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Level of Development - LOD - as a Lifecycle BIM tool

When owners and facility managers want to get started with BIM for FM the focus quickly turns to model content and quality. Are my current models suitable? Can I ask for improvements within my current contract? What improvements should I ask for? How do I specify better in the next project?

Owners need to review their requirements both for ongoing projects and for new projects.

For new projects the project team needs to agree what information at what detail and quality level needs to be provided at what stage of the project. Both coordination needs and phase handover needs are to be considered.

For existing ongoing projects where BIM for FM requirements have not been contractually specified the owner need to engage with the project. Together they need to assess what info is already generated in the BIM models and what “traditional” data and documentation are contractually required for handover. Based on this the parties should discuss what a suitable “change order” should be.

To support these processes and open discussions there is a need for both sides of the table to have a good understanding and a common ground for discussions.

In this article we will look at the “Level Of Development” (LOD) concept and see how it can be used both as a basis for contract documents and as a tool to communicate needs and wants. We will look how this is implemented in two common “BIM frameworks” (UK BIM standards and the US national BIM standard). The discussion should be relevant also for other regional implementations of the concept.

Basic concept of the LOD family of specifications

Level of development (LOD) is a concept with varying definitions and implementations. Core to the concept is that the level of development defines the content and reliability of BIM elements at different stages or milestones. With “content” we mean geometric information, structured data and linked documentation. With reliability we mean for what uses and to what extent the downstream users of the information can trust the accuracy and quality of that content.

The LOD “classes” that we will present will, if correctly used, help you understand the usability and limitations of the modelled elements. And it is not just about the amount of or granularity of information that has gone into a model. It is as much about the quality and trustworthiness of the information that can be extracted from the models.

The LOD Specifications can be used in various ways. You can use them as attachments to contracts and as supplements to BIM Execution plans. As we will see they are also used as references in common standards to describe information management practices.

As a basic premise the LOD specifications acknowledges the different phases for lifecycle BIM, and help specify the needs at every stage. It tries to answer questions like: What are requirements for geometric content and model data for each discipline and each “level”. What is needed for design intent, what is needed for construction collaboration and what is needed for facility management? What information can be carried over between phases and what information needs to be replaced? What links to historic data needs to be kept?

Example implementations in BIM frameworks.

The US National BIM standard - NBIMS v3

Probably the most common way the industry is using the Level Of Development term is by defining LOD “numbers” like LOD 100, LOD 200 or LOD 300.

This system was originally developed by Vico Software as a way to define the reliability and suitability of model elements for use in automatic model-based costing. Originally Vico named the system “Level of Detail” and gave it the acronym LOD.

The system has since been made more generic and standardized by the American Institute of Architects (AIA). When AIA picked it up (and trademarked it) they renamed it to “Level of Development”. The change of name emphasised the importance of this not just being about geometric features but about all the BIM content for each of the elements.

BIMForum (the current US chapter of buildingSMART) have expanded on the basic AIA LOD specification and have published a “Level of Development Specification”. This spec is embedded in the US National BIM standard.
This system is therefore interesting both as a part of the NBIMS guide and as an often used system either standalone or as a foundation for other systems. Practitioners should know the basic definitions of the LOD levels and they should know about the contents of the BIMForum LOD specification and its practical use as a reference tool, contract document and progress measurement tool.

The basis for the LODXXX system is that numbers describe different “levels of development” assuming model elements pass through these levels and therefore increasing the numbers as the geometry and content gets more specific and final/ specific/ trustworthy. The main levels are described below

LOD 100 - Concept - Here there are no geometric info in the model elements, only symbols with attached approximate info

LOD 200 - design development - Now the elements are generic placeholders for elements and equipment to be - They may be recognisable objects or space allocations for coordination between the disciplines

LOD 300 - documentation - This level should be suitable for design intent to support processes like costing and bidding. These models will be used to generate construction documents and shop drawings. You should now be able to take measurements from the models and drawings and locations should be accurate

LOD 350 - This level defines proper cross trade coordination and will include connections and interfaces between disciplines

LOD 400 - construction - This level supports detailing, fabrication and installation/ assembly. The contractor will be able to split construction requirements and assign to sub contracts

LOD 500 - facilities management - This level will have suitable geometry and information to support operations and maintenance. Geometry and data should be as-built and field verified

To support the LOD definitions for different disciplines and different building element types the BIMForum LOD specification is very specific in giving examples. An example showing progression of geometric detail are included below.

BIM Level Of Development example

In addition the specification includes spreadsheets listing property requirements for different element types for different levels of development

Level of Development Attribute Table from BIMForum spec

NBIMS specifies that there is no LOD classification for the whole model. It assumes that at any stage the model will contain elements and systems at different levels of development

The BIMForum LOD specification does not prescribe what Levels of Development are to be reached at what point in a project but leaves the specification of the model progression to the user of the document. To support this process the BIMForum specification includes a very detailed spreadsheet to monitor and track development progress for models.

Level of Development Model Element Table from BIMForum spec

We do not have experience ourselves from using these tools. If any of the readers have such experience to share we would love to learn about your experience in the comment section below.

UK BIM

In the UK BIM “movement” LOD is an acronym for “Level of Definition”. The concept of this LOD is primarily the same but we will look at some of the differences. The UK LOD is defined in the “information management” standard PAS - 1192-2. This is important as this is currently being used as a foundation for the coming ISO 10950 standard.

The PAS is focused on both the geometry and the structured data, and acknowledges that the balance between them vary across the lifecycle of the model element. The PAS does so by splitting the LOD into two components

  1. Level of model detail - this defines how detailed the geometric part of the BIM elements are
  2. Level of model information - this defines the development and trustworthiness of the structured data (non graphical) part of the model elements

The PAS is more focused on phases than the BIMForum LOD spec, harmonising with the RIBA plan of work. The main levels of the LODs in the PAS are:

Design - Here we have a 3D representation with specification attached. The geometric detail should be minimum space allocation for operational and maintenance space

Definition - Generic geometric representation of element. Specification properties and attributes to allow selection of manufacturer product

Build and commission - Replace generic product with product procured from manufacturer. Reattach/ relink object to replacement object. Should also include final positioning of pipework and ductwork.

Handover and closeout - All necessary information about the product should be included - operations and maintenance documentation - The model should now be a “digital twin” - as constructed.

Operational and in use - Update asset information model with maintenance records, replacement dates, replaced equipment

The PAS 1192-2 do include a matrix that describe the different levels and their characteristics. In addition to the rows shown below rows include “outputs”, “parametric information” and “critical information and logic” among others.

Levels of model definition table from PAS 1192-2

Implications and advice for owners

As we see a lot of work has been done to specify and describe model content, detail and suitability. Owners can use these standards and guidelines as communications tools and references when defining, expressing and discussing their needs. Now they have baselines to support their discussion when they ask if their models are suitable for facilities management. When their needs differ from the standards they can specify those differences instead of starting from scratch. For professionals working with multiple clients they now only need to consider the deviations from what they know and not start on a custom made spec each time. (please note however that as with any other BIM standard or tool the main focus is on design and construction, not so much on facility management)
We generally advice owner to focus on their requirements on the end result - what do they need post handover. Then they should let the project decide the process needed for design and construction collaboration. At the same time the project should consider the owner needs as soon as possible and consider how that will affect their BIM execution strategy. E.g. if the owner have specific requirements the project should decide who should generate the data and how it should be validated. Structured and validated data should be included in the model “as soon as possible”. Data availability will add value to the processes and data use will ensure quality.

There will be a lot of information generated during the lifecycle of the design and construction process. Owners should focus on a minimum set of info that should be present and validated during handover (e.g. correct positioning, correct level of detail for geometry, core product data and linked O&M instructions.) Then they should be able to archive the additional information generated during the process but should also have a way to assess the trustworthiness of that info.
We think that an important step to handle this is is for the owner to approve BIM element handover on a per object and per system basis and preferably as an ongoing effort during commissioning. As soon as equipment and building elements are finalised and confirmed “as built” they should be submitted for owner approval. The approval should include the required data and attached additional info and history.

The main takeaway is for both the project and the owner to understand the difference between a design intent model, a construction model and a facilities management model. To help visualise the differences both in geometric detail and data content and reliability the LOD specifications can be used.

Currently most BIM model elements “end” their lifecycle at design intent. They are used to automate the creation of design documents. Generic object are not replaced by manufacturers objects and manufacturer's product data are not linked to the model elements. Contractors do not improve the models to the LOD 400 level. For us it is important to point out that you can generate LOD 500 models (models for facility management) without evolving the models to the full LOD 400 spec. We feel that there should be a level of definition “in between” where generic objects are replaced by specific data and the model geometry and correctness is verified without having to aspire for the geometric detail that LOD 400 describes.

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