The IfcMaterialLayerSetUsage determines the usage of IfcMaterialLayerSet in terms of its location and orientation relative to the associated element geometry. The location of material layer set shall be compatible with the building element geometry (that is, material layers shall fit inside the element geometry). The rules to ensure the compatibility depend on the type of the building element.

EXAMPLE  For a cavity brick wall with shape representation SweptSolid, the IfcMaterialLayerSet.TotalThickness shall be equal to the wall thickness. Also the OffsetFromReferenceLine shall match the exact positions between the two shape representations of IfcWallStandardCase, that is the IfcShapeRepresentation's with RepresentationIdentifier="Axis" and RepresentationIdentifier="Body".

NOTE  Model view definitions or implementer agreements may provide more instructions on matching between building element geometry and material layer set usage.

The IfcMaterialLayerSetUsage is always assigned to an individual occurrence object (and only to relevant subtypes of IfcElement). The IfcMaterialLayerSet, referenced by ForLayerSet, can however be shared among several occurrence objects. If the element type is available (in other words, an instance of the relevant subtype of IfcElementType exists), then the IfcMaterialLayerSet can be assigned to the element type. The assignment between a subtype of IfcElement and the IfcMaterialLayerSetUsage is handled by IfcRelAssociatesMaterial.

Attribute use definition

The IfcMaterialLayerSetUsage is primarily intended to be associated with planar building elements having a constant thickness. With further agreements on the interpretation of LayerSetDirection, the usage can be extended also to other cases, for example to curved building elements, provided that the material layer thicknesses are constant.

Generally, an element may be layered in any of its primary directions, denoted by its x, y or z axis. The geometry use definitions at each specific type of building element will determine the applicable LayerSetDirection.

The following examples illustrate how the IfcMaterialLayerSetUsage attributes (LayerSetDirection, DirectionSense, OffsetFromReferenceLine) can be used in different cases. Normative material use definitions are documented at each element (how these shall be used).

Figure 379 shows an example of the use of IfcMaterialLayerSetUsage aligned to the axis of a wall.

EXAMPLE  For a standard wall with extruded geometric representation (vertical extrusion), the layer set direction will be perpendicular to extrusion direction, and can be derived from the direction of the wall axis. With the_DirectionSense_ (positive in this example) the individual IfcMaterialLayers are assigned consecutively right-to-left or left-to-right. For a curved wall, "direction denoting the wall thickness" can be derived from the direction of the wall axis, and it will remain perpendicular to the wall path. The DirectionSense applies as well.

NOTE  According to the IfcWallStandardCase material use definition the LayerSetDirection for IfcWallStandardCase is always AXIS2 (that is, along the y-axis), as shown in this example.

Figure 379 — Material layer set usage for wall

Figure 380 shows an example of the use of IfcMaterialLayerSetUsage aligned to a slab.

EXAMPLE  For a slab with perpendicular extruded geometric representation, the LayerSetDirection will coincide with the extrusion direction (in positive or negative sense). In this example, the material layer set base is the extruded profile and consistent with the IfcExtrudedAreaSolid.Position, with the DirectionSense being positive, the individual IfcMaterialLayers are built up from the base towards the positive z direction in this case.

NOTE  According to the IfcSlabStandardCase material use definition the LayerSetDirection for IfcSlabStandardCase is always AXIS3 (that is, along the z-axis).

Figure 380 — Material layer set usage for slab

Figure 381 shows an example of the use of IfcMaterialLayerSetUsage aligned to a roof slab with non-perpendicular extrusion.

EXAMPLE  For a slab with non-perpendicular extruded geometric representation, the guidelines above apply as well. The material layer thickness and the OffsetFromReferenceLine are always measured perpendicularly, even if the extrusion direction is not perpendicular. Therefore the total material layer thickness is not equal to the extrusion depth of the geometry.

Figure 381 — Material layer set usage for roof slab