Friday, March 08, 2013

Concrete Volume between Slab and Beams

A message via Linked In arrived the other evening asking about the volume of concrete when slabs and beams are involved. The hopeful question hinged on the slab reducing its volume by however much the beam overlapped the slab. The answer is a qualified "yes" because it is aware when Join Geometry is used but in the opposite direction, beam reduces volume not the slab.

Here's a floor slab that is 20'-0" x 20'-0" x 6" thick and a 12" Wide x 24" Deep x 20'-0" long concrete beam reporting a volume of 200 CF and 40 CF respectively, before using Join Geometry (slab properties shown).

Here's the properties of the beam before Join Geometry as well, reporting a volume of 40 CF.

Here's the properties of the beam after Join Geometry, now reporting a volume of 30 CF.

I didn't bother capturing an image of the floor slab, its volume didn't change, still 200 CF. I hope you'll take my word for it. The point of the exercise is to determine how much concrete is really needed, at least in my mind. The floor slab volume is intact (for this particular condition at least) but any beams that are joined with the slab will reduce their volume appropriately.

That means a two schedules, one for slabs and one for beams, should provide a reasonably close total if you look at both schedules together. It might be cool if the slab could inherit the beam volume and report a total that included the beams or the reverse subtract the beams from the slab.

In the field the beam and floor, minus any topping slab, are often poured together so the reinforcing ties it all together. Curiously, the way Revit deals with beam sizes and types is generally in conflict with how engineers think of the beam size as it relates to the slab. It's never simple or easy is it?


cbaze said...

One thing I have not figured out is how to extract an accurate form contact area without either modeling the formwork or including a parameter for floor thickness in the beam. Form contact area is another critical value from the model that concrete contractors need.

Unknown said...

I'm swapping the b and h parameters of beams with shared parameters in the family. If I need the volume of beams I use =cut length * shb * shh.

Julien BENOIT said...

I also use SP for beam dims, with formulas in my schedule to have the prefab volume of the beam (Revit computed volume), and also the full volume of the beam (AxBxCutlength) to have the reinforcement estimated (volumexratio). I have a calculated value to check whether the geometry is attached or not. I'm fine with Revit on this subject in fact :)

Unknown said...

I too use shared parameters for b & h. But yes, it is difficult to get that SFCA , square foot contact area for form work takeoffs, because the side area will be reduced by the slab thickness. Right now, I'm creating an instance parameter for slab thickness right in the beam family, not that intelligent, but gets the job done.