IFC - Industry Foundation Classes

An IFC file being viewed in FreeCAD

IFC (Industry Foundation Classes), is an open-data schema and set of formats used to store OpenBIM data. It is developed and maintained by buildingSMART. IFC data can digitally describe many concepts, including:

• Physical objects in our built environment (walls, slabs, columns, pipes)
• 2D and 3D geometry that represents objects or annotate objects
• A diverse set of properties and attributes spanning many domains
• Materials attributes and display colours
• Construction planning, resource allocation, and scheduling
• Quantification of elements
• Roles and responsibilities of organisations and individuals
• Design strategies and legal constraints
• Analytical models for structural analysis, energy analysis, and light analysis

The majority of BIM programs can read and write IFC data. However, the quality of vendor support for IFC data varies significantly between software.

IFC versions

There are currently two commonly supported versions of IFC: IFC2X3 and IFC4. IFC4 contains many new features compared to IFC2X3, such as improved geometry representations, geolocation support, and more element categories. However, IFC4 support is less prominent than IFC2X3, though this is changing. IFC2X3 has been an ISO standard since 2005, and IFC4 has been an ISO standard since 2013.

IFC formats

IFC data is most commonly found in a plain text file format with the file extension .ifc. This is one of many possible ways to store IFC data. Other methods include:

• .ifc IFC-SPF format, a commonly used plain text format based on STEP
• .ifczip IfcZIP format, where a single .ifc file is compressed into a ZIP package
• .ifcxml IfcXML format, a plain text format
• .json JSON format, a plain text format
• .hdf HDF5 format, a binary format
• .sqlite SQLite format, a binary format

In reality, currently only .ifc and .ifczip sees common usage.

IFC classes

In IFC, a single concept is known as an IFC class. There are hundreds of IFC classes. Examples of IFC classes are IfcWall, IfcBuilding, and IfcTask. Classes can have attributes, for instance, the IfcWall can have a Name attribute. Classes can also have relationships to other classes, for example an IfcWall can be related to an IfcBuilding by being spatially contained within the IfcBuilding.

An example hierarchy of IFC classes

Classes can inherit from other classes, building up a hierarchy of classes. If a class inherits from another class, it inherits all of its attributes and relationships. For example, the IfcProduct class has a Representation attribute, which can store 3D geometry that represents that class. Because the IfcWall class inherits from the IfcProduct class, it also has a Representation attribute to store 3D geometry. However, the IfcPerson class does not inherit from the IfcProduct class, and so it does not have a Representation attribute.