Felt the need to post this solution as I had alot of pain getting to the bottom of this and this in the end was the only way I found that worked and felt it worth sharing.For those not familiar with 3D dwg’s in revit. They don’t respect the cut planes for plans / elevations / sections. A result of this is they become practically un-usable for many views that we would need. There is a few ways around this if you look online (e.g. linking a DWG to a mass in place is one way). But this tends to work with smaller less complicated DWGs. For a recent job we had a particularly complex glazed roof structure. The consultant only worked in 3D in autocad and the model was quite complex, including every bolt and screw. This was a significant amount of trouble as we had to produce a number of sections/elevations that couldn’t work using the DWG we were provided.
The consultant produced a glazed roof structure in autoCAD using 3D blocks for all the various components. In autoCAD it appears as the attached image  "roof-correct-appearance.png") Which looks correct.
However when it is imported into revit (and other 3D design programs) parts are missing or become warped. (see image "Roof-after-import.png", this is after linking the model into revit)
I believe the unusual angles are caused because the blocks in autocad are using nodes points to create the shape. This looks correct in autoCAD but the nodes imports into revit and faces become joined to it. This is just my suspicion as each block was quite complex and was distorted even more when brought into other 3D packages (image attached called "zbrush.png" is from an import of the same file into zBrush where the distortion is even more extreme).
Should anyone have any similar issues to this. After some trail and error I came across a solution.
- Open the DWG in Autocad, zoom extents and delete any irrelevant info.
- Exploded blocks until only faces are left.
- Type CONVTOSURFACE then select all items. Press enter. To convert 3d faces to surfaces.
- Type UNION to join surfaces together. Select all items. Press enter. To join the surfaces together
- Type MESHSMOOTH to create a mesh from the resulting faces.
- Then purge the file and save.
When trying this method should the resulting file become quite a large I would recommend linking it into revit rather than importing directly in.
The above method will tidy up the cad and remove unusual vertices. It can then be brought into other 3D modelling softwares without distortion as well. But we still have the issue of a DWG not respecting cut planes in revit. From what I’ve read, I understand this is because an imported DWG isn’t in a category in revit (e.g. like wall / generic mass / roof / etc) and when something’s not in a category it won’t be cut.
In Google Sketchup 2015 the full version has an IFC export option. If you import the a DWG into sketchup then export a 3Dmodel as an IFC this creates something that can be brought into revit and will respect cut planes. Again I’d recommend linking the file if it is rather large, rather than importing.
A couple things to note. If the file is quite large. It will take a while to import into sketchup. Also the IFC export from sketchup will only export faces. Not lines. So if you have a site grid file in the DWG to help place the item into revit. The grid will be lost. One way to resolve this is just extruded some small cubes at grid intersections. Then when the IFC is imported into revit you can then align these cubes in the IFC with the grid in the revit model. Then hide them if they get in the way of any views (I placed them at 0.0 – sea level so the chances of them appearing in view was minimal and it helped me to see where to place the IFC interms of the Z axis).
If anyone has similar issues / questions / more efficient ways of doing the above feel free to comment.
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