Apply a face pressure or traction

Put a distributed surface load — wind, snow, water, a uniform "pull" — on a face (or, in 2-D, an edge) physical group. Declare it pre-mesh against the group name with g.loads.surface; it resolves to equivalent nodal forces at get_fem_data and is opt-in at the typed bridge — import its case into a pattern with p.from_model(case).

Recipe

from apeGmsh import apeGmsh
from apeGmsh.opensees import apeSees

g = apeGmsh(model_name="pressure_demo")
g.begin()
# ... geometry, parts, the "Roof" / "FacadeW" surface PGs, mesh ...

with g.loads.case("snow"):
    # Pressure: scalar, perpendicular to each face. Positive magnitude
    # pushes INTO the face, so it follows a sloped/curved surface without
    # you resolving components by hand.
    g.loads.surface.pressure("Roof", -3.0e3)

with g.loads.case("wind_X"):
    # Traction: a vector applied the same way on every face regardless of
    # orientation (a uniform "pull").
    g.loads.surface.traction("FacadeW", (1.2e3, 0, 0))

# Resolve: surface loads become equivalent nodal force records on the broker.
fem = g.mesh.queries.get_fem_data(dim=3)

# Build OpenSees. The g.loads.* surface loads are opt-in: import each
# case into a pattern with p.from_model(case) -- nothing auto-emits.
ops = apeSees(fem)
ops.model(ndm=3, ndf=3)
# ... ops.nDMaterial / ops.element / ops.fix / ops.mass ...
with ops.pattern.Plain(series=ops.timeSeries.Linear()) as p:
    p.from_model("snow")     # Roof pressure
with ops.pattern.Plain(series=ops.timeSeries.Linear()) as p:
    p.from_model("wind_X")   # FacadeW traction
ops.run()

Notes / gotchas

• surface.pressure is pressure; surface.traction is traction. Pressure is perpendicular to each face (right for wind/snow/water on a sloped or curved surface). Traction takes a vector and ignores face orientation. Sign: positive magnitude pushes into the face.
• Loads are opt-in — no double-count trap. A surface load on g.loads.* does not auto-emit; it reaches the deck only when a pattern imports its case with p.from_model(case). The deck is authoritative — a case you don't import is simply not applied.
• Element pressure= is a different thing — and it can bite a benchmark. The 2-D quad/tri elements (FourNodeQuad, Tri31, SixNodeTri) take a pressure= constructor arg. That is OpenSees' element-native uniform normal pressure applied around each element's perimeter edges — it does not know which boundary you meant. On a uniaxial tension/pressure benchmark it loads the interior element edges too and gives the wrong resultant. For a clean traction on a named boundary face, use g.loads.surface.traction(pg="...") (resolved nodal traction) and leave element pressure= unset. Reach for element pressure= only when you genuinely want OpenSees' per-element edge pressure behaviour.
• Higher-order faces want reduction="consistent". Tributary (default) splits p·A equally to corner nodes — exact for tri3/quad4. For tri6/quad8/quad9, pass reduction="consistent" so the curved Gauss-point normal and mid-side weighting are integrated correctly.
• Sanity-check ΣF. Sum force_xyz over fem.nodes.loads.by_pattern(pat) and compare to p·A by hand — a flipped sign or a load on the wrong face shows up immediately.

See also

• Concept: Loads guide §7 — surface pressure vs traction, the resolve pipeline, and reduction / target_form.
• Bridge: OpenSees bridge guide — how to import g.loads.* cases via p.from_model vs what you re-declare.
• Tutorial: Plate in tension — a surface traction driving a 2-D continuum model end to end.
• API: g.loads — surface, line, gravity, and the rest of the load factories.

---

Next: Add a point load.