Fiber Section

Collects fibre points, layers and patches into one section.

🔧 Grasshopper component

Fiber Section (Alpaca4d) — Alpaca4d ▸ MomentCurvature_βeta, nickname FiberSection

Inputs

PointFiberPointFiberlist(empty)Single fibres, typically bars — from Fiber Point.

LayerLayerlist(empty)Rows of fibres — from Layer Fiber.

PatchPatchlist(empty)Filled areas — from Patch.

GJNumber1e8Torsional stiffness, in kN·m².

Elastic and uncoupled from the bending response.

Outputs

FiberSectionFiberSectionFor Moment Curvature.

Local axes

The component draws the section's local axes at the area centroid of its fibres, steel included: the point OpenSees takes the section's moments about, and the Centroid of Interaction Domain.

Arrow	Axis	World direction	Moment about it
Green, labelled y, My	local y	world X	My
Blue, labelled z, Mz	local z	world Y	Mz
Red ⊙, labelled x	the beam's axis, x = y × z	world Z, out of the drawing	—

That is the order OpenSees writes a fibre in. Draw the section in the world XY plane and the mapping takes care of itself; the arrows are there to confirm it.

My is the moment about y, Mz the moment about z, by the right-hand rule with x coming out of the drawing, towards you in the Top view. The double arrowheads are a moment vector's. So:

Positive	Stretches	Squeezes
My	the fibres on the +z side	the −z side
Mz	the fibres on the −y side	the +y side

This is the convention of Moment Curvature, whose Direction 0 bends the section about y and 90 about z, of Interaction Domain and Domain Check, and of the views, which draw My along X and Mz along Y, as the arrows here lie.

The preview also shows every fibre the section contains, so a patch whose mesh is too coarse or a layer pointing the wrong way is visible before anything is run.

📈 When to use it

Use it when you want a section whose non-linear behaviour comes out of its materials.

Do not use it when the element is a beam — fibre sections cannot yet be assigned to a ForceBeamColumn, which takes an elastic section. Today this is a section-level tool.

Note

GJ is a fixed elastic number. A fibre section integrates axial and bending response from its fibres but has no torsional stiffness of its own, so it has to be given one. The default 1e8 kN·m² is deliberately large: with the section under pure bending, torsion never engages, and the value only matters if the section is ever used somewhere that torsion does.

🔗 Relation to OpenSees

section Fiber $secTag -GJ $GJ {
    fiber $y1 $z1 $A1 $mat1
    fiber $y2 $z2 $A2 $mat2
    ...
}

Patches and layers are expanded into individual fiber lines. Use Serialize to see the result.