Creating a Cone Tent Model
The bottom system points are drawn in using a 3D polyline. The system points will usually be picked off from a supplied architects drawing.

You can draw in any units you like, as long as you stay consistent in the drawing. We are using 1 on the screen equals 1 inch in the real world.
Figure A
The top ring is harder to get right, so it helps to draw it in too large, and then scale it down. It needs exactly the number of points to let the meshes stitch to it, so we have to decide on each mesh's density.

The mesh will eventually be turned into panels, each defined by a number of points, so it helps to refer to a mesh as having say 6 panels by 12 panel points.  In cone tents it is usual for the panels to run up towards the top ring.

In this case we will be using Double Mesh later, so the final determination of the mesh density can be postponed. We just need a coarse model having roughly even mesh density around the model, so we will use a 3 panel mesh on the west side, and 4 panel meshes on the east, north and south sides. (The west side is shorter).  

Adding all the panels gives  3+4+4+4=17 so use the AutoCAD polygon command to draw a 17-sided polygon. Rotate the polygon so that the panels going up to it won't be twisted, and elevate it to the required top ring height.
Figure A2
Once you have the geometry correct, you can scale the top ring to the required size.

There is one irritation to take care of… AutoCAD draws a polygon as a LWPolyline whilst MPanel needs a 3Dpolyline. Convert it using the MPanel Convert Lines tool with the option set to convert to 3D poly.
Figure B
Now we add the meshes in the flat. There are several ways in AutoCAD to do this, but the way shown uses the AutoCAD 3D Mesh command.
(Draw… Surfaces…. 3D Surfaces…. Mesh).

Make the first mesh edge line (the warp line) go from near a system point to near the top ring.  There is some potential confusion over the Msurf and Nsurf values… on the west side we want a mesh with 3 panels each defined by 10 side points: that means a 4*10 mesh, i.e. Msurf = 4 and Nsurf = 10. The other three meshes are 5*10 meshes.

The exact shape and placement of the meshes doesn't matter, but it is important to keep the orientation correct, i.e. all the meshes going from the system points to the top ring. The MPanel info button will report the principal mesh direction to help in checking this.
Figure C
The meshes are snapped to the system points and top ring, making sure that there are no gaps. Adjacent meshes share common nodes on the top ring. Make sure that AutoCAD has OSNAP on and set to endpoints.
Figure D
The system of meshes and 3D polylines is relaxed using the MPanel tool Multi Mesh In this case there is too much weft tension, so the structure has necked.
Figure E
Setting the MPanel option Weft/Warp Ratio to 0.5 produces an acceptable result. Relax it again to make sure that the system is fully relaxed.
Figure F
The model has a coarse mesh density. So we use the MPanel tool Double Mesh, selecting all the meshes and the top ring, to increase the mesh density. Double Mesh can be used repeatedly to achieve the required density, which is determined by:

!Is the model for visualization or for paneling?
!You need enough “mesh panels” to keep edge errors low. This is the error caused by approximating the cable edge curves by polylines.
!If you are paneling directly from the mesh, the panel width needs to be less than your roll width.

In this case we just use Double Mesh once, which gets the panels comfortably less than 36 inches.

Often at this point the model will be shaded or rendered and sent to the client for approval
Figure G
We will panel one side of the  structure to show the method, and we will use simple paneling, where the panels come straight from the mesh. Just select the mesh and click on the MPanel Mesh to Panel tool.
Figure H
Arrange the panels in a set by using the MPanel Production tool with options Mark Nodes off and Panel Name blank.
Figure J
Apply compensation to the panels. How much compensation is determined by the fabric characteristics. The mesh shape requires twice as much warp tension as weft tension.

Look up in the fabric data sheets and find values for warp and weft compensation that produce twice as much warp tension as weft, and produce a reasonable fabric stress.

Here 1% in warp and 0.5% in weft is used, representative of a fairly soft “indoors” fabric.
Figure K
Now apply a seam allowance, here using 1 inch for the top, left and right and 3 inches for the bottom to allow the bottom to be tuned up into a cable pocket.

Note that the compensation has reduced the panel, the seam allowance has enlarged the panel, but all by fairly small amounts.
Figure L
Finally label the panels, using the MPanel Panel Production tool with option Mark Nodes off.  

Any additional detailing can be drawn in using normal AutoCAD methods, and the panels can be printed directly to any AutoCAD supported plotter.

If your plotter needs a dxf file then save the selected panels as a dxf.
Figure M
We would need to do some material estimating:

If a mesh is selected the MPanel Info tool will report the mesh area, which will give a fabric cost estimate.

If a panel is selected, the info tool will report the panel area and width, so you can check that the panel fits on the roll.

Another significant cost is the cabling:  use the MPanel Mesh to Lines tool to break the mesh into two groups of polylines. Select just the polyline that represents the cable ( by setting AutoCAD to Group Off  using Ctrl Shift A ) and the MPanel Info tool will report it’s length and mean curvature.
Figure N
For a stress estimation, we would start with the average warp and weft stress, as determined by the compensation. Let’s say that your figures are 10 lbs/inch warp, and 5 weft. The maximum fabric stress is at the top ring, where the mesh lines are 3 times closer than the average. So the maximum fabric stress is 30 lbs/in warp at the top ring. Can your fabric handle this safely? If not you would have to reinforce in this area. The minimum fabric stress would be at the cable edge, where the mesh lines are 3 times further apart than the average, giving a stress of 3.5 lbs/in.

The length of the south edge is around 250 inches, so the total edge loading on the south edge is 3.5*250 = 875 lbs. The model was designed with a cable tension ratio of 1, so the cable tension is 875 lbs.  If two edge cables are selected, the MPanel Info tool will prompt for the cable tensions and report the anchor load at the corner. In the southwest corner these came out to 1300 lbs.
This concludes this design note. To keep it brief, the following important subjects were omitted:

Modeling turnbuckles at the corners to get some tolerance on the design.
Using even stress relaxation to even out the stress variations in the mesh.
Creating panels that are not directly linked to the mesh, i.e. geodesic and cross section panels.
Using de-compensation to fit the panels onto a top ring easily.


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Copyright Meliar Design 2008