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.

• ShellDKGT: 3‑node triangular shell element (and its nonlinear variant ShellNLDKGT 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 → ShellDKGT (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 ShellDKGT) 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:

element ASDShellQ4  $eleTag $n1 $n2 $n3 $n4 $secTag
element ShellDKGT   $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 ShellDKGT / ShellNLDKGT) is chosen internally based on face type and nonlinearity settings in Alpaca4d.