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BIM 101 – I Use CAD But What Is BIM?

8 September,2020

BIM 101 – I Use CAD But What Is BIM?

For many CAD users out there BIM seems to be a fancy buzz word for 3D CAD, but BIM is far from a fancy 3D tool.
What is confusing people is that they hear that Revit is a BIM tool, so they think that this means Revit is a total BIM solution. No, BIM would be better described as a process or workflow. Let us start at the beginning with a description of CAD and then describe the BIM process in simple terms so as to hopefully clear things up.

A Review Of CAD

CAD is the acronym for “Computer-Aided Design”. Whilst other people refer to it as CADD which means “Computer-Aided Design and Drafting”. CAD is simply the use of a computer system to assist with design and drafting. AutoCAD is a prime example of a successful CAD (drafting tool) that is used around the world to draw everything from Buildings to Machines. It is essentially used to draw Lines, Circles, and Arcs to create whatever it is you want to draw.

People using CAD at a high level will have CAD Standards in place (Layers, Annotation Styles, Templates etc.), as well as Libraries of standard details and Symbols (Blocks) that they use extensively, they most probably also have a lot of Customization (Programs, Menus, Macros, Scripts etc.) in place to speed up repetitive tasks. Others may also have additional and/or third party software programs that sit inside their CAD program that are specific to the work that they do and assist greatly with the design and drafting of the projects they work on (Autodesk examples of this would include AutoCAD Architecture, AutoCAD Civil 3D, and AutoCAD Plant 3D to name but a few).

Now Let Us Look At BIM

BIM is an acronym for “Building Information Modelling”. So, whilst “BIM tools” such as Revit are technically 3D CAD programs, the term “CAD” is used more for 2D drafting programs.

ISO 19650:2019 defines BIM as:

“Use of a shared digital representation of a built asset to facilitate design, construction and operation processes to form a reliable basis for decisions.”

So BIM is an intelligent workflow, based on a 3D model using a series of “BIM tools” to perform a variety of design, construction and administration tasks for buildings and infrastructure. It is a process for creating information models containing both graphical and non-graphical information in a Common Data Environment (CDE), which could be described as a shared repository for digital project information.

To help clarify this more let us look at a simplified Building Project and how BIM fits in. Basically, we are dealing with three (3) main phases of a building and its lifecycle, the buildings “Design”, the actual “Build”, and the ongoing day-to-day “Operation” of the building. I have put together an example workflow that in reality will vary from project to project, and will also vary depending on what companies are involved and what tasks they are assigned.

Within each Main Phase there are a series of tasks that are being completed by different people.

So how does BIM help us with manage the lifecycle of a Building? Well, BIM process starts out with a 3D model, which is used throughout the whole process. Let’s look at this process/workflow in some detail:

Business Information Modeling


  • Programming
  • Conceptual Design
  • Detailed Design
  • Analysis
  • Documentation


  • Construction (4D [work out timeline], 5D [work out costs])
  • Construction Logistics


  • Operation and Maintenance
  • Renovation (when we are here the whole process starts again)

So how does BIM help us with manage the lifecycle of a Building? Well, BIM process starts out with a 3D model, which is used throughout the whole process. Let’s look at this process/workflow in some detail:

1. Architects

Using a 3D modelling product, such as Revit, the Architect will:

  • Do their design in 3D.
  • Give presentations to clients and interested parties so that they can fully understand what the Architect is proposing.
  • Using the 3D Model extract their construction documentation.

2. Engineers

Normally the Model will be “Shared” so that the Engineers can work on the same model that the Architect is working on. There could be one (1) or many Engineering companies involved doing the following tasks:

  • Structural Design using the Structural capabilities of Revit to add their 3D design to the model and extract their construction documentation. They would also use Design software such as Robot Structural Analysis on the 3D model.
  • Civil Design using a product such as AutoCAD Civil 3D and “Linking” it with the Revit 3D model to add their design and extract their construction documentation.
  • Then comes the MEP (Mechanical, Electrical and Plumbing) teams that can use the MEP capabilities of Revit to add their 3D design to the model and extract their construction documentation. They could then use MEP Analysis software on the 3D model to check their systems.

3. Quantity Surveyors

Again, using the “Shared” 3D model, and associated data the Quantity Surveyor can perform the tasks of:

  • Material Take-Offs can be done by extracting the information from the “Shared” 3D model. This of course will depend on what “Level Of Development” the 3D model has been taken to… simple terms, how much of the building has actually been modelled from the largest to the smallest items.if items are not modelled but shown in 2D drawings as details then the Quantity Surveyor has to do some tasks manually.
  • Cost Analysis be done using additional software using the Material Take-Off data extracted from the “Shared” 3D model. For your own information this process is often referred to as part of the 5D (Costing) task within the BIM workflow.

4. Project Engineers

Using the “Shared” 3D model and associated data the Project Engineer can:

  • Analyse the “Shared” 3D model to work out what gets built and in what order using products such as Microsoft Project. Which will help them with important aspects of the project such as “Critical Path”. This helps them plan the whole building process against a timeline.
  • Use the Phasing capabilities of Revit to help work out what parts of the building get built when, and in what order over a timeline (4D). For your information 4D is a term used within the BIM workflow for Construction Sequencing. Navisworks is used extensively for this task as it allows you to used the “Shared” 3D model and link its building phases with products such as Microsoft Project so as to get a visual result (video) of the whole building process.
  • Using these tools the Project Engineer can put together all necessary reports and documentation that they require.

5. Contractors

Contractors can access the “Shared” 3D model and associated data to perform tasks relevant to them.

  • Before commencing building the Contractor can use the “Shared” 3D model for tasks such as “Clash Detection”. This can be done using both Revit and Navisworks.
  • They can also use products such as Microsoft Project along with the data used from the Project Engineer to assign the tasks to different people involved in the building process.
  • Contractors involved in the Design/Manufacture of items used within the building (often MEP related) will use a software product such as Inventor Professional to perform tasks such as Prototyping, 3D Design, and 2D Documentation of the required item. This 3D model can be taken back into the “Shared” 3D model.

6. Clients

The client will then take the “Shared” 3D model and associated data to manage the ongoing lifecycle of the building to perform tasks such as:

  • Facilities and Asset Management by linking the “Shared” 3D model and associated data with their selected Facilities Management software package such as MEX, MyBos etc.
  • If they later need renovations/alterations done to the building they can put together a Project brief using all of the data that they currently have to start a whole new BIM cycle again.

In Conclusion

If you are moving from a 2D CAD solution you may move to Revit as a 3D CAD solution. Here you have access to 3D Object driven tools that allow you to quickly create 3D models of buildings using Walls, Doors, Windows, Roofs etc. You can then quickly create linked 2D documentation/drawings that will automatically update with any changes to the 3D model. You also have access to many other tools inside Revit that allow you extract data for a wide variety of Schedules, you can create Walkthroughs, high level Rendered Images, Sun Studies etc.

However, remember that by moving to Revit you are also moving to a vital BIM tool that will open a lot more capabilities and projects to you and your company. Welcome to the world of BIM!!

Interested? Talk to Our Team Member