How HVAC LOD Levels Impact Your 3D BIM Model
It’s a term commonly used in BIM projects: LOD. The required LOD may seem like a simple number, but its interpretation can have a major impact on the size of your model, as well as the amount of hours and effort needed. A clear understanding of the different levels, will help you deal with LOD in a LEAN way.
First, what is LOD?
The term LOD stands for Level of Development and has been conceived to improve the communication within construction projects. The Level of Development is often mentioned in BIM-related documents such as the BIM implementation plan. The Level of Development can best be understood as a reference to the reliability of your model. It’s the degree to which an element’s geometry and attached (non-graphic) information have been thought through. The higher the Level of Development, the more your project team members can rely on your model. Note that LOD does not make any statements about the required amount of information and that both graphical and non-graphical information can contribute to the reliability of a model.
Discover where the term LOD comes from and why it has been conceived in The LOD on LOD.
How LOD levels impact your HVAC model
To clarify the different levels ranging from LOD 100 to LOD 500, the BIMForum created the Level of Development Specification. These levels can be used by AEC practitioners as guidelines, to set expectations and to make estimates about the workload. The higher the required LOD, the more time and effort you’ll likely need to spend on your HVAC models. Here’s what the different LOD’s mean:
LOD 100 - Conceptual design
At this level, elements are not represented geometrically: the exact location, shape or size of an element cannot be derived from the model. Instead, elements are represented with symbols or another generic representation. This definition is of little use for modelling HVAC systems.
LOD 200 - Design development
According to the BIMForum specification, elements at LOD 200 are represented with generic placeholders. The approximate size and shape of elements and systems are shown. At this level, your team will get an idea of the shape and direction of pipes. However, it’s not possible to make any claims about the exact length of a pipe for example.
LOD 300 - Documentation
At this point, the model includes information about the size, shape, location, orientation, and quantity of the elements - and this information can be measured directly from the model. It’s not necessary to look up the dimensions in notes or call-outs. Measurements can be derived directly from the geometry. At LOD 300, a project origin has been defined and the location of elements are accurate with respect to this origin.
LOD 350 - Model coordination
For modeling and coordination, LOD 350 is often required. At this level, parts that are needed for coordination with other building systems are included in the model. This means that interfaces with other building systems are graphically represented and supports and connections are included. For example, at this stage you will include hangers to show how a system connects to the ceiling, or incorporate wall openings at places where a pipe intersects a wall. This information can already be used on the construction site, to lay out points for example.
LOD 400 - Ready for construction
At this level, specific system information is included and elements are modelled with the necessary level of detail and accuracy for the fabrication of the components. Graphic and non-graphic information is included that will guide the fabrication, assembly, and installation processes. At this point, prefabrication sheets and cutting lists can be derived from the model, and components can be ordered. A model at this LOD level is ready for construction and fabrication.
LOD 500 - Field verified
If your model will be used for facilities management, you will most likely be asked to model your systems at LOD 500. This level means that the elements in the model have been verified in the field. The dimensions, location, quantity, and orientation of the HVAC elements have been checked in the actual building after completion. As-built models fit in this category. Note that this does not mean that the systems have been modelled at a higher level of information or with more detail. At LOD 500, a model can look the same as a model at LOD 400. The difference is that at LOD 500, the model has been verified on site - and therefore has a higher level of reliability.
Find out how you can manage the information as your model progresses in How to Use Your BIM Model in All Project Stages.
How to deal with LOD in a LEAN way
Note that across LOD definitions, both graphical and non-graphical information are mentioned. In order to avoid unnecessarily heavy model files, it can be useful to make a distinction between the Level of detail and the Level of Information:
1. Level of detail [LOD]
Level of Detail can be thought of as the graphical representation of the element. An element with a low level of detail, for example, has a simple geometric shape such as a cube or cylinder. At a high level of detail, you’ll be able to see the texture, manufacturer brand, and other visual details.
2. Level of information [LOI]
This term is used to indicate the level of information linked to an element. An element with a high level of information includes, for example, manufacturer-specific data sheets and maintenance instructions.
Thinking beyond the Level of Development will prevent data (graphic or informative) being added to the model that is not used. In a project where LOD 500 is required for the sake of maintenance, it may be useful to model the level of detail at a lower level than the level of information. This way, mechanics can derive detailed maintenance data from the model, without being slowed down by the file size of a model with highly detailed graphics.
While LOD may be useful for making initial agreements about what’s expected in BIM projects, the definitions do not state which information needs to be included exactly. Therefore, it is wise to write down in the BIM execution plan what is covered by the various levels of the LOI and LOD. This way, it becomes clear which information really matters, what is expected from each party, and BIM collaboration can take off.