Domains (plane vs plate)

Every xdfem2D model lives in one of two domains. The choice is made in New model, and it is fixed as soon as the model has geometry — because every support, load and result changes meaning between them, nodes, bars and areas cannot be switched from one to the other. While the model is still empty (no nodes, bars, areas or objects) the domain can be changed: see below.

Domain	What it is	Elements	DOFs per node	Typical use
Plane	a frame or wall in its own plane	frame bars, membrane triangles (CST / Allman / ES-FEM)	ux, uy, θz	frames, trusses, walls
Plate	a slab or grillage bending out of plane	grillage beams, plate-bending triangles (MITC3 / DKT)	w, θx, θy	slabs, beam grillages

Both domains carry three degrees of freedom per node — the solver machinery is shared — but the three slots mean different things, so the same storage reads as ux, uy, θz in a plane model and w, θx, θy in a plate one.

Choosing the domain

New model shows the two domains as clickable icon cards — a portal frame with a shear wall for Frame / Wall (in-plane), and a supported slab field for Slab / Grillage (out-of-plane). Click a card to select it (the border highlights); a double-click selects and confirms in one gesture. A coloured domain badge in the toolbar (blue Plane / green Plate) then shows which one is active, and the window title carries the document name.

Changing the domain of an empty model

Click the coloured domain badge in the toolbar while the model has no geometry; the same two cards as in New model open, on the current domain. What does not depend on the domain (materials, beam sections, load cases) stays; the support definitions and area sections, which do, are removed and named in the status bar. Undo brings them back. Once the first node, bar or area exists the badge stops being a button, and its tooltip says the domain is fixed; use File ▸ New to work in the other domain. In a script, model.set_domain('plate') does the same and raises an error if the model has geometry.

What follows from the domain

The interface is domain-aware throughout — the same primitives read correctly in either domain (see Topology):

Aspect	Plane	Plate
Line element role	Beam	Grid member
Area element role	Panel (membrane)	Slab (MITC3 / DKT)
Supports	PIN, FIXED, ROLLER-X/Y, GUIDED-X/Y, BLOCK	SIMPLE, CLAMPED, CLAMP-X/Y, SYM-X/Y, BLOCK
Loads	Fx, Fy, Mz; edge tractions	Fz, Mx, My; area pressure pz; Winkler kz
Triangle results	membrane stresses σ (with an N=σ·t force toggle)	slab forces: moments, shears, Wood–Armer
Dynamics	in-plane modes; P-Delta available	vertical (out-of-plane) modes; no P-Delta

Each topic page covers both domains: Modelling, Loads, Analysis, Results and Design. For a plate-focused orientation and worked scripts, see Slabs and Grillages.