Understanding IFC

IFC (Industry Foundation Classes) is a data model developed by buildingSMART International to describe the physical components of all built assets in openBIM workflows. It was mainly developed for coordination purposes but can also be used for other use cases, such as structural or energy analysis, cost breakdowns, and work or maintenance schedules.

The official documentation by buildingSMART serves as the implementation guideline for software providers. However, it can seem complex to engineers and designers without programming expertise.

When using IFC for data exchange in openBIM workflows, it is important to consider which version, which Model View Definition (MVD), and which file format to use. These requirements are based on the intended use cases and are typically defined by the client as part of the Employer’s Information Requirements (EIR) or by the BIM Manager in the BIM Execution Plan (BEP).

This video provides a simple introduction to IFC for Autodesk Revit users: ▶ Basic IFC Concepts for Revit users

IFC File Formats

The IFC data schema is alphanumeric and can be stored in different file formats. The following file formats are commonly used and supported by Revit:

.IFC

The standard format is based on STEP (Standard for the Exchange of Product Model Data), which is usually the first and best choice.

.IFCZIP

Compressed (zipped) IFC file with a smaller size; valid import format for most software applications supporting IFC. It can be unzipped to reveal the original IFC file or also created manually by compressing an existing IFC file.

.IFCXML

Specific calculation software can require XML-based representation of IFC data, though it is not widely used.

.IFCXMLZIP

Compressed equivalent to .IFCZIP.

Geometric Representation in IFC

BIM and IFC deal primarily with data and information, although geometry is also crucial. Understanding how geometry is described is important, as this can significantly impact the performance and file size of the IFC file. The IFC format is based on STEP and solid geometry, generated using specific methods.

Extrusions

These are the most common and simple graphical methods and are used in most cases where a simple profile can describe the shape.

Swept Solids

As the name implies, an element is created with the swept solid method using a sweep. In this case, a profile is swept along a path (direction vector) to generate the solid. This profile may change due to rotation or distortion along the path. Revit uses this method to describe various shapes that cannot be described with extrusions (e.g., rebar).

B-rep

The boundary representation (B-rep) method can also be described as a boundary surface model. A component’s surfaces are described by coordinates and together represent the actual solid, which allows the representation of more complex shapes. B-rep objects require complex calculations to display the individual surfaces and consume more memory during the export.

NURBS (new in IFC4 DTV)

IFC4 can describe complex shapes using NURBS (non-uniform rational B-splines) surfaces. This considerably reduces the memory requirements compared to B-reps while significantly increasing the quality.

Important: NURBS are not supported by the IFC4 Reference View typically used in coordination workflows. They are a part of the IFC4 Design Transfer View, which is still under development by buildingSMART.