Section Selection Skill

You are a specialist in selecting the appropriate structural section from the SkyCiv section library to be used in the S3D model object.

You will likely be given information from a user with (a) country where the structure is being built and (b) the preferred material. You may also be given specific sections to use. The name will likely not perfectly match the nomenclature of the SkyCiv library.

You should then be very efficient at location the appropriate section from the SkyCiv database and injecting it into the S3D model JSON.

Typical Section Format (for S3D Model)

You will mainly load in sections using this format:

    "sections": {
        "1": {
            "load_section": ["American", "AISC", "W shapes", "W14x22"],
            "material_id": 1
        }
    }

The strings in the array come directly from the names in section_tree.json, and the lookup is an exact string match — a near-miss doesn't fuzzy-resolve, it fails.

Building an S3D App or left-menu panel? Don't ship section_tree.json and don't make the user type the path. SB.library.getTree() returns the live library client-side and synchronously, in this same 4-level shape — drive four cascading dropdowns off it. See "Choosing sections" in s3d-apps. Server-side, the equivalent is the S3D.SB.getLibraryTree API function in s3d-api. section_tree.json here is the captured copy — the right tool for agent-time lookups and test fixtures.

Common Section Types and their Use Cases

Region → Standard Mapping

Choose the outermost two path components from the table below based on the project country:

Finding the Exact Section Name

The load_section array is always four strings in the exact order they appear in section_tree.json:

[  Region,  Standard/Publisher,  Category,  Section name  ]

Examples across common regions:

Exact string match required. Copy names character-for-character from section_tree.json. A mismatched space, capitalisation, or unit suffix will silently fail to load.

Injecting into the S3D Model

Sections object

Each section entry needs a material_id that must already exist in the model's materials object.

"sections": {
    "1": { "load_section": ["American", "AISC", "W shapes", "W18x35"], "material_id": 1 },
    "2": { "load_section": ["American", "AISC", "Rectangular HSS", "HSS6x4x1/4"], "material_id": 1 }
}

Matching material class to section library

When using load_section, the material properties (E, Fy, density) still come from the materials entry — the section library only supplies the geometric properties (A, Iz, Iy, J, etc.). Always define matching material properties alongside.

Typical material entries (imperial)

"materials": {
    "1": {
        "name": "Steel A992",
        "density": 490,
        "elasticity_modulus": 29000,
        "poissons_ratio": 0.3,
        "yield_strength": 50,
        "ultimate_strength": 65,
        "class": "steel"
    }
}

Typical material entries (metric)

"materials": {
    "1": {
        "name": "Steel 350 Grade",
        "density": 7850,
        "elasticity_modulus": 200000,
        "poissons_ratio": 0.3,
        "yield_strength": 350,
        "ultimate_strength": 480,
        "class": "steel"
    }
}

Concrete Sections

Concrete sections have no entry in the section library — always define them as template shapes using the info object. Do not use load_section for concrete.

Template shape format

"sections": {
    "1": {
        "material_id": 1,
        "info": {
            "shape": "rectangle",
            "dimensions": { "h": 600, "b": 400 }
        }
    }
}

S3D computes all geometric properties (A, Iz, Iy, J, shear areas) automatically from the dimensions. All dimension values are in the model's section_length units (default mm for metric, in for imperial).

Inside S3D (an S3D App or in-app script): S3D builds the section on the server, so the resolved properties only exist in a callback: S3D.API.API2S3D(apiModel, null, callback) or S3D.structure.set(model, null, true, callback). Never read area/Iz from a synchronous return. The same applies to load_section paths. See s3d-apps/references/resolving-sections.md.

Supported shapes and their dimension keys

Dimension keys are case-sensitive and must match exactly. A wrong key (for example d instead of D for a circle) does not resolve, and the section comes back without properties. r is the corner or root radius; use 0 for none.

Convention: h is the overall depth in the plane of bending. If the member bends about its strong axis, h should be the deeper dimension.

Steel sections that are stocked in the library (W, UB, SHS, RHS, CHS, …) should still use load_section. Use the template shapes for concrete, and for steel or cold-formed sizes the library doesn't stock.

Worked examples (metric, mm)

"sections": {
    "1": {
        "material_id": 1,
        "info": { "shape": "rectangle", "dimensions": { "b": 300, "h": 600 } }
    },
    "2": {
        "material_id": 1,
        "info": { "shape": "circle", "dimensions": { "D": 500 } }
    },
    "3": {
        "material_id": 1,
        "info": { "shape": "rectangle", "dimensions": { "b": 500, "h": 500 } }
    }
}

Worked examples (imperial, in)

"sections": {
    "1": {
        "material_id": 1,
        "info": { "shape": "rectangle", "dimensions": { "b": 12, "h": 24 } }
    },
    "2": {
        "material_id": 1,
        "info": { "shape": "circle", "dimensions": { "D": 18 } }
    }
}

Concrete material properties

Pair each section with a material whose class is "concrete". elasticity_modulus depends on the design code as well as the grade, so take it from the table for the user's code. yield_strength and ultimate_strength are set to the characteristic strength (fck or f'c). poissons_ratio is 0.2. Density is 2500 kg/m³ for reinforced concrete.

Eurocode, EN 1992-1-1 (MPa). The grade is cylinder / cube strength, so C30/37 has fck = 30.

Australia, AS 3600 (MPa).

US, ACI 318, metric (MPa). Ec = 4700√f'c.

"materials": {
    "1": {
        "name": "Concrete C30/37 (EN 1992)",
        "density": 2500,
        "elasticity_modulus": 33000,
        "poissons_ratio": 0.2,
        "yield_strength": 30,
        "ultimate_strength": 30,
        "class": "concrete"
    }
}

Imperial (ksi, lb/ft³):

The 150 lb/ft³ density is for reinforced concrete; plain concrete is about 145.

"materials": {
    "1": {
        "name": "Concrete 4000 psi",
        "density": 150,
        "elasticity_modulus": 3605,
        "poissons_ratio": 0.2,
        "yield_strength": 4,
        "ultimate_strength": 4,
        "class": "concrete"
    }
}

Design Guidance — Selecting the Right Section

• Columns under axial + bending: prefer W/UC/HD wide-flange shapes or square HSS — doubly symmetric, efficient for combined loading.
• Primary beams (strong-axis bending): W/UB/IPE I-sections — maximise Iz.
• Bracing diagonals: round or square HSS, angles, or rods — compact, symmetric, efficient in tension/compression.
• Purlins / girts (secondary, single-span): cold-formed C or Z; for heavier loads use PFC/MC channels.
• Truss chords: W or WT tees; for lighter trusses use angles (back-to-back or single).
• Concrete beams: "rectangle" template, b = width, h = depth; deeper is stronger in bending.
• Concrete columns: "rectangle" for square/rectangular; "circle" (key D) for circular; keep the smallest dimension ≥ 300 mm (12 in).
• When the user specifies a section by name: look it up verbatim in section_tree.json — don't guess the category.
• When the user specifies only depth (e.g. "a 200mm deep I-beam"): pick the lightest standard section at or above that depth in the appropriate regional library.