The abstract entity IfcStructuralItem is the generalization of structural members and structural connections, that is, analysis idealizations of elements in the building model. It defines the relation between structural members and connections with structural activities (actions and reactions).

Relationships between elements in the building model and structural items as their idealizations can be expressed by instances of IfcRelAssignsToProduct.

HISTORY  New entity in IFC2x2.

IFC4 CHANGE  Use definitions and informal proposition added.

Coordinate Systems:

The following coordinate systems are distinguished:

• The so-called global coordinate system is the coordinate system shared by all items and activities which are grouped in a common IfcStructuralAnalysisModel. This coordinate system is established by SELF\IfcProduct.ObjectPlacement. (This coordinate system is not necessarily the same as the IfcProject's world coordinate system.)
• The so-called local coordinate system is a coordinate system local to a structural item (connection or member). This coordinate system is established by a Representation (attribute inherited from IfcProduct) in conjunction with further use definitions and attributes of subtypes of IfcStructuralItem.

Representation items in topology representations are always given within the ObjectPlacement, i.e. in so-called global coordinates (global with respect to the IfcStructuralAnalysisModel to which this item belongs).

The usage of local coordinate systems is further defined in subtypes.

Topology Use Definitions:

Instances of IfcStructuralItem shall have a topology representation. It includes a placement and a product representation. The IfcProductRepresentation shall be given by an item in a Representation of type IfcTopologyRepresentation.

Local Placement

The local placement for IfcStructuralItem is defined in its supertype IfcProduct. It is defined by the IfcLocalPlacement, which establishes a global coordinate system which shall be common to all items and activities in an IfcStructuralAnalysisModel.

Topology Representation

Instances of IfcStructuralItem shall have a topology representation given by an instance of a subtype of IfcTopologicalRepresentationItem, which should be the single item of IfcTopologyRepresentation.Items. Depending on the dimensionality of the structural item, one of the following types of toplogical representation items shall be used:

• Point connections shall be represented by an IfcVertexPoint with an underlying IfcCartesianPoint. The Cartesian point is the reference point of the connection in the so-called global coordinate system. The following labels are used in the IfcTopologyRepresentation:
  • RepresentationIdentifier: 'Reference'
  • RepresentationType: 'Vertex'
  
  
• Curve members and curve connections shall either be represented by an IfcOrientedEdge, IfcEdgeCurve, or IfcEdge. The curve to which the IfcEdgeCurve (or an IfcOrientedEdge's underlying IfcEdgeCurve) refers to is the reference curve of the structural item in the global coordinate system. Start and end vertex of the edge shall be IfcVertexPoints with underlying IfcCartesianPoints. The following labels are used in the IfcTopologyRepresentation:
  • RepresentationIdentifier: 'Reference'
  • RepresentationType: 'Edge'

  NOTE  While an IfcEdge (or IfcOrientedEdge with underlying IfcEdge) does not provide an explicit underlying curve geometry, it may be used to imply an underlying straight line as reference curve with the origin of the curve parameter at the start vertex point.

• Surface members and surface connections shall be represented by an IfcFaceSurface. The underlying surface defines the reference surface of the structural surface item in the global coordiante system. All edges in the bounds of the face shall conform to the rules for edge representations of structural curve item. The following labels are used in the IfcTopologyRepresentation:
  • RepresentationIdentifier: 'Reference'
  • RepresentationType: 'Face'

The reference point, reference curve, or reference surface partially or completely defines the local coordinate system of the represented structural item according to the following rules. In all cases, The local x,y,z directions form a right-handed Cartesian coordinate system.

Structural point items

• The reference point in the representation is the origin of the local coordinate system of the structural item.
• The axes of the local coordiante system are either parallel with and directed like the so-called global coordinate axes, or are oriented according to definitions at the respective subtype of IfcStructuralItem.

Structural curve items

• The u parameter origin of the reference curve in the representation is the origin of the local coordinate system of the structural item.
• The local x axis is parallel with the tangent on the curve and directed like the u parameter direction.
• The local y and z axes are oriented according to definitions at the respective subtypes of IfcStructuralItem.

Structural surface items

• The u,v parameter origin of the reference surface in the representation is the origin of the local coordinate system of the structural item.
• The local x and y directions follow the tangents on the surface and are in parallel with and directed like u and v respectively.
• The local z direction is in parallel with and directed like the surface normal.

Informal Propositions:

1. The ObjectPlacements of all structural items which are grouped into the same instance of IfcStructuralAnalysisModel shall refer to the same instance of IfcObjectPlacement.

NOTE  This rule is necessary to achieve consistent topology representations. The topology representations of structural items in an analysis model are meant to share vertices and edges und must therefore have the same object placement.

NOTE  A structural item may be grouped into more than one analysis model. In this case, all these models must use the same instance of IfcObjectPlacement.