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| schema: foundry-doc-v1 | schema: foundry-doc-v1 |
| title: "BIM objects — substrate" | title: "BIM objects — substrate" |
| slug: bim-objects-substrate | slug: bim-objects-substrate |
| short_description: "BIM Objects anchor to the IFC 4.3 entity hierarchy, Uniclass 2015 classification, and bSDD URIs, organizing into eight primitive categories that encode spatial, elemental, material, assembly, system, performance, and climate-zone specifications." | short_description: "BIM Objects anchor to the IFC 4.3 entity hierarchy, Uniclass 2015 classification, and bSDD URIs, organizing into eight primitive categories that encode spatial, elemental, material, assembly, system, performance, and climate-zone specifications." |
| category: architecture | category: architecture |
| type: topic | type: topic |
| status: active | status: active |
| bcsc_class: public-disclosure-safe | bcsc_class: public-disclosure-safe |
| last_edited: 2026-05-17 | last_edited: 2026-05-17 |
| editor: pointsav-engineering | editor: pointsav-engineering |
| cites: [ifc-4-3, uniclass-2015, bsdd-v1, ids-1-0, dtcg-w3c] | cites: [ifc-4-3, uniclass-2015, bsdd-v1, ids-1-0, dtcg-w3c] |
| paired_with: bim-objects-substrate.es.md | paired_with: bim-objects-substrate.es.md |
| --- | --- |
| The [[building-design-system-bim|Building Design System]] [[bim-objects-what-they-are|BIM Object]] library anchors every object to a node in the IFC 4.3 entity hierarchy, classified by Uniclass 2015, and published as a dereferenceable bSDD URI. This article describes how the [[bim-objects-three-layers|three-layer reference system]] works, what the eight BIM Object primitive categories encode, and how a BIM operator reads and applies them when authoring an IFC file. | The [[building-design-system-bim|Building Design System]] [[bim-objects-what-they-are|BIM Object]] library anchors every object to a node in the IFC 4.3 entity hierarchy, classified by Uniclass 2015, and published as a dereferenceable bSDD URI. This article describes how the [[bim-objects-three-layers|three-layer reference system]] works, what the eight BIM Object primitive categories encode, and how a BIM operator reads and applies them when authoring an IFC file. |
| ## Three Layers of Reference | ## Three Layers of Reference |
| ### Layer 1 — IFC 4.3 entity hierarchy | ### Layer 1 — IFC 4.3 entity hierarchy |
| IFC 4.3 was published as ISO 16739-1:2024 in April 2024. The standard defines a hierarchy of named entity classes — `IfcSite`, `IfcBuilding`, `IfcBuildingStorey`, `IfcSpace`, `IfcWall`, `IfcSlab`, `IfcDoor`, and hundreds more — each with typed Property Sets (Psets) specifying what attributes a compliant implementation must support. | IFC 4.3 was published as ISO 16739-1:2024 in April 2024. The standard defines a hierarchy of named entity classes — `IfcSite`, `IfcBuilding`, `IfcBuildingStorey`, `IfcSpace`, `IfcWall`, `IfcSlab`, `IfcDoor`, and hundreds more — each with typed Property Sets (Psets) specifying what attributes a compliant implementation must support. |
| Every [[building-design-system-bim|Building Design System]] BIM Object maps to a specific IFC entity class or Pset. The spatial BIM Object `bim.spatial.storey` anchors to `IfcBuildingStorey`; the performance BIM Object `bim.performance.door-fire-exit` anchors to `Pset_DoorCommon.IsFireExit`. When a BIM Object value changes, a validator can trace the change back to the IFC Pset that the BIM Object represents. | Every [[building-design-system-bim|Building Design System]] BIM Object maps to a specific IFC entity class or Pset. The spatial BIM Object `bim.spatial.storey` anchors to `IfcBuildingStorey`; the performance BIM Object `bim.performance.door-fire-exit` anchors to `Pset_DoorCommon.IsFireExit`. When a BIM Object value changes, a validator can trace the change back to the IFC Pset that the BIM Object represents. |
| ### Layer 2 — Uniclass 2015 classification floor | ### Layer 2 — Uniclass 2015 classification floor |
| Uniclass 2015 is the unified classification system for the UK construction industry, maintained by the NBS. Building Design System BIM Objects carry a Uniclass 2015 reference in their `$extensions.uniclass` field, establishing a classification floor that maps to the national procurement and specification context. | Uniclass 2015 is the unified classification system for the UK construction industry, maintained by the NBS. Building Design System BIM Objects carry a Uniclass 2015 reference in their `$extensions.uniclass` field, establishing a classification floor that maps to the national procurement and specification context. |
| The classification is non-normative for jurisdictions outside the UK but serves as the consistent structural vocabulary for the object catalogue. Where a North American equivalent exists — OmniClass, UniFormat II, or MasterFormat — the object carries a cross-map annotation in `$extensions.omniclass`. | The classification is non-normative for jurisdictions outside the UK but serves as the consistent structural vocabulary for the object catalogue. Where a North American equivalent exists — OmniClass, UniFormat II, or MasterFormat — the object carries a cross-map annotation in `$extensions.omniclass`. |
| ### Layer 3 — bSDD URIs | ### Layer 3 — bSDD URIs |
| buildingSMART's Data Dictionary (bSDD) publishes machine-readable JSON-LD definitions for building properties and classifications, each with a stable dereferenceable URI such as `https://identifier.buildingsmart.org/uri/buildingsmart/ifc-4.3/prop/IsFireExit`. Building Design System BIM Objects carry these URIs in their `$extensions.bsdd_uri` fields, linking the object to an authoritative definition that a validator or authoring tool can resolve at runtime. | buildingSMART's Data Dictionary (bSDD) publishes machine-readable JSON-LD definitions for building properties and classifications, each with a stable dereferenceable URI such as `https://identifier.buildingsmart.org/uri/buildingsmart/ifc-4.3/prop/IsFireExit`. Building Design System BIM Objects carry these URIs in their `$extensions.bsdd_uri` fields, linking the object to an authoritative definition that a validator or authoring tool can resolve at runtime. |
| ## Eight BIM Object Primitive Categories | ## Eight BIM Object Primitive Categories |
| The [[building-design-system-bim|Building Design System]] groups BIM Objects into eight categories, each mapped to a distinct layer of the IFC entity model. | The [[building-design-system-bim|Building Design System]] groups BIM Objects into eight categories, each mapped to a distinct layer of the IFC entity model. |
| ### 1. Spatial (`bim.spatial.*`) | ### 1. Spatial (`bim.spatial.*`) |
| Site, Building, Storey, and Space — the `IfcSpatialStructureElement` hierarchy. BIM Objects in this category encode floor-to-floor heights, minimum net area per use type, and occupancy limits. These are the objects that drive building massing at the earliest design stage. | Site, Building, Storey, and Space — the `IfcSpatialStructureElement` hierarchy. BIM Objects in this category encode floor-to-floor heights, minimum net area per use type, and occupancy limits. These are the objects that drive building massing at the earliest design stage. |
| Source file: `spatial.dtcg.json` | Source file: `spatial.dtcg.json` |
| ### 2. Elements (`bim.elements.*`) | ### 2. Elements (`bim.elements.*`) |
| Walls, slabs, columns, beams, curtain walls, doors, and windows — `IfcProduct` subclasses. BIM Objects encode structural fire ratings, acoustic performance classes, maximum glazing ratios, and the door-width minima required by ADA 2010 and CSA-B651. | Walls, slabs, columns, beams, curtain walls, doors, and windows — `IfcProduct` subclasses. BIM Objects encode structural fire ratings, acoustic performance classes, maximum glazing ratios, and the door-width minima required by ADA 2010 and CSA-B651. |
| Source file: `elements.dtcg.json` | Source file: `elements.dtcg.json` |
| ### 3. Materials (`bim.materials.*`) | ### 3. Materials (`bim.materials.*`) |
| Thermal conductivity, density, acoustic absorption coefficients, and compressive strength — `Pset_MaterialCommon` and material-specific Psets. Material BIM Objects encode the properties of the substrate, not the assembly. Assembly-level properties (U-values for a wall assembly) are encoded in the Assemblies category. | Thermal conductivity, density, acoustic absorption coefficients, and compressive strength — `Pset_MaterialCommon` and material-specific Psets. Material BIM Objects encode the properties of the substrate, not the assembly. Assembly-level properties (U-values for a wall assembly) are encoded in the Assemblies category. |
| Source file: `materials.dtcg.json` | Source file: `materials.dtcg.json` |
| ### 4. Assemblies (`bim.assemblies.*`) | ### 4. Assemblies (`bim.assemblies.*`) |
| Composite element constructions — wall assemblies, floor-ceiling assemblies, roof assemblies — where the performance property belongs to the layered system, not any single material. Thermal transmittance (U-value) and sound reduction index (Rw) live here, anchored to ISO 10077 and ISO 717-1 respectively. | Composite element constructions — wall assemblies, floor-ceiling assemblies, roof assemblies — where the performance property belongs to the layered system, not any single material. Thermal transmittance (U-value) and sound reduction index (Rw) live here, anchored to ISO 10077 and ISO 717-1 respectively. |
| Source file: `assemblies.dtcg.json` | Source file: `assemblies.dtcg.json` |
| ### 5. Systems (`bim.systems.*`) | ### 5. Systems (`bim.systems.*`) |
| HVAC zones, electrical circuits, and plumbing networks — `IfcSystem` and `IfcDistributionSystem`. The central BIM Object in this category is the climate-zone-autonomy principle: one Tile equals one HVAC zone. The BIM Object `bim.systems.tile-climate-autonomy` encodes this invariant so any downstream validator can confirm a proposed floor-plate composition satisfies it. | HVAC zones, electrical circuits, and plumbing networks — `IfcSystem` and `IfcDistributionSystem`. The central BIM Object in this category is the climate-zone-autonomy principle: one Tile equals one HVAC zone. The BIM Object `bim.systems.tile-climate-autonomy` encodes this invariant so any downstream validator can confirm a proposed floor-plate composition satisfies it. |
| Source file: `systems.dtcg.json` | Source file: `systems.dtcg.json` |
| ### 6. Performance (`bim.performance.*`) | ### 6. Performance (`bim.performance.*`) |
| Energy, daylight, accessibility, and fire-safety thresholds — regulatory minima drawn from EN 12464-1 (workplace daylight), EN 17037 (daylight in buildings), ADA 2010, IBC 2021, and ASHRAE 90.1. Performance BIM Objects are the regulatory floor that the spatial and element objects must satisfy. | Energy, daylight, accessibility, and fire-safety thresholds — regulatory minima drawn from EN 12464-1 (workplace daylight), EN 17037 (daylight in buildings), ADA 2010, IBC 2021, and ASHRAE 90.1. Performance BIM Objects are the regulatory floor that the spatial and element objects must satisfy. |
| The most architecturally consequential performance BIM Object is `bim.performance.max-workstation-to-window`, set at 6.0 metres from EN 12464-1:2021. This single value drives the Zone 1 Habitat depth in every professional office floor plate. | The most architecturally consequential performance BIM Object is `bim.performance.max-workstation-to-window`, set at 6.0 metres from EN 12464-1:2021. This single value drives the Zone 1 Habitat depth in every professional office floor plate. |
| Source file: `performance.dtcg.json` | Source file: `performance.dtcg.json` |
| ### 7. Climate zones (`bim.climate-zones.*`) | ### 7. Climate zones (`bim.climate-zones.*`) |
| ASHRAE 90.1 climate zones 1 through 8, encoding the heating-degree-day and cooling-degree-day thresholds that govern mechanical-system sizing across North American jurisdictions. These BIM Objects are consumed by service-codes when a building site is assigned to an ASHRAE zone and the energy model is validated. | ASHRAE 90.1 climate zones 1 through 8, encoding the heating-degree-day and cooling-degree-day thresholds that govern mechanical-system sizing across North American jurisdictions. These BIM Objects are consumed by service-codes when a building site is assigned to an ASHRAE zone and the energy model is validated. |
| Source file: `climate-zones.dtcg.json` | Source file: `climate-zones.dtcg.json` |
| ### 8. Identity codes (`bim.identity-codes.*`) | ### 8. Identity codes (`bim.identity-codes.*`) |
| Use-type codes (FFE series: M1, M2, B1, L1, A1, C1), tile codes (Tile A through Tile H), and key-plan index ranges. These are the opaque identifiers that planning documents use when describing floor-plate compositions. The BIM Object library is the lexicon that maps each code to its geometric and programme meaning. | Use-type codes (FFE series: M1, M2, B1, L1, A1, C1), tile codes (Tile A through Tile H), and key-plan index ranges. These are the opaque identifiers that planning documents use when describing floor-plate compositions. The BIM Object library is the lexicon that maps each code to its geometric and programme meaning. |
| Source file: `identity-codes.dtcg.json` | Source file: `identity-codes.dtcg.json` |
| ## How BIM Objects Are Delivered | ## How BIM Objects Are Delivered |
| The BIM Object library is distributed as a set of DTCG-format JSON files. DTCG (W3C Design Token Community Group format) is a JSON structure where each BIM Object carries a `$value`, `$type`, and `$description`, with optional `$extensions` for standards references and provenance. | The BIM Object library is distributed as a set of DTCG-format JSON files. DTCG (W3C Design Token Community Group format) is a JSON structure where each BIM Object carries a `$value`, `$type`, and `$description`, with optional `$extensions` for standards references and provenance. |
| A BIM authoring tool or validator reads the DTCG files at startup and constructs an in-memory BIM Object index. Any dimension, threshold, or classification referenced in a design session resolves to a BIM Object value rather than a hard-coded number. When a BIM Object changes — because a code amendment raises the minimum corridor width, or a new Uniclass edition replaces a classification code — the consuming tool receives the update via a library version bump, not a manual re-entry of values across hundreds of model elements. | A BIM authoring tool or validator reads the DTCG files at startup and constructs an in-memory BIM Object index. Any dimension, threshold, or classification referenced in a design session resolves to a BIM Object value rather than a hard-coded number. When a BIM Object changes — because a code amendment raises the minimum corridor width, or a new Uniclass edition replaces a classification code — the consuming tool receives the update via a library version bump, not a manual re-entry of values across hundreds of model elements. |
| ## Relationship to the IFC File | ## Relationship to the IFC File |
| The DTCG BIM Object library does not modify IFC files directly. It is an authoring-time reference — the source of truth for what values to write into an IFC Pset when a model element is created or modified. A BIM operator who follows the [[building-design-system-bim|Building Design System]] writes the IFC file with the BIM Object values; IDS 1.0 contracts then validate that the IFC file conforms to the published thresholds. | The DTCG BIM Object library does not modify IFC files directly. It is an authoring-time reference — the source of truth for what values to write into an IFC Pset when a model element is created or modified. A BIM operator who follows the [[building-design-system-bim|Building Design System]] writes the IFC file with the BIM Object values; IDS 1.0 contracts then validate that the IFC file conforms to the published thresholds. |
| The BIM Object-to-IFC pipeline is therefore: | The BIM Object-to-IFC pipeline is therefore: |
| ``` | ``` |
| BIM Object value (bim.performance.max-workstation-to-window = 6.0 m) | BIM Object value (bim.performance.max-workstation-to-window = 6.0 m) |
| ↓ applied at authoring time | ↓ applied at authoring time |
| IFC Pset_SpaceOccupancy attribute in IfcSpace | IFC Pset_SpaceOccupancy attribute in IfcSpace |
| ↓ validated at delivery time | ↓ validated at delivery time |
| IDS 1.0 rule: IfcSpace.MaxWorkstationToWindow ≤ 6000 mm | IDS 1.0 rule: IfcSpace.MaxWorkstationToWindow ≤ 6000 mm |
| ``` | ``` |
| This separation — author with BIM Objects, validate with IDS — is the substrate that makes [[city-code-as-composable-geometry]] possible at the next layer. | This separation — author with BIM Objects, validate with IDS — is the substrate that makes [[city-code-as-composable-geometry]] possible at the next layer. |
| ## See also | ## See also |
| - [[bim-objects-what-they-are]] — the pre-constraining thesis and how BIM Objects differ from IFC property sets | - [[bim-objects-what-they-are]] — the pre-constraining thesis and how BIM Objects differ from IFC property sets |
| - [[bim-objects-three-layers]] — the Specification, Regulation, and Climate Zone layers and their composition rule | - [[bim-objects-three-layers]] — the Specification, Regulation, and Climate Zone layers and their composition rule |
| - [[building-design-system-bim]] — the broader Building Design System for the built environment | - [[building-design-system-bim]] — the broader Building Design System for the built environment |
| - [[flat-file-bim-leapfrog]] — the flat-file architecture that underpins the BIM Object vault | - [[flat-file-bim-leapfrog]] — the flat-file architecture that underpins the BIM Object vault |