- | ------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| AlpacaModel | `AlpacaModel` | Model   | —       | The analysed model.                                                                                                                                                  |
| History     | `History`     | Boolean | `false` | Read every recorded step instead of one. **ReactionForce** and **ReactionMoment** become trees `{step}`; **SupportPosition** stays a flat list. **Step** is ignored. |
| Step        | `Step`        | Integer | `0`     | Analysis step.                                                                                                                                                       |

### Outputs

| Name            | Nick              | Type          | Description                                                                                                                           |
| --------------- | ----------------- | ------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
| SupportPosition | `SupportPosition` | Plane (list)  | Where each support is, **and the axes its reactions are given in**. Its origin is the support position; its axes are the frame below. |
| ReactionForce   | `ReactionForce`   | Vector (list) | Reaction force, in `kN`, in the support's own axes.                                                                                   |
| ReactionMoment  | `ReactionMoment`  | Vector (list) | Reaction moment, in `kN·m`, in the support's own axes.                                                                                |

The three lists are parallel: item *i* of each belongs to the same support.

## 💡 Why the support's own axes

For a support placed on a **Point** the plane is world-aligned, so the reactions are the global components — exactly as before.

For a support placed on a **Plane**, the reactions are resolved onto that plane. This is what makes a released direction read as the zero it actually is: OpenSees reports reactions in global components whatever the support is turned to, which for a skewed support spreads a reaction running along one local axis across all three global ones.

{% hint style="info" %}
A skewed support carries its restraint on a coincident auxiliary node — see [Support](/docs/basics/elements/support.md#relation-to-opensees) — so that is where OpenSees puts the reaction, and the support node itself reads zero. This component reads the right node automatically; you never see the auxiliary node.
{% endhint %}

## 📈 When to use it

**Use it when**

* You need foundation loads to pass on to a footing design.
* You are checking global equilibrium: the sum of the vertical reactions should match the applied load, and if it does not, something in the model is not connected.
* You want to confirm a support is doing what you meant — a released direction should read zero.

## 💡 Notes

* Reactions are what the **support applies to the structure**. Sign convention follows the plane's axes.
* Restrained directions of a skewed support are held by a penalty spring, so their reactions are accurate to about one part in 10⁶ rather than exact.