Canadian Consulting Engineer

Fixing Interferences with BIM

January 1, 2011
By Jay Polding, CAD Microsolutions

How did we miss that?!"

How did we miss that?!”

Hopefully this statement or question is asked at a coordination meeting before the drawings head out to the construction site. It’s hard to quantify exactly how much is saved by finding the interference in advance – but in many cases the amount will be substantial.

The BIM (building information modeling) process enables designers to pre-build before a shovel goes into the ground. If something is wrong, it stands out for all to see. The insurance industry knows that for every interference there is a potential claim. This is one reason why their industry is taking BIM seriously, in some cases offering discounts to firms who use BIM.

Some BIM technologies that are available are Autodesk Civil3D for civil engineering and Autodesk Revit for buildings. Bentley has both BIM and Collaboration software. ArchiCAD has BIM capabilities. Keep in mind that no matter what BIM software you use, chances are it can be imported into Navisworks. Also, the IFC file format is enabling BIM models to be shared across platforms.

Fixing BIM interferences rests upon three main pillars; modeling, collaboration and communication. The principles apply to any digital design process.

Modeling first

The building must be modeled digitally first. The process may involve a team building the structural frame in Revit Structure, a team working in Revit Architecture building the walls, ceilings and stairs, and another team working in Revit MEP (mechanical/electrical/plumbing) making the building systems. As these teams work, their models are cross-linked.

Revit will automatically warn you when doors, windows and walls are intersecting. There is an Interference Checker tool inside Revit that compares beams to walls, or stairs to columns and other combinations. The tool can also compare objects in the various linked structural, architectural and MEP files. The Copy/Monitor tool will notify you when certain key building elements, like grids and levels, have changed. Of course, the user can usually just see where the problems are because of the 3D nature of the software.

Competing software

But what if the structure is done in Tekla, the architecture in Revit Architecture and the MEP systems in Bentley MEP? Also, these BIM models are rich with information and can get very large. Is there any way to just focus on collaboration and filter out some of the extra information, even if there are different types of software involved?

For many years the aerospace and automotive industries have required their factories to be designed in 3D. In an automotive project you may have different software for the design of the building, manufacturing systems and processes, as well as the final product. Yet, everything and everyone must fit under one roof, without bumping into each other. For this type of challenge, Autodesk’s Navisworks imports all major CAD, BIM and Laser Scan formats. Also, its file size is amazingly small, often just a fraction of the original source file.

You are able to query the imported models based upon material, layer, name or other criteria. Then you can group these queries into sets. The Clash Detective tool can then compare the interferences of sets of objects, even objects which move, like tower cranes. Navisworks only offers true interference checking in its Manage version. The Navisworks Manage Clash Detective can provide tolerances, coordinates of clashes, and images. This information can then be exported in a detailed HTML report.

All Navisworks versions have something called Collision Detection. This is used while “walking” though the model. You can set your avatar to a preset height so that you bump your head on height obstacles as you walk through the model. This, along with the gravity setting, creates a very realistic effect while walking stairs.  

Responsibility for errors

But who is responsible for all of this interference checking? The answer is anyone and everyone. Once these problems are found they need to be fixed on the spot if it is your responsibility, or communicated to the person who should do this. SnagIt (Techsmith) is a very useful and inexpensive technology for quickly documenting an error and then sending it over email. Online screen sharing technologies like GoToMeeting can offer teams a real-time coordination meeting.

When you have teams of designers finding interferences as they work it can feel like this is slowing things down. It forces cross-discipline communication. A few years ago a junior Revit user found that a design error was causing headroom problems in the entire stairwell. This issue triggered a major redesign which made it appear that he had slowed down the project. Of course, he saved everyone money, time and reputation. Now, he’s an office-wide BIM project manager, leading jobs of his own. While the technology helped him to find the problem, it was up to him to report it. cce

Jay Polding, LEED AP is with CAD Microsolutions in Toronto. He has 15 years of design and drafting experience and is an Autodesk Certified Implementation Expert (ICE). He is the founder and moderator of the Ontario Revit Users Group (ORUG), and writes a blog, “Revit in Plain English,” which has had over 50,000 visitors to date.


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