Friday, 23 March 2012

Why Windows in Revit Don’t Cut it.

Brok Howard, BIM manager for our St Louis office gives some advice on why windows don’t cut.

We ran into an issue last week where a window family was not cutting in plan as would be expected. For example purposes I will demonstrate what was happening and why this might happen with your window content. I will use the out of the box content that ships with Revit if you want to follow along. It also starts to address how this might be useful for some instances.

In the example below we have three windows, each at different sill heights.

The first one cuts as expected, the second is going through the sill and the third is above the cut plane of the view. The view is set to 4’-0” and in the elevation above I am showing where this is cut in the view.

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Again, this would be as expected. If we move the cut plane in the view up, so it only cuts the second two windows, we no longer see the window below in plan.

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No, for the interesting part of how windows work. The assumption we have made so far is that when you move the view range cut plan that is what is changing the way the window is being cut. Let’s investigate this further by opening up this family.

Opening the family and then looking at the Floor Line view shows us this.

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Notice on the Properties pallet we do not see any information about a cut plane. clip_image008

Now click where the Family:Windows drop down menu in the this section and change it to Floor Plan:Floor Line, now you are seeing the view propery settings.

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Now click on View Range. You will find that the default Cut Plane for this view is set to 4’-0”, same as the model.

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Let’s now change this to 7’-0”.

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The other thing we are going to change is we are going to remove the symbolic lines in the family so we are seeing the real solid geometry.

And finally we are going change the default sill height to 6’-0”. You should end up with something that looks like this.

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When we load this new family into the project we end up losing the second windows in plan. This is due to the fact that the symbolic lines were really what was being represented in plan, not the window. What we are seeing is the opening from the family.

Now, we will go back to the family and change the geometry to now show in plan by selecting them and changing the settings to show in plan.

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Now we are seeing the family in its true modeled elements.

Note that now we are seeing the third window. Why are we able to see the window with the cut plane set to 4’-0”?

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Because windows are not really cutting. They use the internal cutplane in the family in combination with symbolic lines.

But why is this? If you go to the Visibilty/Graphics Settings it clearly shows the option to cut them?

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Now we will change the cut plane in the model to match the cut plane of the one in the family.

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We see the geometry but not what you would think, now select one of the windows. All the detail is there, but we only see it as if it was not cutting.

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Now change the default sill height in the family back to 3’-0” and the cut plane back to 4’-0” and reload the family back into the project. Now change the floor plan view range back to 4’-0”. What you get is this.

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Until window families are really, truly cutable geometry this is what you will get. And if you have ever created window content from scratch, this will be important to keep in mind because you will need to make all your window geometry content not show in plan and not cutable, then use symbolic lines to make them show correct in plan.

This is the farthest family from BIM that I have found so far, it would be good if Autodesk addresses issues like then rather than create workarounds. But at least now you are equipped with the WHY on windows.

7 comments:

Clay Starr said...

We've been adding vertical invisible lines to our windows and doors for years to address this problem. All Revit needs is to cut 'something' in order for the family to show properly. By cutting the line, it will even show proper floor plan symbolic lines.

akg980 said...

Thanks for this post, its a very common problem that needs sloving

Andres Franco said...

Great tip, I try to follow up all the steps, and as a certain way I made the changes as I need, the only thing is the level of representation, I try to make my windows equal as you said but I don't know where I lose myself because the windows still appearing with a minimum level of detail!!!, well I guess I still studying Revit.

Andres Franco said...

I try to do that but I can't hack it!!! my windows still appearing without a good level of detail.

Andres Franco said...

Great tip, I try to follow up all the steps, and as a certain way I made the changes as I need, the only thing is the level of representation, I try to make my windows equal as you said but I don't know where I lose myself because the windows still appearing with a minimum level of detail!!!, well I guess I still studying Revit.

Dave Baldacchino said...
This comment has been removed by the author.
Dave Baldacchino said...

Window families are not the only category with this behavior. Basically, a "symbol" for cut representation is stored in the family and used in the project environment if the project view cut plane crosses the family. This symbol is represented based on the family built-in plan view cut plane. If the family plan view cut plane does not cross any geometry (ex: below the opening), geometry such as frame extrusions will show in projection in the project plan view when crossed by the cut plane (which is an incorrect representation). This behavior is further exacerbated when these families have slanted sides. Only structural columns have a family property whereby cut representation can be set to be by project ("Show family pre-cut in plan"). Current behavior was set in order to increase performance. For more information, take a look at this AUGI thread, paying particular attention to David Conant's response:

http://forums.augi.com/showthread.php?58323-quot-Magic-Height-quot-for-cutting-in-plan-What-is-it