Materials and sections
Open the Materials panel from the right edge of the canvas to set the section for the whole model, or override individual members.
What the properties mean
| Symbol | Property | What it controls |
|---|---|---|
| E | Young's modulus Pa | Axial stiffness of the material. Sets how much a member stretches or shortens under load. |
| ν | Poisson's ratio unitless | How much the material narrows sideways when stretched. Must be between 0 and 0.5; structural steel is 0.30. Together with E it sets G. |
| G | Shear modulus Pa | Shear stiffness of the material, derived from E and ν as G = E / (2(1 + ν)). You can type a G directly and ν is back-calculated to match. |
| I | Second moment of area m⁴ | Bending stiffness of the cross-section about its strong axis. The single biggest driver of how much a member deflects in bending. |
| A | Cross-sectional area m² | Sets axial stiffness together with E, and shear stiffness together with G and κ. |
| A_s | Effective shear area m² | The part of the cross-section that effectively resists shear, A_s = κ × A. Together with G it sets shear stiffness. Some references tabulate A_s directly; type it here and κ is back-calculated to match. |
| κ | Shear correction factor unitless | The fraction of the area that effectively carries shear. Thin-walled boxes sit near 0.5, W-shapes near the web-area ratio, solid sections near 1. |
How does the section impact the answer?
For a statically determinate structure, it doesn't. Reactions and member forces follow from equilibrium alone, so swapping steel for aluminum changes nothing. The section starts to matter for indeterminate structures, where load distributes according to relative stiffness. There, the ratio of I between members is what moves the answer. Stocky members can also be affected where κ, the shear factor, becomes significant; see How It Works for the formulation.
When a member is too stocky: the SECTION SIZE badge
ForceCanvas checks every member’s slenderness, its length compared with the size of its section, and raises an amber SECTION SIZE badge in the footer when a member is too stocky for the beam theory you’re using.
L / r where r = √(I / A)
- L
- member length, measured from the sketch
- r
- radius of gyration of the section
- I, A
- from the per-member override, else the model section
Euler-Bernoulli mode: L / r < 10
EB ignores shear deformation. That’s a safe simplification for slender members, but for stocky ones EB results start to drift from the more complete Timoshenko answer. To clear it, switch back to Timoshenko in Canvas Settings or use a smaller section.
Timoshenko mode: L / r < 2
Timoshenko already accounts for shear, so only extreme cases are flagged: members so short for their section that they behave more like a solid block than a beam, and a beam model stops being meaningful. This usually means the section is far too large for the sketch’s scale. Use a smaller section.
In both modes, the badge only appears on indeterminate structures. On a determinate structure, reactions come from equilibrium alone, so the section can’t change them. Construction lines are skipped. The tooltip tells you how many members are affected, and clicking the badge opens the Materials panel. For how large the EB error can get, see How It Works.