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The Revit Cat and the Parametric Stars



Some time ago, My friend Jeff Hanson, from Autodesk, posted the picture of this cat in a thread at the Autodesk Revit forum, to express that, in Revit, there is usually more than one solution to solve the same problem, saying that “there is more than one way to skin a cat“. Jeff and I have participated in some threads in which he presents a solution and I present another. So, we have baptized that picture that he posted as “The Revit Cat”. Well, today, I want to dedicate this blog article to another “Revit Cat” situation, in which I want to post another solution to the challenge presented by my friend Kelvin Tam in his blog “Revit Swat“. Kelvin has been posting different ways to create a parametric star. He came up with the idea during the last Christmas season, of course. Well, since December up to now, Kelvin has posted not one but five very interesting, nicely illustrated solutions for the parametric star, and I have enjoyed reading them. If you have not read them, I invite you to visit Revit Swat and look for a series of posts entitled “Dashing with the Stars” which began with the article at this link. Well, I promised Kelvin that I would post my solution to the challenge here in my blog, and he is very interested in reading my solution, too.


My solution

My solution for the parametric star is based on my previous blog article about the parametric polygon. But this time, the idea of the parametric polygon is going to be adjusted for the generic adaptive template and the divided path with repetitions. Note: This solution does not apply for generic templates, or 2d families such as profiles or detail items; in those cases we need to work with arrays, because the repetition feature is not available in those templates. Therefore, for this solution I will be using two families that start with the generic adaptive template, like this:

Family 1

Begin with a generic adaptive template, make it “always vertical”. Draw some reference planes as shown here, with one parameter, “side“, and one vertical model line, constrained by “side“. For repetitions, Revit assumes that the vertical reference plane is tangent to divided path, therefore this line is the side of the polygon.



Family 2

Begin with a generic adaptive template, and create a circle with a reference circle. Create a “radius” parameter to control the size of the circle. Create a “side” parameter to control the length of Family 1. Select the circle, and use “Divide Path”. The default division is 6. Tie the “number” parameter of the divided path to an integer parameter, such as “n“. The “side” parameter needs to have a formula, to make all the endpoints of the sides meet at their endpoints. The formula shown in this image is for a circumscribed polygon, in meters. For feet, simply change the “m” to be the apostrophe for feet (‘).



Nest Family 1 into Family 2

Nest family 1 into Family 2, and then place an instance of Family 1, by face, on one of the points of the divided path. Select family 1, and tie its “side” parameter to the local “side” parameter of family 2. Then, select Family 1, and click on the “repeat” tool. Now we have a parametric polygon.



Modify Family 1 and reload into Family 2

Select the nested Family 1, and do Edit Family. Now, using the same constraints, delete the line and create any geometry, to your liking, to be repeated along the circle. The geometry to the left of the vertical reference plane goes outside the circle. The points at the vertical line will be tangent to the circle, and the geometry to the right of the vertical reference plane goes inside the circle.



Tie parameters from Family 1 to local parameters in Family 2

Once Family 1 has been reloaded, it will take whatever shape you made, as shown on the left in this image. But we need to tie the parameters from Family 1 to existing or new local parameters in Family 2, so that we take control over the dimensions of the star.



Repeat away !

Now that we have control over everything, click on the nested Family 1, and from the ribbon, click on the “repeat” tool. Now you have a parametric star!. Load into project, and create as many stars as you want, with whatever radius you want, whatever number of sides you want (well, up to 200), whatever length you want, and whatever height you want. Note: the cat is not included in the family. 🙂



Applications

Here are some famous structures that could be modeled in Revit with the help of the techniques used in the “parametric star” or the “parametric polygon” : Filippo Brunelleschi’s Duomo of Florence, Oscar Niemeyer’s Cathedral of Brasilia, and Pier Luigi Nervi’s Palazzetto dello Sport.



Epilogue

Well, as Jeff Hanson says, “there are multiple ways to skin a cat“. This is a “6th” solution, just to add another alternative to the 5 other solutions already posted by Kelvin in his awesome blog, Revit Swat.
By the way, both Kelvin and I will be speakers at RTC, Revit Technology Conference, in Auckland, New Zealand. Check out the RTC Australasia website to learn more about our classes.


See you in our next blog…

2013 Planta1.com, inc. , Alfredo Medina | permission to reproduce this article is granted if the name of the author and the URL of this article receive proper credit.

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