Why LOD Matters for Constructibility

April 18, 2019

The intended purpose of “level of development” (LOD) was always to judge the components that make up 3D BIM models by how reliable they are. Without getting caught up too much in semantics, we can all agree that a more reliable model is more valuable to every stakeholder on a project.

For example, a sheet metal fabricator charged with producing a length of ductwork needs to have that component modeled at LOD 400 to allow them to extract all the correct dimensions and fixing details in order for them to do their job. At the same time, HVAC installers putting that ductwork in place can succeed with modeled MEP systems at LOD 300. But, would it hurt the installers to work from a model that’s clearly more reliable? Of course not. And, there are clear benefits to the whole project for all components of the model to be accessible on one platform and in one format.

So, modern construction pros want to streamline the workflow while still favoring reliability. That’s where LOD really matters if you’re interested in moving beyond BIM and into true constructibility.

What changes with each successive LOD?

When a 3D structural model is first created — like every creative effort — it starts with simple, broad strokes to get the idea across before being filled in with additional detail. If a concrete foreman on the jobsite were given access to just the initial concept model, it would be of almost no value because it wouldn’t contain enough useful or reliable information to offer any real guidance.

But, as geometric detail and non-geometric information are added to the components making up the model throughout the engineering and detailing phases of project planning, the model gains valuable attributes that make it more reliable — and therefore, more useful — to stakeholders all along the project timeline.

It’s also important to note here that when we’re discussing LOD, we’re never describing an entire federated BIM model. As noted in the ductwork example above, various components need to be modeled to a given LOD at various stages throughout a project in order for work to progress. And, while we’ll get to the value of higher LOD as a standard, there are many portions of a model that will never need to go beyond, say, LOD 350, and doing so anyway would be a waste of time, effort, and money. So, we’re only using “model” for simplicity in the descriptions below, but what we’re referring to is each digital component or piece of content that makes up the federated model:

  • LOD 200 - At this level, basic geometric measurements are included and primary equipment shows up as boxes that cover the maximum space required for the component. The model still isn’t build-ready, but the general outline is there.
  • LOD 300 - At this level, much of the ambiguity of the LOD 200 model has been rectified as all components are modeled with specific quantity, size, shape, location and orientation. Some components are still too small to be accurately rendered in the model at this LOD, but it’s “good enough” to move forward with some fabrication and most installation. (Important Note: This is the LOD required by most contracts that demand BIM deliverables. It is the industry standard for optimal LOD.)
  • LOD 350 - Taking an LOD 300 model one important step further, this level incorporates how components and complete building systems represented in the model interface with other building systems. (Important Note: This is the maximum LOD available when working with the most popular 3D modeling program on the market, AutoDesk Revit.)
  • LOD 400 - In addition to the specific detailed geometric data and visuals provided at LOD 350, models developed to this level will include detailing, fabrication, assembly, and installation information. This metadata adds tremendous value at all phases of the project for clash avoidance and reduction of rework.
  • LOD 500 - The highest possible LOD, as it is field verified to be “as-built” — a digital twin to the physical component.

Some engineers, detailers, and other construction professionals don’t even recognize LOD 500. Revit and most other popular modeling solutions can’t offer it, and it’s actually of little value from a design and delivery standpoint since it references “as-built” model components that have been field verified. As you can imagine, however, being able to graduate even some of the model components to an “as-built” level of development and have them available for long-term reference can be a huge boon for owners, facility managers, and future contractors hired to work on that structure in the future.

So, why does LOD matter for constructibility?

One of the greatest challenges facing every construction project is uncertainty.

When a model lacks a crucial bit of data, or the model’s measurements or placement information is even slightly ambiguous, it introduces doubt for contractors and fabricators alike. No one wants to risk making a costly mistake under these circumstances, so doubt drives requests for information (RFIs) back to the engineers or detailers, which cost time, energy, and money to respond to. This, in turn, leads to bottlenecks in the project timeline, budget overruns, and loss of profit margin. Or, if fabricators and contractors move ahead without questioning the model, costly rework and other waste can lead to the same results.

On the other hand, modern modeling solutions and field technology has progressed to the point that laser scanners and the point clouds they produce can be integrated seamlessly with existing 3D models to produce field-verified “as-built” components at LOD 500. A fully reliable digital twin with zero ambiguity.

The higher the LOD of each component modeled, the more every project stakeholder can rely on it. The most value is in a component that is modeled to the highest level of development that’s both possible and practical. That value is realized by:

  • The engineers and detailers who avoid RFIs and rework
  • The fabricator who has no problem feeding the data into the machine and producing the component
  • The contractor who can quickly and confidently install it without last-minute problem solving
  • The GC whose project moves ahead on schedule and on budget because mistakes and delays are minimized
  • And, finally, the owner — who ends up with a quality structure without the inflated cost and timeline the above issues would have produced if they hadn’t been mitigated.

Every bit of data that is incorporated into a constructible model is another step closer to optimal speed, efficiency, accuracy, and profitability. When it’s possible and practical to produce field verified, “as-built” components in 3D model form, why wouldn’t you? That’s why LOD should matter to every construction professional pursuing success in today’s ultra-competitive market. And, why moving toward higher LODs where practical is going to be a key factor for success into the future.

To explore the only BIM solution capable of supporting constructible models at every LOD, get to know the Tekla suite of structural modeling solutions. And, to learn how more reliable 3D models fit into the future of construction, click here to explore The Constructible Process.

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