This blog piece looks at an important concept of Building Information Modelling – Level of Detail (or simply LOD). With both LEGO and construction projects clients will be familiar with what the final constructed ‘building’ looks like. However the final model or building undergoes a series of developments before the design is finalised. This process means that the design can be developed in the most efficient way possible. In very simple terms the more detailed a model is the more time it takes to produce. It also means more time is required to modify more detailed models. Getting the right Level of Detail at the right stage of projects is critical to not introducing waste to the process.
Level of Detail
Level of Detail has different ways of measuring LOD in different parts of the world. For example the US uses LOD100, 200, 300 etc whilst the UK uses LOD2, LOD3, LOD4 etc (embedded in the NBS BIM Toolkit). The problem with these scales is they tend to ignore the early stage models that need to be produced. The key UK document that provides an overview to Level of Detail is PAS1192-2:2013 and is covered in Section 9.9. This more of an overview and this is more useful to explain to clients what Level of Detail means broadly.
RIBA Stage 1: Preparation and Brief (PAS1192-2:2013 9.9.1 refers to this stage as Brief)
PAS 1192-2:2013 describes a “graphical model will either not exist or will inherit information from the AIM (Asset Information Model)”. So in simple terms with our LEGO project, we start with no model or we have an existing LEGO model to build upon.
Of course in order to develop a model from scratch we need to think about the design. With LEGO we could just begin putting pieces together but for construction projects we need to design the idea in stages so we create what is in the mind of others, rather than simply modelling what we want. Often this will start even before model authoring tools are even used with pen and paper (and no these are not a dirty words with a BIM process!) to develop initial sketches.
Image: An early sketch design [click image to enlarge]
RIBA Stage 2: Concept Design (PAS1192-2:2013 9.9.2 refers to this stage as Concept)
PAS 1192-2:2013 describes “at concept design stage the graphical design may only show a massing diagram or specify a symbol in 2D to represent a generic element”.
A concept model is typically used early in a design process or for providing context to a detailed model (i.e. surrounding buildings where a high level of detail is not required). These models can vary in terms of detail, from very simple block models through to more developed massing. The important thing to understand about these models is they are not made up of individual pieces. So for example in the image below the first floor is only one element.
These types of models may appear primitive but they serve a purpose and can be used to understand the size of the building and how it might fit on a site. Whilst they have a low level of detail they can still produce information to be used for high level cost plans. The main advantage of these models is that they are quick to assemble and therefore a number of variations can quickly be developed for discussion with key stakeholders such as clients and planning authorities.
Image: An example low level of detail concept model [click image to enlarge]
Image: A example concept model developed further from initial massing [click image to enlarge]
This level of detail can be developed still further but there are still relatively few components to manipulate. Materials and colours can also be shown if required to give more visual information to others.
Image: A more developed concept model [click image to enlarge]
RIBA Stage 3: Developed Design (PAS1192-2:2013 9.9.3 refers to this stage as Definition)
PAS 1192-2:2013 describes a model at Design stage as “object(s) shall be based on a generic representation of the element”.
Concept models are an extremely effective way to develop a design during the early phases of a project but they don’t contain individual parts (like the parts in a LEGO set). So moving beyond a concept model, models are typically developed as a generic Developed Design model. This means the parts could be bought from a variety of companies. This allows a contractor for example to compare costs of different parts that meet the same requirements. We might specify LEGO but the contractor still has the freedom to pick an alternative product (even if we would prefer to see LEGO as the installed product).
So in the model below this model has the same number of components as the final built model but there is less detail. The blocks below are very simple representations of the final installed products. This model is typically not something that would be constructed as it doesn’t have enough geometric information to construct the final building.
Image: A developed design model [click image to enlarge]
RIBA Stage 4: Technical Design (PAS1192-2:2013 9.9.4 refers to this stage as Design)
PAS 1192-2:2013 describes a model at Technical Design stage as “objects shall be represented in 3D with the specification attached”.
Developing the above further the model is typically developed so that the model could allow the model to be constructed. The model we have built for these blog posts is this level of detail. Whilst in the first post I told you this was a LEGO model, the reality is that the model could be made up of another manufacturer’s pieces. As you will see the model below is still more than enough detail to explain how this model/building goes together.
Image: A technical design model [click image to enlarge]
RIBA Stage 5: Construction (PAS1192-2:2013 9.9.5 refers to this stage as Build and Commission)
PAS 1192-2:2013 describes a model at Construction stage as “any generic object shall be replaced with the object procured from the manufacturer. Any essential information to be retained shall be reattached or relinked to the replacement object”.
Going even further beyond the model we have created above we could have used even more detailed parts. This includes manufacturer specific level of detail, in this case it includes the word ‘LEGO’ on all the relevant bricks. Some would say this provides more information but at all times you have to ask what value adding more information brings. Does adding more geometry detail add value or just make the model ‘look nicer’.
Whilst PAS sets out a general process there are major questions about whether this part of the process is truly necessary. For example a box can represent a boiler. Does it need all the detail applied to it? Who is that detail going to benefit? I would argue it isn’t needed for either the construction process or facilities management. Remember we are only talking about geometry in this post – data or information is a completely different subject, and one for another post.
Image: A construction level of detail model [click image to enlarge]
RIBA Stage 6: Handover and Close Out (PAS1192-2:2013 9.9.6 refers to this stage as Handover and close-out)
PAS 1192-2:2013 describes a model at Handover and Close Out stage as a “model shall represent the as-constructed project in content and dimensional accuracy”.
So basically this is the same model as Stage 5 but adjusted to match what was actually constructed. Of course theoretically the design should match the construction but this is rarely the case. There are methodologies for verifying the proposed design against the actual built model which we will cover in another post.
RIBA Stage 7: In Use (PAS1192-2:2013 9.9.7 refers to this stage as Operation and in-use)
PAS 1192-2:2013 describes a model at In Use stage as “objects that have been changed or replaced with different equipment shall be updated accordingly”.
So if someone comes along and adjusts our LEGO model, then the client (or someone acting on their behalf) needs to update the computer model to match so that there is an accurate record of what exists at that moment in time.
Agreeing the Levels of Detail (LOD) to be modelled is an important part of planning a BIM project, whichever scale you use. It is crucial to only deliver the right level of detail at the right point in the project. The model will get more detailed as a project progresses and it is important to understand the final level of detail required and also who is going to produce that model.
From a personal point of view I believe that many manufacturer specific models offer too much detail and we don’t generally use manufacturer specific content as we believe that these highly detailed models don’t provide any additional value to either contractors or clients. We see accurate data as far more valuable and we will look at data/information in further posts.
Rob Jackson, Associate Director, Bond Bryan Digital
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