The abstract entity IfcStructuralActivity combines the definition of actions (such as forces, displacements, etc.) and reactions (support reactions, internal forces, deflections, etc.) which are specified by using the basic load definitions from the IfcStructuralLoadResource. 

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 SELFIfcProduct.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. 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. 

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

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

Instances of IfcStructuralItem 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 is used: 

• Point connections are 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 can 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 is _IfcVertexPoint_s with underlying _IfcCartesianPoint_s. The following labels are used in the IfcTopologyRepresentation:
  • RepresentationIdentifier: 'Reference'
  • RepresentationType: 'Edge'
• Surface members and surface connections is 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 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 coordinate 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.

changes 

• IFC4 changes:
  • Type of the inverse attribute AssignedToStructuralItem changed from

"IfcRelConnectsStructuralActivity FOR RelatedStructuralActivity" to "SET OF [0:1] IfcRelConnectsStructuralActivity FOR RelatedStructuralActivity"