Materials & sections

What Young's modulus, strength, area and moment of inertia mean, and how each cross-section shape gets there.

Last updated 2026-09-14

Quickstart

1. Open a member's Material and Section dropdowns in the Member tool panel — each has an Add new… option.
2. For a material, start from a preset (Rubber, Spaghetti, Balsa wood, Timber, Aluminium, Steel) or dial in your own values.
3. For a section, pick a shape — solid round, pipe, solid rectangle, hollow rectangle, or custom — and enter its dimensions. Area and moment of inertia are worked out for you automatically.
4. Use the eyedropper tool to pick up a member's material and section, or the apply tool to paint them onto other members, instead of setting the dropdowns one member at a time.
5. To see all materials or sections at once, click the icon next to a dropdown (Open library), or click Materials or Sections in the Statistics panel.

What it is

Every member has two separate properties: a material (what it's made of) and a section (the shape and size of its cross-section). They're independent — the same steel material can be used with a thin rod or a thick tube, and the same tube shape can be made of steel or aluminium. Both are shared: a material or section lives once in your project's library, and any number of members can point at it, so changing one value in the library updates every member using it at once.

The library

The library lists every material and section in your project, on a Materials and a Sections tab.

• Rename and recolour right in the row. Use an item's edit button to change its values, or Add material / Add section to create a new one.
• Default marks the material or section new members are drawn with. Pick another row to change it.
• Used by shows how many members use the item. The button next to the number selects those members on the canvas; click it again to deselect them.
• Hover a section's shape to see its dimensions.

Material properties

A material has two numbers that drive the analysis:

• Young's modulus (E) — how stiff the material is. A bigger E means less stretch under the same pulling force. This is what the deformed shape and displacements are computed from.
• Strength (fMax) — the maximum stress the material can take before it breaks. This is the limit the strength check compares every member's stress against.

A material also has a density — mass per volume. It doesn't feed into the structural analysis; it's what the Total mass figure in the Statistics panel is computed from.

A few built-in presets (Rubber, Spaghetti, Balsa wood, Timber, Aluminium, Steel) fill in typical E, fMax, and density values as a starting point — pick one to see its numbers, then edit them freely, or skip the presets and type your own.

Cross-sections & their formulas

A section's shape decides two things every calculation needs: its cross-sectional area (used by the stress and strength checks) and its moment of inertia — specifically the smallest one, I_min, since a member always buckles about its weakest axis (see Buckling). For four of the five shapes, both are computed automatically from the dimensions you enter:

• Solid round (diameter d): area = (π/4)·d², I_min = (π/64)·d⁴
• Pipe (outer diameter d, wall thickness t): area = (π/4)·(d² − inner²), I_min = (π/64)·(d⁴ − inner⁴), where inner = d − 2t
• Solid rectangle (width, height): area = width · height, I_min = (min side) · (max side)³ / 12 — the smaller side is always cubed, since that's the weak-axis direction
• Hollow rectangle (width, height, wall thickness): the solid rectangle's area/I_min minus the same formulas applied to the inner (wall-thickness-reduced) rectangle
• Custom: there's no shape to compute from, so you type area and moment of inertia (I_min) directly — useful for a cross-section that doesn't match any of the four standard shapes

Behavior notes

Deletion is protected. A material or section can't be deleted while any member still uses it, or if it's the only one left in your project — a truss always needs at least one of each.

Names are informational, not identifiers. Renaming a material or section doesn't break anything. Sections get a name from their shape and dimensions (e.g. Ø20, 30x30-3), and changing the shape or a dimension updates it again, even if you renamed the section before.

Editing updates every member at once. Since members reference a shared material/section rather than owning their own copy, changing E, fMax, or a dimension immediately recalculates every member that uses it.