Folding Geodesic Dome

Geodesic domes are light, strong structures with huge interior space that is very pleasant and inviting.  Unfortunately they tend to take many man-hours to erect.  I wanted a dome that could be easily put up by one or two people in short order. Hence the Folding Geodesic Dome, also known as the Democracy Dome since it's made from recycled political campaign signs.  With another election coming we may get some new raw materials soon...  Unfortunately this concept is not yet strong and windproof enough for the desert, but testing is underway for a more secure way to attach the segments.  More on this later in the Instructable.

It takes 105 triangles to make the particular dome shape I wanted (called a 3V), which could be a time-burner to assemble.  The Folding Geodesic Dome simplifies things  with tape hinges that pre-connect many of the triangles into 16 segments, each of which can fold into a small stack and connects to its neighbors via tabs and clips. The more hinges used the less time spent connecting triangles in the field.  The dome goes up with 2 people in 2 hours or less. Takedown time is 30 minutes.

There are great Instructables for light domes made from coroplast or cardboard which negate the need for a support frame. This dome was made with an eye for economy, using 20 surplus 4’x8’ 4mm coroplast political campain signs found on Craigslist for $100.  The panels can be made from any stiff material  (cardboard, coroplast, thin plywood, plexiglass) that is rigid enough to resist bowing.

This project was made possible by the resources at desertdomes.com including their dome calculator, dome formulas and the dome diagram that I use in this Instructable. (If you build a dome, please donate to desertdomes.com as I did.)

This dome is a light-duty ‘backyard’ dome, done as a proof-of-concept; it isnt’t waterproof or windproof enough for open country yet.  My next version will utilize a combination of thin plywood for the hexagons and coroplast for the pentagons (to let light in) and will address weatherproofing.  This project was a runner-up in the ShopBot Contest - most exciting!  Thanks to everyone who took the time to vote.

A “3V” geodesic or polyhedral dome is made of hexagons and pentagons, which means you will need 2 basic triangle shapes, called BAA and BCC. Start your project with a visit to desertdomes.com, where you will find dome diagrams, dome formulas and an invaluable dome calculator.  This Instructable is for a dome with a 7’ radius; this is a “5/8” 3V dome meaning taller vs flatter, over 8’ tall in the middle. The dome likes a flat surface to sit on making it ideal for the open desert playa, not so suitable for uneven terrain. The dome base isn't exactly flat, but close; the flexible nature of the coroplast bows and yields a little when you deploy it to make up for any uneven contact (Note:  the prototype pictured here has a radius of 7.2’ but wasted more material from a 4x8 sheet. My next dome and this Instructable use the dimensions for an 84” radius - 7')

The dome is composed of 3 basic folding segments: a pentagon (5 triangles), a hexagon (6 triangles) and a “hexagon plus 3” (9 triangles for the base row ).  Each segment unfolds and is clipped into a convex shape with binder clips or a clamp made from pvc pipe. 

To calculate the dimensions of the 2 triangles (BAA and BCC), use the desertdomes.com dome calculator and enter the dome radius in inches to get get the lengths of the A, B and C triangle sides. 

For the radius of 84” (7 feet)  we get
A= 29.284”  (roughly 29 1/8”)
B= 33.898” (roughly  33 7/8”)
C= 34.642” (roughly  34 5/8”)

I used those numbers to calculate:
Height of BAA =  23.881” (roughly  23 7/8”)
Height of BCC= 30.213” (roughly  30 1/4”)
.5xB = 16.949” (roughly 16 15/16”, for the midpoint of the B edge)

For the 3V, there are 30 “BAA” triangles (to make 6 pentagons) and 75 “BCC” triangles (to make 10 hexagons and 5 semi-hexagons).   1” wide tabs are placed along B edges to connect segments together, and on some of the C and A sections too.  The triangles along the base of the dome have 4” tabs with holes for the anchor stakes.  Edges that don’t have tabs will have tape hinges that connect to their neighbors.  I’ve mapped out where the tabs need to be for every triangle needed in the dome, adding up to 12 different shapes you’ll cut - download the pdf Cutlist for shapes and quantities.

Tools:
- metal yardstick or metal straightedge.
- carpenters square
- carpet knife and some extra blades
- dextrous gloves with tough palms- coroplast can give a mega-papercut
- ladder or stool for erecting
- hammer for stakes
- spring clamps for easier deployment
- (staple gun for wood slats)
- (saw to cut pvc tubes)

Materials
- 15 - 20x coroplast sheets 4x’8’, 4mm (use stiffer panels for stronger structure)
- strong duct tape (use gaffer’s tape for stronger structure)
- 200x 1” binder clips (use 1-foot long x 1.5” pvc pipe tubes with a slit for stronger clamps)
- 20x 1’ long round tent stakes (minus the plastic dealy)
- 20x washers to fit the stakes (bigger the better)
- 5x 6’ long round garden poles
- 5x 8’ long round garden poles
- 10x rubber tips that fit the poles
- bag of thin bamboo “garden” sticks 6’ long for vents
- spray tack adhesive
- (15x 4”x24” thin wood slats for stronger base, hole in center, glue to affix)
- (light schedule 1-1/4” pvc pipe to make tube-style clamps)

You’ll make 2 templates for the two types of triangle, BCC and BAA. Making the template is complicated to describe, so use the pictures for a better understanding.  The base dimension  (B) is 33 7/8” for both triangles.  For BAA, mark points for the base corners of the B edge at 33 7/8” apart, and mark the midpoint of the two.  The estimated apex of the triangle is a point 23 7/8” above the midpoint of B.   The apex needs to be 29 1/8” from each base corner; and the project needs both edges to be exactly the same length for it to assemble well.  You’ll likely have to adjust the estimated apex until you find the sweet spot.  Draw your A edges from apex to base corners and cut out your template with a straight edge.  Repeat this for the BCC triangle with the appropriate dimensions.  Optionally you can include tabs along the template edges as shown.

From the 2 templates you will draw 12 different shapes as shown on the pictures and listed in the cutlist.  The shapes vary by which edges have 1" or 4" wide tabs.

You will use the cutlist and the templates to mark your material for cutting.

To mark cutlines on your material, place the template and make a mark at the 3 corners of the triangle, and marks to denote edges with tabs.  Use a straightedge to connect the dots for your cutlines.  Important: use a dotted line to draw where your hinges are to be scored - this will keep you from cutting off a tab by mistake. Have all your lines in place for the whole sheet before you cut to minimize mistakes.  You’ll find that many triangles can share a cut with a neighbor to save cutting.   

Important:  when drawing cutlines, use a pencil to denote the B edge of every triangle and write which number shape it is.  You’ll save a lot of time during fabrication of the segments.   You can erase the pencil marks later.

These templates assume that the ‘up’ face of the sheet you’re about to cut will be the outside of the dome, and that the tabs fold away from you.  Double check all cutlines for accuracy against the cutlist one last time, then use the razor knife and straight edge to cut out your shapes from the coroplast sheets.  I regret not having photos of the sheets being cut.  The numbers on these pictures reference the 12 shapes on the cutlist.  You will score this top face of the material along each dotted line to make the hinges for the tabs.

Important:  Use gloves when handling coroplast - it gives mean ‘paper’ cuts.

Important: please don’t cut any material until you’ve made a scale cardboard model of the dome and understand it’s assembly first.  Plus it’s fun.  Use the dome calculator and pick a radius, then do this Instructable with cardboard boxes and make sure you’ve got it right before spending money. 

Coroplast has two skins connected by thin ribs just like cardboard.  I  cut the triangles using a razor knife and straight edge.  This made for easy production without real tools.   Hinged tabs with the coroplast are easy - just cut one face of the sheet  with a shallow razor and fold it back with the other face as the hinge.  The tabs connect together with large binder clips during assembly.

Optional: to make stronger clamps for the tabs, cut 12” lengths of light-duty 1-1/4” PVC pipe. Cut a slit lengthwise into each pipe.  Cut a notch at each end of the slit to allow the clamp to slip onto the tabs.

To make a segment lay out its pieces (5, 6 or 9) on the floor in the “anatomical” shape with the outer face up, tabs down.  Make tape hinges along alternating edges to connect the triangles to their neighbors.  The hinges fold in alternating directions to allow the accordion effect, using flat strips of tape on the inside face of the hinges.  Once the inside hinges are laid down, fold them and apply tape to the still-exposed outer edges of the hinge  Lastly, use temporary tack spray adhesive from a craft store to add “grip” to the tab faces to prevent them from slipping apart when the dome is under strain.

Use a sturdy staple gun and glue to affix the wood slats to each footer.   The slats will need a hole for stake.

Vents can be cut into a triangle with a flap to cover the hole and a stick to prop the flap.  Make a BCC #1 or BAA #10 for the flap.  The flap is removable using velcro so that it doesn’t get crunched when the segment is folded.  The flap for the vent is held open by a thin bamboo garden stick, with the nodes of the bamboo serving as convenient stops for holding the flap up.   I made a small notch to hold the pole at the vertex of the opening.  On the triangle with the vent hole, you will need to adjust where you score your B edge in order to make room for the second tab of the cover flap.  This is a good step to practice on your cardboard model first.

The door is a variation of one of the base hexes, and it is actually detached from the neighboring half-hex.  You will cut the 9-piece segment for the door a little differently from the other 4 - see the diagrams and the cutlist. The hinges are made to allow the door to fold up and out for a diamond-shaped entry - no crawling to get in.  To give structural strength to the doorway, 2 of the 6’ poles get re-purposed into supports, and extra clips are placed on nearby tabs.   Extra tape around the joints on the outside also helps.

To do this most easily have all supplies stacked in the center of a tarp marked with a 14’ circle, and build from the inside.  Optionally draw a circle in the sand if you don’t use a floor tarp.  Use a stepladder to save time.

The dome will be quite floppy during deployment and hard to manage until you get the hang of using the support poles.  You will need to adjust the poles during the process to allow all edges to meet their neighbors and get the true dome shape.  Moving one pole can make 2 other poles fall down.

Partially unfold the 9-piece base segments and clip them to their neighbors; as you adjust the ring to fit the circle it should hold itself up. See photo. A couple of chairs help.  Once the ring is even, nail down all the footers with the stakes.

Pre-assemble all the pentagons.  If you have a helper, they can put together pentagons and hexagons while you clip them in place from within.  Hang the first 5 pentagons off the outside of the base ring. 

Open a base hexagon up and clip it into its convex shape, then support it with a pole.  Flip up a pentagon beside it and connect them with the tabs; support the pentagon with a pole.   Go around the perimeter and flip up alternating hexes and pents.  See photo.  Finish with one 6’ pole holding up each of the 5 pentagons - things should balance in a sagging fashion.

Next pre-assemble all 5 upper hexagons, hang them all off the outside by their tabs, then flip them up in turn using 8’ poles to support them as you clip them together.  As  you go around, the 6’ poles will fall away and you will need to adjust the tall poles frequently.   Once you’ve added the last hexagon the dome gets firm and the 8’ poles fall down.  Add the top pentagon to complete the structure.

Put in the light bamboo sticks to hold up the vents, and put two poles on either side of the door to support the opening.  For tough conditions use tape on the outside to reinforce connections between segments as desired.  You can put extra clips inside around the door frame.

Updated May 7 '012: The prototype is great for backyard use but not wind-proof enough for open desert.  To make it more weatherproof, I have just taped all the tab hinges to cover the flutes and keep water out.  The tape covers much of the face of each tab to give it extra friction along with spray tack adhesive on all tabs, but in testing this setup still blows apart in strong gusts/dust devils.  Using clips to secure the tabs still allows too much slippage and the panels come apart from each other.

Next method to test: heavy-duty staples to secure tabs to each other, which of course would need a staple-puller to minimize damage to tabs.  This seems like a fast way to erect the dome, but it would slow the deconstruction process.

Once deployed I add short strips of tape across all joints on the outside of the dome to bolster it against slippage, and around the door joints.  I add extra clips to the tabs near the inside of the door.

There is room for improvement in all facets of the “Democracy Dome”, including clips, materials, hinges,  weatherproofing joints, etc. 

The 4mm coroplast is a little soft, allowing a couple of the lower hexes bow just a little to accommodate surface unevenness.  This is also a result of the dome base having 5 'low points' that bend under the load.  One could use thicker stock for the base triangles if desired.  My next version will use thin plywood for hexes and coroplast for pentagons wlong with some tweaking of the dimensions of the lower panels to make it sit truly flat.  I'll post the details once I get a chance to figure it out in SkecthBook.

Putting this up in windy conditions would be problematic; morning while it’s cool and calm is the ideal time.

The dome ventilates well and doesn’t heat up in the sun due to the high reflectivity of the white exterior. With the tall sides there is standing room in most of the interior.  More than enough room for queen airbed and a table.  The light filtering through the blue and orange gives a stained-glass quality to the interior.  Using pure white coroplast would make a really bright interior.

Comments welcome, good or bad.