The projection element is a specialization of the general feature element to represent projections applied to building elements. It represents a solid attached to any element that has physical manifestation. An IfcProjectionElement has to be linked to a element (all subtypes of IfcElement) by using the IfcRelProjectsElement relationship. Its existence depends on the existence of the master element. The relationship implies a Boolean union operation between the volume of the projection element and the volume of the element. 

The IfcProjectionElement does not participate in the containment relationship, i.e. it is not linked directly to the spatial structure of the project. It has a mandatory ProjectsElements inverse relationship pointing to the IfcElement that is contained in the spatial structure.

• The inverse relationship ContainedInStructure should be NIL.

The geometric representation of IfcProjectionElement can be defined using the swept area solid geometry. The following attribute values for the IfcShapeRepresentation holding this geometric representation are used:

• IfcShapeRepresentation.RepresentationIdentifier: 'Body'
• IfcShapeRepresentation.RepresentationType: 'SweptSolid'

The following constraints are recommended:

• IfcShapeRepresentation.Items can include a single, or multiple, instances of IfcExtrudedAreaSolid.
• IfcExtrudedAreaSolid.SweptArea supports IfcRectangleProfileDef, IfcCircleProfileDef and IfcArbitraryClosedProfileDef.
• IfcExtrudedAreaSolid.ExtrudedDirection extrudes the profile in a direction non-parallel to the element it is projecting. This can be horizontal for wall projections or vertically for floor projections.
• If multiple instances of IfcExtrudedAreaSolid are used, the extrusion direction of each extrusion should be equal.

As shown on the image below, the following interpretation of dimension parameters applies for rectangular projection:

• IfcRectangleProfileDef.YDim interpreted as projection width
• IfcRectangleProfileDef.XDim interpreted as projection height
• IfcExtrudedAreaSolid.Depth is interpreted as projection depth

The general geometric representation of IfcProjectionElement can also be defined using the Brep or Tesselation geometry. The Brep or Tessellation representation allows for the representation of complex element shapes. The following attribute values for the IfcShapeRepresentation holding this geometric representation is used:

• IfcShapeRepresentation.RepresentationIdentifier: 'Body'
• IfcShapeRepresentation.RepresentationType: 'Tessellation' or 'Brep'

The local placement for IfcOpeningRecess is defined in its supertype IfcProduct. It is defined by the IfcLocalPlacement, which defines the local coordinate system that is referenced by all geometric representations.

• The PlacementRelTo relationship of IfcLocalPlacement should point to the local placement of the same element, to which the projection adds, i.e. referred to by ProjectsElement.RelatingBuildingElement.

This concept can be applied to the following resources:

• Qto_ProjectionElementBaseQuantities