How do building owners make sure they get the 3D model they need? How do architects and engineers make sure they’re on the same page when bidding on projects where BIM is a requirement in the RFP, which nevertheless offers no detail of what exactly should be in the model? How does a general contractor or construction manager know if he can use the model for quantity take-off – and if he can, to what level of detail?
The AEC industries have reached the point where BIM can mean a huge range of models – everything from a massing model to a fully coordinated construction model, or maybe a specialized model for structural purposes or energy simulation. For the structural engineer, this often means new deliverables. Building officials are beginning to look at structural models as the deliverable for automated code checking. To deal with the ever-growing potential of BIM as a process and as a virtual 3D model, VICO software created a Level of Detail framework in 2005. This framework was adopted and augmented into specific levels of development (LOD 100, LOD 200, LOD 300, LOD 400 and LOD 500) by the American Institute of Architects and turned into the contract document AIAE202-2008 BIM Protocol Exhibit, released in 2008. AIAE202 also defines the extent to which downstream model users, such as contractors and fabricators, can use and rely on the model for scheduling, pricing, fabrication and construction. It is important to note that LOD only applies to an individual model element and not an entire model. For example, there is no such thing as an LOD 300 model when defining scope. Additionally, a model element author is defined as the party responsible for developing the content of a specific model element to the LOD required for a particular phase of a project.
In early 2011, a working group from the AIA and AGC BIM Forum emerged to explore how to further expand the definitions in AIAE202 to make working LOD definitions for BIM projects. At the BIM Forum’s San Antonio meeting last April, attendees were treated to a progress update by members of the LOD working group, including James Vandezande, an HOK principal and firmwide BIM leader, Jim Bedrick, FAIA, of consulting firm AEC Process Engineering representing the AIA, and Will Ikerd, P.E., of IKERD Consulting in Dallas, a structural engineer. There are many different opinions on this topic as to who should model what and to what level of development.
“The problem the workgroup was formed to deal with was to tackle the fact that the single-sentence definition of those levels – LOD 100, LOD 200 and so on – is vague,’ Vandezande says. “If you’re on a project what do you use to determine if it is built to that level? That’s what the catalog is meant to provide the background for. If you’re on a project and you’re filling out a table for your project plan of who is going to model what, and you’re using a code of 100, 200 and 300, the authors can then understand what that means as far as input. That’s the difference between detail and input a minimum amount of detail to meet the level of development.’
The catalog is a collection of definitions of model elements that includes descriptive input requirements, information requirements and graphic/model examples of what LOD 100, 200, 300, 400 and 500 would be for building elements.
Definition of the intended use of the model elements is also possible in the E202, which addresses use cases. Currently, the dominant use case for structural engineers is the creation of 2D documents. Other structural use cases are estimating, 3D spatial validation (clash detection), automated (CNC) fabrication, and others, such as automated structural code checking in building departments.
LOD definitions in terms of model elements:
LOD 100: Conceptual, overall building massing indicative of area, height, volume, location and orientation may be modeled in three dimensions or represented by other data.
LOD 200: Generic placeholders, modeled elements are modeled as generalized systems or assemblies with approximate quantities, size, shape, location and orientation. Non-geometric information may also be attached to the model elements.
LOD 300: Specific assemblies, model elements are modeled as specific assemblies accurate in terms of quantity, size, shape, location and orientation. Non-geometric information may also be attached to the model elements.
LOD 350 (proposed): The model element being graphically represented within the model with the detail necessary for cross-trade coordination and construction layout. Non-geometric information may also be attached to the model elements.
LOD 400: Model elements are modeled as specific assemblies that are accurate in terms of size, shape, location, quantity and orientation with complete fabrication, assembly and detailing information. Non-geometric information may also be attached to model elements.
Model elements at LOD 100 may be graphically represented with a symbol, but they do not have any indication of actual physical geometry. For structural modeling, there is seldom any content modeled at LOD 100. Information related to the model elements (cost-per square-foot, steel or reinforcing pounds-per-square-foot) is often derived from a textual structural narrative that the engineer provides to accompany the architect’s early mass model of the building. At LOD 200, an element is graphically represented as a generic system, object, or assembly with approximate quantities, size, shape, location, and orientation. From this point, the model elements progress to LOD 300, where they are graphically represented as a specific system, object or assembly that is accurate in terms of quantity, size, shape, location, and orientation. For full trade coordination, however, additional element development is often needed beyond LOD 300.
For this reason, Ikerd has authored and advocated on the joint BIM Forum/AIA working group on LOD for a newly proposed LOD 350, which is “Assemblies for Coordination.’ LOD 350 is defined as the model element being graphically represented within the model with the detail necessary for cross-trade coordination and construction layout. LOD 400 “Detailed Assemblies,’ requires information that may not yet be available.
“At LOD 400 the model element is graphically represented as a specific system, object or assembly that is accurate in terms of size, shape, location, quantity, and orientation with detailing, fabrication, assembly, and installation information,’ Ikerd says. “For permit drawings which will be required for the foreseeable future on building projects, model elements are typically only modeled to LOD 300. However, for full trade coordination, higher LOD is required that is short of fabrication LOD. This is where the LOD 350 comes into place between element development at permit drawings and fabrication level element development.’
In structural design models, main structural members and systems are normally modeled with standard modeling tools for the creation of 2D construction documents. Higher levels of structural element information are usually prescribed with typical details. Examples of these structural elements are gridlines, levels, columns, beams, slabs, walls, main gravity systems and main lateral systems. Construction drawings made from the LOD 300 model are accompanied by additional 2D information such as general notes, typical details, specific details and specifications to define higher level information not typically shown in 1/8-inch scale plans or modeled for permit drawings. Other use cases for Level 350 have come up in the mechanical and interior enclosures subgroups.
Most main structural member elements are at LOD 300. However, structural engineers must be mindful LOD 300 requires elements to be in the correct location. Thus, sloping roof members that are modeled flat are not at LOD 300, even though the 2D plans made from them appear correct. Ikerd points out that design level open web bar joists cannot be at LOD 300 as they will not have the manufacturer’s web profiles that would be needed for trade coordination with MEP passing through the joists. Situations such as this make a compelling case for using LOD 350 as a half-step between specific assemblies and assemblies for coordination with manufacturer’s specific content needed for trade coordination. Other areas of misunderstanding are structural elements shared with architecture, such as tilt walls, slabs and load bearing masonry walls. The structural engineer’s scope should address who is responsible for modeling items such as floor depressions, openings, top of wall heights of parapets, etc.
“Few fail to see that BIM use is expanding in structures far beyond just a tool for creating 2D documents’ Ikerd sa. “The significance of the LOD concept is that it is an important tool to help define the structural engineer’s scope in BIM.’
Visit www.SEIBIM.org for more resources on this topic.
“For permit drawings which will be required for the foreseeable future on building projects, model elements are typically only modeled to LOD 300. However, for full trade coordination, higher LOD is required that is short of fabrication LOD. This is where the LOD 350 comes into place between element development at permit drawings and fabrication level element development.’
Jeff Yoders is the technology editor at Zweigwhite. He can be contacted at email@example.com.