What is a BIM File Format? BIM vs. CAD.

The history of CAD programs (computer-aided design) is relatively long, but it provides the ability to create detailed models in both 2D and 3D. Because of that, many companies have been relying on CAD software for years. However, with the rise of BIM (building information management) in recent years, many thought that the transition between the two would be as simple as converting the files from one format to another.

Unfortunately, it’s not that easy. Since the differences between BIM and CAD extend far beyond the file formats, making the transition is much more complicated than just the data conversion process.

CAD and BIM: the definition

Computer-aided design (CAD) refers to utilizing computer technology to create both design files and documentation. It’s often used for projects that require multiple different parts and components to fit seamlessly together. Both 2D and 3D models can be created using CAD, as the software has evolved throughout the past thirty years, making it easier and faster to work with more complex projects.

The vast majority of industries nowadays are still using CAD for a myriad of different purposes, including civil engineering, manufacturing, plant design, industrial, and so on. The most popular file formats when it comes to CAD are DXF, DWG, IGES, STEP, SAT, and more.

Building information management (BIM), on the other hand, is an entirely new process of collaboration between different parties to design and build a project using the same model from the same database. The extent of visualization that BIM offers allows departments to analyze and visualize various design choices, together and before the construction process begins in the first place.

One of the major advantages of BIM is that BIM files are capable of showing a digital representation of various would-be facilities with information-rich models, including electrical systems, HVAC, various aesthetical parts like windows and doors, and so on. The biggest cornerstone of BIM is collaboration, first and foremost.

Even though the definition of BIM as we know today happened fairly recently, there’s already a lot of demand from owners to provide an as-built BIM model at the end of the construction process – forcing companies to make the transition sooner rather than later.

BIM vs. CAD file specifics

It’s not uncommon for CAD to be used for all kinds of industrial design of various assemblies, including smartphones, computers, vehicles, airplanes, and so on. BIM, on the other hand, is a more specific construction-related tool that’s often used to design and construct buildings, including schools, airports, offices, etc., but it’s quickly becoming the new industry standard in general.

Additional information included in these files allows for collision detection, problem discovery, and several other features that can ease the construction process as early as the design stage.

For example, knowing a specific part’s pressure ratings allows detecting the lack of the correct material for this particular part to handle that kind of pressure. Understandably, various characteristics of models, especially performance characteristics, take up a lot of space in the context of a CAD file and generally are deleted in the process of CAD-to-BIM conversion.

Another example of context-specific information is the behaviour of the actual components. A light fixture “knows” it’s supposed to be connected to an electrical system, an HVAC duct knows it needs to be installed within a wall, and so on. The ability of these components to adjust themselves in accordance with such information is what makes this example so useful.

A significant difference between CAD models and BIM is their adaptability. CAD models, more often than not, are only capable of presenting a highly detailed image of a model at a specific zoom level. BIM, on the other hand, is all about zooming in and out, expanding, contracting, and such. For this exact reason, the addition of a CAD model into a BIM system is bound to have some problems as soon as you need to zoom in or expand out.

CAD & BIM file formats and data types

The answer to the question “What is a BIM file?” is closely tied to the variety of different file formats that various BIM platforms can or cannot work with. It’s easier to digest by separating all of the formats into two groups: proprietary and non-proprietary.

Proprietary file formats are the ones that can only be read by a specific company’s software. Since the BIM software market is relatively big, there’s a lot of these different formats. Let’s go over some of the most popular ones so far:

• NWD is the proprietary BIM format for Autodesk Navisworks, it can only be opened in either Navisworks Manage or Navisworks Freedom. Two file formats with similar meaning are NWC and NWF.
• RVT is the proprietary format of the Autodesk Revit, also includes RTE and RFA file formats.
• AutoCAD files are also in their own league with the DWG file format, but this one is also one of the most popular CAD file formats period, capable of being opened in the majority of CAD-based software appliances.

A popular myth about DWG format is that it can only work with 2D models. This, of course, is not true since 3D objects can also be contained in this format either via basic planes or using full components/blocks. There’s also DXF format (Drawing Interchange Format) that works with BIM drawings. It is similar to DWG but is somewhat larger yet it has the same level of interoperability with most CAD platforms.

Proprietary data formats in the industry are creating expected coordination problems when it comes to interacting with several different proprietary data formats. This problem can be solved by converting the file to one of the non-proprietory formats, using compatibility plugins, and so on.

Speaking of non-proprietary formats, these are vendor-neutral, often open-sourced, and developed via international community collaboration. A few examples of those are:

• COBie (construction operation building information exchange) is a BIM format that allows the sharing of asset data, rather than geometric or graphical data. This one can be used to transfer documents through different project stages, from design to construction.
• IFC (industry foundation classes) is the most popular non-proprietary BIM file format out there, supported by a lot of programs, including Revit, Navisworks, Allplan, BricsCAD, and so on. The problem is that this file format is read-only and not suitable for editing. Two file formats similar to IFC are ifcXML and ifcZIP, they are an XML file with the information from the IFC data file, and the compressed IFC file, respectively.

Conclusion

Of course, these are not the only examples of different BIM file formats. However, this is a good example of a decent amount of variety between different file formats, and a competent information source about the differences between BIM and CAD systems, their files, and principles.