A few weeks ago Steve Stafford wrote a blog post about the use of voids in families. It appears the Autodesk documentation for Revit recommends to “avoid voids” because of performance issues. I found this interesting as I tend to use this functionality a lot when making sure modeled objects cut each other as they should according to construction. The automatic joins and Join Geometry gives me pain in over 50 % of the situations where I need to do this, and when Autodesk introduced the “Cut with Voids When Loaded” option in the Family Editor they opened up a lot of efficient 3D detailing potential.
Of course this requires that someone sets up all families with voids, and this can be tedious work. Especially with constraints (you want the voids to follow the same rules as you solids) and nesting. But once you’re done, the models can reflect reality way faster and more intuitive than before.
If you are into 3D Rebar, you will notice that using voids to cut will produce Rebar Cover references for your reinforcement modeling. That’s a good thing.
I use this functionality on more objects than Structural Foundations. For instance prefabricated slabs embedded in cast in place structures, or columns cutting slabs with a tolerance. The options are endless.
So, what does not work very well when you’re playing around with this? First, there is a graphical bug in Revit 2013 that occur “sometimes” when you do not use a voided family for cutting. In some cases Revit views will display the yellowish void instead of geometry. This can badly affect your drawings if you print with color, so beware. Second, you always have to Cut Geometry manually. There is no automated procedure to do this. Again though, the upsides are greater than the downsides.
One can always hope Autodesk adds some new functionality in the future, and high up on my list is the ability to use family solids for cutting. The void cutting is okay, but in most situations I am looking for my solid geometry to cut something. An option to enable automatic cutting like in Wall-, Floor- and Face-based families would also be favorable.
I’ve always thought Revit lacks a way to control the construction procedure of elements, and something like a hard coded “operation sequence number” (almost like Phases) could provide helpful. Try to imagine a cut priority connected to placement procedure. Pile = 1, Concrete Foundation = 2, Foundations Slab = 3, and so on. And the lowest number will always cut the higher number. From a structural and construction point of view that would make sense.
Last I’d like to use this opportunity to show a small detail I’ve added to all our circular elements (foundations, columns and openings). It’s a tiny cone that is hidden by default in most view, but gets exported to IFC so that site engineers and land surveyors can use our models to pinpoint the concentric global position of each element.
With the “avoid voids” mantra in mind, lets hope the Autodesk documentation needs a few revisions rather than my project models.
Download one of the Steel Piles from Content.