Alpaca4d

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Shell Forces ​

The stress resultants on every shell element: membrane forces, bending moments and transverse shears, all per unit width.

🔧 Grasshopper component ​

Shell ForcesAlpaca4d ▸ 08_NumericalOutput
Shell Forces component
Inputs
AlpacaModelModelThe analysed model.
HistoryBooleanfalseRead every recorded step instead of one.
The outputs become trees {step; element}. Step is ignored.
StepInteger0Analysis step.
ElementIdTextlist(empty)Read only some of the shells: an ElementId, a wildcard over them, a tag, or a regular expression.
Empty reads them all.
Outputs

One branch per shell element, values at the element's integration points.

pxxNumbertreeMembrane force along local x (axis 1), in kN/m.
pyyNumbertreeMembrane force along local y (axis 2), in kN/m.
pxyNumbertreeIn-plane shear, in kN/m.
mxxNumbertreeBending moment whose stresses run along local x (axis 1) — it bends the shell about local y — in kN·m/m.
myyNumbertreeBending moment whose stresses run along local y (axis 2) — it bends the shell about local x — in kN·m/m.
mxyNumbertreeTwisting moment, in kN·m/m.
vxzNumbertreeTransverse shear on the x face, in kN/m.
vyzNumbertreeTransverse shear on the y face, in kN/m.
ElementElementlistThe shells read, in the order of the branches.
Deconstruct them for the tag and the geometry.

📈 When to use it ​

Use it when

  • You are designing slab or wall reinforcement — mxx, myy and mxy are what go into a Wood–Armer calculation.
  • You need membrane forces in a shear wall or a tank.

Do not use it when

  • You want a contour → Model View, with Shell forces ticked.
  • You want the principal directions → Principal Stress Lines traces them as curves.
  • You want the forces through a line across the shells, or their totals → Shell Section.

Warning

These are local-axis results, and the local axes are per face unless you set them. Two adjacent faces of the same slab can report mxx about different directions, which makes the numbers meaningless to compare or to contour.

Set Local X Axis on the ASD Shell component for any model whose shell results you intend to read.

💡 Sign convention ​

All quantities are per unit width. Local x, y and z are axes 1, 2 and 3 of Local Axes in Model View — red, green and blue. Positive membrane force is tension. A positive mxx or myy puts the face on the positive local z side — the side the blue arrow points to — in tension: the Top layer of Shell Stresses. On a cantilever slab whose mesh faces up, mxx is positive over the support.

Which side is +z follows the order of the mesh's vertices, not the world: a mesh whose faces point down has its Top layer underneath.