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IFC4 RV Scope

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by courtesy of CAD Stelle Bayern

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IFC4 Reference View

The IFC4 Reference View targets all work flows that are based on reference models. Read more:

 

General scope of the IFC4 Reference View

The general scope defines the main functionalities of the IFC4 Reference View as an overview. It includes a complete listing of the model elements and model element types that are included in the IFC4 Reference View Model View Definition.

NOTE  The Model elements are referred to as "Root Concepts" within the Model View Definition specification, being the individual root elements, that contain the attributes, geometric shapes, dynamic property sets and other semantic information that are combined and expressed as "Concepts". The common definition of a "Concept", that is applicable to many "Root Concepts" is called a "Concept Template".

The detailed scope of the IFC4 Reference View is determined by the concept templates that are included. A detailed description of each concept template is provided by Chapter 4 "Fundamental concepts and assumptions" listed in the MVD specification delivery.

Model elements included in the IFC4 Reference View

Model elements

The main components of the IFC4 Reference View are the semantic model elements that carry a predefined meaning. The complete breakdown of all model elements declared in IFC4 are also known as the IFC4 built-in classification following an element by function classification.

In addition to each of the model elements shown here in the subsequent tables, each Model element maybe further specialized by its "PredefinedType", or even a user defined type.

EXAMPLE An IfcFireSuppressionTerminal is a specific model element, that may be further specialized using its PredefinedType enumeration being: a sprinkler, a hose reel, a fire hydrant, or a breeching inlet. If a proper predefined type is not yet included in the specification, a user defined type can be assigned as well.

Model element types

Model element types are part of the IFC4 Reference View. They enable to describe and share common model element information that are shared by multiple occurrences of the same type. Sharable type information includes:

  • Geometric shape representation;
  • Property information;
  • Material information.
EXAMPLE A particular air outlet as an article, with its shape, its material and its manufacturer information being described once as a type and then having several occurrences, each placed within the building, referencing the same type and hence its shape, material and properties.
MVD concepts included in the IFC4 Reference View

Object attribute

All model elements, listed in the previous section, are defined by several generic and direct object attributes, some specific model elements do carry additional direct attributes. The usage of the direct generic attributes is defined within the following concept templates (see also Chapter 4 "fundamental concepts and assumptions"):

  • "Software Identity", it defines how to apply the Globally Unique Id and how to compress it during exchange;
  • "User Identity", it defines the meaning of Name, Description, LongName, and Tag attributes;
  • "Object Occurrence Predefined Type", it defines how to use the PredefinedType and in case of user defined types, how to assign the user defined type information within the ObjectType attribute for occurrences of model elements;
  • "Element Type Predefined Type" it defines how to use the PredefinedType and in case of user defined types, how to assign the user defined type information within the ElementType attribute for types of model elements.

Project context

There is a single instance of IfcProject within each IFC data set. It sets the context for the exchange, including units, geometric context, global positioning and classification systems used within the data set. The usage of the project context is defined within the following concept templates (see also Chapter 4 "fundamental concepts and assumptions"):

  • "Project Units", declaration of the units used within this data set, all geometric representations are forced to use the global units (e.g. for length measure and plane angle measure), property values may override global unit declarations;
  • "Project Representation Context", declaration of the 3D and 2D representation contexts, including precision factors;
  • "Project Global Positioning" (new in IFC4), positioning the project engineering coordinate system (right handed Cartesian coordinate system) within a global coordinate reference system;
  • "Project Classification Information", providing the name and version information about the classification systems used within the data set.

Object definition

A main objective of the IFC4 Reference View is to enable rich information content for each model element. Model element occurrences can refer to their model element types for sharing common information. General properties are attached to model elements as property sets, either directly to the model element occurrence, or to its type. Individual model element occurrences can hold their quantities, if those are pre-calculated by the sender of the IFC data set. The usage of the object definition is defined within the following concept templates (see also Chapter 4 "fundamental concepts and assumptions"):

  • "Object Typing", associates the Model element occurrences with the corresponding element type;
  • "Property Sets", assigns dynamically defined property sets, holding set of individual properties, to the model element occurrences of element type;
  • "Quantity Sets", assigns dynamically defined quantity sets, holding set of individual quantities, to the model element occurrences.
Object typing

The concept template describes the mechanism of associating model element occurrences to the corresponding type and the way and restrictions of overriding type based properties by properties directly assigned to the model element occurrences.

Property sets

Property sets hold, as the name suggests, a set of properties grouped by a common theme. Each individual property has:

  • a name
  • an optional description
  • a value, or number of values, of a given datatype
  • a unit or the information of being unitless

The semantic meaning of each property is provided by its name. Properties, that are semantically declared within the scope of the IFC4 Reference View, are based on a property definition template that is published as an instrinct part of the Model View Definition. Extensions to the property definitions can also be defined outside the property definition scope of the IFC4 Reference View, however the name prefix for property sets "Pset_" is restricted to properties defined within the original scope of IFC.

There are two ways to declare property templates:

  • using the Property Set Definition PSD schema, an XML schema developed independent of the IFC specification,
  • using the newly introduced IFC4 property set template and property template
NOTE The PSD Schema has been used since many earlier versions of the IFC standard and has a broad legacy. The newer property set template definitions are now part of the IFC schema and can therefore be embedded within an IFC data set directly. Both schemas can be mapped without information loss.
Quantity sets

Quantity sets hold, as the name suggests, a set of quantities pre calculated for the model element occurrence. Each individual quantity has:

  • a name
  • an optional description
  • a value of a given datatype corresponding to the quantity measure (length, area, volume, weight, time
  • a unit
  • a quantity formula, describing how the quantity value was calculated

The semantic meaning of each quality is provided by its name. Quantities, that are semantically declared within the scope of the IFC4 Reference View, are based on a quantity definition template that is published as an instrinct part of the Model View Definition. Extensions to the quantity definitions can also be defined outside the quantity definition scope of the IFC4 Reference View, however the name prefix for quantity sets "Qto_" is restricted to quantities defined within the original scope of IFC.

Object association

In addition to the Property sets and the Quantity sets, also a classification reference to an external classification system can be assigned, and material as either single material, a material constituent sets or an material layer sets or material profile sets combining material information with dimensions can be associated to one or many model elements.

NOTE Material dimensions are layer thicknesses, or profile geometries for e.g. a column with an embedded steel profile and a concrete protection. Within the IFC4 Reference View, such material dimensions are used exclusively as alphanumeric information, and not as part of a dimension driven parametric shape representation.

The usage of the object association is defined within the following concept templates (see also Chapter 4 "fundamental concepts and assumptions"):

  • "Material Association"; assigns a material (or material set - constituent, layer, profile) to one or several model elements (either to element occurrences, or as shared material information to element types).
  • "Classification Association"; assigns a classification reference to one or several model elements.

Product shape

The first main objective of the IFC4 Reference View is to enable the exchange of highly accurate, not non-parametric, geometric representations of the model elements. Each model element is placed directly or indirectly within the project coordinate system, defining its own object coordinate system. The geometric representation items describing its shape are positioned within this object coordinate system.

In order to minimize the effort for receiving and interpreting the geometric representations by the receiving software systems, in terms of development effort, processing power and loading times, the complexity and variety of geometric models has been minimized for the IFC4 Reference View.

Product placement

Each model element defines its own object coordinate system. The placement is defined by the following concept template:

  • "Product Local Placement", creating an object coordinate system for the shape representation of the model element, either absolutely to the project engineering coordinate system, or relatively to another object coordination system.
Product geometric representation

In scope of geometric shape representations of the 3D body geometry of physical and spatial elements are the following concept templates:

  • "Body Tessellation Geometry", using tessellated geometry in form of triangulated tessellations for describing the body shape of the model element;
  • "Body SweptSolid Geometry"; using extruded solid geometry or revolved solid geometry for describing the body shape of the model element;
  • "Body AdvancedSweptSolid Geometry"; using advanced swept solid geometry of circular cross sections for describing the body shape of the model element, only the swept disk solid is in scope;

It is the default geometric representation of all model elements, allowing for a surface model representation with an indicator for closed shells (and therefore true volumes). The tessellated representation offers a very efficient way of exchanging 3D shape date, both for data set sizes and for processing time. Optionally the face normals can be exchanged as well.

Since curved shapes would lead to very densely triangulated areas, the following swept solid based representations are also in scope of the IFC4 Reference View, balancing simplicity and compactness of representation:

All other geometric models are out of scope of the IFC4 Reference View, in particular Boolean operations required for Constructive Solid Geometry CSG.

NOTE The IFC2x3 Coordination View included CSG capabilities, the IFC4 Reference View therefore imposes a more restricted geometric representation of model elements. The IFC4 Design Transfer View should be used, if more complex geometric representations are required by the workflow. In particular, if a dimension-driven parametric representation, used by the IFC4 standard case elements, is needed.

The geometric shape representation can either be directly assigned to a model element, or to its type. In case of type-based geometry, a the following representation type is used at the model element using the following concept template:

  • " Mapped Geometry", mapped representation defined at the corresponding element type. A mapped representation uses Cartesian transformation operations to place the type-based geometry within its object coordinate system.

As an exception, the following elements, IfcGrid, IfcSpace, and IfcSpatialZone may have an additional foot print 2D geometry (in case of IfcGrid this is the only geometric representation. It is described in the following concept template:

  • " FootPrint Geometry", defining a 2D shape representation within the XY plane of the object coordinate system.
Geometric presentation

Visual appearance is an important factor for the communication process using BIM data. The objective is not to support photo-realistic rendering of reference models, but to use color, basic rendering, and texture information to add visually accessible meaning to the model elements.

In scope of presentation capabilities for the appearance of model element shapes are the following partial concept templates:

  • " Surface Style Shading", applying a single coloring for each solid;
  • " Surface Style Rendering", applying a single rendering (color, transparency, reflection, etc.) for each solid;
  • " Surface Style Textures", applying a single texture for each solid according to a texture mapping based on the solid type;
  • " Suface Style Tessellation", applying a color and/or texture for each face of a tessellated solid.

The visually adequate presentation of model elements is constraint by the shape representation

  • for tessellated geometry: color per face, texture per face
  • for swept solid geometry: color and rendering information per solid, texture applied to solid using standard mapping

Object Composition

The object composition functionality describes the product breakdown structure of model elements within an IFC data set, with separate breakdown structures for physical elements and spatial elements. Physical element structures describe parts and assemblies, spatial element structures describe vertical structures (for buildings) and horizontal structures (for other assets - as a stub in this release). A specific type of decomposition is the voiding - a subtraction of a void from a physical element. Another specific type is the nesting of ports within a distribution elements.

The usage of the object association is defined within the following concept templates (see also Chapter 4 "fundamental concepts and assumptions"):

  • "Element Decomposition", creating a hierarchical product breakdown structure relationship between assemblies and parts;
  • "Spatial Decomposition", creating a hierarchical spatial decomposition relationship between spatial structure elements;
  • "Element Voiding", creating a voiding relationship between a physical element and penetrating voids - within the scope of the IFC4 Reference View this relationship is a logical relationship, the void is already part of the geometry of the physical element,
  • "Port Nesting", creating an 1:N relationship between the physical element and one or many ports defining inlets or outlets - used for distribution elements.

Object Assignment

The object assignment defines the assignment of objects, such as a link between model elements to groups, tasks or resources. Only the grouping assignment is in scope of the IFC4 Reference View and defined within the following concept template:

  • "Group Assignment", Assignment of one or several model elements to a group. It includes the more specific assignments of Grouping General, Grouping to System, and Grouping to Zones.

Object Connectivity

The object connectivity defines the interlinkage between model elements. Examples are the link between physical elements and the spatial structure, where they are located, of the connection between the two ports of two consecutive distribution elements.

The usage of the object association is defined within the following concept templates (see also Chapter 4 "fundamental concepts and assumptions"):

  • "Spatial Structure", defines the containment of a physical element within a spatial container;
  • "Port Connectivity", defines the connection and the direction of flow between two ports of consecutive distribution elements;
  • "Building Service Connectivity", links a spatial or distribution system to a spatial structure (such as a building section);
  • "Element Filling", links a filling (usually a door or window) to an opening (usually in a wall or slab).