🎛️ Sections
A section pairs a geometry with a material and hands the resulting stiffness properties to an element.
Beam sections are uniaxial: they take a Uniaxial material and reduce to an area, two second moments, a torsion constant and two shear factors. Shell sections are multi-dimensional: they take an nD material and a thickness.
Components in this tab
| Component | Nickname | For | Defined by |
|---|---|---|---|
| Rectangular | RectangleCS | Beams | Width, Height |
| Circular | CircleCS | Beams | Diameter, Thickness (0 = solid) |
| Rectangular Hollow | RectangleHollowCS | Beams | Width, Height, Web, two flange thicknesses |
| I Section | I Section | Beams | Height, two flange widths and thicknesses, Web |
| Double L-Angle | 2L | Beams | Height, Width, Thickness, Gap |
| Generic Section | Generic Section | Beams | A, Izz, Iyy, J, AlphaY, AlphaZ entered directly |
| Steel Section Library | Section Library | Beams | A catalogue name — IPE200, HEA300, and 600 more |
| Plate Fiber Section | PFS | Shells | Thickness |
Choosing a section
| You know | Use |
|---|---|
| A catalogue steel profile by name | Steel Section Library |
| The dimensions of a parametric profile | the matching parametric component |
| Only the stiffness properties — a homogenised deck, a section computed elsewhere | Generic Section |
| A shell thickness | Plate Fiber Section |
Note
All parametric beam sections reduce to the same OpenSees command — section Elastic — with A, Izz, Iyy, J, AlphaY and AlphaZ computed from the dimensions. They differ in how you describe the geometry and in what Model View draws when extruded preview is on.
For a genuinely non-linear section, build a fibre section instead: Fiber Section.
Local axes
Izz is bending about the local z axis; Iyy is bending about the local y axis. The orientation of those axes on a beam comes from the ZAxis input of the ForceBeamColumn component, not from the section.
A section's depth — the height of a rectangle, an I or a hollow section — is laid along local y, so Izz is the strong axis and gravity on a floor beam shows up as Vy and Mz. Left without a ZAxis, a beam gets local y pointing up, so its sections stand the right way up; a vertical member gets local y along global X.