Apply gravity / self-weight¶

Add the body weight (ρ · g) of a meshed solid as a load. Reach for this whenever a model carries its own dead load — a footing, a dam, a soil column, an RC volume — instead of (or alongside) a hand-applied nodal load.

Gravity is a body load with a convenience wrapper: you give it a volume target by name, a gravity vector g, and a density. Like every g.loads.* factory it is declared pre-mesh under a load case and resolved at get_fem_data; on the bridge you then opt in with p.from_model(case) inside a pattern — you never write eleLoad for it, and nothing emits without your pattern.

Recipe¶

from apeGmsh import apeGmsh
from apeGmsh.opensees import apeSees

g = apeGmsh(model_name="gravity_demo")
g.begin()
# ... build a part, mesh a volume into the PG "rc_volume" ...

# Self-weight of the concrete volume, grouped in a dead-load pattern.
with g.loads.case("self_weight"):
    g.loads.gravity("rc_volume", g=(0, 0, -9.81), density=2400)

# density=(0,0,-9.81) is the default g; the one-liner is just:
#     g.loads.gravity("rc_volume", density=2400)
# density=None tells the bridge to read ρ from the assigned material
# (only valid with target_form="element").

# Resolve: the gravity def becomes per-node body-force records on the broker.
fem = g.mesh.queries.get_fem_data(dim=3)

# Build OpenSees through the typed bridge, then opt the case in:
ops = apeSees(fem)
# ... ops.section / ops.element / ops.fix / ops.mass ...
ts = ops.timeSeries.Linear()
with ops.pattern.Plain(series=ts) as p:
    p.from_model("self_weight")     # replay the resolved case as nodal loads
ops.run(...)

Notes / gotchas¶

Don't double-apply. Import each case with from_model in exactly one pattern, and do not also hand the same elements a body force through the bridge (a raw eleLoad -bodyForce / element body_force= for the identical volume). That stacks two copies of self-weight and doubles the dead load — a silent, order-of-magnitude error in the reactions. Declare it in exactly one place.
Target volumes only. Gravity must resolve to 3-D entities. Targeting a surface or curve is a no-op — the volume iterator silently skips non-3D dim-tags. (Self-weight of a shell belongs on the shell section's thickness × density, not on a load.) If a gravity def produces zero records, you almost certainly targeted the wrong-dimension label.
g is unit-sensitive. (0, 0, -9.81) is for SI-metre models. For a kg-mm-s model use (0, 0, -9810). A unit mismatch in g or density (kg/m³ vs g/cm³) shows up as an order-of-magnitude error in ΣF.
density=None needs element form. Reading ρ from the material is only available with target_form="element"; the default target_form="nodal" requires an explicit density and raises ValueError without one.
Sanity-check ΣF. After get_fem_data, sum force_xyz over the pattern and compare against ρ · g · V by hand — this is the cheapest way to catch a flipped sign or a doubled load.

Apply gravity / self-weight¶

Recipe¶

Notes / gotchas¶

See also¶