IFC file structure: the IfcObjectDefinition

The IFC file format is not only an exchange open file format, but also as a true interoperability standard with a precise data structure.

In practice, the IFC schema is consider a storage system organizing and transferring digital data to facilitate interoperability between the various BIM professionals within a given workflow.

You can use an IFC viewer online to view these files with any device using a simple browser.

Here are the links to the 3 contents on the IFC architecture article on:

IfcObjectDefinition;
IfcRelationship;
IfcPropertyDefinition.

The schema of an IFC model

An IFC model integrates with hundreds of entities that are organized in a hierarchical manner.

Some examples of these entities are:

• construction elements, such as IfcWall, IfcWindow , IfcDoor;
• geometric elements, such as IfcExtrudedAreaSolid (solid extrusion from an area);
• basic constructs, such as IfcCartesianPoint.

The IFC logic is based on 3 fundamental concepts, which represent the structure itself:

• The IfcObjectDefinition defining the entities themselves (objects);
• The IfcRelationship defining the relationship between entities;
• The IfcPropertyDefinition defining the properties associated with the entities.

In this week’s article we’ll be focusing on entities, while in the upcoming posts we’ll also cover the remaining concepts. These 3 links refer to the IFC architecture contents:

1. article on IfcObject (IFC schema part 1);
2. article on IfcRelationship (IFC schema part 2);
3. article on IfcPropertyDefinition (IFC schema part 3).

The IFC Data schema

IfcObjectDefinition

The IfcObjectDefinition class contains all the useful objects to fully describe a construction asset.

The word object refers to an abstract or tangible entity that represents the description of a construction part that we are digitizing. Let’s see some examples..

The IfcObject can include:

• physically tangible objects, such as walls, beams or roofs
• physically existing objects, such as spaces or conceptual objects, like grids or virtual borders
• processes, such as work tasks and controls, like cost items, actors or people involved in the design process, etc.

Going into detail, the schema also identifies 6 fundamental concepts that are directly connected to the IfcObject class:

1. The IfcActor defines all the actors involved in the construction process. It easily identifies actors and it includes information such as name, address, organizations and roles
2. The IfcControl is the abstract generalization of all the concepts that control or limit the use of products, processes or resources in general. Therefore, it represents the design constraints. For example, within this class we can include the elements that influence project planning, for instance:
   • client’s requests;
   • project costs;
   • regulations and standards;
   • delivery times.
3. IfcGroup is a generalization of any arbitrary group. A group is a logical collection of objects, it does not have its own position, nor it can contain its own shape representation. Therefore, a group is a topological aggregation. For example, we can associate a selection of elements of a structural frame in a group that are warped according to a specific direction.
4. The IfcProcess represents activities, events and procedures that occur over time. The IfcProcess is defined as a single activity or event, that is:
   • ordered in time;
   • has sequence relationships with other processes;
   • transforms input into output.
5. IfcResource contains the information needed to represent costs, planning and other impacts from the use of an asset in a process. For example, for cost estimating purposes, construction equipment such as vehicles or earthworks machinery, which are not currently modelled within the IFC, these can be considered as an IfcResource.
6. The IfcProduct includes any entity that can be related to a geometric / spatial context. Each entity has a specific location in space when it has a geometric representation assigned. This representation exists in relation to other entities, but it will certainly refer with respect to the project coordinate system. In addition to real entities, the IfcProduct also includes non-physical elements, such as grids, notes, structural actions.

The IFC file structure | IfcObjectDefinition scheme subdivision

IfcProduct

Looking further into theIfcProduct, we can say that it is the base class for all the entities of a project and is therefore made up of spatial elements, physical elements, structural analysis elements and other concepts. Products can have materials associated, shape representations and position in space. Let’s see some examples:

• Spatial elements include:
  • IfcSite;
  • IfcBuilding;
  • IfcBuildingStorey;
  • IfcSpace.
• Physical construction elements include:
  • IfcWall;
  • IfcBeam;
  • IfcDoor;
  • IfcWindow;
  • IfcStair.

IFC file structure | IfcProduct class distribution scheme

Here’s a screenshot of an example of IfcProduct and of the window entity with its relating properties within the usBIM.browser working space.

Identification of an IfcObject (ifcwindow) and its properties with usBIM