Parametric Arc family limitations.

[Anonymous]

I made this parametric arc family but it has some limitations.

I can't reduce the "Rise" parameter below 55 for a "Width" of 18950 as the "Contraints cannot be satisfied"

It seems as though when the arc radius gets very big revit cannot handle it.

What can I do to get smaller "Rise" values? 

[barthbradley · Accepted solution]

[Alfredo_Medina]

To get that very small rise to work correctly, the arc needs more constraints, not just the length of the beam and the rise, as you did. Make this skeleton first, then enter the value of L for the length of the beam and the value of "s" for the sagitta (rise).

[Anonymous]

Thank you! Could you please try and explain why those formulas work and how they were developed because Im trying to understand them so I can rewrite them for a slightly different use.

[barthbradley · Accepted solution]

This might help you: 

Revit Formulas for "everyday" usage - Revit Forum

Valid Formula Syntax and Abbreviations | Revit | Autodesk Knowledge Network

[Anonymous]

Ah yeah, awesome resource! Thank you.

This website also helped me understand where the original formula came from which helps me undertstand the maths of circular segments.
https://planetcalc.com/1421/

[Alfredo_Medina]

Search for the word "Sagitta", online, and you will find several resources explaining how to find the radius of an arc given a certain rise (sagitta), or how to find the sagitta given a certain radius, or a certain chord, etc. Apart from that, the link posted in the previous message is a good resource for trigonometry formulas, which are very useful to solve this kind of problems.