Approximating Nurbs Surface in Autodesk Architectural Desktop 2005/2006

For all of you that are fascinated with tensile structures, when one way or another architectural fabric ends up being an essential part of the design idea, sometimes it can be quite intimidating to achieve its proper representation by using Architectural Desktop.

This brief tutorial will show you how, with a modest amount of knowledge, to create a Nurbs surface in Autodesk VIZ and bring it into Architectural Desktop to be converted into a Mass element that can be viewed in plan, section or elevation without unsightly tessellation.

The first step is to create an impromptu structure in order to drape a piece of architectural fabric and that will define some kind of shelter.

By accessing the Structural Members Catalog create a Structural Member Style based on AISC HSS and pipe category. From the list of available Steel Pipe profiles, highlight Pipe3Std and pres CTRL+G. This will generate a member’s style of the same name.

In the following example a shelter structure has been designed and the DWG file linked to Autodesk VIZ for the purpose of providing a reference for a new Nurbs surface that will drape over the structure.

A very similar method can be used in conjunction with any surface modeler.

In this instance a (Control Vertices) CV Nurbs Surface has been used to provide a basic geometry that will cover the shelter structure.

   

The next step calls for deforming a surface by resorting to manual vertices adjustment in the case of Autodesk VIZ 2006, or to the fabric plug-in of Max 7.5.

When a look that resembles a fabric in tension has been achieved a section tool will be used in order to get spline segmentation of the surface. Rotating the section gizmo around y – axes for 90 degrees gives us a vertical slice through the entire model.



Needless to say, this is a very useful tool for capturing basic information from a massing study that can be part of a schematic design done in VIZ. In a small regression from this topic, I still believe that both Max and VIZ are by far the best modeling tools out there within Autodesk’s family, and it is long overdue to bring some of the polygonal and Nurbs modeling capability into other design oriented applications.

Now back to our surface approximation:

    

After hiding everything but the Nurbs surface and section gizmo, go ahead and array the gizmo in the direction that will give you slices through the entire surface. For those of you that have Autodesk VIZ 2006, you will find the new improved Array tool and the preview feature to be extremely useful.

The finer the array offset is the closer the approximation of the Nurbs surface will be. Following this step, go ahead and select all of the newly created section objects and convert into editable splines.

    

Export these newly created splines through the option Export Selected into a DWG file format, and import the same file as a block into the original ADT file by inserting it at 0,0,0.

    

Explode the block and invoke the drape command, which that will prompt you to select the contours. Select the profile lines and create a non-regular and non-rectangular topo surface.

    

Copy the newly created surface to the location that is 1” in the negative Z direction and subtract the second mass element from the first one through Boolean subtraction. Make sure that DISPSILH variable is set to 1 and take a look at the approximated draped fabric in both section and elevation views.

Here are some of the sample surfaces I have created for the purpose of demonstrating the effectiveness of the previously described method.