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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:

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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