by Richard McPartland
Last time we looked at what the standard is and why standardisation is so important, this time we delve into the guidance relating to Section 1: General requirements.
This article is an abridged version of the guidance that accompanies the BIM object standard; NBS BIM Object Standard - Section 1: General requirements (Requires NBS ID).
See also: About the NBS BIM Object Standard
A guide to terminology
(1.1.1) Throughout the NBS BIM Object Standard the following words are used to express mandatory and optional requirements:
||Expresses clear, absolute requirements of the standard
||Expresses permissibility (For example, as an alternative to the primary recommendation of a clause)
|Can||Expresses possibility (For example, a consequence of an action or event).
Terms and definitions are generally defined in PAS 1192-2 and PAS 1192-3 - outlying terms and definitions are explained in an online glossary.
Types of objects
Generic objects and manufacturer objects
(1.1.2) Objects may be referred to as an entity, a construction entity or element and take one of two forms - as a generic object or manufacturer object:
- Generic objects are intended for use in stages of design when the finalised solution has not yet been 100% resolved.
- Manufacturer objects (sometimes called proprietary objects or product objects (in BS 8451)) are intended to represent an obtainable product provided by a manufacture or supplier.
Component objects and layered objects
(1.1.3) Objects can be modelled as a component object or a layered object:
- Component objects (For example, doors, windows or sanitaryware) can be defined as static or parametric components. The former will be available in one size, the latter in a range of predetermined sizes (or as determined by the designer).
- Layered objects (For example, walls, floors, ceilings) are constructed from a number of layers without fixed geometry (this being determined by the designer). Thickness of object layers may be determined by manufacturers (For example, you would expect insulation board to be available in a set number of thicknesses).
(1.1.4) Component and layered objects can be aggregated together to form an assembly (as defined in PAS 1192-2) to show the context in which the object is being used (1.1.5).
This section describes the general requirements for BIM objects. The scope of this section includes general requirements such as object categorisation, IFC object type and predefined type requirements. In addition, this section defines the level of detail within the BIM object.
Care must be taken when an assembly comprises multiple objects (each with their own performance criteria) that may be unrelated to the assembly as a whole. For example, a manufacturer’s wall insulation BIM object may be shown within a generic wall build-up, even if the insulation manufacturer does not supply any other objects within the wall.
Objects within an assembly should have a minimum 'schematic level of detail' as defined by BS 8541-3. For example, in a generic wall build up the brick outer leaf, and block inner leaf would have a minimum schematic detail.
BIM authoring tools, Construction Operation Building information exchange (COBie) and Industry Foundation Classes (IFC) offer the capability to group components into functional systems.
See also: What is COBie?
See also: IFC - Is it simply misunderstood?
|Assigning a System to an Attribute of a Component
||Component ‘Light Fitting LF 001’ has a property ‘Circuit’ set as ‘Ground Floor Lighting’.
|Assigning a Component to a previously defined System
||System ‘Ground Floor Lighting’ includes Component ‘Light Fitting LF 001’.
|Using Components whose assignment to a System is unambiguous
||IfcLightFitting may implicitly imply ‘Lighting Installation’.|
In some instances, when aggregated together to form an assembly, some component information may become irrelevant (For example, a door handle that comes as part of an overall door assembly).
Component assemblies are documented using the aggregation relationship between IFC elements.
Type assemblies for construction are documented using the IFC Material Layer relationship between Material Layer Set and IFC Materials, which includes layer thickness. (Note: Material Layer Sets contain Materials, so only one level is possible).
1.2 Level of Detail
(1.2.1) Generic objects should have a 'schematic level of detail' as defined by BS 8541-3.
Manufacturer objects should have either a 'schematic level of detail' for mechanical, electrical, plumbing or civil engineering infrastructure or a 'coordinating level of detail' for other domains defined in BS 8541-3.
(1.2.2) Generic objects should include nominal or expected level of measurement where actual dimensions are unknown (as defined in BS 8541-3).
Manufacturer objects should include actual dimensions of the construction product.
1.3 Object Type
(1.3.1)The BIM objects should be identifiable within your BIM platform and assigned using the appropriate IfcTypeObject and IFC predefined type. (These are documented in the BuildingSMART International IFC 2xEditon 3 Technical Corrigendum 1 (IFC2x3 TC1) schema (ISO/PAS 16739)).
Object type is known in other publications as object class, template, style, functional type, library part or subtype.
The IfcTypeObject defines the specific information about a type that is common and shared by multiple object occurrences; it is represented by a set of property set definitions. Types can also have shape representations and associated quantity and property sets. The type is further developed through its IfcPredefinedType.
The IfcTypeObject defines the specific information about a type that is common and shared by multiple object occurrences; it is represented by a set of property set definitions. Types can also have shape representations and associated quantity and property sets.
Where an appropriate type does not exist, a proxy should be used:
||USERDEFINED predefined type, in upper case.
Type name should follow the IFC naming convention, CamelCase with IFC Prefix.
The IfcBuildingElementProxyType defines a list of commonly shared property set definitions of a building element proxy and an optional set of product representations. It is used to define an element specification (i.e. the specific product information that is common to all occurrences of that product type).
(1.3.2) The BIM object can include additional information from IFC4 (BS ISO 16739) in addition to IFC2x3 (see clause 2.8, 'Supplementary') as long as this is not to the determent of IFC2x3.
(1.3.3) The BIM object should include all necessary IFC properties to allow complete export to IFC from the BIM platform by filtering relevant model subsets. In order to do this, objects must be categorised correctly. Some BIM platforms automatically assign IFC information based upon the IFC schema while others require additional properties (e.g. Revit IfcExportAS and IfcExportType).
(1.3.4) The BIM object shall be publishable in a format that enables the transfer of information from person to person, application to application as documented in BS 8541-1 which provides file header and footer structure examples.
Previous: Exploring the NBS BIM Object Standard
Next: Exploring the BIM Object Standard: Section 2: Information Requirements
About the NBS BIM Object Standard
We introduced the NBS BIM Object Standard in part one of this series.
The NBS BIM Object Standard can be viewed online or downloaded as a .pdf document from www.nationalbimlibrary.com/nbs-bim-object-standard.
The online version includes comprehensive NBS guidance with background information, technical help and supporting content to help provide clarity and competency when creating BIM objects to the NBS BIM Object Standard.
Register for an NBS Account for free to access both the online version and to benefit from the additional NBS guidance.
BIM Object Standard Requirements are detailed in five sections (general requirements, information requirements, geometry requirements, functional requirements and metadata requirements). We'll be looking at these sections in more detail in subsequent parts of this series.
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