Limitations of converting AutoCAD to Revit

You often need to convert AutoCAD to Revit. At times to convert "Standard Details" to Revit or use DWG files to build a Revit Model. This article focuses on using DWG files to build a Revit Model as well as limitations of the process. Revit Model that is built up using AutoCAD will not be an exact replica in terms of "Graphic Representation". This post will help you be aware of such limitations.

AutoCAD to Revit model is not automatic. It is used as an underlay to create the Revit Model. When you start building the model, there may be a mismatch between DWG wall lines and those in Revit.

AutoCAD Drawings are composed of geometric elements and contain no information about building elements. Revit models are composed of building elements, such as walls, floors and so on. Conversion process requires importing or linking DWG floor plans into Revit and then using imported CAD lines are used as guides to build Revit walls, openings and staircases etc.  Many of these Element thicknesses may not graphically match up.

Modeling Building Elements

DWG file is imported in Revit and used as a reference to model elements. Problem occurs when things do not match up.
As an example, an 8” CMU is drawn using parallel lines offset at 8”. In reality, an 8” CMU is "7’-5/8” thick. In Revit, one can easily define a wall with a core and all the skins.  Similarly, stud walls may be drawn using nominal sizes with materials and skins along with thicknesses of each. This may not have any serious implications in AutoCAD. 

In contrast, Revit models are often used to generate quantities. Inaccurate definitions would result in wrong quantities. As a result, wall lines do not match up.
The best option is to use center-line dimensions to build the model instead of aligning with wall lines. Never trace the imported DWG. Instead, always read dimensions if available in AutoCAD. If not, create Grids and read critical dimensions to remove any discrepancies.

Blocks & Families 

Most of your Plans, elevations, sections look almost identical to your typical AutoCAD drawings. However, there may be small differences, such as Doors & Windows in Plan, Furniture Elements and so on. even though they look similar in elevation. AutoCAD uses Blocks, Revit uses Families. And they may not match up. It is relatively easier to handle Doors & Windows. Most of these are available for download from Autodesk or other sites. In case of furniture however, modelling these to match DWG plans or elevations can be difficult.

Fonts

Earlier versions of AutoCAD used shape fonts or SHX fonts. Later versions can use both SHX and True Type fonts. Revit does not use SHX fonts and if your drawings have used SHX Fonts, they need to be replaced with “True Type” fonts and then also the two are not an exact match.

Units

AutoCAD AEC (Architecture, Engineering & Construction) disciplines use drawings drawn to full scale in either Feet & fractional inches or millimetres. The drawing units in AutoCAD should either be “Architectural” or “Metric”. There may be times when you come across drawings where dimensions may show imperial units, whereas actual unit settings in AutoCAD may be metric or engineering. When you import DWG in Revit, scaling may go hay wire. The imported DWG may look very large of extremely small. By default, Revit reads default DWG units and uses this to scale drawings.
Check this out before importing. Type the following command in AutoCAD: 

Lunits → Enter → “4”  or "2"

if unit settings are Feet & Fractional inches. If they are in “MM”, it results in “2”. Make sure you the units in DWG and Revit Match up. At times you may need to specify import units.

In the next post I’ll discuss Tips on importing AutoCAD in to Revit