.IFC File Format – Kernel and spatial structure

IfcRoot

IfcRoot is the abstract supertype of every entity outside the resource schemas. Its subtypes can stand on their own in a file; entities of the resource schemas, which do not descend from IfcRoot, have no identity and exist only when something rooted refers to them. Two resource instances with equal values mean the same thing, and sharing one instance between users is recommended but not required.

Kernel entity tree

IfcRoot has three direct subtypes: object definitions (things, occurrences and types), property definitions, and relationships. Objects divide into six kinds: products (physical or spatial things with a place and shape), processes (actions in time), controls (constraints and requirements), resources (the use of something in a process), actors (people and organisations involved) and groups.

IfcRoot
├── IfcObjectDefinition
│   ├── IfcObject
│   │   ├── IfcActor
│   │   ├── IfcControl
│   │   ├── IfcGroup
│   │   ├── IfcProcess
│   │   ├── IfcProduct
│   │   │   ├── IfcAnnotation
│   │   │   ├── IfcElement
│   │   │   ├── IfcLinearElement
│   │   │   ├── IfcPort
│   │   │   ├── IfcPositioningElement
│   │   │   ├── IfcSpatialElement
│   │   │   ├── IfcStructuralActivity
│   │   │   └── IfcStructuralItem
│   │   └── IfcResource
│   ├── IfcTypeObject
│   │   ├── IfcTypeProcess
│   │   ├── IfcTypeProduct
│   │   │   ├── IfcElementType
│   │   │   └── IfcSpatialElementType
│   │   └── IfcTypeResource
│   └── IfcContext
│       ├── IfcProject
│       └── IfcProjectLibrary
├── IfcPropertyDefinition
│   ├── IfcPropertySetDefinition
│   │   ├── IfcPreDefinedPropertySet
│   │   ├── IfcPropertySet
│   │   └── IfcQuantitySet
│   │       └── IfcElementQuantity
│   └── IfcPropertyTemplateDefinition
└── IfcRelationship
    ├── IfcRelAssigns
    ├── IfcRelAssociates
    ├── IfcRelConnects
    ├── IfcRelDeclares
    ├── IfcRelDecomposes
    └── IfcRelDefines

GlobalId compression

IfcGloballyUniqueId holds a GUID, an automatically generated 128-bit number, compressed to a fixed string of 22 characters. Each character carries six bits, taken from this 64-character alphabet:

      1     2     3     4     5     6
 0123456789012345678901234567890123456789012345678901234567890123
"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$"

The documentation warns that this differs from common base64. The 16 bytes are encoded in order, most significant bits first:

1. The first byte is encoded in the first two characters (2 + 6 bits). The first character can therefore only be 0, 1, 2 or 3.
2. The remaining 15 bytes are encoded in five groups of three bytes, each group becoming four characters.

The documentation’s example: the GUID f70dd363-bfe3-495d-84a0-2c02dcb7d4d2 compresses to 3t3TDZl_D9NOIWB0BSjzJI. Worked through:

Decoding reverses the steps: the first character gives the top two bits of byte 0, and every following group of four characters gives three bytes. The documentation refers to buildingSMART’s technical website for sample code.

Owner history

IfcOwnerHistory (resource schema IfcUtilityResource) records who owns an object, which application created it and the last change. It is attached directly to rooted objects, relationships and property sets.

IfcTimeStamp is an INTEGER count of seconds since 1 January 1970, 00:00:00 UTC. Rule CorrectChangeAction: if ChangeAction is given but LastModifiedDate is not, ChangeAction must be NOTDEFINED. If the date is given without ChangeAction, the action is taken as undefined; if both are given, the action applies to that date.

IfcStateEnum

IfcChangeActionEnum

The documentation describes the intended use: an application publishing a model for others to edit sets NOCHANGE as a baseline; a receiving application sets ADDED or MODIFIED on what it changes; a model server resets the values to NOCHANGE before republishing. MODIFIEDADDED and MODIFIEDDELETED were removed in IFC4.

The user and the application

Objects, products, types and the project

Each entity below adds its attributes after those of its supertypes; positions continue from IfcRoot’s four. IfcObjectDefinition adds only inverse attributes, so it takes no position.

IfcProject and IfcProjectLibrary add nothing to IfcContext, so an IfcProject has nine attributes. Rules on the project: a file has at most one (global rule IfcSingleProjectInstance); its Name must be given (HasName); it may not decompose another object (NoDecomposition); and its RepresentationContexts may not directly include a sub-context (CorrectContext). The project is the root of the spatial tree, which hangs from it through IfcRelAggregates, and of other trees such as work plans, which it declares through IfcRelDeclares.

An object occurrence gets its type through IfcRelDefinesByType and its own property sets through IfcRelDefinesByProperties; property set names on one object must be unique (rule UniquePropertySetNames). When the same property appears in a type’s and an occurrence’s property set, the occurrence value overrides the type value.

Objectified relationships

Most links between objects are not attributes of the objects but separate relationship instances, subtypes of IfcRelationship. This keeps relationship data on the relationship and lets the objects find them through inverse attributes. Every relationship starts with IfcRoot’s four attributes. The documentation names six kinds:

Every relationship entity in IFC 4.3

The relationships most files use

IfcRelationship, IfcRelDecomposes, IfcRelDefines and IfcRelConnects add no attributes of their own. In the selects, IfcPropertySetDefinitionSelect lets one relationship carry either a single property set definition or a set of them.

Spatial structure

The spatial structure is a hierarchy that organises the project. It is built with IfcRelAggregates, one level at a time, with IfcProject at the top: the project aggregates the root of the structure, which may be an IfcSite, an IfcBuilding (when no site is given) or an IfcSpatialZone. Elements such as walls and pipes join the structure through IfcRelContainedInSpatialStructure. The documentation’s informal rules: the hierarchy is acyclic; a site should not be under a building, storey or space; a building not under a storey or space; a storey not under a space. Every spatial structure element must be aggregated into another one or into the project (rule WR41).

IfcSpatialElement
├── IfcSpatialStructureElement
│   ├── IfcSite
│   ├── IfcFacility
│   │   ├── IfcBuilding
│   │   ├── IfcBridge
│   │   ├── IfcMarineFacility
│   │   ├── IfcRailway
│   │   └── IfcRoad
│   ├── IfcFacilityPart
│   │   ├── IfcBridgePart
│   │   ├── IfcFacilityPartCommon
│   │   ├── IfcMarinePart
│   │   ├── IfcRailwayPart
│   │   └── IfcRoadPart
│   ├── IfcBuildingStorey
│   └── IfcSpace
├── IfcSpatialZone
└── IfcExternalSpatialStructureElement
    └── IfcExternalSpatialElement

A spatial zone is different: a non-hierarchical, possibly overlapping division of the project for some purpose, such as a thermal, lighting or fire zone. Positions below continue from IfcProduct’s seven attributes.

IfcSite, IfcBuilding and IfcBuildingStorey have no PredefinedType in IFC 4.3.

IfcElementCompositionEnum

When one facility or facility part aggregates another of the same kind, the parent has the higher composition type: COMPLEX above ELEMENT, ELEMENT above PARTIAL.

IfcSpaceTypeEnum

Zone and external element types

Facilities and facility parts

IFC 4.3 generalises the building into IfcFacility, any built facility: building, bridge, marine facility, railway or road. A facility is placed relative to the site and broken down into IfcFacilityParts, which are placed relative to their facility. IfcFacility adds no attributes; each facility other than IfcBuilding adds a PredefinedType at position 10.

A facility part has two attributes after CompositionType:

IfcFacilityUsageEnum

Facility part types

Values are as in the UML model. Two documentation pages differ from it: the railway part page spells DILATATIONTRACK, and the bridge part page does not list SURFACESTRUCTURE.

Property and quantity sets

Most information about an object beyond its identity and shape is held in property sets. An IfcPropertySet is a named container of properties, each interpreted by its name; sets defined by the standard are named Pset_…, and quantity sets Qto_…. Sets are attached to occurrences with IfcRelDefinesByProperties, and to types through IfcTypeObject.HasPropertySets. Changes to the standard sets do not change the structure of files.

Properties and quantities are resource entities, without a GlobalId:

The simple properties, subtypes of IfcSimpleProperty, are IfcPropertyBoundedValue, IfcPropertyEnumeratedValue, IfcPropertyListValue, IfcPropertyReferenceValue, IfcPropertySingleValue and IfcPropertyTableValue; IfcComplexProperty nests properties. Each simple quantity adds a value and a formula after the unit:

IfcPhysicalComplexQuantity groups quantities. The other subtypes of IfcPropertySetDefinition are IfcPreDefinedPropertySet, whose subtypes (door and window lining and panel properties, permeable covering properties and IfcReinforcementDefinitionProperties) have fixed attributes, and the templates under IfcPropertyTemplateDefinition, which define the syntax of custom property sets.

Units

The project’s UnitsInContext sets the default units, declared once for the whole file. They apply to the geometry, to every attribute whose type is a measure, and to properties and quantities that give no unit of their own. A unit may be an SI unit with an optional prefix, a conversion-based unit with a factor to an SI unit (inch, foot), a derived unit built from unit elements (W/m², for example), or a monetary unit naming a currency.

In IfcSIUnit the inherited Dimensions is redeclared as derived: it is computed by the function IfcDimensionsForSIUnit rather than stored. The SI unit of mass is the gram when no prefix is given, even though the SI base unit is the kilogram. IfcDimensionalExponents holds seven INTEGER exponents in this order: LengthExponent, MassExponent, TimeExponent, ElectricCurrentExponent, ThermodynamicTemperatureExponent, AmountOfSubstanceExponent, LuminousIntensityExponent. The other named units are IfcContextDependentUnit and IfcConversionBasedUnitWithOffset (a conversion with an offset, a subtype of IfcConversionBasedUnit); IfcDerivedUnit and IfcMonetaryUnit are units that are not named units.

IfcUnitEnum

ABSORBEDDOSEUNIT, AMOUNTOFSUBSTANCEUNIT, AREAUNIT, DOSEEQUIVALENTUNIT, ELECTRICCAPACITANCEUNIT, ELECTRICCHARGEUNIT, ELECTRICCONDUCTANCEUNIT, ELECTRICCURRENTUNIT, ELECTRICRESISTANCEUNIT, ELECTRICVOLTAGEUNIT, ENERGYUNIT, FORCEUNIT, FREQUENCYUNIT, ILLUMINANCEUNIT, INDUCTANCEUNIT, LENGTHUNIT, LUMINOUSFLUXUNIT, LUMINOUSINTENSITYUNIT, MAGNETICFLUXDENSITYUNIT, MAGNETICFLUXUNIT, MASSUNIT, PLANEANGLEUNIT, POWERUNIT, PRESSUREUNIT, RADIOACTIVITYUNIT, SOLIDANGLEUNIT, THERMODYNAMICTEMPERATUREUNIT, TIMEUNIT, VOLUMEUNIT, USERDEFINED.

IfcSIUnitName

AMPERE, BECQUEREL, CANDELA, COULOMB, CUBIC_METRE, DEGREE_CELSIUS, FARAD, GRAM, GRAY, HENRY, HERTZ, JOULE, KELVIN, LUMEN, LUX, METRE, MOLE, NEWTON, OHM, PASCAL, RADIAN, SECOND, SIEMENS, SIEVERT, SQUARE_METRE, STERADIAN, TESLA, VOLT, WATT, WEBER.

IfcSIPrefix

IfcDerivedUnitEnum

The unit types of IfcDerivedUnit: ACCELERATIONUNIT, ANGULARVELOCITYUNIT, AREADENSITYUNIT, COMPOUNDPLANEANGLEUNIT, CURVATUREUNIT, DYNAMICVISCOSITYUNIT, HEATFLUXDENSITYUNIT, HEATINGVALUEUNIT, INTEGERCOUNTRATEUNIT, IONCONCENTRATIONUNIT, ISOTHERMALMOISTURECAPACITYUNIT, KINEMATICVISCOSITYUNIT, LINEARFORCEUNIT, LINEARMOMENTUNIT, LINEARSTIFFNESSUNIT, LINEARVELOCITYUNIT, LUMINOUSINTENSITYDISTRIBUTIONUNIT, MASSDENSITYUNIT, MASSFLOWRATEUNIT, MASSPERLENGTHUNIT, MODULUSOFELASTICITYUNIT, MODULUSOFLINEARSUBGRADEREACTIONUNIT, MODULUSOFROTATIONALSUBGRADEREACTIONUNIT, MODULUSOFSUBGRADEREACTIONUNIT, MOISTUREDIFFUSIVITYUNIT, MOLECULARWEIGHTUNIT, MOMENTOFINERTIAUNIT, PHUNIT, PLANARFORCEUNIT, ROTATIONALFREQUENCYUNIT, ROTATIONALMASSUNIT, ROTATIONALSTIFFNESSUNIT, SECTIONAREAINTEGRALUNIT, SECTIONMODULUSUNIT, SHEARMODULUSUNIT, SOUNDPOWERLEVELUNIT, SOUNDPOWERUNIT, SOUNDPRESSURELEVELUNIT, SOUNDPRESSUREUNIT, SPECIFICHEATCAPACITYUNIT, TEMPERATUREGRADIENTUNIT, TEMPERATURERATEOFCHANGEUNIT, THERMALADMITTANCEUNIT, THERMALCONDUCTANCEUNIT, THERMALEXPANSIONCOEFFICIENTUNIT, THERMALRESISTANCEUNIT, THERMALTRANSMITTANCEUNIT, TORQUEUNIT, VAPORPERMEABILITYUNIT, VOLUMETRICFLOWRATEUNIT, WARPINGCONSTANTUNIT, WARPINGMOMENTUNIT, USERDEFINED.

Worked example

A project with a site, a building, one storey and a wall in that storey, with the relationships that join them. Owner history, placements, geometry and units are left out to keep it short; the identifiers are made up for the example.

#1=IFCPROJECT('0YvctVUKr0kugbFTf53O9L',$,'Project',$,$,$,$,$,$);
#2=IFCSITE('2Ws3aT1Fz1ERmYc1Lr7xgO',$,'Site',$,$,$,$,$,.ELEMENT.,$,$,$,$,$);
#3=IFCBUILDING('1hOSvn6df7F8_7GcBWlRGQ',$,'Building',$,$,$,$,$,.ELEMENT.,$,$,$);
#4=IFCBUILDINGSTOREY('0eBbL5mQf0vBhWl2VZbVDn',$,'Ground floor',$,$,$,$,$,.ELEMENT.,$);
#10=IFCRELAGGREGATES('3Kb2YwKbXEMfJm6ox3H_kx',$,$,$,#1,(#2));
#11=IFCRELAGGREGATES('1VQ5n5$rL6UgGHOFp6Q2Wb',$,$,$,#2,(#3));
#12=IFCRELAGGREGATES('0j5VvM$kD7vg5OwrfJ$rgq',$,$,$,#3,(#4));
#30=IFCWALL('1kTvXnbbzCWw8lcMd1dR4o',$,'Wall 1',$,$,$,$,$,.SOLIDWALL.);
#40=IFCRELCONTAINEDINSPATIALSTRUCTURE('2SKbDu1jX0MRo6l$mKz9ex',$,$,$,(#30),#4);

Reading it: #1 has the nine attributes of IfcProject, with the name required by rule HasName. #2 has the fourteen of IfcSite, all optional ones unset except CompositionType. #3 has the twelve of IfcBuilding and #4 the ten of IfcBuildingStorey; their deprecated elevation and address attributes are left unset. Each IfcRelAggregates names the whole at position 5 and the set of parts at position 6, building the chain project → site → building → storey. The wall is placed in the storey by #40, whose position 5 is the set of contained elements and position 6 the storey.

Not confirmed

• The EXPRESS name of the select type used for IfcFacilityPart.PredefinedType. The UML model gives the attribute no type of its own; its values come from the part enumerations listed above, and the select is produced when the schema is exported. The exported EXPRESS file was not read.
• Whether IfcBridgePart, IfcRoadPart and the other parts redeclare PredefinedType in EXPRESS: the UML model attaches the enumeration to each part without an attribute position.
• The spelling DILATIONTRACK or DILATATIONTRACK, and SURFACESTRUCTURE: the UML model and the documentation pages disagree, as noted above.
• The full attribute lists of the property, quantity, unit and template entities not shown here, of IfcDerivedUnit and IfcMonetaryUnit, and of the actor role and address entities.

Sources

• IFC4.3.x-development – buildingSMART’s IFC 4.3 source repository, master branch: schemas/IFC.xml (the UML model: attributes, their order, types, optionality, generalisations and enumeration values), docs/schemas/core/IfcKernel/README.md, docs/schemas/resource/README.md, the entity pages IfcRoot, IfcObjectDefinition, IfcObject, IfcProduct, IfcContext, IfcProject, IfcTypeObject, IfcOwnerHistory, IfcApplication, IfcRelAggregates, IfcRelNests, IfcRelDeclares, IfcRelDefinesByProperties, IfcRelDefinesByType, IfcRelContainedInSpatialStructure, IfcSpatialElement, IfcSpatialStructureElement, IfcSpatialZone, IfcSite, IfcBuilding, IfcBuildingStorey, IfcSpace, IfcFacility, IfcFacilityPart, IfcFacilityPartCommon, IfcBridge, IfcBridgePart, IfcRoad, IfcRailway, IfcMarineFacility, IfcPropertySet, IfcElementQuantity, IfcPropertySingleValue, IfcUnitAssignment, IfcSIUnit, IfcLocalPlacement, the type pages IfcGloballyUniqueId, IfcTimeStamp, IfcStateEnum, IfcChangeActionEnum, IfcElementCompositionEnum, IfcSpaceTypeEnum, IfcFacilityUsageEnum, IfcFacilityPartCommonTypeEnum, IfcBridgeTypeEnum, IfcBridgePartTypeEnum, IfcRoadTypeEnum, IfcRoadPartTypeEnum, IfcRailwayTypeEnum, IfcRailwayPartTypeEnum, IfcMarineFacilityTypeEnum, IfcMarinePartTypeEnum, IfcSpatialZoneTypeEnum, IfcExternalSpatialElementTypeEnum, IfcSIPrefix, and the concept templates Project Context and Project Units; content/introduction.md (naming of property and quantity sets, deprecations)
• ifc-gherkin-rules – buildingSMART’s rule IFC102 (deprecated attributes such as RelatedObjectsType)