# Force Beam Column

A **Force Beam Column** is a frame element that uses a **force‑based (distributed plasticity)** formulation.\
Section response is integrated along the member length, which allows Alpaca4d to capture **spread of yielding** and nonlinear curvature distributions more accurately than simple hinge models.

## 🔧 Grasshopper component

The `Force Beam Column (Alpaca4d)` component creates a force‑based beam–column element from a line, section and (optionally) a geometric transformation.

* **Inputs**
  * **Line**: Centreline of the element.
    * Type: `Curve`
    * Units: length
  * **Section**: Cross‑section assigned to the element.
    * Type: Alpaca4d uniaxial section (e.g. steel, concrete, timber section components)
    * Controls: stiffness, strength, mass density, etc.
  * **GeometricTransformation** (`GeomTransf`, optional): Local axis definition and 2D/3D beam formulation.
    * If not provided, a **linear** transformation is automatically created from the line and a default local ( z ) axis.
  * **ZAxis** (optional): Vector controlling the local ( z ) axis orientation of the element.
    * If omitted, Alpaca4d computes a perpendicular frame from the line.
  * **Colour** (optional): Display colour of the element in Grasshopper/Rhino.
* **Outputs**
  * **Element**: Alpaca4d `ForceBeamColumn` element ready to be connected to the assemble/model component.

## 📈 When to use a force‑based beam column

* **Use it when**
  * You want to model **nonlinear behaviour along the length** of beams/columns (distributed plasticity).
  * You need realistic curvature and rotation demands for **seismic** or **pushover** analysis.
  * You are using **advanced sections** (e.g. fibre sections) where section response is integrated along the member.
* **Do not use it when**
  * A simple **linear elastic** beam is sufficient → a simpler elastic element may be enough (depending on your Alpaca4d setup).
  * You only need very coarse global stiffness representation and want to minimise computational cost.

## 🔗 Relation to OpenSees

Alpaca4d’s `ForceBeamColumn` writes an OpenSees `forceBeamColumn` command.\
In OpenSees Tcl this has the form:

```tcl
element forceBeamColumn $eleTag $iNode $jNode $transfTag $integrationTag -mass $massDens
```

In Alpaca4d, the **GeomTransf** and **beam integration** objects (e.g. `NewtonContes`) are created and tagged automatically; the material and section data come from the **Section** input of the Grasshopper component.


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