Wednesday, 26 January 2011

Steel Weight - A Conversation, Further Thought and Another Method

  • A Conversation
Mike: "So, the Revit weight schedule calculates using volume?"

Me: "Yes"

Mike: "So when you put a service hole in a beam, it subtracts it from the volume?"

Me: "Yes"

Mike: "But when you buy steel, it doesn't come with holes in. Shouldn't the weight takeoff reflect this?"

Me: "Ahh... Good point"

  • Further Thought
In order to overcome this, the weight can be calculated using kg/m multiplied by length. In my mind, this presents a couple of issues:

  1. How to get a steel length from a column
  2. How to make it multi-category, so it all appears on one schedule
Fortunately, our families all contain cross-sectional area figures 'A', calculated automatically by formula based on the parametrics of the profile. The kg/m value 'M' is calculated from this:

  • Another Method

The solution to both problems is to introduce a new shared parameter, 'Section Length' to all the steelwork families. This is set to 'instance' and set as a reporting parameter. A dimension is then run from one end of the beam/column to the other and is set to this parameter. Because it is a reporting parameter, whatever length the beam/column is, its value is fed back to 'Section Length'. This can then be used to calculate the weight. The following video demonstrates this concept


Anonymous said...

Here is what we do. Most of the beam and columns have a parameter called "W" for weight/foot. So I made it a shared parameter. Then in you schedule you can create a calculated value of (Lenght*W)/1'. you could also use the "cut lenght" parameter if you wanted to get more precise. Although there are some cases where the start and end extensions aren't correct. We also figured it's better to be more conservative in our tonnage.

Darren Snook said...

Thanks for the comment Dennis. What you have said is equally valid. However, the drawback with using the Length/Cut Length parameters are that these are only available to 'Structural Framing' and are hard-coded into Revit. The shared parameter 'Section Length' in the post can be used for any family category, so it allows for a consolidated, multi-category schedule. I agree that Structural Framing does have particular problems with accuracy, so it may be better report the overall length using the 'Section Length' shared parameter and adopt a more conservative approach as you've stated

BIM-practice & project workflows said...

Hi. What about cutting beams with reference planes? In this case we've got incorrect Section Length value...