Introduction: The Geiger Cable Dome
The Kalani High School Engineering Class was given an assignment to publicly display knowledge of science, technology, engineering and math. To complete this project, the group of twenty-six students constructed a Geiger Cable Dome, a.k.a. the Tension Dome, in the campus. Using knowledge gained from a The Great Courses lecture about structure (conducted by Stephen Ressler, M.D), the students were able to finish the 20' diameter dome in twenty-four hours of brainstorming, designing and construction. The class was challenged to use wood and sisal rope instead of concrete and steel cabling in real world examples. The design would have to be easily deconstructed, too, so screws or nails would have to be used instead of wood adhesive.
Step 1: What Is a Tension Dome?
A tension dome is a structural system that uses the tensile strength of materials rather than the compression qualities of usual domes. The Tension-Dome is consisted of five main components: compression ring, centerpiece, cables, stents, and hoop cables.
Compression Ring (a.k.a. outer ring)
-resists the pulling force of the spanning cables
-usually held up by a base
-a piece that acts in tension that resists the pulling force of the spanning cables
Spanning Cables (8)
-connects the center piece to the compression ring
-multiple cables distribute the weight of the dome to the outer ring
-segments attached to the spanning cable
-act as compression members
Hoop Cables or Cable Rings (2)
-act as tension members which pull in the bottom of the stents towards the center
A great real world example of a Geiger Cable/Tension Dome is the Seoul Olympic Gymnastics Dome in Korea (seen in picture above). This dome is able to cover 13,000 spectators in the 1988 Seoul Summer Olympics. Besides the difference in size and the covered roof of the stadium, the principles of this 390 foot diameter dome are exactly the same as our own 20 foot diameter.
Step 2: Materials
2'' x 4''
(16 ct.) 10 feet
(16 ct.) 8 feet
(8 ct.) 3 ½ foot segments
(24 ct.) 2 foot segments
(16 ct.) 1 foot, 10 inch segments
(3 ct.) 7 inch segments
(2 ct.) 1' plywood circles
Sisal Rope (3/8" x at least 500')
(160 ct.) 3" wood screws
Things to know:
Step 3: Fabrication
10 ft. segments:
1. Obtain sixteen 10' segments of 2x4.
2. On each segments, cut 22.5º on the width of the wood at both ends to create a trapezoid.
1. Obtain sixteen 2' segments of 2x4.
2. On each segment, cut 45º on the width of the wood on both ends to create a trapezoid.
1. Obtain sixteen 2' segments of 2x4.
2. On each segment, cut 22.5 degree such that the sides are not too angled but still form a triangle.
1. Obtain the two circular pieces, 1' diameter.
2. On both circular pieces, drill out eight 3/4" diameter holes at every 45º.
3. Obtain 2 x 4 segments that are 7" in length.
4. Screw the top and bottom of the circular pieces to the top and bottom of the 2x4 pieces. The final product of the center piece should look a wheel.
1. Obtain eight 2' 2x4 segments.
2. Drill out a 1" hole 1.5" from the edge along the middle of the plank.
3. Drill out a 1" hole 1.5" from the edge on the opposite side of the plank.
4. Repeat for all planks.
5. Obtain eight 3.5' 2x4 segments.
6. Drill out a 1" hole 1.5" from the edge along the middle of the plank.
7. Drill out a 1" hole 1.5" from the edge on the opposite side of the plank.
8. Repeat for all planks.
Step 4: Frames
To create the frames, we will need:
(16) 10' segments
(16) 8' segments
1. On each of the prefabricated 10' segments, mark two 1' margins from each end of the longest side of the trapezoid.
2. Between two parallel 10' segments, place the end face of two 8' segments perpendicularly where the margins are located. Screw the pieces together.
3. Repeat step 2 until all the frames are "squares".
4. Using For the trussing, create a 45º right triangle using the 2' segments. Make sure that both trusses are connected at the top two corners of the square. Screw the trusses onto the frames.
Step 5: Connecting Frames
To successfully connect the frames, we will need:
(8) completed frames
(16) Frame Connecters
1. Bring two frames together so that the diagonal faces of the 10 foot segments are meshed to create a 135 degree angle.
2. Screw the two frames together at the top and the bottom, then repeat until all the frames connect to form an octagon.
NOTE: Do not let go of the frames until at least four are successfully connected!
3. At each corner of the octagon, place and screw two frame connectors between the 8' segments so that it forms three equal rectangles.
Step 6: Roofing
For the roofing, we will need:
(8) 2' Stent
(8) 3.5' Stent
1. Eye-splice a 17' rope to each hole on one face of the centerpiece.
2. Attach a 2' stent to each of these cables 4' away from the centerpiece. To make this attachment, use a small piece of rope through the hole of the stent and splice both ends onto the 17' cables.
3. Attach a 3.5' stent to each of these cables 8' 5" away from the center. Attach using the same method in step 2.
4. Eye-splice a 6' rope at the bottom hole of all 2' stents and joint-splice the ends to the 17' rope where respective the 3.5' stent is attached.
5. Eye-splice a 7' rope to all 3.5' stents.
Step 7: Attaching the Roof
To attach the roof, we will need:
1. Measure and mark 6' on the 17' rope from the top of the 3.5' stent. Measure and mark 5' on the rope attached to the bottom of the 3.5' stent. Joint-splice the two ropes together at those marks.
2. Repeat step 1 until all eight "legs" of the roof are joint-spliced.
Note: Place the center piece on an elevated surface so that the ropes are loose.
3. Once the ropes are spliced together, tie rope to the corners of the octagon. The marks on the combined rope should meet at the edge of the corner.
4. Repeat step 3 for each "leg" of the roof.
Step 8: Tightening the Dome
To tighten the dome, we will need:
1. Using a long piece of rope (around 100'), feed the rope through the eye-splices at the bottom of the 3.5' stents.
2. Once you are able make at least a complete circle, pull the ends of the long rope. Notice that when the dome is tight enough, the stents will become vertical.
3. When the rope is tight, use a joint-splice to seal the rope together to complete the cable ring.
4. Repeat the same method from steps 1-3 for the ring of 2' stents.
Step 9: How to Improve Your Dome?
To take the design of the dome further, there are many different ways to upgrade the structure for more functional uses, but here are two to get you started!
Gazebo (pic 1)
Since the structure of the dome is relatively straight, meaning that there are no curved surfaces, a cover can be easily attached to the dome. Using tarp or any large waterproof material to cover spaces between the cables can ensure a nice, covered dome for guests inside. The lower cable ring, the ring connecting the outer stents, can be used to hold up a light source!
Garden Structure (pic 2)
The idea of a garden structure is to basically provide an atheistically pleasing and Eco-friendly structure that fits nicely into a garden. The exposed structural skeleton gives the dome the illusion of being a greenhouse. Cables and horizontal beams can be used to support planters. It is not recommended to use glass for the roof since, one, the weight of glass can overcome the strength of the sisal rope, and two, placing glass panels is rather difficult on the dome.
Note: The images shown are different types of domes, but there are no recorded instances of a Geiger Cable dome being used for these small, practical projects.
Step 10: Our Reflection
Due to the poor use of the short time allotted for planning, the Kalani Engineering class noticed that there were many issues with the dome during and after the construction period. The most noticeable problems are listed here:
1. Rope Measurements
Probably one of the more obvious issues we have encountered was the inaccurate measurements of sisal rope. There were many factors that had to be accounted for when measuring the length of rope, including: the amount of material used for splicing, how much the rope will stretch, and the how much the rope length will be affected by different sizes of stents. Many of the ropes used were too long, so they can be easily tightened. After a few days of standing, ropes began to stretch and become lose, which resulted in collapsing stents seen in the picture above. To avoid this, make sure all ropes are tight and all stents are vertical.
2. Wood Measurements
The next issue occurred when pieces of wood were not cut at a proper length. Minor errors where pieces do not mesh together were caused by only millimeters of inaccuracy. Also, the outer ring of stents were considerably long, which resulted in the low roof upon entering the structure.
3. Warped Materials
The wood we used for the project was sometimes twisted, knotted, or bent. To cope with this, wood would have to be twisted slightly as the screws were attached, connections would require more screws to attach knotted wood together, and there was no way to avoid bent members.