Load Combinations

How to put load groups, load cases and load combinations on an s3d_model so that S3D.model.solve gives code-correct results and the S3D UI classifies every group correctly.

Prerequisite: these objects live inside the s3d_model passed to S3D.model.set — see s3d-api for the full model schema and solve flow, and skyciv-api-v3 for auth and the { auth, options, functions } envelope. The load magnitudes come from you or load-gen-api; reading the solved combinations is analysis-results.

Units: factors are dimensionless. Load magnitudes carry the units set by s3d_model.settings.units ("metric" or "imperial"); combinations never change them.

How the pieces fit

Every load (point_loads, distributed_loads, moments, pressures, area_loads, self_weight, …) carries a load_group string. A combination is a factor per group; the solver sums group × factor.

Minimal working example

{
  "self_weight": { "1": { "enabled": true, "x": 0, "y": -1, "z": 0, "load_group": "SW1" } },
  "distributed_loads": {
    "1": { "member": 2, "x_mag_A": 0, "y_mag_A": -5,  "z_mag_A": 0, "x_mag_B": 0, "y_mag_B": -5,  "z_mag_B": 0, "position_A": 0, "position_B": 100, "axes": "global", "load_group": "D1" },
    "2": { "member": 2, "x_mag_A": 0, "y_mag_A": -10, "z_mag_A": 0, "x_mag_B": 0, "y_mag_B": -10, "z_mag_B": 0, "position_A": 0, "position_B": 100, "axes": "global", "load_group": "L1" }
  },
  "point_loads": {
    "1": { "type": "n", "node": 2, "x_mag": 10,  "y_mag": 0, "z_mag": 0, "load_group": "Wx+" },
    "2": { "type": "n", "node": 3, "x_mag": -10, "y_mag": 0, "z_mag": 0, "load_group": "Wx-" }
  },
  "load_combinations": {
    "1": { "name": "1: 1.40D", "criteria": "strength", "SW1": 1.4, "D1": 1.4 },
    "2": { "name": "2: 1.20D + 1.60L", "criteria": "strength", "SW1": 1.2, "D1": 1.2, "L1": 1.6 },
    "3": { "name": "3: 1.20D + 1.00L + 1.00W (+x)", "criteria": "strength", "SW1": 1.2, "D1": 1.2, "L1": 1.0, "Wx+": 1.0 },
    "4": { "name": "4: 1.20D + 1.00L + 1.00W (-x)", "criteria": "strength", "SW1": 1.2, "D1": 1.2, "L1": 1.0, "Wx-": 1.0 },
    "5": { "name": "5: 0.90D + 1.00W (+x)", "criteria": "strength", "SW1": 0.9, "D1": 0.9, "Wx+": 1.0 },
    "6": { "name": "6: 0.90D + 1.00W (-x)", "criteria": "strength", "SW1": 0.9, "D1": 0.9, "Wx-": 1.0 }
  },
  "load_cases": { "ASCE-7-2022-LRFD": { "SW1": "D", "D1": "D", "L1": "L", "Wx+": "W", "Wx-": "W" } },
  "load_combination_settings": { "country": "United States", "code": "ASCE-7-2022-LRFD", "criteria": "All" }
}

Gotchas (all verified against the live API and S3D UI)

• load_cases values are the code's short symbols (D, L, W, G, Qdo, Wu, …) from the tables below, keyed by the code id. The "Dead: dead" style shows every group as Dead in the UI. So does a symbol that doesn't belong to the code in load_combination_settings.code, or to the default ASCE code if that's missing.
• Set load_combination_settings.country and .code for any code other than ASCE-7-2022-LRFD, or the UI opens on ASCE and classifies Australian/European symbols as Dead.
• One wind (or seismic) direction per combination. Model each direction as its own group (Wx+, Wx-, Wy+, Wy-) and repeat the wind rows once per direction, as combos 3–6 above do. Putting Wx+ and Wx- in the same combination largely cancels them out. The simultaneous generator pattern does exactly this, so use independent (see below). Even with separate combos, the automatic load_case result for W sums every W group, so it is meaningless when opposite directions share a symbol; read the combinations instead.
• Leaving a group out = factor 0. The results are identical, so don't pad every combination with zeros.
• A combination with no self-weight key applies no self-weight. Put the self-weight group in every combination, with the same factor as the dead load. Self-weight defaults to group SW1: a flat self_weight: {enabled, x, y, z} and keyed entries without load_group both behave as SW1.
• Keys that match no group are silently ignored ("NOPE": 5 → no error, no effect). Group names are case-sensitive exact matches, so check them against your loads.
• Identical combinations only trigger a warning ("Load Combination #1 and Load Combination #2 are identical"), but each one still costs solve time. The generator's own sets repeat some combos (below); delete them.
• Results come back keyed by position, not by your ids. User combos are type: "user_defined", followed by load_case, load_group and envelope entries. Their ids depend on how many of each exist, so match results by name/type.
• Solve time grows with the combination count. Only include cases the model actually has.

Building combinations from the tables

1. Pick the code and write its id into both load_cases and load_combination_settings.
2. Map every load group to a symbol from that code's list. Dead loads and SW1 both get the dead symbol (D / G).
3. For each table row, write one combination per direction group (row × Wx+, Wx-, …). Where a row's case has several groups (e.g. D1 + SW1), give each of them that row's factor.
4. Drop rows whose cases the model doesn't have, then drop any duplicates this creates.
5. Name each combination with the row formula, set criteria in lowercase, then solve.

Factors below are copied from the live 7000-load-combination-generator output (2026-09-23, remove_unnecessary: true, filter ",f,u"). u rows treat dead load as unfavourable. f rows treat it as favourable and use the reduced dead factor.

United States — ASCE 7-22 LRFD (ASCE-7-2022-LRFD, country United States)

Symbols: D dead · L live · Lr roof live · S snow · R rain · W wind · Wt tornado · Eh/Ev seismic · H/Hf lateral earth · T · Ak · N notional.

United States — ASCE 7-22 ASD (ASCE-7-2022-ASD, country United States)

Same symbols as LRFD.

The generator also emits 1.0D + 0.6W twice; delete the duplicate. Names round to 2 dp (0.53S); the stored factor is 0.525.

Australia / New Zealand — AS/NZS 1170.0 (AS-1170.0-2002, country Australia; AS/NZS-1170.0-2002, country New Zealand)

Both ids share the same symbols and factors; the UI lists them as separate standards.

Symbols: G dead · Qd?/Qc? live, where the letter after d/c is the occupancy: Qdd domestic, Qdo offices, Qdp parking, Qde retail, Qds storage, Qdx other, Qdr/Qdy roofs, Qcb balustrades, Qcm machinery · Wu/Ws wind ultimate/serviceability · Su snow/liquid/rain · Eu/Es seismic · Ld?/Lc? long-term live · N. Use ultimate symbols (Wu, Eu) and serviceability symbols (Ws, Es) as separate groups.

The table below is for Qdo. The combination factors depend on the Q type (ψc = 0.4, ψE = 0.3, ψs = 0.7 for offices), so regenerate for other occupancies or look up Table 4.1.

Canada — NBCC 2020 (NBCC-2020, country Canada)

Symbols: D · L · Ls sustained live · S · W · E · H/Hf · T · N.

Europe — EN 1990 (EN-1990-2002, country Europe; no national annex)

Symbols: G · Qa–Qh imposed by category (Qa domestic, Qb office, Qc congregation, Qd shopping, Qe storage, Qf/Qg traffic, Qh roofs) · Sl/Sh snow below/above 1000 m · W · T · Ad/Ae accidental/seismic · N. The UI also offers the EN-1990-2002-NA-UK and -NA-FR annexes, whose factors differ; regenerate if you use one.

Each set is one pattern: a leading variable action × γQ, plus every accompanying one × γQ·ψ0. The generator used ψ0 = 0.7 (Qb), 0.5 (Sl), 0.6 (W) and ψ1/ψ2 = 0.5/0.3 (Qb), 0.2/0 (Sl), 0.2/0 (W). ψ depends on the Q category.

Don't read EQU (1.1G) rows as the design set; STR is 1.35G. The full generator run (G, Qb, Sl, W) is 85 combinations. Filter multi_dropdown_2 to the sets you need (see below).

India — IS 800:2007 (IS-800-2007, country India)

Symbols: D · L · W · E · A accidental · N (IS-456-2000 has no A).

Generating a set — 7000-load-combination-generator (optional)

Use this to regenerate the tables above for other occupancies, cases or national annexes. Call it through run-quick-design (POST https://qd.skyciv.com/run, own auth, not the S3D envelope). The payload below mirrors the request the S3D UI's own Load Combination Generator sends (captured from the live UI, 2026-09-23).

Also send "calcs_only": true next to input, as the UI does.

New Zealand. The model and UI use code AS/NZS-1170.0-2002 with country New Zealand, but the calculator has one AS/NZS 1170.0 schema, filed under AS-1170.0-2002. The S3D UI itself sends country_input: "australia", standard_input: "AS-1170.0-2002" when New Zealand is selected. Do the same: passing AS/NZS-1170.0-2002 as standard_input fails. Then write AS/NZS-1170.0-2002 / New Zealand into load_cases and load_combination_settings, as the UI's import does. The factors are the ones in the AS/NZS table above.

Patterns (col3): with col2 > 1, independent puts one group per combination, which is correct for wind directions. simultaneous puts them all in every combination: right for the dead groups (D1, SW1), wrong for directions (the cancelling bug above). mirror, paired, sweep and checker exist but haven't been verified here.

{ "uid": "7000-load-combination-generator", "auth": "you@example.com", "key": "YOUR_API_TOKEN",
  "pdf_report": false, "calcs_only": true,
  "input": { "country_input": "united-states", "standard_input": "ASCE-7-2022-LRFD", "criteria_input": "All",
    "combination_naming_input": "ID + Factor + Name", "multi_dropdown_1": ",f,u", "remove_unnecessary": true,
    "load_case_table": [
      { "col1": "D", "col2": 2, "col3": "simultaneous", "col4": 1, "col5": 1, "col6": "D1,SW1" },
      { "col1": "L", "col2": 1, "col3": "simultaneous", "col4": 1, "col5": 1, "col6": "L1" },
      { "col1": "W", "col2": 2, "col3": "independent",  "col4": 1, "col5": 1, "col6": "Wx+,Wx-" } ] } }

The output is not load_combinations. Read data.results.combination_obj.value: { "3": { "name": "…", "criteria": "Strength", "schema_row": "4a-W1-u", "groups": [ { "id": "D1", "label": "D1", "value": 1.2, "load_case": "D" }, … ] } }. The group's name is in label: id is internal (G1, G2) when you name the groups in col6. criteria is capitalised. If you used "auto", there is no self-weight group, so add it with the dead factor (as below).

function toModel(combinationObj, code, country, { selfWeight = 'SW1' } = {}) {
  const load_combinations = {}, cases = {};
  Object.values(combinationObj).forEach((c, i) => {
    const combo = { name: c.name, criteria: c.criteria.toLowerCase() };
    for (const g of c.groups) { combo[g.label] = g.value; cases[g.label] = g.load_case; }
    const dead = c.groups.find(g => g.load_case === 'D' || g.load_case === 'G');
    if (dead && !(selfWeight in combo)) { combo[selfWeight] = dead.value; cases[selfWeight] = dead.load_case; }
    load_combinations[i + 1] = combo;
  });
  return { load_combinations, load_cases: { [code]: cases }, load_combination_settings: { country, code, criteria: 'All' } };
}

Pass the model code and country (AS/NZS-1170.0-2002, New Zealand for NZ), not the calculator's. After converting, rename f-variant names as described in the tables and delete duplicates.