Friday, January 13, 2012

Designing Revit Content - What is Interference?

Since today is a superstitious day and I'll be pretty busy watching out for black cats, walking around ladders and such...I thought I'd bump something I wrote in August 2009 up to the front. I thought of this one because of a couple posts by Doug Bowers.

For the most part Revit will help you find when various elements interfere with each other. A window that gets bumped into by a perpendicular wall will generate a warning. A wall that overlaps another will too. A desk copied on top of another will but only if it is in the exact same location.

There are instances that do not generate warnings at all. The same window that complained about a wall won't complain if another window overlaps it. Then there is a door that doesn't mind something encroaching on the swing area or the accessibility requirements. Put a desk so that it crosses into the swing area and use Interference Check between doors and furniture and you'll find no interference reported. Is Revit blind? In a way yes!

A typical door family doesn't have a real element representing the swing or panel in a plan view, it is just symbolic lines. Therefore no interference. The only solid geometry in most door families is the panel and glass which is usually confined to the extents of the wall interior and exterior faces. That desk will need to cross into that space to be a conflict.

Is all of the content for Revit missing this intelligence. Yes, nearly all of it. Why? Because except for a few instances this intelligence isn't so simple. The clearance requirements for content becomes highly specific very quickly. Even more specific when you start examining MEP equipment. Even doors that have seemingly simple push/pull clearance requirements have subtle exceptions depending upon where in the world the door is installed and the relevant code(s). Thus far the content we use ignores this issue for the most part.

The next step is for content to begin to address these design considerations and that's how content becomes more powerful and relevant. More powerful when it not only helps us model and document a design but it begins to make sure that our decisions will meet codes and design best practices. Does your content help your firm in this way? If it does then bravo, if it doesn't it could. How?

One way is to include solid geometry that represents the clearance requirements for the element. This means defining a boundary, usually parametric too, that will represent whatever clearance/interference issues a family might have. This could be a bounding box surrounding the entire element or a box defining an access door's swing clearance for maintenance.

Incidentally, with Naviswork's Clash Detective it is possible to test for Hard and Soft clashes and even define a clearance value that can be applied during a test. Revit lacks this subtlety so a family needs to provide something for it to use. That something is solid geometry.

Practically speaking this means more in each family. This extra solid will also have to be managed otherwise you'll be seeing a lot of boxes in your views.

Autodesk could help us by defining a new sub-category for all elements called Clearance or similar. This would mean that Revit could then learn how to detect a user defined clearance sub-category element and even have a default visibility behavior or setting allowing us to flip a switch to show or hide clearances. Until such time we have to do it by adding it ourselves and ensuring these solids are properly assigned and done consistently for our content.

Keep in mind that the obvious way to manage visibility by using Detail Level won't help us for now. Why? Detail Level doesn't work with Interference Checking, the solid has to be "visible". If you assign the clearance solid to use a specific Detail Level the Interference Check tool fails to see the solid at all even if you change the view to the correct detail level.

Bottom line, can't use Detail Level to manage the visibility of clearance solids. You must use sub-categories or Yes/No parameters. Using sub-categories is a broader brush solution while Yes/No is more involved because you have to manage them at the family level. When you use these methods you can turn off the visibility of a clearance solid and the Interference Check tool will still find them.

Just when you thought your content was great you find out there is something else you could do to make them even better! A toast to making content better still!


Anonymous said...

You can also combine both a boolean parameter for individual family control and also a subcatagory for broad control like before you go to print etc. Thats our approach to utilize both.

Ill toast to making content better still! Salude!

Julien BENOIT said...

Thanks Steve to underline these.
I was wondering if the clearance solid could be in a nested family.
It would help to speed up family's update in my office! I'll give a try soon.

ShawnF said...

Interesting discussion, this comes up a lot.

FYI: The USACE Attachment F contact language requires the modeling of interference envelopes, and more agencies are looking to AttF as a starting point for their language.

In other tools, they have view properties that can turn off/on element classes. I don't think you necessarily want an individual control per geometry piece, I think a binary "all on" or "all off" would work best, based on the view.

again, interesting discussion