Which shapes are strongest? Did you know that a triangle is stronger than a square? The triangle is the only shape that cannot be deformed without changing the length of one of its sides. The arch, a variation of the triangle, distributes forces more evenly. If you think about it, a dome is simply made up of many arches. Can you think of any bridges or buildings that use triangles or arches to make them stronger? In the dome you are about to build, the arches will be made from 24 pipes. Naturally, the more pipes you have, the stronger the dome will be.

• Table saw
• Drill press
• Band saw
• Pencil
• Tape measure
• Protractor
• Circle scribe
• 1" wood bit
• 5/16" wood bit

• 24 10' lengths of 3/4" PVC
• 12 4' long 2 x 4's
• 3 16" x 16" x 3/4" plywood
• 50 5" long 1/4" threaded bolts
• 1 5" long 1/4" eye bolt
• 50 wing nuts
• 100 washers

1. Using the protractor, measure two 75 degree angles, 36" apart, on the 2 x 4's as shown in figure 1.



2. Cut the boards along the lines by using the table saw.
3. Next, cut the remaining one-foot lengths of 2 x 4's into 8" lengths cut at 90 degrees.
4. From one of the 3/4" plywood boards, cut 12 pieces of wood that are 8" long by 3.5" wide to match up with the 2 x 4's that were cut earlier.
5. Cut the 24 pipes to 10' lengths.
6. Number each of the twelve 8" 2 x 4 and 3/4" blocks along with the angle cut 2 x 4's from 1 to 12.
7. Take the two corresponding numbered blocks and mark off two locations on the 3/4" board that are 1/5" from each of the long ends and along the centre.
8. Place the 3/4" board on top of the 2 x 4 and drill through both at the same time.
9. Countersink a hole 1/4" deep into the 2 x 4 blocks with the 1" wood bit (later this will hide the bolt heads so they do not scratch the floor in case the dome is to be used inside).
10. Print a number 1 on the second board on the end that is nearest the first printed number 1.
11. Butt a second 36" 2 x 4 against the first number 1 so they begin to form a circle.
12. Print a number 1 on the second board on the end that is nearest the first printed number 1.
13. Place the 3/4" board over the seam so it is centred, and use the pencil to mark two locations - one on each of the two 36" 2 x 4's that you just marked.
14. Repeat this procedure for all 12 sets of blocks until the base circle is complete.
15. Next, measure 10.5" along centre from each of the ends of the twelve 36" 2 x 4's.
16. At each of these locations, drill a 1" diameter hole - the pipes will be put through these holes later. Note that if the dome is being used for a star lab, you may want to space these holes apart on one of the boards to create a door when the dome in finished. This concludes the construction of the base.
17. Use the circle scribe to make a 16" diameter circle on the two remaining 3/4" boards. Be sure to mark where the centre of the circle is so you can drill a hole later.
18. Cut these boards on the band saw. You can cut them both at the same time, if it's easier.
19. Use the protractor to create lines on the circles every 15 degrees through the centre of the circle.
20. Along each of these lines from centre, measure 6" out, which should be 2" from the edge.
21. Place this marked circle on top of the second circle and clamp together if possible.
22. On the edge, draw a line across each board in two places. Mark that A and B on each board. This is to ensure all the holes line up correctly during assembly.
23. At each of these 24 marks, drill a hole with the 5/16" drill bit.
24. Drill a hole with the 5/16" drill bit 1.5" from the end - on only one end of the pipe.

Note: Here's how to make screwdrivers from scrap ends of 2 x 4's.

1. Drill a 5/16" hole 3" in the middle of the cut end.
2. Cut a slot on the edge, wide enough for the head of a wing nut. These can be used to tighten bolts without hurting your fingers.

You can use the Dynamic Domes to investigate many different topics. The following procedure can be used for studying structures and pulleys. Note: Be sure to have a large area in which to perform this activity. Outdoors is preferable, but the dome can be built indoors if there is sufficient space.

1. Arrange the 12 angled boards in a circle.
2. Place the corresponding numbered blocks by each pair of numbers from 1 to 12 on the angled boards.
3. Place 4 washers, 2 bolts, and 2 wing nuts at each of the 12 numbered locations.
4. Put a washer on each of the bolts, push them through the 2 x 4 blocks from the countersunk side, and place this on the ground with the bolts in the air (see figure 2).
5. Place the two angled boards with corresponding numbers over the two bolts.
6. Place the 3/4" board over the bolts to hold in place from the top.
7. Place a washer over each of the bolts and tighten with a wing nut. All wing nuts can be tightened with the screw drivers made according to instructions in the construction section.
8. Once the base is complete, put washers on 24 bolts and place them through one of the circles.
9. Tip the circle with bolts over and place it on top of two of the wooden screwdrivers.
10. Place one pipe over each of the 24 large holes with the hole going over a corresponding bolt in the centre.
11. When all holes have been placed on the bolts, place the top circle on so the A and B are aligned. Push the bolts up through the second circle (see figure 3).
12. Place washers and wing nuts on all the bolts.
13. You'll need a person at the end of each pole. Have two people lift the centre while each person places his pole down into the corresponding holes.
14. The dome is now in place, and you can try hanging from it if you wish.
15. Place the eye bolt through the centre hole and hang the pulleys.
16. First one pulley is used, and you can try lifting a weight and then pulling it with the single pulley. It does not get lighter to lift, but you can now pull in a different direction.
17. Set up a compound pulley using 4 pulleys.
18. Try to lift the weight again. Why has the compound pulley made it a lot easier to lift? You cannot push on a rope (you can only pull on it), so the force in the rope is constant throughout. The compound pulley made use of four ropes to lift the weight instead of a single rope. As a result, it was four times easier to lift the weight, but you had to pull four times as much rope.
19. Dismantle the dome in the reverse order to which it was assembled.

Compression is a pushing force - it squashes materials. Look for materials in compression in tree trunks, pyramids, and arch bridges.

Tension is a pulling force - it stretches materials. Look for materials in tension in telephone wires, suspension bridges, steel elevator cables, and inflated stadium domes.

Have you ever broken an egg by squeezing it between your fingers? Perhaps, if you squeezed in the middle, but try squeezing the ends evenly between your hands. You can think of an egg as two domes glued together - a sturdy structure indeed!