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

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

Unfortunately, it is not so easy. Since the differences between BIM and CAD extend far beyond the file formats, the transition is much more complicated than just the data conversion process. Even the history of these two terms is entirely different, and history is what we will start with.

History of BIM

BIM has existed as a technicality since the 1970s, around the same time building modeling software tools appeared. However, they were expensive and unsuitable for widespread use. The term “BIM” was first used in 1992 in a paper by F.P. Tolman and G.A. van Nederveen. BIM owes the beginning of its wave of popularity to Autodesk, since the software company published a white paper titled “Building Information Modeling” in 2002.

Although BIM is not as old as CAD, it has always had its niche: architecture. Architects are the prime target audience for BIM, as it provides advanced features and simplifies the entire design process. The other benefits of BIM are a consequence of its original purpose, which has proven to work well. BIM’s popularity is growing and it regularly influences how construction projects are designed and executed.

History of CAD

CAD has a much longer history than BIM, although there is no single event attributed to its creation. Two events are often cited, one in 1957 and the other in 1960. In 1957, Dr. Patrick Hanratty released Pronto, the first programming system that used numerical control. Dr. Hanratty is often called “the father of CAD” because this event is the earlier of the two. In 1960, MIT student Ivan Sutherland created Sketchpad, the first program that allowed one to create technical drawings with a computer.

The debate about which event is the true origin of CAD is ongoing, but both contributed significantly to the industry and the creation of CAD software. CAD software continues to evolve and change today, with aerospace technology being just one example of how important CAD is for modern-day construction tasks. CAD has become a de facto standard for any form of engineering and is a job requirement for engineers.

CAD and BIM: the definition

CAD

Computer-aided design (CAD) is the use of computer technology to create design files and documentation. It is 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 over the past thirty years, making it easier and faster to work with more complex projects.

The widespread adoption of CAD software began quite a while ago. One of the first deciding factors was the long list of manufacturing guidelines from the automotive and aerospace industries that forced manufacturers to adapt their processes to keep up with the demand. This started about two decades ago, and CAD is now a requirement for players in the industrial market to be at least somewhat competitive with others.

Some of the most significant advantages of CAD as a standalone toolset, and without the BIM-CAD comparison in mind, are:

• Easier communication – it is an easy way for different teams to communicate with each other about specific parts of a project and with the help of an existing 3D model.
• Feedback and user input – the ability of CAD to take the most unusual concepts and turn them into comprehensive designs in three dimensions is a massive advantage in the design phase that cannot be ignored, and it allows different teams and specialists to give feedback in their specific fields of work.
• Visualization – while this ability offers many different advantages, the mere fact that project ideas can be visualized at the earliest stages of project realization allows for much more perspective and insight.
• Structural engineering – CAD’s ability to offer specific capabilities for various industries allows it to be a highly versatile tool that can cover multiple approaches to design and planning.
• Comprehensive tools – CAD offers many features that help with better project visualization and, at the same time, provide an unprecedented level of customization.

As with almost any existing technology or system, CAD also has disadvantages, some of which are also on the list of differences between BIM and CAD. For one, CAD is quite a resource-intensive technology, even though it has been around for a while. One of the prerequisites for CAD is a cloud server, which makes the cost of implementing CAD far higher for more conservative companies.

Additionally, despite all of its attempts at user-friendliness, CAD software is still relatively tricky software to get used to. There is a learning curve, and there is a training cost. This also limits the number of CAD experts in the field. Another problem of CAD is the disconnect with reality when it comes to assembling different parts. It can be done in CAD with ease, but in reality, these parts must be welded or fastened together, which is quite different from just connecting one detail to another in CAD software.

CAD is still used for many different purposes today in the vast majority of industries, including civil engineering, manufacturing, plant design, industrial design, and so on. The most popular file formats for CAD are DXF, DWG, IGES, STEP, SAT, and others.

BIM

Building information management (BIM), on the other hand, is an entirely new process of collaboration between different parties to design and build projects using unified models in unified databases. 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.

Technically, the Great Recession of 2008 was one of the most significant contributors to today’s widespread adoption of BIM. It resulted in the defunding of many commercial building projects, and even more were put on indefinite hold, stopping the entire massive industry. This was when AEC firms took the initiative and started reshaping their operational processes to include BIM as a new approach to design and construction with a lot of emphasis on collaboration. That change also led other project participants to update their approaches to construction to keep up with the amount of information that the AEC industry could now provide while also reaping all of the other positives of BIM as a completely new approach to the construction process.

One significant advantage of BIM is that BIM files can show digital representations of various would-be facilities with information-rich models, including electrical systems, HVAC, various aesthetic parts like windows and doors, and so on. The most significant cornerstone of BIM is collaboration, first and foremost.

Of course, BIM provides other advantages, again without thinking of the comparison of BIM and CAD, such as:

• Error spotting – it is far easier to analyze the initial design with the help of different teams with BIM since it gives all of the various crews access to the same project design.
• A project as a process – BIM software presents the entire process of creating a building as a series of steps, allowing for different measurements and considerations to be done as early as possible to prevent the need to reconstruct the entire project.
• Clash detection – on a more automated note, BIM provides clash detection for multiple different materials and object types, down to the soil materials, roots, and rocks in the ground that might interfere with the creation of the project.
• Beyond 3D – BIM solutions can include multiple project dimensions in a single working process, as well as cost calculation, time constraints, thermal and acoustic properties, and more.

Like CAD, BIM has its share of shortcomings and issues. The most significant challenge is the lack of widespread adoption in the industry. Although BIM has become a more popular approach to construction in recent years, many companies still hesitate to adopt it. This reluctance can create unnecessary issues during inter-company interactions on any project.

At least part of this hesitation is understandable, since it can be expensive for companies that have never used BIM before to adopt it. While the long-term benefits of adopting BIM are worth the investment, the one-time massive cost is a major deterrent for many companies.

Another potential issue for BIM adopters is that it is not yet the industry standard. The lack of experts in the field of BIM, particularly when compared to older industries like CAD, makes it challenging to find BIM specialists to train personnel.

CAD vs BIM

Of course, there would be no comparison between BIM and CAD if one of them were perfect in every way. Even BIM has potential problems that may arise in the future, starting with the problem that every new technology has in any field: compatibility.

What is the biggest issue for BIM?

BIM is not as new of a technology as it was a few years ago. The widespread adoption of this approach combined with the variety of advantages it provides is quickly changing the overall opinion of the industry. And yet, BIM software still has plenty of issues to go over, including one of its biggest problems: interoperability. There is still no universal compatibility for all possible branches of the construction industry, even though efforts in this regard are made regularly.

Why is BIM preferable over CAD for large-scale projects?

Even though BIM as we know it today arose quite recently, there is already a lot of demand from owners to provide an as-built BIM model at the end of the construction process, forcing companies to transition sooner rather than later.

Companies themselves have been building their processes around CAD software for decades now. However, the rise of BIM as a methodology has led plenty of clients in the field to understand how big of a difference in efficiency and performance can be gained. As such, the customer side of the construction market has begun to raise its standards regarding project speed, budget levels, and so on.

At this point, it is exceptionally challenging to stay competitive while not using BIM methodology in one way or another. There is a limit to how effective a company can be while only using traditional workflow management approaches, and the capabilities of a proper BIM integration are much greater than any CAD-based system can offer.

What are the major technical differences between CAD and BIM models?

The viewing experiences of CAD and BIM models differ quite a lot. First of all, the primary purpose of CAD models is to offer highly detailed representations of structures using a somewhat static viewing angle. A BIM model, on the other hand, is made to be dynamic from the start, creating opportunities for zooming-in, zooming-out, and other dynamic actions that may be difficult to perform with CAD models.

The main reason for this difference is the context of different model parts. CAD models do not have the knowledge of how elements can be connected to one another outside of the fundamental static perspective, meaning that things do not stay the same when zoomed in or expanded.

A BIM model, on the other hand, includes the precise context, since it deals with objects instead of geometric elements, and these objects have some form of “logic” behind them (doors are attached to walls, floors are attached to foundations, etc.), making it possible for this logic to be considered when attempting to create a different perspective of the same product model.

Is it possible for CAD models to be converted to BIM models?

While the conversion itself is not impossible, it is definitely tricky. The biggest issue is compatibility between the different approaches used to create CAD and BIM models. Even CAD models with rich data sets may not always be translated entirely into the BIM workflow purely because the CAD model’s data formatting is incompatible with the BIM workflow.

It is worth noting that there are plenty of exceptions to this rule. One of the most notable examples is the software from Autodesk. Transferring files from AutoCAD in the Autodesk Construction Cloud (ACC) is relatively painless, and the process results in little to no information loss. The same logic can be applied to other software from the same developer, and there are also separate conversion services, but they are often not as effective.

Direct comparison of CAD and BIM approaches

Comparison of CAD and BIM software

The table above offers relatively basic information about the differences between CAD and BIM. However, many other differences can be showcased only using specific BIM and CAD software solutions. We can try to highlight these differences using two groups of examples:

• Revit vs SketchUp

Revit is far more complex in comparison with SketchUp. Both can work with 3D models, but only Revit can add information to different model elements (HVAC, plumbing, etc.), creating a BIM model out of a CAD model. SketchUp is easier to pick up at first, but learning its full capabilities takes a while, and it does not offer any kind of BIM-adjacent features by default. Those capabilities can be introduced with plugins to a certain degree, but none are enough to create a complete BIM competitor to Revit out of SketchUp.

• ArchiCAD vs AutoCAD

Both solutions offer drafting and modeling capabilities in both 2D and 3D, but the other features differ greatly. AutoCAD is a well-known CAD application with a steep learning curve and a high level of design and rendering features. ArchiCAD is a BIM solution with a slightly gentler learning curve and numerous collaborative capabilities, including workflow management, data sharing, etc.

While CAD and BIM are different systems, it is common for them to coexist. The relationship between the two is symbiotic, since BIM needs detailed models to work with, and CAD cannot provide as much information about every aspect of a model as BIM can. Almost every BIM software application includes CAD tools, and most CAD software now has some BIM features, which has resulted in a close connection between the two systems. This connection is likely to become even closer in the future.

Specifics of BIM and CAD files

It is 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 specifically construction-related tool which is often used to design and construct buildings, including schools, airports, offices, etc., but it is quickly becoming the new industry standard in general.

The additional information 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 the pressure ratings of a certain part will allow the detection that the part is not made of the correct material to handle the pressure to which it will be exposed. Understandably, various characteristics of models, especially performance characteristics, take up a lot of space in the context of CAD files and are generally deleted in the process of CAD-to-BIM conversion.

CAD & BIM file formats and data types

The answer to the question “What is a BIM file?” is closely tied to the various file formats that BIM platforms can and cannot work with. It is easier to digest by dividing all formats into proprietary and non-proprietary formats.

Proprietary file formats

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

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

A popular myth about the 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 by using full components/blocks. There is also the DXF format (drawing interchange format), which works with BIM drawings. It is similar to and somewhat larger than DWG, but it has the same interoperability level as most CAD platforms.

Regarding proprietary file formats, sharing data between different software solutions can be somewhat complicated, especially if both solutions lack native support for the same file format. As such, there are four possible approaches to collaboration in this case:

• Attempt to find a plugin that provides interoperability between two specific BIM solutions, if such a plugin exists. These plugins are not only developed by software providers but also by independent programmers or companies.
• Export the BIM model in a different file format if there is a file format that both the sending software and the receiving software support.
• Remodel the necessary parts from scratch using different BIM software.
• Convert the BIM model to a non-proprietary file format such as IFC. This step may lead to the loss of some of the more sophisticated elements of the model during the conversion if the IFC format does not support them.

This last approach leads us to change the subject to non-proprietary file formats.

Non-proprietary file formats

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

Non-proprietary (open-ended) formats are vendor-neutral, often open-source, and developed via international community collaboration. A few examples are:

• COBie (construction operation building information exchange) is a BIM format that allows the sharing of asset data, rather than geometric or graphical data. It can be used to transfer documents through different project stages, from design to construction. COBie files are created by converting Excel sheets with all the information that has been embedded in a BIM model. It simplifies the information delivery process while also boosting collaboration due to the fact that the file type is non-proprietary.
• IFC (industry foundation classes) is the most popular non-proprietary BIM file format and is 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. IFC files offer many different BIM data categories, including shapes, materials, geometries, and spatial data. Two file formats similar to IFC are ifcXML and ifcZIP, which are XML files with the information from IFC data files and compressed IFC files, respectively.

That is not to say that these are the only examples of BIM file formats. There are many other examples of proprietary extensions in different BIM software applications, and the non-proprietary sector is still developing. The .BIM file format is a very recent addition to this list, for example.

Dotbim, or .BIM, is a BIM file format that was initially released in 2022. It is a simple and open-source file format that is also completely free, and it promotes interoperability and collaboration across different solutions. Its documentation is one page long and includes everything that may be relevant or worth conveying between different iterations of BIM models: geometry and information.

The geometry element of the .BIM file format is transferred only using triangulated meshes. This decision was intended to simplify interaction and export from one solution to another. Many BIM solutions support different types of geometry, and the supported geometry types rarely align with each other. Using a single very common geometry type makes it a lot easier to evade potential compatibility issues in exporting (such as parts of the geometry disappearing because one of the solutions does not support a certain geometry type).

The way .BIM files handle transformations is also relatively simple: there are only two types of transformation available in the first place. “Vector” describes the placement of the mesh, and “rotation” rotates said mesh into the correct position.  This simplicity also saves space regarding overall file size, since the same mesh can be reused and placed throughout the model multiple times and .BIM can just retrieve the same mesh multiple times to build a model.

Information stored in a BIM model using .BIM is also not particularly difficult. It is possible to attach data to every separate element of the model or to the .BIM file itself. The data is stored using a simple dictionary, and a basic “key-value” system attaches specific information to a singular element (or multiple elements).

The last interesting element of .BIM that is worth discussing is the fact that there are no future development plans (unlike for IFC or COBie). The goal of .BIM was to create a simple file format that could be opened by most BIM solutions, and trying to add more features would only result in this goal being more difficult to provide due to compatibility issues.

Going back to the topic of CAD and BIM, there is something else worth mentioning here regarding compatibility and conversion. While converting CAD files to BIM is technically possible, it can result in the partial or complete loss of data, especially when working with advanced CAD files with rich data sets. To ensure the safest conversion, it is best to use software from the same developer. For example, Autodesk provides an easy conversion process from AutoCAD to Autodesk BIM 360 that does not result in any loss of the data in the original CAD model.

The future of BIM and CAD

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

It is easy to envision CAD and BIM continuing to develop alongside one another, incorporating more advanced technologies to streamline the design process. It is possible that there may come a time when drafting a project from scratch is no longer necessary. Instead, a system might automatically pre-render several building options if it has information such as load capacity and optimal footprint.

Recent advancements in artificial intelligence such as ChatGPT make this future more possible. Technology companies are currently competing to add AI to various parts of their products and services. For instance, the most recent version of ChatGPT (version 4) can simultaneously operate with text requests and pictures. As a result, the idea of an AI that can provide a detailed CAD model of a building in seconds no longer seems far-fetched.