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Building Information Modeling (BIM)
June 8, 2009

Header Image

Many types of geometry can be used to represent the shape of a building. We have previously shown how simple point cloud data can be collected from a laser scanner to display the locations of points on surfaces and objects in the building. A more common way to represent three-dimensional geometry in a digital file is to connect a few points together to create simple polygons like triangles and rectangles. In this way, a large surface can be represented by just 3 or 4 points. Typically, the geometry is tagged by a user to further describe its properties such as the color and transparency level of the surface. To properly describe a model of a building, additional properties are needed to describe the real physical materials that will be used when the building will be constructed. For example, a rectangle may be tagged as being a "wall" or as a "door". In this way, the building model can be made of known objects with known properties. When a model contains these higher-level building-specific tags, it is called a Building Information Model (BIM).

Revit Screenshot

To create a BIM for a specific building project, advanced software is needed to help the user author sensible models. For example, the software will know that a "door" exists within a "wall" and that it can open in a certain direction. These applications are sometimes called "parametric" because the entire model can be easily changed by modifying a few parameters. In the same manner that a spreadsheet program can recalculate all the totals when a single value is changed, BIM software can update the whole building model when a parameter, like a wall type for example, is changed. Some BIM applications are also connected to vast databases of building materials catalogs. This supports the advanced feature that when the user adds a "door" to the model, for example, the program can present a catalog where the user can choose a real door from a specific supplier and the correct shape and size will be used in the building model. Also, an ongoing parts list can be maintained automatically by the software so that, at any time, the user can see how many sheets of drywall are needed or how many doors to order and how much they will cost.

Revit Screenshot

Beyond tags for materials and parts, BIM can include structural and mechanical data, information about the location and orientation of the building, or construction staging and scheduling. Essentially, BIM is a digital representation of the physical and functional characteristics of a design. In contrast to most common 3D applications that present a building through a set of simple geometrical components, BIM facilitates the assembly of actual building components by incorporating intelligent and contextual semantics such as floors, beams, pipes, roof, etc. Software applications, like Autodesk Revit, are specifically designed to support BIM and the collaborative BIM process. The process of creating a BIM dataset typically involves a number of experts in different disciplines to create various parts of the building model. BIM is essential for large construction projects. Without BIM, small changes could mean hundreds of floor plans must be redrawn. Also, as mentioned above, coordinating many simultaneous users would be impossible. However, when using BIM software and methodology, updates from multiple users can propagate through the model instantly. Furthermore, using BIM, another major aspect of modern building design becomes feasible: simulation.

BIM Image

Simulation software can import a BIM database and it will have all the information it needs to analyze lighting, heating, water usage, and many other emergent properties to which a certain building design may lead. Simulation can be used to help optimize the use of resources to create a more sustainable building. From Revit, the BIM can be exported to the Green Building XML (gbXML) format. Specific simulation products, like Autodesk Green Building Studio and Autodesk Ecotect, can then import the gbXML file and begin simulation to measure and analyze the sustainability of the design. Linking these products to Revit gives architects quick feedback on design alternatives early in the design process.

Energy analysis requires spatial information – it is essentially a simulation of energy movement in, out, and through the rooms and volumes within a building. Which surfaces are exposed to the outside? How many are exposed to sunlight? What are the number, size, and orientation of openings in each space? How much heat is generated by internal lighting and equipment? In the past, this information was manually calculated from 2D drawings. An engineer would use building plans, elevations, and details to collate spaces (type, area, volume), surfaces (including adjacency and thermal properties), and shading. All this information is latent in a Revit model, and in a form that is much easier to interpret than 2D drawings. And, if the project is consistently structured, software such as Autodesk Ecotect Analysis can be used repeatedly right from the beginning of the project. This is a very important contribution to the design process at a stage when change is still possible. [excerpt from Ecotect User Guide]

gbXML

To effectively use the simulation tools, similar spaces in the building should be blocked out as "zones". These building volumes are individual units, or thermal zones, in the simulation. Perimeter rooms facing the same direction should be grouped together. Core zones with little or no exterior exposure should be grouped together. Similarly, unconditioned support spaces, such as restrooms, stairwells, elevator shafts, and storage spaces, should be grouped together. If need be, a mechanical engineer can provide guidance on the zone layout.

ZIP
Building Information Model (BIM) - Sample Revit File
ZIP - 6.237MB


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