Detail Family needing to lock lines to intersection of reference lines (can't use reference planes)

[trevor.clarkM88RA]

I'm making a detail family that will be a zigzag line spanning between a pair of arcs - this will be for a component of a seismic joint at fire wall detail. The family is a set of zigzal lines that accordions to meet the gap width, which varies.

I have the bones of a family set (attached), with formulas that squish and stretch a pair of reference line arcs and a set of equally spaced rays, also reference lines, from arc center.

The issue is that I can't anchor the actual linework to the intersections of the arcs and subdividing rays as they're both reference lines. I can't use reference planes as they're arcs and lines that rotate.

Does anyone have a way to either:

• Anchor linework to reference line intersections?
• Have a different method that can create a zigzag line that span between a pair of flexible arcs?

*Edit to add family, sorry!

[SteveKStafford]

Family isn't attached. In your view turn on Automatic Sketch Dimensions (V/G Annotation tab). They show up for elements that Revit is attempting to constrain when we haven't explicitly done something to control them with a dimension, parameter, align/lock etc.

[trevor.clarkM88RA]

Shoot, really thought I had added it - here it is! I just tried turning on the automatic sketches and shifting them to the points at the intersection, but the linework still comes apart when stretched. See below before and after images in case the family still isn't posting (Revit 2024).

[SteveKStafford · Accepted solution]

The attached version of your family flexes okay in my testing but it's origin moves and I didn't have time today to ponder rebuilding it to keep it at the same location.

[azad.Nanva]

do you want to have the Fix Center of arcs or what ?

[trevor.clarkM88RA · Accepted solution]

Sorry, wasn't clear in my original post, the flex of the reference lines/arcs were working correctly, the conflict was that I couldn't get the detail lines zigzagging between arcs to stay attached to the intersections when it flexes.

Unfortunately, I don't have Revit 2025 yet, so I can't check the family @SteveKStafford posted, but deadlines don't wait, so I used a different approach entirely to solve it: polar array of line segments that are spaced to be continuous zigzag. Feels a bit Rube Golberg, but bonus is that the array can allow a parametric number of "accordions". See attached for the set-up and math if you need such a contraption yourself (R2024)

[SteveKStafford]

The version I posted the zig zag segments follow the flexing too but the whole thing moves when you change the size.

[azad.Nanva]

your last family is smart , but you have width and you have the angle why you don't use the sin cos and tag ? do you need the center of arcs?

[trevor.clarkM88RA]

Thank you! I've not found a definitive source of how to make these kinds of very computation heavy families, so I just have my own personal best practices:

• I've found someone who did testing that that trig calculations are computationally the most intense, even more than square roots, so I try to avoid them when other math options are available. I've not found anyone doing this testing in Revit, so it's all I've got right now:  A simple benchmark of various math operations
• For the center of the arcs, they're definitely needed for the polar array, but if you were just using reference arcs, I think I'd still use the center constrain method, although I'm not sure it's technically needed. The Sagitta formulas need the arc radius - without the radius the formulas can only be approximate and it's still hard at that: https://math.stackexchange.com/SagittaWithoutRadius. Since you need it in the math, might as well use it in the family as I find center points and radius easier to constrain than 3 point arcs in Revit.
• I've also really struggled with using the equal dim tool for angle dimensions, they seem to break a lot. So I'll typically choose to do a Revit calculation instead.

I'm guessing other people who have more academic experience in Revit probably understand the ins and outs of which kinds of constraints should be used, but I the above is my experience on keeping arc/circle based families from breaking.