Discussions on BIM are not always simple; the impacts of BIM may even have the potential to alter the roles of engineer and contractor. BIM has already begun to change the construction process. The question is: How will construction take place by the time we’re fully building in a BIM world? Early indications show that for the right projects, BIM has become a very compelling proposition even at this early stage when most people are still learning and refining their BIM skills.

Today, full 3D models are built in CAD as well as in BIM, and adding to this confusion is the fact that many BIM functions are actually achieved in CAD. This is because certain BIM functions are still more feasible in software like Autodesk’s AutoCAD than in BIM software like Revit, also by Autodesk. For example, there are many more tools available for driving sheet-metal fabrication from AutoCAD MEP than from Revit MEP.

However, here are ways in which BIM exceeds CAD.

A BIM model is more parametric; that is, changing a parameter of a BIM object (like a piece of equipment or a run of pipe) will affect any view of that object throughout the model. In CAD, multiple views of the same components might require manual updating (for example, elevation vs. cross-section views) and therefore are prone to discrepancies caused by human error.

More so than CAD, BIM is designed for other parameters which stakeholders may wish to add to the model objects, such as physical, price, procurement, performance, or even maintenance data.

Beyond CAD, BIM refers to processes as well as tools which support an evolution of the building industry toward Virtual Design and Construction or VDC. A premise of VDC is to "build it twice", meaning to complete a model (to the required specifications) first virtually on-screen before beginning to build it physically on-site. On certain projects, the savings in physical construction outweighs the cost of virtual construction, and these savings increase with the complexity of the project.

The BIM concept is sound, efficient, and productive. The creation of a virtual model before the real building goes into production presents many advantages. Problems that would normally surface during the construction stage and cause costly changes and delays will be uncovered and resolved while building the model. A virtual model can be built for a modest amount of money and it can be easily modified.

In spite of its benefits, the more substantial upfront investment and diverse skills/knowledge required to build the virtual model can hamper the proliferation of BIM. Add to this the fact that owners are used to paying smaller fees for drawings and specifications, and some like to conveniently leave the drawings partly unfinished so that they can make changes as late in the process as possible and better accommodate the needs of their tenants.

We can also appreciate that engineers like the current process better and some may hesitate to participate in BIM because they know that limiting the detail of their work will keep them less liable. As for skills and knowledge, the BIM process truly pushes the equation to the next level; we are now dealing with modeling and no longer with just detailing. The detailing process, for valid reasons, is somewhat incomplete at present, since engineers are not given time, information, or enough money to produce a fully detailed set of drawings. Detailing is simpler than modeling in that the symbols on the drawings are just symbols, whereas in the model, we have objects that contain real-world data.

When it comes to estimating, it may be some time before the virtual model includes enough building objects for estimators to complete their takeoffs without the need for additional 2D or CAD drawings. And unless the designer works for the contractor, designers generally do not want liability for the accuracy of the quantities, leaving such details to the estimators.

It is too early to speculate if BIM will move beyond supplementing CAD to someday replacing it. Many continue to find it more cost effective to build the model (or parts of it) in AutoCAD and then import those parts into Revit as needed. It is unlikely that BIM will ever literally make 2D drawings obsolete, or fully replace CAD for use on simpler projects.

While electrical and mechanical contractors continue getting more adept at modeling geometrically in CAD, the concept of virtual modeling is relatively new, and today, there are more BIM mechanical software tools available than electrical. However, given the vast market potential, it is reasonable to expect substantial progress in the development of more powerful tools in the foreseeable future. It stands to reason that, as we get better at it, greater benefits from BIM will be attained and it will become the industry standard building process for medium to large scale projects.

Illustration by Angelo Katsaros