Issue:
Gating and hatching included in a 3D models is not visible in Navisworks.
Grating added by the Plant 3D grating tool shows only the grating frame. Hatching is not visible at all.
3D Model view (Plant 3D)
Navisworks View
Reference:
Autodesk Forum - Gratings not visible in Navisworks
Causes:
This issue has been recognized and there is no plan to resolve this at this time.
Solution:
There are two main work flows to have grating and hatching visible in Navisworks.
NWCOUT
The NWCOUT command will export the model to a Navisworks NWC file. Grating will be displayed as is is in the model.
To make this option most functional it is recommended that grating be drawn in a grating model that is an XREF in the models. This can then easily be exported to NWC and the combined Navisworks model updated. This does not work for hatching.
If the NWCOUT command does not work refer to this article - NWCOUT command is unavavailable in AutoCAD
Explode to elements
The second option is to explode the grating or hatching to elements.
Grating will be required to be exploded twice. The first changes the grating to a hatch pattern, the second explode changes the hatch pattern to lines.
Once exploded to elements the hatch pattern will be visible in Navisworks as a flat series of lines.
To make the lines look like a grating or a 3D item, use the extrude command to add depth. 
Alternatively add depth to the elements by changing the element property 'thickness' to an appropriate depth.
Extruded elements in Navisworks
