Last edited 08 Jul 2016

Level of detail for BIM

Building Information Modelling is a very broad term that describes the process of creating and managing digital information about a built asset such as building, bridge, highway, tunnel and so on.

The level of detail of a building information model increases as the project proceeds, often based in the first instance on existing information, then developing from a simple design intent model through to a detailed virtual construction model, then an as-constructed asset information model (AIM). Different aspects of the model may develop at different rates, may originate with different members of the project team, and their development may pass from the employer, to consultants, to the contractor and suppliers and ultimately back to the employer.

It is important therefore that the employer defines the level of detail that is required at each stage of development of the project. This not only ensures that the design is developing in sufficient detail, but also that the information required by the client to make decisions about the project development and then to operate the completed project efficiently, is actually provided. It also gives an indication of the reliance that can be placed on information.

The employer defines the level of detail that is required in the Employer's Information Requirements (EIR). The Employers Information Requirements might be appended to a BIM protocol, incorporated into the contract by addition of a 'model enabling amendment', making the delivery of required information a contractual obligation.

A summary of level of detail requirements and responsibility for model development might be provided in a Model Production and Delivery Table.

PAS 1192-2 (Specification for information management for the capital/delivery phase of construction projects using building information modelling.) defines two components to the 'level of definition':

  • Levels of model detail (LOD), which relates to the graphical content of models.
  • Levels of model information (LOI), which relates to the non-graphical content of models.

In fact, the two are closely aligned as it is normal for graphical and non-graphical content to develop alongside one another.

The levels of model detail and model information are generally defined for key stages of the project, at which 'data drops' (information exchanges) take place, allowing the employer to verify that project information is consistent with their requirements and enabling them to decide whether to proceed to the next stage. This is analogous to a stage report on a conventional project.

As present, there is no standardised definition for the timing of data drops or for levels of model detail and model information, other than the suggestion that they should be aligned to employer decision points and should be consistent across all appointments. This is because it is thought they will vary depending on the nature of the project. However, some very broad guidance is given in PAS 1192-2:

  • Brief: If a graphical model exists it is likely to have been developed from an existing asset information model. Other information might relate to existing buildings and structures (there may also be schedules of requirements).
  • Concept: The graphical design may show massing diagrams and 2D symbols to represent generic elements.
  • Definition: Objects are based on generic representations, and specifications and attributes allow the selection of products.
  • Design: Objects are represented in 3D with the specification attached along with information about space allocation for operation, access, maintenance, installation and replacement.
  • Build and commission: Generic objects are replaced with manufacturers objects, with essential information re-linked to the replacement objects and manufacturer information added.
  • Handover and close-out: The model represents the as-constructed project and all necessary information is included in handover documentation, including maintenance and operation documentation, commissioning records, health and safety requirements and so on.
  • Operation and in-use: Performance is verified against the Employer's Information Requirements and the project brief and if changes are necessary, the model is updated. Information about maintenance, replacement dates, and so on may be added.

The NBS BIM toolkit, developed following a government competition can be used to help define information requirements for projects aligned to specific project stages.

The American Institute of Architects (AIA) has also published a LOD framework for the AIA G202-2013 Building Information Modelling Protocol Form. Here LOD refers to the 'Level of Development' required for model element content. The term 'level of development' is used rather than 'level of detail' in recognition of the fact that a visually very detailed element might in fact be generic and despite appearances might be at a low level of design development.

The AIA suggest that the LOD framework recognises that different elements of the project will develop at different rates and '…allows the Project Participants to efficiently communicate to one another the extent to which a Model Element has been developed … It also allows the Project Participants to communicate the extent to which a Model element may be used and relied on…'

The LOD framework defines the following model element content requirements:

  • LOD 100: The Model Element may be graphically represented in the Model with a symbol or other generic representation, but does not satisfy the requirements for LOD 200. Information related to the Model Element (i.e., cost per square foot, tonnage of HVAC, etc.) can be derived from other Model Elements.
  • LOD 200: The Model Element is graphically represented within the Model as a generic system, object, or assembly with approximate quantities, size, shape, location, and orientation. Non-graphic information may also be attached to the Model Element.
  • LOD 300: The Model Element is graphically represented within the Model as a specific system, object, or assembly in terms of quantity, size, shape, location, and orientation. Non-graphic information may also be attached to the Model Element.
  • LOD 400: The Model Element is graphically represented within the Model as a specific system, object or assembly in terms of size, shape, location, quantity, and orientation with detailing, fabrication, assembly, and installation information. Non-graphic information may also be attached to the Model Element.
  • LOD 500 The Model Element is a field verified representation in terms of size, shape, location, quantity, and orientation. Non-graphic information may also be attached to the Model Elements.

Ref AIA, Guide, Instructions and Commentary to the 2013 AIA Digital Practice Documents.

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