Wednesday, August 1, 2012


You may seek him in the basement, you may look up in the air--
But I tell you once and once again, Macavity's not there!  (T S Elliot)

This post is all about drawing things that aren't there, or shouldn't be there.  How to make things disappear perhaps.  And it's about cavities: cavity walls, service ducts, boxed out bits of nothing.  At GAJ we do quite a lot of resorts, often with a middle eastern village feel.  They tend to have thick walls with niches all over the place.  The external shell  is probably concrete block, cavity wall construction with wet-trade plaster inside & out.  Awkward junctions can result.

The internal walls that are boxing out to form niches are breaking through into the external cavity wall and preventing the corner from healing nicely.  The "edit wall joins" tool fails to find a good solution, so I resort to "disallow join", then drag the offending piece of wall back, and "join geometry".  Not perfect, but much better.

We don't want plaster on the inside of the duct.  I could make wall types that are only plastered on one side, but this means cutting walls up into short lengths, so let's try a different solution.

I could use a masking region, but this is like going back to CAD drafting ... separate regions in each affected view.  What if I place the mask into a generic model family?  If this family has a vertical model line (use invisible lines) the masking region will show up in any plan where the cut plane intersects the model line.

Extending this idea into 3 dimensions I can create a family with 3 model lines (X,Y,Z) and 3 masking regions.  Give it instance parameters to control the size of this "virtual box" and it will mask out the finish layers for any view that cuts through the duct at right angles.

Another issue with cavity walls is how to show the finishes returning at openings.  I realise that this type of construction will be unfamiliar to North American readers, but it's fairly common in other parts of the world.

You can make the door and window families force materials to turn the corner.  This will affect those layers of the wall that are outside the core.  You can also make this the default behaviour for the wall. 

But I have to be honest & say that Revit is not brilliant at doing this kind of construction.  We usually end up with a few intractable junctions that just won't clean up nicely.  You can resort to drafting, you can try to embed complex sill & jamb details into window families.  But at times it all seems a little forced. 

The truth is that this kind of old-fashioned, wet-trades building work is a tough nut for BIM software to crack.  Things that are quite easy for a tradesperson on site to solve on an ad-hoc basis are problematic for the rules-based, automation of a software system.  How do you allow for all those one-off situations ?

Maybe one day we will have BIM programmes that show individual masonry units and have the intelligence and flexibility to allow me to easily explore different bonding solutions, to remove the plaster on the inside of ducts with a quick hand-gesture or vary the joint size of a particular course using a slider-bar. 

1 comment:

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