BIM objects — three composition layers
A BIM Object has three layers: Specification, Regulation, and Climate Zone. All three are embedded data in the object. None of the three layers is a runtime user-selectable option — a designer does not "switch" between climate zones any more than they switch building codes. The three layers are displayed simultaneously as static reference tables, each showing all registered overlays for the BIM Object's element type. This structure reflects a physical reality: a built element has a fixed type (Specification), exists in a fixed jurisdiction (Regulation), and performs in a fixed climate (Climate Zone). All three are facts about the element's physical context, not user preferences.
Why Three Layers
Software design system tokens typically have two concerns: what a value IS (its semantic role — primary colour, heading size) and what value it resolves to (its computed output — #164679, 24px). BIM Objects address a fundamentally different problem space that requires three concerns.
Three questions every element specification answers
A built-environment element specification must simultaneously answer:
- What is this element? — its type in a neutral, tool-independent schema (IFC), its classification in a neutral reference system (Uniclass), and its semantic identity in a jurisdiction-spanning dictionary (bSDD). This is stable across all deployments.
- What does the jurisdiction require of it? — the specific regulatory requirements imposed by the law of the place where the building is located. These vary by jurisdiction, change when regulations are updated, and may include geometric constraints (setbacks, clearances, fire compartment boundaries) that cannot be expressed as numeric values.
- What does the climate require of it? — the thermal, moisture, and structural performance requirements imposed by the physical climate of the site. These vary by climate zone, are expressed in energy codes (ASHRAE 90.1, NBC Part 11, EN ISO 52000), and change as climate zone mapping is updated.
The three-layer structure gives each concern a clean home and a clear composition rule.
Layer 1 — Specification
The Specification layer is the object's permanent identity. It does not vary by jurisdiction or climate. Every deployed instance of the object reads the same Specification layer regardless of where the project is located.
Fields:
| Field | Content | Format |
|---|---|---|
ifc_class |
IFC 4.3 entity class | IfcWall, IfcSlab, etc. |
ifc_predefined_type |
IFC predefined type where applicable | SOLIDWALL, FLOORPLATE, etc. |
uniclass_ref |
Uniclass 2015 code | Ss_20_05_30_75 |
uniclass_title |
Uniclass 2015 title text | "Masonry walls" |
bsdd_uri |
bSDD concept URI | https://identifier.buildingsmart.org/... |
description |
Plain-language element description | Free text, max 280 chars |
applicable_psets |
Applicable IFC Property Sets | Array of Pset names |
dtcg_type |
DTCG type extension | bim-element, bim-material, etc. |
The Specification layer is authored once per BIM Object and promoted through the standard design-system pipeline (vendor → customer → deployment). Changes to the Specification layer require a version bump and changelog entry.
IFC entity hierarchy breadcrumb. Every Specification record includes the full IFC inheritance path from IfcRoot to the specific class. For IfcWall:
IfcRoot → IfcObjectDefinition → IfcObject → IfcProduct → IfcElement → IfcBuiltElement → IfcWall
This breadcrumb enables authoring tools to present the element's position in the IFC hierarchy without reference to a separate schema document, and enables inheritance-based rule application.
Layer 2 — Regulation
The Regulation layer holds jurisdiction-specific requirements. It is a table of registered overlays — one row per jurisdiction per constraint — not a single value. The table shows all registered overlays simultaneously.
Why a table, not a dropdown. A regulatory requirement is a fact about the jurisdiction where a building is located, not a choice a designer makes. The object does not ask "which jurisdiction are you in?" and display a single jurisdiction's requirements. It displays all registered jurisdictions' requirements as reference data, the same way a technical standards datasheet shows multiple national standards rows side by side.
Jurisdiction overlay rows
Overlay structure:
| Column | Content |
|---|---|
| Jurisdiction | ISO 3166-1/2 jurisdiction code (e.g., CA-BC, US-VA, DE, SG) |
| Standard | Identifying reference for the regulatory document |
| Constraint type | Numeric / Geometric / Classification / Approval |
| Parameter | The specific property being constrained |
| Required value | The threshold or required value |
| Unit | SI unit or categorical value set |
| IDS file | Path to the IDS 1.0 constraint file encoding this requirement |
| IFC fragment | Path to IFC geometric exclusion fragment, if applicable |
| Source URI | URI to the authoritative regulatory document |
| Effective date | ISO 8601 date from which this overlay is in effect |
Example rows for IfcWall (exterior wall, residential):
| Jurisdiction | Standard | Parameter | Required value | Unit |
|---|---|---|---|---|
| CA-BC | NBC 2020 Part 11 | Thermal resistance (opaque wall) | ≥ RSI 3.85 | m²K/W |
| DE | EnEV 2020 | Wärmedurchgangskoeffizient (U-value) | ≤ 0.28 | W/m²K |
| SG | SGBC BCA Green Mark | Thermal transmittance (OTTV) | ≤ 45 | W/m² |
| US-VA (federal) | ASHRAE 90.1-2022 | Assembly U-factor (climate zone 4A) | ≤ 0.124 | Btu/h·ft²·°F |
Empty state and geometric exclusion fragments
Empty state. At v0.0.1, most BIM Objects have no registered overlays — the overlay structure is defined but unpopulated. The v0.0.3 milestone is planned to deliver the first overlay set: BC RS-1 residential zoning requirements for exterior walls, slabs, and windows. The empty state is displayed explicitly: "No regulatory overlays registered. BC RS-1 in development (v0.0.3)."
Geometric exclusion fragments. Where a regulatory requirement has geometric expression — fire compartment boundaries, accessibility clearances, setback envelopes — the overlay row includes an IFC fragment: a solid geometry encoded in IFC that defines the spatial constraint. Geometric exclusion takes unconditional precedence over numeric constraints in the composition rule.
Layer 3 — Climate Zone
The Climate Zone layer holds climate-based performance requirements. Like the Regulation layer, it is a table of registered climate zone rows — all zones shown simultaneously, not selected by dropdown.
Why Climate Zone is distinct from Regulation. Climate zone classifications are physical geography — they are determined by latitude, altitude, precipitation, and temperature range. Building energy codes frequently reference climate zones as performance multipliers, but the climate zone itself is not a building code. It is a geographic classification that energy codes reference.
BIM Object performance specifications use Climate Zones exclusively. The eco-region concept (WWF biome classification) is not used in built-environment regulatory contexts.
Classification systems and example zone rows
Climate zone classification systems used:
| System | Jurisdiction coverage | Reference standard |
|---|---|---|
| ASHRAE 90.1 climate zones (1A–8) | US, international reference | ASHRAE Standard 90.1-2022 |
| National Building Code climate zones | Canada | NBC 2020 Part 11 |
| EN ISO 52000 energy performance zones | EU member states | EN ISO 52000-1:2017 |
| Köppen-Geiger simplified | Global cross-reference | Kottek et al. 2006 (updated 2021) |
Example rows for IfcWall (exterior wall):
| Zone ID | Parameter | Required value | Unit | Source |
|---|---|---|---|---|
| ASHRAE-4A | Assembly U-factor (steel-framed wall) | ≤ 0.064 | Btu/h·ft²·°F | ASHRAE 90.1-2022 Table 5.5-4 |
| ASHRAE-5C | Assembly U-factor (mass wall) | ≤ 0.104 | Btu/h·ft²·°F | ASHRAE 90.1-2022 Table 5.5-5 |
| NBC-Zone-6 | Effective thermal resistance (above-grade wall) | ≥ RSI 4.91 | m²K/W | NBC 2020 |
| EN-Dfc | U-value requirement (continental subarctic) | ≤ 0.15 | W/m²K | EN ISO 52000 |
Composition Rule
When Regulation and Climate Zone both specify a numeric constraint on the same performance parameter, the effective requirement is the more restrictive value.
effective_value = max(regulation_requirement, climate_zone_requirement)
This is a lower-bound composition: both layers express performance minima (higher values are always acceptable; lower values are not). The maximum of the two minima is the binding requirement.
Priority stack and precedence rules
Priority stack for Regulation overlays:
- Municipal (local) — highest authority for land use, zoning, setbacks.
- Provincial/State — structural, fire, energy performance.
- Federal — ITAR-restricted facilities, federal standards (ASHRAE via GSA).
- Accessibility — ADA, EN 301 549, equivalent national accessibility standards.
Geometric exclusion unconditional precedence. An IFC geometric exclusion fragment in any overlay cannot be overridden by any numeric constraint.
Fail-open. When a layer has no registered data for a given element type in the project's jurisdiction and climate zone, the object remains valid. The Specification layer is always present; the Regulation and Climate Zone layers are optional additions.
The Four-Zone CMS Authoring Model
When a new BIM Object is authored, the authoring interface follows a four-zone model that maps directly to the three object layers plus a publishing workflow zone.
Zone 1 — Specification. IFC class selector, Uniclass 2015 lookup, bSDD URI field, plain-language description (max 280 chars), applicable property set selector. This zone is completed once per new element type.
Zone 2 — Regulation. Add-overlay form: jurisdiction selector (ISO 3166-1/2), standard name field, constraint type selector, parameter name, required value, unit, IDS file upload, IFC fragment upload (optional), source URI, effective date. Multiple overlays per object. Each overlay is versioned independently.
Zone 3 — Climate Zone. Add-zone-row form: zone system selector (ASHRAE / NBC / EN ISO 52000), zone identifier, parameter name, required value, unit, source standard reference. Multiple zone rows per object.
Zone 4 — Publishing. Validation summary, preview of BIM Object JSON output, approval workflow, commit message field, publish action (git commit to object vault).
The CMS authoring model is the intended interface for app-console-bim (planned, v0.1.x). At v0.0.1, BIM Objects are authored directly as DTCG JSON files and committed via git.
How This Differs from IFC Property Sets
An IFC Property Set (Pset_WallCommon) is a list of property definitions: property name, data type, unit. It declares what properties an element of a given type may carry. It does not declare what values those properties must have. Pset_WallCommon includes ThermalTransmittance of type IfcThermalTransmittanceMeasure. It does not say that ThermalTransmittance must be ≤ 0.28 W/m²K in Germany. That constraint lives in the Regulation layer of the BIM Object.
The BIM Object Regulation layer consumes the IFC Property Set structure but adds the values that the Property Set specification omits: the required value, the jurisdiction for which it applies, and the IDS 1.0 file that formally encodes the constraint in a machine-executable form.
See also
- bim-objects-what-they-are — what a BIM Object is and how it differs from IFC entity types, proprietary BIM Family formats, and property sets
- bim-objects-substrate — the eight BIM Object primitive categories and their DTCG delivery format
- open-bim-regulatory-acceptance — how jurisdictions are adopting open BIM standards
- design-system-bim — the broader Building Design System for the built environment