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# Create a Tensile Fabric Roof

In this article, I want to provide some guidelines about modeling a tensile fabric roof, as the one shown in this image:

To understand the process, I will explain first how to model one portion of the roof. That portion corresponds to a quarter of one of the "pyramids" that we see in the image above. Once we understand how to model that portion, the rest is easier to understand, since it will be just a repetition of the same method.

With different combinations of points and reference lines, different solutions could be modeled, depending on the location of the supports, anchors, posts, etc. In this example, we assume that the roof has posts at the four corners of this portion, having the main high post at the upper right corner of the square, looking at the plan view.

Start with a family, with the generic adaptive template. From the plan view, create a skeleton of reference planes. All these planes can be controlled by parameters; for clarity, those dimensions and parameters are not shown in these illustrations. From the Front view, create reference planes to control the heights, and name those reference planes, such as H1, H2, H3, and H4. Then, at the floor plan view, create 3 reference splines with 2 segments each. Then select the 3 reference splines, and use "Create form". Now we should have a flat surface, as shown in this image.

Each one of these 3 reference splines serves a different purpose. Let's suppose that the reference spline at the South of the floor plan view is # 1. The one in the middle will be # 2, and the one on the North will be # 3.

• Spline # 1 will define the edge of the surface, the arc that is formed from one support (anchor or post) to the next support.
• Spline # 2 works as a profile, providing points to adjust the curvature of the form in the direction that is perpendicular to the splines. Without this profile, the elevation from the short sides (East and West in this exercise) will be made of straight lines, like a pyramid.
• Spline # 3 defines the curvature from the edge to the top of the form.

Then, start the process of lifting points to their correspondent named reference plane. It does not matter which points are lifted first than other. Therefore, let's begin with the points that are in between the posts, on the exterior edges of the square. Select those points (highlighted in blue in the image), and, from the Options bar, use the Host drop down menu to lift those points to the H1 named reference plane, as shown:

Then, proceed to select the other 2 points of Reference Spline # 2, (highlighted in blue in the image), and lift them to the reference plane named as H2.

Then, proceed to select the mid point of Reference Spline # 3, (highlighted in blue in the image), and lift them to the reference plane named as H3.

Then, proceed to select the right endpoint of Reference Spline # 3, (highlighted in blue in the image), and lift them to the reference plane named as H4.

Note: In this image, the plan view is a 3d view from the top. If you are using a floor plan at reference level, in this image, there will be an arc on the upper right quadrant of the square, due to the cut plane.

Finally, from the floor plan view (or 3d top view), select the points at the mid points of the exterior edges of the square, in between the supports, and shift them towards the inside. This will make the points at the supports to be pushed to the outside, as a result of the tension in the fabric.

That is, in a few words, the process. The rest is repetition, adjustments of points in plan or elevation, adjustments of dimensions by parameters, etc.

To make a tensile fabric roof like the one shown at the beginning of this article, we need 5 series of Splines. Series # 5 is a repetition of # 1 (the edge Spline), and series # 4 is a repetition of # 2 (the profile Spline). Series # 3 is unique, and that is the Spline that defines the main curvatures at the axis of the main posts. The five series of splines are created in segments, stopping every third point, and restarting the Spline command to begin a new Spline, and repeating this as many times as necessary. This image shows the layout of points, located first flat at the reference level.

We begin by selecting Series 1, 2, 3, and using "Create form". Then Series 3, 4, 5, and "Create form" again. Now, we have 2 flat tents on the ground, which we are going to lift. Actually, in a way that is similar to what happens in real life when a group of workers are going to put up a tent.

This diagram shows the profile of Series 1, 2, and 3. Notice that in the profile of Series # 3, the top points of the curvature, wher the main posts are, have been shifted to the left and to the right, to represent the effect of tilted posts in opposing directions.

The height of all these points can be adjusted easily by moving the named reference planes, or by changing values in the parameters that control those reference planes. Other offsets in elevation and plan could be controlled by dimensions as well.

These are views of the completed tensile fabric roof, with the reference splines on, and shown in red. Notice the role of the 3 series of Reference Splines, which is very evident in this last image. Spline # 1 (and 5) defines the edge. Spline # 2 (and 4) is a profile to control the curvature in the elevation that is perpendicular to the Spline, and Spline # 3 defines the curvature from the edge to the main post, parallel to the splines.

Well, at the end, you may want to put a cap on top of the posts.

Once the form is completed, the family is loaded into a project. Create a wall type, very thin, and apply Walls by Face on the surfaces of the form. Then you need to model the posts and cables, which is a lot easier

See this technique in action.

This article was originally authored by Alfredo Medina and published at his blog http://planta1.com/forum/blog.php