The reliable sequence
Quick start: calculate a custom section from beginning to end
Follow this order whether the section begins with catalogue steel, parametric parts, a polygon, or DXF geometry.
- 1Choose coordinate units
Select millimetres or inches before entering dimensions, coordinates, snap spacing, or importing geometry.
- 2Create the solid material
Add catalogue or parametric parts, draw closed polygons, load a preset, or import a clean 2D DXF.
- 3Assemble the section
Edit dimensions and exact X/Y positions, then move, rotate, duplicate, or mirror components. Solids may touch or overlap.
- 4Subtract openings
Add void components or closed opening polygons completely inside solid material. Check that no opening touches or crosses a boundary.
- 5Calculate and review
Select Calculate, resolve geometry messages, and verify the final outline, overall dimensions, centroid, axes, and expected symmetry.
- 6Use the required properties
Review geometric and plastic results first, then confirm whether the section is connected before relying on mesh-based shear, torsion, or warping results.
- 7Save, report, or continue
Save the editable workspace at any time. Calculate before downloading its PDF or making the section available to Beam Calculator and 2D Structural Analysis.
Overlapping solids are not double-counted. A void removes material only where it lies inside the combined solid region.
When to use this workspace
Built-up, fabricated, irregular, and asymmetric sections
Use Section Builder when a cross-section needs several components, a custom boundary, an internal opening, CAD geometry, or properties beyond one standard shape formula.
Workspace and view
Orient the canvas before editing geometry
Coordinate units
The selected unit applies to component dimensions, X/Y coordinates, polygon points, snap spacing, overall geometry, and dimensional results. Area, section modulus, inertia, and warping constant use the corresponding powers of that length unit.
Changing the unit converts the working geometry; it does not reinterpret the same numeric coordinates as a different physical size. After any unit change, confirm one known dimension and recalculate.
Snap, navigation, and Focus mode
With Snap enabled, dragged geometry and arrow-key movement follow the entered increment. Turn Snap off for free placement or enter exact values in Properties. Choose Pan or press H for persistent panning. Hold Space for temporary panning, or middle-drag at any time; touch-drag empty canvas space pans when zoomed in. Zoom, pan, Fit, and Focus mode change only the view; they do not change section coordinates or results.
Centroid and shear-centre coordinates are reported in the same X/Y system shown on the canvas, not from the section’s lower-left corner unless the geometry was positioned there.
Starting geometry
Add catalogue members, parametric parts, or presets
Steel library
Choose AISC, CISC, EN, UK, or AS/NZS; filter a family, search a designation, and add the selected shape as one editable component.
Parametric parts
Add plates, rectangles, circles, rings, I-sections, channels, angles, tees, or a starter void, then edit labelled dimensions.
Presets
Load an I-section, channel, angle, or box-with-void starting model, or start blank and enter polygon mode.
- 1Add rather than replace
The Add and Steel workflows append a component to the current section. This makes cover plates, back-to-back shapes, and multi-part fabrications straightforward.
- 2Check the geometry badge
Source geometry, published standard rules, catalogue-derived approximations and schematic outlines are labelled in the steel browser. AISC double-angle separation and orientation come from the designation; CISC double angles default visibly to contact and offer every published gap. Sections missing critical dimensions stay unavailable instead of receiving a guessed outline.
- 3Keep catalogue properties authoritative
The steel database retains the publisher's area and moments of inertia. Reconstructed outlines are independently checked against those values and carry their verification result and source assumptions.
- 4Edit the selected component
Open Properties and enter its name, material role, dimensions, position, and rotation. Dimension labels depend on the component type.
- 5Make the intended connection explicit
Move parts until solids touch or overlap if the physical section is intended to act as one connected region.
Area and ordinary geometric properties still include all regions, but advanced warping and shear properties cannot describe an unspecified physical connection.
Arbitrary boundaries
Draw and edit solid polygons and openings
Select Polygon or press P, keep the boundary type set to Solid, then click points in perimeter order. Enter exact Next X and Next Y values when a corner must land at a known coordinate.
Choose Opening as the boundary type or use Draw opening. Place the closed boundary fully inside solid material. The opening is subtracted during Boolean composition.
- 1Place ordered corners
Trace one perimeter continuously. Do not jump across the shape or create crossing edges.
- 2Close the boundary
Click the first point, double-click, select Finish polygon, or press Enter after at least three points.
- 3Refine the vertices
Open Points, select or drag a corner, type its exact X/Y coordinates, or select a + handle on an edge to insert another point.
- 4Add tangent radii where required
Enter corner radii only where adjacent edges are long enough. Recheck the generated boundary and overall dimensions because curves are discretized for calculation.
An opening must remain within a solid and must not touch or cross another boundary. Use Undo point or Backspace while drawing if the latest corner is wrong.
CAD input
Prepare and import a reliable 2D DXF
- 1Flatten the profile to the XY plane
Create one closed outside boundary and a separate closed boundary for every opening.
- 2Remove drawing-only content
Delete dimensions, annotations, hatches, centre lines, title blocks, duplicate edges, and construction geometry.
- 3Use supported entities
Export supported polylines or connected LINE, ARC, CIRCLE, and ELLIPSE geometry. Explode or convert blocks, splines, and unsupported profile objects first.
- 4Match model-space and DXF units
In CAD model space, make 1 drawing unit equal the intended physical unit and set the DXF
$INSUNITSdeclaration to match. Page, layout, or annotation units do not establish the section scale. The importer accepts ASCII DXF files up to 5 MB. - 5Import and verify before calculating
The importer reports the declared unit and imported width × depth. Compare at least one known model-space width or thickness before Calculate. Unitless files use the builder unit selected before import and cannot be automatically scale-verified. Import replaces the current canvas, but Undo can restore it.
The builder converts supported $INSUNITS values and rejects unsupported declarations, but only comparison with a known model-space dimension confirms that the CAD coordinates represent the intended section size.
The browser parses the original DXF locally; the file itself is not uploaded. When Calculate is selected, the derived boundary coordinates are submitted for analysis.
DXF arcs, circles, ellipses, and bulges use adaptive chord-error sampling before conversion to straight boundary segments. The target maximum deviation is 0.01 mm (0.000394 in), tightened relative to the radius for very small curves. Tangent radii are also segmented for calculation.
Selection and transforms
Place each component precisely
Select and inspect
Select a component on the canvas or in Parts. Drag empty canvas space from left to right to select only fully enclosed components, or from right to left to include components touched by the crossing box. Hold Shift while clicking or dragging to build a multiple selection. A single selection exposes component dimensions and transforms; a multiple selection can be moved, duplicated, mirrored, or deleted as a group.
Move and position
Drag selected geometry, use arrow keys, or type exact X and Y values. The reference point depends on the component type and is identified in Properties; do not assume every shape uses its lower-left corner.
Rotate
Enter any angle or use the ±90° actions. Positive angles rotate counter-clockwise and negative angles rotate clockwise. With a component selected, Shift+Left Arrow rotates +90° and Shift+Right Arrow rotates −90°.
Mirror
Mirror about an exact vertical line, horizontal line, or line through two points. Leave Keep original selected to create a mirrored copy; clear it to replace the selected geometry.
Stale results: A geometry edit clears the current calculated result. Select Calculate again after changing dimensions, points, positions, roles, rotations, or units.
What Calculate does
Compose the material region, then solve its properties
Boolean composition
Catalogue and parametric parts become closed boundaries. Solids are united so overlaps count once, then valid voids are subtracted.
Geometric analysis
Boundary integration obtains area, centroid, inertia, radii, elastic moduli, and principal properties; equal-area searches obtain the plastic axes and moduli.
Mesh analysis
A triangular finite-element mesh solves connected-section torsion and warping for J, Cw, effective shear areas, and shear centre.
J is reported as a component sum under an equal-twist assumption. Cw, shear areas, and shear centre remain unavailable because the physical connection and load transfer are undefined.
If advanced hosted analysis is unavailable, supported non-touching polygon regions can fall back to browser boundary integrals. Advanced warping values are then left blank rather than estimated.
Calculation output
Read each group of section properties
Let L mean the selected coordinate unit. Dimensional outputs use L, L², L³, L⁴, or L⁶ as shown.
Geometry and centroid
| Output | Units | Meaning |
|---|---|---|
W, D | L | Overall horizontal width and vertical depth of the final material region. |
A | L² | Net area of unioned solids minus valid openings. |
Cx, Cy | L | Geometric centroid coordinates measured from the builder origin. |
ctop/bottom/left/right | L | Centroid-to-extreme-fibre distances used in elastic section moduli. |
Centroidal inertia and gyration
| Output | Units | Meaning |
|---|---|---|
Ix, Iy | L⁴ | Second moments about the displayed horizontal x and vertical y centroidal axes. |
Ixy | L⁴ | Product of inertia; a non-zero value indicates coupling for the displayed axes. |
Ip | L⁴ | Polar second moment, Ix + Iy. It is not generally equal to torsion constant J. |
rx, ry | L | Radii of gyration √(I/A) about x and y. |
Elastic and plastic bending
| Output | Units | Meaning |
|---|---|---|
Sx,top / Sx,bottom | L³ | Ix divided by the corresponding vertical extreme-fibre distance. |
Sy,left / Sy,right | L³ | Iy divided by the corresponding horizontal extreme-fibre distance. |
Zx, Zy | L³ | Plastic section moduli for one homogeneous yield stress. |
PNAx, PNAy | L | Equal-area plastic-axis positions from the bottom and left bounds. |
Principal properties
I1 and I2 are the major and minor centroidal moments after rotation to axes where the product of inertia is zero. θp is the displayed angle from global x to the major principal 1-axis. For a symmetric section, small numerical values can appear where an exact zero is expected.
Shear, torsion, and warping
| Output | Units | Meaning |
|---|---|---|
J | L⁴ | Saint-Venant torsion constant used in uniform torsional rigidity GJ. |
Cw | L⁶ | Warping constant for a connected analyzable section. |
As,x / As,y | L² | Effective shear areas from the warping analysis, not literal cut areas. |
Sc,x / Sc,y | L | Shear-centre coordinates in the builder coordinate system. |
Coordinate convention
Axes, angles, and dimensional units
| Quantity | Metric selection | Imperial selection |
|---|---|---|
| Coordinates, dimensions, centroid, radii | mm | in |
| Area and effective shear area | mm² | in² |
| Elastic and plastic section modulus | mm³ | in³ |
| Inertia, polar moment, and J | mm⁴ | in⁴ |
| Warping constant Cw | mm⁶ | in⁶ |
| Angles | degrees | degrees |
Downstream axis check: Beam Calculator uses the transferred Ix for its bending model. Confirm that the builder’s x-axis matches the intended beam bending axis before handoff.
Projects and deliverables
Recover, save, report, and reuse the section
Local recovery and activity
The workspace maintains browser-local recovery for the working section and records edits, calculations, applied AI changes, saves, checkpoints, and restores in Activity. Local recovery is browser-specific and may be lost when site data is cleared.
Cloud saves, checkpoints, and restore
Eligible signed-in plans can save the complete editable workspace, including components, solid and void boundaries, transforms, units, material, and snap settings. Calculation is optional: an uncalculated workspace is stored as a draft, while current calculated properties make it reusable in analysis libraries. Save updates the current section; Save As creates a separate copy. Create a checkpoint before a major edit, then use Activity & versions to inspect and restore available revisions.
PDF report
Calculate the current geometry before downloading the report. The number-format selector changes decimal and thousands separators in the displayed output. Open the PDF and check the section name, unit system, geometry, properties, assumptions, and page layout before sharing it.
Continue in Beam or 2D analysis
Analyze in Beam Calculator transfers the calculated area, depth, centroid, and Ix into a new beam workflow. Use in 2D Structural Analysis imports the calculated area and Ix as the default section for new members while retaining any recovered 2D model and its existing member assignments. Every cloud-saved workspace can be reopened in Section Builder; only sections with properties calculated for their current geometry appear in the Beam and 2D analysis libraries.
If geometry changed after the last run, the previous result is invalidated. Do not rely on a PDF or downstream transfer until Calculate succeeds for the current model.
Assisted modeling
Use Optimal AI for proposals, then verify the geometry
- 1Open Optimal AI
Use the app-bar action or Assistant tab. No sign-in is required; public request limits protect the service from automated abuse.
- 2State geometry explicitly
Include the unit system, overall dimensions, thicknesses, opening sizes, offsets, rotations, and the intended component arrangement.
- 3Review the proposal
Geometry-changing commands are validated and shown for approval. Inspect every addition, edit, deletion, transform, and unit before applying it.
- 4Apply and calculate
The assistant changes the editable workspace only after approval. The deterministic geometry and analysis solvers—not the language model—calculate numeric section properties.
- 5Check the activity trail
Applied AI changes appear in Activity and participate in the ordinary undo, recovery, and version workflows.
AI requests, section context, responses and usage metadata are retained to understand and improve how the assistant is used. Do not include confidential or personal information in a prompt.
Verify component roles, dimensions, connections, openings, axes, and the final composed outline exactly as you would for manually entered geometry.
Analysis scope
Understand what the section model assumes
Included in the model
- Closed planar boundaries with non-zero area.
- Union of solid components and subtraction of valid voids.
- Gross geometric properties of one homogeneous section.
- Fully connected action where components touch or overlap.
- Mesh-derived advanced properties where a connected mesh succeeds.
Not modeled
- Transformed-section behavior for different materials or moduli.
- Weld, bolt, adhesive, slip, or partial-interaction flexibility.
- Residual stress, local buckling, yielding progression, or code resistance.
- Member stability, loads, boundary conditions, or structural system response.
- Full stress-field contour plots or fabrication tolerances.
Apply the appropriate material properties, connection model, resistance equations, safety factors, detailing rules, and governing design standard outside this calculation.
Keyboard reference
Section Builder shortcuts
Field safety: Plain-letter and deletion shortcuts do not run while you are typing in ordinary form fields. Use the visible controls when a dialog or editor has focus.
Before relying on the output
Section review checklist
Common problems
Troubleshooting the Section Builder
Calculate reports invalid geometry+
Check for fewer than three unique points, zero-area slivers, repeated or crossing edges, an unclosed DXF chain, or an invalid opening. Simplify the boundary and calculate again.
An opening is ignored or rejected+
Confirm the component role is Void, place the entire closed boundary inside the combined solid region, and leave positive clearance from every other boundary. A void that only overlaps part of a solid is not a valid internal opening.
Area is not the sum of the component areas+
Overlapping solids are united, so shared material is counted once. This is expected. If the overlap was accidental, reposition the components and recalculate.
A DXF does not import+
Use a 2D ASCII DXF no larger than 5 MB. Explode or convert blocks and splines, remove annotations and hatches, join boundary chains, delete duplicates, and ensure every required perimeter closes.
Imported geometry has the wrong size+
In CAD model space, check that 1 drawing unit equals the intended physical unit and that $INSUNITS matches. Compare the importer's reported width × depth with one known width or thickness. If the source is unitless or ambiguous, correct the CAD export rather than accepting an unverified outline.
Cw, shear areas, or shear centre are missing+
Inspect the final section for gaps. These values require one connected analyzable material region and a successful mesh solve. Disconnected regions intentionally omit them; J is then only a component sum under the documented equal-twist assumption.
Report or Analyze remains disabled+
Complete valid solid geometry and select Calculate. Any subsequent geometry or unit edit invalidates that result, so run Calculate again. Workspace saving remains independent of calculation but requires the applicable sign-in and plan access.
A property is orders of magnitude too large or small+
Check the coordinate unit and one known dimension first. Remember that area scales with length², S and Z with length³, I and J with length⁴, and Cw with length⁶.
The centroid or principal axis is unexpected+
Inspect component roles, offsets, rotations, openings, and hidden duplicate geometry. Then compare symmetry pairs and calculate a simple area-weighted centroid estimate. A non-zero Ixy is expected for many asymmetric orientations.
A saved or recovered section is not the expected version+
Open Activity & versions to identify saves, checkpoints, and restores. Local recovery belongs to the current browser and can disappear with cleared site data. Use Save As or a named checkpoint before major changes when cloud history is available.
An AI proposal does not match the request+
Do not apply it. Restate units and every controlling dimension, or make the edit manually. If an applied proposal is wrong, use Undo or restore an appropriate checkpoint, then recalculate.
Quick reference
Section Builder terms
- Component
- One editable catalogue, parametric, polygon, DXF, solid, or void part in the working model.
- Solid
- A component that contributes material to the composed section.
- Void
- A closed component that subtracts material where it lies inside the solid region.
- Boolean union
- The operation that combines overlapping or touching solids and counts shared area once.
- Boundary
- An ordered, closed perimeter used to describe solid material or an opening.
- Centroid
- The area-weighted geometric centre of the net section.
- Second moment of area
- A geometric measure of how area is distributed about an axis; used with E in flexural rigidity.
- Section modulus
- An inertia-to-extreme-fibre ratio for elastic bending, or an equal-area first moment for plastic bending.
- Principal axes
- Centroidal axes rotated to make the product of inertia zero.
- J
- Saint-Venant torsion constant used with shear modulus G in uniform torsional rigidity.
- Cw
- Warping constant for non-uniform or restrained warping behavior.
- Shear centre
- The point through which transverse load can act without inducing twist in the idealized connected section.
- Mesh
- A triangular finite-element discretization used for torsion, warping, and shear-property analysis.
Responsible use
Section properties support judgment; they do not replace it.
The user remains responsible for geometry, units, axes, connection assumptions, material behavior, mesh suitability, independent checks, governing design requirements, and decisions affecting safety or construction.