The IFC Model Format

IFC

IFC (Industry Foundation Classes) is a platform neutral open file format developed by buildingSMART . It provides a framework and rules for construction and building data.

IFC - Industry Foundation Classes The open format means that the way the file is built-up, the structure and the terminology are available to view and use.

It's specific to the construction industry. Using terms such as building, storey and space.

The IFC schema is specified using the EXPRESS data definition language. IFC data files are text files following the STEP physical file format.

EXPRESS data definition language - A standard for defining the way that data is built-up and referenced

STEP physical file format - A common text language for exchanging data

Model View Definitions

IFC is a framework for exchanging construction related data. This can include data for material quantities, FM, programming and H&S. IFC Model files are developeed to contain just a part of the overall IFC framework.

MVD - Model View Definition, a defined model data set

As IFC is a framework, models are exported as a subset of IFC. The subsets are referred to as Model View Definitions (MVD). For example COBie maps to a MVD, the FM Basic Handover. When using IFC2x3 the usual MVD for exchanging design models is the Coordination View.

IFC Versions

The most widely used and implemented IFC version is IFC2x3, although the latest version is IFC4 (which is now onto the second addendum).

IFC2x3 released 2006 IFC4 Addendum 2 released 2016

The format is relatively stable with few updates. It aims to be backwards compatible with newer release mainly adding additional capabilities and fixing bugs. The schema structure is also saved within the ifc export itself, this means that format is well placed to be a long term accessible method for storage of model information.

IFC Exports

Design authoring software, such as Autodesk Revit and ArchiCAD, use their own proprietary file formats.

Exchanging model data between organisations using the same authoring software is easy as the native files can be read by the applications.

However, not everyone involved in a project will have access to the originating design authoring software and neither should the design authoring software be imposed upon a team. This where the open IFC file format fills an important role.

Projects rules (through a well defined EIR - Employer's Information Requirements and BEP - BIM Execution Plan) should detail the required file formats for exchanging model data.

Where IFC files are required (all the projects that I work with require IFC), the designers exchanging information will provide an export of their design model as an IFC file. This can be a complex task as the data from the original model will require mapping to the IFC format. Although, this has now been largely automated either natively through the authoring software or through add-ins. Some mapping may be still be required for unique data parameters.

IFC Structure

IFC is a text file so the data is readable. Although, when first viewed it may not make complete sense.

An IFC has some distinct sections:

  • File Header
  • File Data
  • Project Spatial Data
  • Element or Object Data

The file header contains information such as the file name, the IFC version and the IFC MVD. The file data section provides author information and defines the units. Project spatial data and object data are contained within the same overall section. But they are distinct in what they provide. Spatial data defines the shape, location and physical characteristic of a graphical object. Object data provides parametric data, such as colour, material properties and thermal properties.