Double Trouble: AutoCAD solid modeling

Doubly curved surfaces are hard to imagine, hard to model, and hard to fabricate, but have become the signature of many "star" architects. AutoCAD is a great tool for making digital models of doubly curved surfaces and stripping them down into something that can be fabricated.

Many architects, particularly those who style themselves “designers”, know that it is a fact that AutoCAD is a necessary evil for drafting but is incapable of supporting form exploration.

They are wrong.

It puzzles me how they can cling to their ignorance. AutoCAD has incorporated a world-class 3D solid modeling system since the mid-1990’s. It uses the “constructive solid geometry” approach to modeling which depends upon operations such as extrude, revolve, sweep, loft, union, subtract and intersect to allow a designer to make very complex and sophisticated shapes.

I will identify the concepts and commands that AutoCAD accepts for making a doubly curved shape with arbitrary curvature. It’s easy, although a bit tedious. I will also describe how to use AutoCAD to disassemble the shape into planes that can be cut on a CNC router. Furthermore I will describe one approach to converting the shape into triangle faces that can also be cut on the router and assembled into a faceted construction of the original shape.

This is not a tutorial as the word has come to mean in the software industry. It does not describe each step, each click, each menu choice to complete the creation of a given form. Instead this blog entry describes the concepts and the AutoCAD commands that implement the concepts. If you want to make a shape sort of like the one that I illustrate, you can use the Help function to look up the commands and fool around until you get something you like.

The concept is to draw several profiles of the object that you want to make, ”loft” them into a 3D form, section them to make ribs and purlins, and apply triangular facets to define the form. This process only requires a few commands: PLINE, PEDIT, LOFT, SECTION, 3DFACE, and ALIGN. Other commands are essential but they are not distinctive, such as MOVE, VIEW, UCS, OSNAP.

The picture illustrates the process. You can see the profiles, stacked vertically. Next to it is the lofted solid object. The ribs and purlins make up a “waffle” grid. Finally, there is a 3D face on the waffle grid and another one rotated down onto the “floor”, ready to send to the CNC router.

If you are new to AutoCAD 3D, then the first step is to get very familiar with 3D viewing (very easy and intuitive in AutoCAD 2009) and the User Coordinate Systems (UCS). By default AutoCAD has a World Coordinate System that defines an origin and direction vectors for X, Y, and Z axes. The UCS command allows one to move, rotate, and align the working coordinate system to any position and orientation in the 3D space. You can also use the VIEW commands to quickly align views with the UCS to give you a plan view looking straight at the UCS. You can name and save a UCS for quick retrieval. These commands are absolutely critical to drawing in 3D space.

The first step is to draw profiles of the form that you want. I drew four closed polylines to represent the bottom face, two intermediate sections, and the top face of my wall. You can use the polyline command, the line command, the spline command, and pedit command to give you a lot of control over the profiles. I used the MOVE command to move the intermediate and top profiles vertically in space. You can use keyboard commands for moving or you can set your UCS to be a vertical plane such as an elevation and drag the profiles vertically.

The next step is to use the LOFT command to convert the profiles into a 3D solid. I suggest that you get out the Solid Modeling toolbars. Right click on any toolbar and pick the solid modeling, solid editing, UCS, and View toolbars. Before you use the LOFT command, I suggest making a copy of your profiles in 3D space. You may want to edit the profiles and compare multiple 3D forms. With the LOFT command, you pick each profile in order starting with the bottom one, and just accept the defaults. You can play with the different options and see how they change the form.

The SECTION command projects the solid onto whatever plane that you want. You will use it to make vertical ribs of your solid and horizontal purlins. I drew some layout lines to make it easy to find the planes for my sections using OSNAP settings. To get the horizontal sections, just use a vertical UCS. You can do it all in the 3D view which is kind of fun because you get to watch each section appear in 3D space.

Finally, the 3DFACE command allows you to apply triangular facets to the ribs and purlins using an intersection snap to get points exactly where the rib crosses the purlin. This gives you the complete model of the “virtual plywood” form. Its tedious, but, hey, you can make a few score faces in about half an hour. This sure beats manual drafting.

You can use the ALIGN command to rotate each facet onto the world coordinate system plane where you can arrange the facets, ribs, and purlins onto virtual sheets of 4x8 plywood. This is a really cool command but it is confusing to use. I’m not complaining – it is a complicated concept and the command is about as simple as it can be. Look at the Help function to see how it works.

Launch all of your virtual plywood off to the CNC router and you have the pieces to a faceted form that approximates the original doubly curved, lofted shape. Of course, since we did not design the joints or how to put it together it may be difficult to assemble it. If you laser cut it out of cardboard, then glue should be fine. I am working on several alternative joinery systems that will be easy to fabricate and easy to assemble.

Wild forms with Autodesk Revit

Ok guys. I asked you to make cool, sexy, curvilinear shapes for train station canopies, studio furniture and houses. And, I want you to use Revit so that we can pull quantity take-offs, GBS anlayses, structural analysis and other evaluative studies off the central BIM. I did not know how to do that stuff, but I knew that we can figure it out. Here is my latest experiment.

The concept is to create a sweep or blended sweep in Revit as a mass family and use void objects to "crop" it into a smaller piece. You then use a "curtain system" to put grid of glass and mullions on it.

Create a New Family in Revit and choose the Mass template. Create a big "landscape" of a curved surface. Make it about 4 to 10 times the size of whatever you are designing. I was working on the canopy for the train station, so I made mine 400 feet by 100 feet. Start out with just a swept surface and don't make the profile or the sweep path too complicated.

It does not matter much how you derive the "landscape". You could use an actual landscape, such as the Brazos Valley, or you could just draw a random surface that looks cool. I am going to play with an extrusion of the form of the Mississippi River simply because the forms and ideas will remind me of my home town. The example pictures are a random surface.

Draw void objects in the mass family that you use to "cut" the landscape into a smaller piece. You make parameters for positioning the voids so that you can use the keyboard to move the voids around the landscape and crop different parts of the landscape away. I set mine up so that they measure from the origin of the mass family object. You can use the Family Types command and play with the parameters to make sure that they are working properly.

Once you save your mass, you can place it into a Project. You can tweak the parameters to shift the voids around to get a shape that you like.

In your Project file, use the curtain system object and attach it to the top face of the mass. The default curtain system has a 5'x10' grid which is way too big for my mass object, so I changed it to a 2'x2' grid. You will have to wait a few minutes, but it should generate an entire glass canopy structure over your mass surface.

You can edit the mass object by just changing the parameters. You will have to "remake" the curtain system but you can quickly try out another form for the canopy.

Here is a picture:

My example is pretty simple. There is one swept surface to create the "landscape". The clipping voids are rectangular boxes. You could create a positive surface out of multiple sweep objects, out of blends, or out of very complicated solid shapes. The clipping voids could be complex shapes themselves, such as curved extrusions, sweeps, cylinders or anything else. Finally, I used the defaul curtain system which is just a mullion and panel system. You could design a spider joint, or an overlapping shingle system, or even a welded steel shell.

Start simple and try it out. Slowly add complexity. Revit often supports substituting more complex elements for ones that you used to sketch an idea. For example, you can redo the Mass family and reload it into your Project to substitute a new form. You can build a new curtain system family and substitute it for the default one to get a new skin over your mass surface. I look forward to seeing what you do.

One more thing. I came up with this approach while trying to create structural ribs for a canopy system. It works very well for that purpose. You can place twenty copies of the mass family in the same location and then use the parameters to move the clipping planes at whatever rib spacing you want. The clipping planes can be set to be 4" apart, or 8" or whatever thickness that you want for the ribs. By making the ribs solid and then using the same mass form for the surface curtain system, you can make an entire structural canopy.

(Sorry that the images are so bad. My Revit files are way too big to upload and I am just learning blogspot. I may switch to a different way of sharing notes.)