Fix supports & boundary conditions¶

Hold a model down by fixing the nodes of a named face — pin, roller, or fully fixed — and, where you need it, push a face to a non-zero prescribed displacement. Reach for this whenever a part would otherwise float free, or when you want to drive a support by an imposed settlement / motion instead of a force.

Supports are single-point constraints (SPs). They are not auto-ingested from the session — you re-declare them on the bridge: homogeneous fixities with ops.fix(...), non-zero prescribed values with p.sp(...) inside a pattern. (Contrast: g.loads.* and g.constraints.* do auto-emit.) Target a physical-group name, never a raw tag.

Recipe¶

from apeGmsh import apeGmsh
from apeGmsh.opensees import apeSees

with apeGmsh(model_name="support_demo") as g:
    g.model.geometry.add_box(0, 0, 0, 2.0, 1.0, 1.0, label="Block")
    g.model.sync()

    g.physical.from_label("Block", name="Block")
    g.model.select(dim=2).on_plane((0, 0, 0.0), (0, 0, 1), tol=1e-6).to_physical("Base")
    g.model.select(dim=2).on_plane((0, 0, 1.0), (0, 0, 1), tol=1e-6).to_physical("Top")

    g.mesh.sizing.set_global_size(0.5)
    g.mesh.generation.generate(dim=3)
    fem = g.mesh.queries.get_fem_data(dim=3)

# ── Bridge — supports are re-declared here, by PG name ──────────────
ops = apeSees(fem)
ops.model(ndm=3, ndf=3)
conc = ops.nDMaterial.ElasticIsotropic(E=30e9, nu=0.2, rho=2400)
ops.element.FourNodeTetrahedron(pg="Block", material=conc)

# Homogeneous SP — every node in "Base" pinned in x, y, z.
ops.fix(pg="Base", dofs=(1, 1, 1))           # 1 = fixed, 0 = free

# Non-zero prescribed displacement — drive "Top" down 0.01 in z.
with ops.pattern.Plain(series=ops.timeSeries.Linear()) as p:
    p.sp(pg="Top", dof=3, value=0.01)        # dof is 1-based

ops.tcl("model.tcl")

ops.fix(pg="Base", dofs=(1, 1, 1)) fans the mask across every node of the Base group at build time and emits one fix $node 1 1 1 line each; p.sp(pg="Top", dof=3, value=0.01) emits sp $node 3 0.01 lines inside the pattern Plain {…} block.

Pin vs. roller vs. fully fixed¶

The dofs tuple is a length-ndf mask (1 = fixed, 0 = free). Pick the pattern; the table is for the common ndm/ndf pairs:

Support	`ndm=2, ndf=2`	`ndm=3, ndf=3` (solid)	`ndm=3, ndf=6` (frame/shell)
Roller (free in x, vertical held)	`(0, 1)`	`(0, 0, 1)`	`(0, 0, 1, 0, 0, 0)`
Pin (translations held, rotations free)	`(1, 1)`	`(1, 1, 1)`	`(1, 1, 1, 0, 0, 0)`
Fully fixed / encastre	`(1, 1)`	`(1, 1, 1)`	`(1, 1, 1, 1, 1, 1)`

For a 3-DOF solid there are no rotational DOFs, so "pin" and "fully fixed" coincide — (1, 1, 1). The distinction only appears once nodes carry rotations (ndf=6).

Notes / gotchas¶

len(dofs) must equal ndf. A mismatch raises BridgeError at build(). Pin a 6-DOF node with (1, 1, 1, 0, 0, 0), not (1, 1, 1).
dofs is a 0/1 mask; dof= in p.sp is a 1-based index. Same word, two conventions: ops.fix(dofs=(1, 1, 1)) is a per-DOF flag vector, while p.sp(dof=3, value=…) names which DOF (the 3rd) to prescribe.
Homogeneous vs. prescribed is the deciding split. Zero-value fixities are model-level → ops.fix. Any non-zero imposed value goes in a pattern → p.sp. Don't try to fake a prescribed displacement with ops.fix.
Supports are NOT auto-ingested. If you declared a face SP on the session via g.displacements.surface(...), it resolves into fem.nodes.sp and feeds the viewer / Results, but it does not reach the runnable deck — re-declare it: homogeneous → ops.fix, prescribed → p.sp.
g.constraints.bc vs. g.displacements ownership. g.constraints.bc owns permanent homogeneous fixes; g.displacements owns prescribed motion (any non-zero value, or a pattern-bound / time-varying value). A zero authored via g.displacements is an allowed pattern-bound hold, not a silent alias for bc.
Target names, not tags. pg= resolves a physical-group or an apeGmsh label (FEM-direct, no promotion needed). Keep PG names dimension-unique so pg= is never ambiguous.
For a stage-bound support that only comes online in a later analysis stage, use s.fix(pg=…, dofs=…) inside a with ops.stage(...) block instead of the flat ops.fix.

Fix supports & boundary conditions¶

Recipe¶

Pin vs. roller vs. fully fixed¶

Notes / gotchas¶

See also¶