Whats wrong with this formula?

[Sahay_R]

What's wrong with this formula? i'm trying to calculate the value of b, which should be (using Pythagoras' Theorem) the square root of Radius squared - a squared. So what is wrong here? Why do I get an incorrect formula warning?

[ToanDN]

You don't need a formula. Just change b to a reporting parameter.

[Sahay_R]

@ToanDN - what i am trying to do is control the length of the arch by relating it to the radius. The arc is already set to sit at the proper Height, and it will rise up to Total height. the objective is to control the length of the arch and still have it sit at the right height and rise to the right height

[barthbradley]

Hey Rina.

Not following you. What are you trying to solve for? 

[Sahay_R]

@barthbradley - I've seen this screenshot too.

Basically, what I want is an arch where the width can be controlled. Input the width and it sits at the right height and goes up to  the right height. The arch that I have in the family does all that, but it's controlled by radius, not width of the arc. 

So what I need to calculate is the length of the arc. It seemed logical using the formula in my screenshot, but Revit does not like it. Aarrrgggghhhhh.

[ToanDN]

You overconstrain. The arch only needs the radius and what you call "a" to constrain the length. You don't need "b".

[Ilic.Andrej]

Can't open the family right now, but there s something I notice by looking at the picture.... Btw, I used similar sqrt() formula before and it works just fine. So I think this is about the parameter a. 

[barthbradley]

Sorry, Rina; I'm still lost. Can you just post the file? 

BTW, I'm looking at this sqrt formula and it's missing "/2".  Should be written = sqrt((6^2)-((6^2)/2))

[Sahay_R]

@barthbradley - I did - it's in my original post..... when I think that I've reached a solution then the circular parameters raise their ugly heads...

[ToanDN]

Okay what does "a" control?  It doesn't seem to do anything.  I thought it match the end of the arch but No.

[Sahay_R]

@ToanDN a is the perpendicular of the triangle where radius is the hypoteneuse and is used to calculate b which is the half of the ar c width

[barthbradley · Accepted solution]

Arrgh! I don't have access to 2019 right now. But, Rina, seriously, if you build it like the picture I posted, you'll have zero problems. I've modeled hundreds and hundreds of arched openings using those parameters/formulas! 

...Take a look at this complex concave wall opening I posted on this forum a couple of times last week. It's built with the same parameters/formulas shown in that picture.  All of the arched openings I build (simple to complex) are driven by the same parameters/formulas. 

[barthbradley]

Thanks for the solution credit, Rina. I’m warning you though: once you get these parameters/formulas working for you and see them in action; you’re going to start loving the opportunity to model anything with an arch.   Could be addictive. 

Cheers!  

[Sahay_R]

@barthbradley - thank YOU for this learning moment! I will study and reverse engineer this family to use it.

[barthbradley]

@Sahay_R: Just poking back in and wondering how you're doing.  Are you having fun yet creating arches?   Are you the Arch Queen now at work?