How to Model Slab Setdowns

You may require slab setdowns for Wet Areas or Floor Boxes. There are a number of different methods for modelling slab setdowns. The best method to use depends on the circumstance. For example:

Create a hole in the main slab and model a separate slab beneath it of the required setdown, or
Use a Void family to partially cut the depth into the slab, or
Create a hole in the main slab and model a separate slab beneath it of the reduced depth to achieve the setdown

Method 1 Use

When the slab has a thickening at the setdown for structural purposes.

Method 2 Use

When the slab soffit is to remain flat. For example when the slab depth is already sufficient or the setdown/cutout is minimal such as recessed floor boxes.

Method 3 Use

Method 3 is just an alternate to Method 2. It achieves the exact same result, but requires more floors. More floors means more attachments of Walls below, more joining geometry, and more coordination effort. As such, Method 2 is recommended instead. It’s only real benefit over Method 2 is the ability to correctly tag the thickness of this floor. Something as Architects we don’t do…

CRITICAL:

As a side note, DO NOT model Concrete beams as floors. Always use Structural Beams. This way the thickness and depth of the beams is managed by type and much easier to coordinate with the Structural Engineer. They will also automatically join with Structural Floors of the same material.

Method 1: Two separate floors

Edit the sketch of the top floor and add a loop to create an opening.
Create another Floor with the Height Offset from Level set to a the required setdown. For example, -50. There will likely be an overlap for the transition of the slab fold, in the below example 450mm is used. The slab fold will be determined by the Structural Engineer.
Select Modify ► Geometry ► Join Geometry
Select the Main Floor then the setdown floor. This will clean up the overlapping parts of the floors.

Slab Setdown with Thickening

NOTE:

If the setdown extent changes you will need to coordinate the lower floor to ensure the slab fold distance remains consistent.

Method 2: Void Family

Load the Family from the Library, Box-Structural Framing-Generic-Void.rfa
Preferably in a Plan view, place the void family. You can place roughly first and then adjust its size using the grips or the instance parameters. It will snap to any walls.
Set the Host Cut Offset parameter to the desired setdown amount.
Select Modify ► Geometry ► Cut Geometry
Select the Floor or Beam to be Cut then the Void Family. Repeat the Steps 4 & 5 for each element needing to be cut.
The Floor has now been cut as required.

Set Host Cut Offset & Align with Walls

Finalised setdown