Industry Foundation Classes (IFC) - Polimi.it

Industry Foundation Classes (IFC)

Industry Foundation Classes (IFC) Intended to describe building and construction industry data Platform neutral and open file format specification An object-based file format intended to facilitate interoperability in the architecture, engineering and construction (AEC) industry A commonly used collaboration format in BIM based projects An official International Standard ISO 16739-1:2018 Managed by BuildingSmart

Industry Foundation Classes (IFC) IFC is a definition of a standard format to describe a Building Information Model IFC defines how information should be provided/stored for all stages of a building projects lifecycle It goes right down to the properties of the individual object It can do from “very little” information to “everything” IfcDoor represents a door modelled in BIM software In IFC structure, it has many properties which can be defined and updated in BIM software

Industry Foundation Classes (IFC) 100Ks of lines about the BIM model. Impossible to read and understand.

Industry Foundation Classes (IFC)

Industry Foundation Classes (IFC) The information in IFC can be grouped in Graphical 3D Model Information Numerical and Textual Information

Industry Foundation Classes (IFC) Each building entity has a Global Unique Identification Number (GUID) BIM applications store and query the specific entities based on GUIDs

Views No application implements the whole IFC specification Data exchange is achieved with subsets, called views Each IFC View Definition, or Model View Definition (MVD) defines a subset of the IFC schema that is needed to satisfy one or many Exchange Requirements

Conceptual data model: EXPRESS IFC defines an EXPRESS based entity-relationship model consisting of several hundred entities organized into an object-based inheritance hierarchy EXPRESS applies the general notions of conceptual models Classes (called Entities) Attributes Associations (called Relationships) Generalization Examples of entities include building elements such as IfcWall, geometry such as IfcExtrudedAreaSolid, basic constructs such as IfcCartesianPoint

Physical Models IFC defines multiple file formats that may be used IFC-SPF (Step file) is a text format defined by ISO 10303-21, having file extension ".ifc" each line typically consists of a single object record it is the most widely used IFC format, (compact size yet readable text) IFC-XML is an XML format defined by ISO 10303-28, having file extension ".ifcXML“: this format is less common in practice

EXPRESS G symbols

EXPRESS: Attribute and data type Attributes are defined as simple data types , e.g. integer, string, logical, boolean or more complex types as, e.g. other entities A solid line shows a mandatory attribute, whereas a dashed line shows an optional attribute

Select data type A data type select allows the possibility to choose exactly one of a defined variety of, e.g. other data types or entities

Relationship A relationship has cardinality and name a relationship is shown as a solid or dashed thin line which is terminated by a circular arrowhead The direction of a relationship is towards the arrowhead, which is important in that the name of the relationship must reflect its direction A solid line indicates a mandatory relationship, whereas a dashed line indicates optional relationship

Cardinality If a relationship has no cardinality, the cardinality is assumed to be exactly one C[m:n], where C is one of S(set), B(bag), L(list) or A(array) A set is an unordered variable length collection of unique items A bag is an unordered variable length collection of not necessarily unique items A list is an ordered variable length collection of not necessarily unique items. There are possibilities to constraint the list to a list of unique items An array is a fixed size collection of not necessarily unique items which can be accessed by an index. If the cardinality is shown as a single number (n), then it is assumed to be S[n:n] ? indicates ”unbounded”

Examples

Inverse Relationship

Generalization and specialization The relationship between a supertype and its subtypes is shown by using a series of thick lines, each one terminated at the subtype end with an open circle

Abstract Supertypes An abstract supertype is a supertype entity which cannot be instanced

Multiple Inheritance

EXPRESS - Textual form SCHEMA Family; ENTITY Person ABSTRACT SUPERTYPE OF (ONEOF (Male, Female)); name: STRING; mother: OPTIONAL Female; father: OPTIONAL Male; END ENTITY; ENTITY Female SUBTYPE OF (Person); END ENTITY; ENTITY Male SUBTYPE of (Person); END ENTITY; END SCHEMA

Wall 3 Project P Site X Space V Representation of a Building Wall 2 Building B odel Representation in IFC Storey S Wall 4 Wall 3 Space V Wall 2 Slab 1 Wall 1 Space V Slab 1 Wall 1 Wall 2 Wall 3 Wall 4 Slab 1 Building B Opening 1 Opening 2 Opening 3 Door 1 Window 1 Window 2 Pset 1 Storey S Space V Pset 2 Wall 1

From EXPRESS to IFC-SPF A step file has a header section and a data section The DATA section contains application data according to one specific express schema The encoding of data follows some simple principles Every entity instance in the file has a unique name in the form "#1234" Instances of each entity data type are represented by writing the name of the entity in capital letters and then followed by the attribute values in the defined order within parenthesis Unset attribute values are given as " “ The elements of aggregates (SET, BAG, LIST, ARRAY) are given in parentheses, separated by ","

Example SCHEMA example; TYPE date ARRAY [1:3] OF INTEGER; END TYPE; TYPE hair type ENUMERATION OF (.); END TYPE; ENTITY person SUPERTYPE OF(ONEOF(female,male)); first name, last name : STRING; nickname : OPTIONAL STRING; birthdate : date; children : SET[0:?] OF person; hair : hair type; . . #10 FEMALE( ‘Sara’, ‘Rossi’, , #20, (#30,#40), .NERO.,); #20 DATE((5,25,66)); #30 PERSON(‘Giacomo’, ‘Rossi’, ‘Giac’, #50, ( ), .BIONDO.); #40 PERSON(‘Elisa’, ‘Rossi’, ‘Lisa’, #60, ( ), .BIONDO., ); #50 DATE((7,2,89)); #60 DATE((10,29,90)); #70 MALE( ‘Giorgio’, ‘Rossi’, , #80, (#30,#40), .CASTANO.); #80 DATE((6,10,65)); #90 MARRIED( #70, #10 ); Giorgio Rossi, born 6,10,65, has 2 children(Giacomo e Elisa), his hair is CASTANO And is married with Sara Rossi

IFC Schema at 4 levels General and abstract definitions Global structure of the Project Site, building, storey, Building elements Wall, slab, roof, door, window, . Geometry

Some fundamental concepts IfcActor represents people or organizations IfcControl represents rules controlling time, cost, or scope such as work orders IfcGroup represents collections of objects for particular purpose such as electrical circuits IfcProduct represents occurrences in space such as physical building elements and spatial locations IfcProcess represents occurrences in time such as tasks, events, and procedures IfcResource represents usage of something with limited availability such as materials, labor, and equipment

IfcRelationship IfcRelationship is the abstract generalization of relationships Objectified relationships are the preferred way to handle relationships among objects This allows one to keep relationship specific properties directly at the relationship and opens the possibility to handle relationship-specific behaviors There are 1-to-1 relationships and 1-to-many relationship The two sides of the objectified relationship are named Relating name of relating object and Related name of related object In case of the 1-to-many relationship, the related side of the relationship shall be an aggregate SET 1:N

Five fundamental IfcRelationship(s) IfcRelDecomposes/IfcRelAggregates captures a wholepart relationship having exclusive containment Building and floors IfcRelAssigns captures assignment relationships where one object consumes the services of another object Task assigned to a building element IfcRelConnects indicates connectivity between objects A pipe connected to a sink IfcRelAssociates indicates external references for an object An external IFC library file where an object is defined IfcRelDefines indicates an instanceof relationship A pipe segment being of a particular type

IfcProduct IfcProduct is the base class for all physical objects and is subdivided into spatial elements, physical elements and other concepts Products may have associated materials, shape representations, and placement in space Spatial elements include IfcSite, IfcBuilding, IfcBuildingStorey, and IfcSpace Physical building elements include IfcWall, IfcBeam, IfcDoor, IfcWindow, IfcStair, etc. Distribution elements (HVAC, electrical, plumbing) have a concept of ports where elements may have specific connections for various services, and connected together using cables, pipes, or ducts to form a system Various connectivity relationships are used for building elements such as walls having openings filled by doors or windows

IfcProject IfcProject encapsulates an overall project and indicates the project name, description, default units, currency, coordinate system, and other contextual information IfcProject is also the uppermost container class instance to which all products (as instances of IfcProduct or more specifically IfcElement) need to relate directly or indirectly IfcProject establishes the representation context for the IfcRepresentation (and the subtype IfcShapeRepresentation) A valid IFC file must always include exactly one IfcProject instance, from which all other objects relate directly or indirectly A project may include multiple buildings, multiple participants, and/or multiple phases according to the particular use

Hierarchical organization

Geometric representation models Boundary Representation (B-rep): each geometry is represented by its boundary Constructive Solid Geometry (CSG): creates a complex object by using Boolean operators to combine simpler objects; In Ifc the elementary objects are defined through: mathematical primitives Swept areas Revolved areas

B-rep Example B-Rep models represent a solid indirectly by a representation of its bounding surface

CSG Example (from Wikipedia) Mathematical definition

Placement Placement may indicate position, vertical angle, and horizontal angle IfcLocalPlacement indicates placement relative to an enclosing element hierarchy IfcGridPlacement indicates placement relative to a grid with user-defined axes Quantities may be defined for takeoff purposes such as Gross Area, Gross Volume, Gross Weight, Net Weight, etc. IFC defines various quantities specific to each element type and the method of calculation according to geometry and relationships

IfcOpenShell-python An third-party library that helps to read and modify IFC Step files Free and open source Can be used to convert IFC files into explicit geometry inputs for CAD-like software Not documented but many operations can be derived from the IFC specification 2x3/TC1/HTML/

Installation Not available on pip Download source files that match your Python setup http://ifcopenshell.org/python Copy the downloaded files into the Python sitepackage folder You can retrieve the folder path by running this script import site print(site.getsitepackages())

Opening a file import ifcopenshell ifc file ifcopenshell.open("example1.ifc")

Main operations on file by type(t): returns all the instances of type t by guid(g): returns an instance with guid equals to g if exists remove(x): removes instance x create entity(t, g): adds an entity of type t with guid g write(p): writes the data onto a new file with path p

Reading files import ifcopenshell # import IfcOpenShell ifc file ifcopenshell.open("example1.ifc") # Open a file products ifc file.by type('IfcProduct') # Retriving entities by type for product in products: print(product) # printing entities (as tuples) print(product.Name) # accessing and printing fields p ifc file.by guid('1Gu3YrB f00eRYf82lGDVO') # Retriving entities by guid