Tuesday, September 09, 2008

Floors - Voids and Various Rumblings

[Note: The 2010 release of Revit has resolved this issue.]

I wanted to use a floor hosted family to create a "waffle slab". Simple idea really, just create a four void blend cluster that could easily be placed in a "thick" floor slab to result in a waffle slab. In experimenting with it I discovered that the floor volume is not altered unless the void cuts both the top and bottom planes of the floor. If the void only removes some of the floor material it does not alter the floor volume at all. Here is an image as an example.

Using Revit now for a little over six years I've managed to convince myself that it wasn't always this way...but I can't be too sure anymore as my memory isn't what I used to remember it to be. I also tried the trick of using it in an In-Place family too...no difference. Ideally a void ought to subtract volume from a floor regardless.

A second issue with this approach is that if it is used in conjunction with a floor slab created by Revit Structure that uses the metal decking profile feature...the void family will kill the display of the profile for the entire slab.

These two issues "killed" this approach for me, shame too as it would have been "easy".

A third oddity before I go, regarding the new Mass Floors feature. If you try to create a Calculated Value that uses the Floor Volume parameter Revit will reject it as an invalid parameter name. Trouble is that it is a system parameter! Here's a couple images that depict the issue.

When you examine the parameter heading value under the Formatting Tab in Schedule Properties you'll find an "extra" space at the end of the parameter name. It's my guess that this is getting Revit annoyed.

Oh all right, since I'm on a roll one more...this is hardly the only such element with this issue but you can't tag a floor with Area, Volume or Perimeter data even though the element clearly knows what they are.

Per a comment I've POSTED the file if you are interested in the "waffle" family though it doesn't really "work" for the reasons that I listed above.


Anonymous said...

Your waffle slab idea is very interesting. I would like to see how you made it. Any chance you could upload it?



Steve said...

Refer to the link at the end of the post.

Aaron said...

re: issues nos. 1 + 3

1. What if you modeled two floors, touching, join geometry; the bottom is just thick enough to be cut through?

3. What of you get your material quantities by material vs. catagory?

Steve said...

1) Total volume is acquired by adding two floor together - works technically but not obvious because you have to schedule and sort to group joined floors so you can see the total volume for a "floor".

2) No difference - Material takeoff reports the same area and volume as the floor.

Anonymous said...

We're wresting with the very samething for Post-Tensioned Waffle mat slabs- would be nice if the voids and floors played properly together! Here's an idea though- what if you were to have a VERY small hole the top of which was aligned to the Reference Level, say in the center of the void, that continued through to the top surface of the slab. The amount of material it removed would be negligible- and the void (in our case a pyramidal frustrum) would then continue through to the top of the slab.

We're testing it now...

Steve said...

Interesting idea which should work but I don't know how much volume it will remove. The other issue is that "negligible" amount would add up over the entire floor area. It would need to be really small not to add up to "something". 8-)

Do post back on your findings!

Steve said...

Sorry, doesn't appear to do anything to help.

rgesner said...

While voids can be used to create a waffle slab on a small scale, on a large scale we find that they are so computationally intensive as to be impractical. We instead use a grid of intersecting beams. Even this is pretty "heavy" for Revit to process, but by putting all inner waffle beams on a separate workset which can be unloaded except when needed, and keeping edge beams on the main concrete workset makes it acceptable.

Steve said...

Yes, seen this approach many times too. The added advantage of doing it is that you can create beams that taper the way a waffle slab really would. It makes for much more convincing section and detail views.