apeGmsh sections¶

A guide to the parametric section builders — creating structural cross-sections directly in the session with automatic labeling of flanges, webs, and end faces.

Grounded in the current source:

src/apeGmsh/sections/_builder.py — SectionsBuilder composite
src/apeGmsh/sections/solid.py — solid-element section geometry
src/apeGmsh/sections/shell.py — shell-element section geometry

All snippets assume an open session:

from apeGmsh import apeGmsh
g = apeGmsh(model_name="frame")
g.begin()

1. What sections are¶

A structural section is a prismatic member defined by its cross-sectional shape (I-beam, rectangle, channel) and a length. apeGmsh builds these as 3D solid or 2D shell geometry directly in the session, with named sub-regions (flanges, web, end faces) that constraints and loads can target by label.

Sections are accessed via g.sections:

col = g.sections.W_solid(
    bf=150, tf=20, h=300, tw=10, length=2000,
    label="col",
)

This creates an I-beam cross-section extruded along Z with: - col.labels.top_flange → "col.top_flange" - col.labels.bottom_flange → "col.bottom_flange" - col.labels.web → "col.web" - col.labels.start_face → "col.start_face" (z=0 end) - col.labels.end_face → "col.end_face" (z=length end)

2. Sections vs Parts¶

Both create geometry with labels. The difference:

	Part	Section
Session	Own isolated Gmsh session	Built directly in assembly session
Persistence	Auto-saves to STEP + sidecar JSON	No file — exists only in session
Reuse	Import many times with transforms	Build once per instance
Labels	Via COM-matching after STEP import	Created natively in session
Use when	Same geometry in many locations	One-off parametric members

Use Parts when you have a column design that repeats 20 times at different locations. Use sections when you have one beam with specific dimensions that appears once.

In practice, sections are convenient for quick prototyping. Parts are better for production assemblies because they survive session restarts (the STEP file is on disk).

3. Available section types¶

W_solid — Wide-flange solid section¶

col = g.sections.W_solid(
    bf=150,      # flange width (mm)
    tf=20,       # flange thickness (mm)
    h=300,       # web height (clear distance between flanges)
    tw=10,       # web thickness (mm)
    length=2000, # extrusion length (mm)
    label="col",
    lc=50,                          # target element size (optional)
    translate=(0, 0, 0),            # position (optional)
    rotate=(1.5708, 0, 0, 1),      # rotation (optional)
)

Creates a 3D solid I-beam extruded along Z. Cross-section built by subtracting two rectangular voids from an outer rectangle, then sliced at the flange-web boundaries to create labeled sub-volumes.

Total section height: 2*tf + h (flanges + clear web height).

Labels created: - {label}.top_flange — upper flange volume(s) - {label}.bottom_flange — lower flange volume(s) - {label}.web — web volume(s) - {label}.start_face — surface at z=0 - {label}.end_face — surface at z=length

rect_solid — Rectangular solid section¶

beam = g.sections.rect_solid(
    b=200,       # width (mm)
    h=400,       # height (mm)
    length=3000, # extrusion length (mm)
    label="beam",
)

Creates a simple rectangular prism. Labels: - {label}.body — the single volume - {label}.start_face — z=0 end - {label}.end_face — z=length end

rect_hollow — Hollow rectangular tube (HSS)¶

hss = g.sections.rect_hollow(
    b=200,       # outer width (mm)
    h=300,       # outer height (mm)
    t=10,        # wall thickness (mm)
    length=3000,
    label="hss",
)

Builds an HSS by cutting a smaller box from a larger one. Labels: - {label}.body — the single hollow volume - {label}.start_face, {label}.end_face — z=0 / z=length

pipe_solid — Solid circular bar¶

bar = g.sections.pipe_solid(
    r=50,        # radius (mm)
    length=2000,
    label="bar",
)

Labels: {label}.body, {label}.start_face, {label}.end_face.

pipe_hollow — Hollow circular pipe¶

pipe = g.sections.pipe_hollow(
    r_outer=80,  # outer radius (mm)
    t=8,         # wall thickness (mm)
    length=2000,
    label="pipe",
)

Built by cutting an inner cylinder from an outer cylinder. Labels: {label}.body, {label}.start_face, {label}.end_face.

angle_solid — L-shape (angle)¶

angle = g.sections.angle_solid(
    b=100,       # horizontal leg width (mm)
    h=150,       # vertical leg height (mm)
    t=10,        # thickness of both legs (mm)
    length=3000,
    label="L",
)

Built by fusing two rectangles, extruding, and slicing along the inside corner so each leg is its own labeled volume.

Labels created: - {label}.horizontal_leg — the X-direction leg - {label}.vertical_leg — the Y-direction leg - {label}.horizontal_leg_face, {label}.vertical_leg_face — outer-skin faces from classify_angle_outer_faces - {label}.start_face, {label}.end_face

channel_solid — C-shape (channel)¶

chan = g.sections.channel_solid(
    bf=80,       # flange width (mm)
    tf=10,       # flange thickness (mm)
    h=180,       # clear web height (mm)
    tw=8,        # web thickness (mm)
    length=3000,
    label="C",
)

Same bf / tf / h / tw parameter set as W_solid, sliced into flange and web sub-volumes.

Labels created: - {label}.top_flange, {label}.bottom_flange, {label}.web - Outer-face labels from classify_w_outer_faces - {label}.start_face, {label}.end_face

tee_solid — T-shape (tee)¶

tee = g.sections.tee_solid(
    bf=120,      # flange width (mm)
    tf=12,       # flange thickness (mm)
    h=180,       # stem height (mm)
    tw=10,       # stem thickness (mm)
    length=3000,
    label="T",
)

Built by fusing a flange rectangle with a stem rectangle and slicing at the stem boundaries.

W_shell — Wide-flange shell section (mid-surfaces)¶

col_shell = g.sections.W_shell(
    bf=150, tf=20, h=300, tw=10, length=2000,
    label="col_shell",
)

Same parameters as W_solid but creates mid-surface shell geometry instead of solid volumes. The flanges and web are represented as surfaces at their mid-plane locations.

Labels created: - {label}.top_flange — upper flange surface - {label}.bottom_flange — lower flange surface - {label}.web — web surface

Use this when meshing with shell elements (dim=2) instead of solid elements (dim=3).

4. Positioning and orientation¶

Sections are built at the origin and can be positioned with translate and rotate:

# Column at (5, 0, 0), rotated 90 degrees about Z
col = g.sections.W_solid(
    bf=150, tf=20, h=300, tw=10, length=3000,
    label="col_A",
    translate=(5000, 0, 0),
    rotate=(1.5708, 0, 0, 1),  # 90 deg about Z axis
)

The rotation uses the same convention as g.model.transforms.rotate: (angle_rad, ax, ay, az) for rotation about an axis through the origin, or (angle_rad, ax, ay, az, cx, cy, cz) for rotation about a point.

5. Element sizing¶

The lc parameter sets the target element size on the section's BRep points. It works alongside g.mesh.sizing.set_global_size():

col = g.sections.W_solid(
    bf=150, tf=20, h=300, tw=10, length=2000,
    label="col", lc=30,  # fine mesh on column
)
# Global mesh is coarser
g.mesh.sizing.set_global_size(100)
g.mesh.generation.generate(3)

The default lc=1e22 imposes no local constraint — the element size is governed purely by the global size.

6. Using labels for constraints and loads¶

The real power of sections is that every sub-region has a name you can target:

# Column section
col = g.sections.W_solid(
    bf=150, tf=20, h=300, tw=10, length=3000,
    label="col",
)

# Fix the bottom face
g.physical.add_surface(
    g.labels.entities("col.start_face"), name="Base"
)

# Apply gravity
with g.loads.pattern("dead"):
    g.loads.gravity("col.web", density=7850)
    g.loads.gravity("col.top_flange", density=7850)
    g.loads.gravity("col.bottom_flange", density=7850)

# Constrain slab to column top
g.constraints.equal_dof("col.end_face", "slab.bottom", dofs=[1, 2, 3])

7. Complete example¶

from apeGmsh import apeGmsh

with apeGmsh("portal_frame") as g:
    # Columns
    left_col = g.sections.W_solid(
        bf=150, tf=20, h=300, tw=10, length=3000,
        label="left_col", translate=(0, 0, 0),
    )
    right_col = g.sections.W_solid(
        bf=150, tf=20, h=300, tw=10, length=3000,
        label="right_col", translate=(6000, 0, 0),
    )

    # Beam
    beam = g.sections.rect_solid(
        b=200, h=400, length=6000,
        label="beam",
        translate=(0, 0, 3000),
        rotate=(1.5708, 0, 1, 0),  # rotate to span X direction
    )

    # Fragment for conformal mesh at joints
    g.parts.fragment_all()

    # Mesh
    g.mesh.sizing.set_global_size(50)
    g.mesh.generation.generate(3)

    # FEM data
    fem = g.mesh.queries.get_fem_data(dim=3)
    print(fem.inspect.summary())