IFC Syntax : Quantities
An IFC model carries measured magnitudes (the length of a wall, the net volume
of a slab, the count of items in an assembly) in a dedicated container :
IfcElementQuantity. Quantities are the data behind quantity take-off and cost
estimation. This skill covers the quantity entities, how they attach to elements,
and how they differ across versions.
Applies to IFC2x3, IFC4, and IFC4.3. Three things changed across versions :
see the Version Differences section.
Quick Reference
Properties vs quantities : the conceptual rule
ALWAYS keep these two apart. They live in different containers.
- A property (
IfcPropertySet) carries a descriptive or design attribute :
fire rating, IsExternal, thermal transmittance, a load-bearing flag.
- A quantity (
IfcElementQuantity) carries a measured magnitude derived
from the element's geometry or specification : gross side area, net volume,
count of items.
NEVER put a measured magnitude in a property set, and NEVER put a descriptive
attribute in a quantity set. A quantity-take-off reader looks only in
IfcElementQuantity containers ; a magnitude hidden in a Pset_ is invisible to it.
The quantity container : IfcElementQuantity
IfcElementQuantity is the container for measured quantities. Verified verbatim
(IFC4.3) :
ENTITY IfcElementQuantity
SUBTYPE OF (IfcQuantitySet);
MethodOfMeasurement : OPTIONAL IfcLabel;
Quantities : SET [1:?] OF IfcPhysicalQuantity;
WHERE
UniqueQuantityNames : IfcUniqueQuantityNames(Quantities);
END_ENTITY;
MethodOfMeasurement names the standard of measurement used (for example a
national take-off standard). It is OPTIONAL.Quantities holds one or more IfcPhysicalQuantity instances. The set is never
empty ([1:?]).- The
UniqueQuantityNames rule forbids two quantities in the set sharing a Name.
Supertype chain (IFC4 / IFC4.3) :
IfcElementQuantity -> IfcQuantitySet -> IfcPropertySetDefinition ->
IfcPropertyDefinition -> IfcRoot. Because IfcQuantitySet is an
IfcPropertySetDefinition, an IfcElementQuantity is attached to elements through
the exact same relationship as a property set.
Attachment : the shared IfcRelDefinesByProperties machinery
A quantity set is not a child of the element. It floats in the file and is linked
by the objectified relationship IfcRelDefinesByProperties :
RelatedObjects : SET [1:?] OF IfcObjectDefinition : the element occurrences
receiving the quantity set.RelatingPropertyDefinition : IfcPropertySetDefinitionSelect : the
IfcElementQuantity being assigned (it qualifies because it is an
IfcPropertySetDefinition).
ALWAYS attach an IfcElementQuantity to an element occurrence with
IfcRelDefinesByProperties. NEVER point that relationship at an IfcTypeObject :
the NoRelatedTypeObject rule rejects it. A type carries quantity sets through the
HasPropertySets aggregation populated by IfcRelDefinesByType. See
ifc-syntax-property-sets for the full attachment machinery shared by both.
The quantity hierarchy
IfcPhysicalQuantity (abstract) attributes : Name, Description
├── IfcPhysicalSimpleQuantity (abstract) adds : Unit (OPTIONAL IfcNamedUnit)
│ ├── IfcQuantityLength LengthValue : IfcLengthMeasure
│ ├── IfcQuantityArea AreaValue : IfcAreaMeasure
│ ├── IfcQuantityVolume VolumeValue : IfcVolumeMeasure
│ ├── IfcQuantityCount CountValue : IfcCountMeasure
│ ├── IfcQuantityWeight WeightValue : IfcMassMeasure
│ ├── IfcQuantityTime TimeValue : IfcTimeMeasure
│ └── IfcQuantityNumber NumberValue : IfcNumericMeasure (IFC4.3 only)
└── IfcPhysicalComplexQuantity groups child quantities by a Discrimination
IfcPhysicalQuantity is abstract with two direct subtypes : IfcPhysicalSimpleQuantity
and IfcPhysicalComplexQuantity. It carries Name (IfcLabel, required) and
Description (OPTIONAL IfcText).
The seven simple quantity subtypes
IfcPhysicalSimpleQuantity is abstract and adds one attribute :
Unit : OPTIONAL IfcNamedUnit. IFC4.3 has seven concrete subtypes ; IFC2x3
and IFC4 have six (IfcQuantityNumber is IFC4.3 only).
| Subtype | Value attribute | Value type | Unit type (WR21) |
|---|
| IfcQuantityLength | LengthValue | IfcLengthMeasure | LENGTHUNIT |
| IfcQuantityArea | AreaValue | IfcAreaMeasure | AREAUNIT |
| IfcQuantityVolume | VolumeValue | IfcVolumeMeasure | VOLUMEUNIT |
| IfcQuantityWeight | WeightValue | IfcMassMeasure | MASSUNIT |
| IfcQuantityTime | TimeValue | IfcTimeMeasure | TIMEUNIT |
| IfcQuantityCount | CountValue | IfcCountMeasure | none (no unit rule) |
| IfcQuantityNumber | NumberValue | IfcNumericMeasure | none (IFC4.3 only) |
Every subtype also has Formula : OPTIONAL IfcLabel (IFC4+). Each measured subtype
(Length, Area, Volume, Weight, Time) carries two WHERE rules :
WR21 : if a Unit is present, its UnitType MUST match the quantity's kind.
An area quantity with a length unit is schema-invalid.WR22 : the value MUST be >= 0. A negative measured quantity is rejected.
IfcQuantityCount has one rule only : WR21 : CountValue >= 0. IfcQuantityNumber
has no WHERE rules.
How a quantity gets its unit
IfcPhysicalSimpleQuantity.Unit is OPTIONAL IfcNamedUnit. If Unit is omitted,
the project-level IfcUnitAssignment supplies the unit for that quantity's measure
type. If Unit is given, it overrides the project unit for that one quantity, and
it MUST be the matching unit type (the WR21 rule). Note Unit is an
IfcNamedUnit, not a generic IfcUnit : a quantity's explicit unit cannot be an
IfcDerivedUnit or IfcMonetaryUnit. See ifc-syntax-units.
IfcPhysicalComplexQuantity
Groups child quantities that share a discrimination (for example one group per
material layer). Verified verbatim (IFC4.3) :
ENTITY IfcPhysicalComplexQuantity
SUBTYPE OF (IfcPhysicalQuantity);
HasQuantities : SET [1:?] OF IfcPhysicalQuantity;
Discrimination : IfcLabel;
Quality : OPTIONAL IfcLabel;
Usage : OPTIONAL IfcLabel;
WHERE
NoSelfReference : SIZEOF(QUERY(temp <* HasQuantities | SELF :=: temp)) = 0;
UniqueQuantityNames : IfcUniqueQuantityNames(HasQuantities);
END_ENTITY;
Discrimination (required) names what distinguishes the group, for example
'layer' or 'steel bar diameter'. NoSelfReference forbids the complex quantity
containing itself.
The Qto_ naming convention
Standardised quantity sets are published by buildingSMART with a Qto_ prefix,
parallel to the Pset_ prefix for property sets. The Name of the
IfcElementQuantity carries that identifier. Verified example :
Qto_WallBaseQuantities applies to IfcWall and defines Length, Width,
Height (Q_LENGTH), GrossFootprintArea, NetFootprintArea, GrossSideArea,
NetSideArea (Q_AREA), GrossVolume, NetVolume (Q_VOLUME), GrossWeight,
NetWeight (Q_WEIGHT).
ALWAYS use the published Qto_ name when a standardised quantity set fits the
element. NEVER give a custom, project-specific quantity set a Qto_-prefixed name :
that prefix is reserved for the buildingSMART library.
Version differences (compact)
| Item | IFC2x3 | IFC4 | IFC4.3 |
|---|
IfcQuantitySet supertype | absent (chain skips it) | present | present |
Formula on quantity subtypes | absent | present | present |
IfcQuantityNumber | absent | absent | present (IFC4.3.0.0) |
| Concrete simple subtypes | 6 | 6 | 7 |
| Standardised set naming | implementer agreement | Qto_ prefix | Qto_ prefix |
In IFC2x3 the supertype chain is IfcElementQuantity -> IfcPropertySetDefinition
-> IfcPropertyDefinition -> IfcRoot : there is no IfcQuantitySet layer.
Decision Trees
Is this a property or a quantity?
What kind of data are you attaching?
|
+-- A measured magnitude derived from geometry or specification
| (length, area, volume, count, weight, duration)
| --> IfcElementQuantity. Pick a subtype from the table below.
|
+-- A descriptive or design attribute (fire rating, IsExternal,
U-value, load-bearing flag, a name, a status)
--> IfcPropertySet. See ifc-syntax-property-sets.
Which simple quantity subtype?
What does the value measure?
|
+-- A 1D length, width, height, perimeter --> IfcQuantityLength
+-- A 2D area (footprint, side, surface) --> IfcQuantityArea
+-- A 3D volume --> IfcQuantityVolume
+-- A mass / weight --> IfcQuantityWeight
+-- A duration --> IfcQuantityTime
+-- A whole-item count (integer of items) --> IfcQuantityCount
+-- A dimensionless numeric, IFC4.3 only --> IfcQuantityNumber
(targeting IFC2x3 or IFC4? use IfcQuantityCount instead)
Occurrence or type attachment?
Is the quantity set the same for every occurrence of a type?
|
+-- It is per-occurrence (measured from this element's geometry)
| --> attach the IfcElementQuantity to the OCCURRENCE with
| IfcRelDefinesByProperties. This is the normal case.
|
+-- It is a type-level default shared by all occurrences
--> add it to the IfcTypeObject.HasPropertySets aggregation.
NEVER point IfcRelDefinesByProperties at a type :
the NoRelatedTypeObject rule rejects it.
Patterns
Pattern : Attach an IfcElementQuantity to an element occurrence
ALWAYS create one IfcElementQuantity, fill its Quantities set with the
individual IfcPhysicalSimpleQuantity instances, and link it to the element
occurrence with an IfcRelDefinesByProperties whose RelatingPropertyDefinition
is the IfcElementQuantity and whose RelatedObjects holds the element.
ALWAYS give every quantity in one IfcElementQuantity a unique Name : the
UniqueQuantityNames rule rejects a duplicate.
NEVER attach the quantity set by pointing IfcRelDefinesByProperties at an
IfcTypeObject : use the type's HasPropertySets aggregation instead.
See references/examples.md for a worked STEP scenario.
Pattern : Pick the matching simple quantity subtype and unit
ALWAYS pick the subtype whose value attribute matches the measured kind
(IfcQuantityArea for an area, IfcQuantityVolume for a volume).
ALWAYS leave Unit unset to inherit the project IfcUnitAssignment, or set it to
an IfcNamedUnit of the matching unit type. An IfcQuantityArea with a length
unit breaks the WR21 rule and the file is schema-invalid.
NEVER write a negative value : the WR22 rule requires value >= 0 on every
measured subtype, and WR21 on IfcQuantityCount requires CountValue >= 0.
Pattern : Choose between IfcQuantityCount and IfcQuantityNumber
ALWAYS use IfcQuantityCount for a whole-item count (the number of bars, panels,
or fixtures). It exists in every version this skill covers.
ALWAYS use IfcQuantityNumber ONLY when the target schema is IFC4.3 and the value
is a dimensionless numeric that is not a whole-item count.
NEVER emit IfcQuantityNumber into an IFC2x3 or IFC4 file : the entity does not
exist before IFC4.3.0.0 and the file will fail schema validation.
Pattern : Group quantities with IfcPhysicalComplexQuantity
ALWAYS use IfcPhysicalComplexQuantity to group child quantities that share a
discriminator, for example one length-plus-area-plus-volume group per material
layer of a wall. Set Discrimination to the discriminator name ('layer').
ALWAYS give the child quantities unique names within the complex quantity : the
UniqueQuantityNames rule applies to HasQuantities too.
A complex quantity is itself an IfcPhysicalQuantity, so it can sit directly in an
IfcElementQuantity.Quantities set alongside simple quantities.
Pattern : Name a standardised quantity set with Qto_
ALWAYS set the IfcElementQuantity.Name to the published Qto_ identifier when a
standardised quantity set fits (Qto_WallBaseQuantities for IfcWall,
Qto_SlabBaseQuantities for IfcSlab, and so on).
NEVER reuse a Qto_ name for a custom set with different quantities : a custom set
MUST use a project-specific name without the reserved Qto_ prefix.
Pattern : Honour the version differences
ALWAYS treat the supertype chain as version-dependent : in IFC4 and IFC4.3
IfcElementQuantity is an IfcQuantitySet ; in IFC2x3 it is a direct
IfcPropertySetDefinition with no IfcQuantitySet layer.
ALWAYS omit the Formula attribute when authoring IFC2x3 : it was added in IFC4.
ALWAYS restrict IfcQuantityNumber to IFC4.3 output.
For IFC2x3 standardised quantity sets there is no schema-level naming rule : the
IFC2x3 spec states recognisable names are agreed in implementer agreements. The
Qto_ prefix is an IFC4 formalisation.
Reference Links
references/methods.md : the complete verified entity definitions :
IfcElementQuantity, IfcQuantitySet, IfcPhysicalQuantity,
IfcPhysicalSimpleQuantity, all seven concrete subtypes with their WHERE
rules, IfcPhysicalComplexQuantity, the attachment relationship, and the
per-version difference matrix.references/examples.md : worked STEP scenarios : a Qto_WallBaseQuantities
set attached to a wall, each simple quantity subtype, a complex quantity grouped
per layer, and an IFC2x3 versus IFC4 comparison.references/anti-patterns.md : the failure modes this skill prevents, each with
the reason it fails and the rule it breaks.
Official sources
Related skills
ifc-syntax-property-sets : the shared IfcPropertySetDefinition and
IfcRelDefinesByProperties attachment machinery, and the property entities.ifc-syntax-units : IfcUnitAssignment, IfcNamedUnit, and how a quantity
value resolves to a concrete unit.ifc-impl-property-extraction : reading quantities back out of a model for
quantity take-off.
