The POD Project - The MiniPOD

1. Introduction
2. Nodes
3. Beams
4. Tension Cables
5. Couch
6. Covering
7. Suspension
8. Utility System
9. Construction
10. Pictures in the Wild

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Introduction

I wanted to make a larger model of the POD to test construction techniques. There were two versions, one with 2' beams and another with 3' beams. The former lived for a time in my attic as an indoor relaxation chamber. The latter was built for outdoors use as a multipurpose tent (standing on ground or hanging from a tree).

This document describes the process of constructing a POD with 3-foot beams, for a total height of about 7 feet. I chose this size because it should provide comfortable dimensions for someone of my size (well over 6 feet tall). I plan to take it on a long cycling journey in lieu of a tent, so it has to be light enough to pull in a bicycle trailer, and be rugged yet easy to assemble and disassemble for daily use.

Materials

Here is what is needed to create one Mini-POD:

1. 48 x 3-foot length, 1", schedule 40 PVC pipe
2. 192 x 1-foot length, 1/4" aluminum rods
3. 24 x 3" long, 1" ring, steel eye bolts and nuts
4. 48 x 1.5" washers
5. 42 1.5" steel rings
6. 104 x 2" steel snap-hooks
7. 8 x 4" high strength steel snap-hooks
8. 1 x 3" high strength steel ring
9. roll of duct tape
10. 2 x foam rubber pilates mats, cut into 24 x 6" squares
11. 96 x 5-foot lengths of 5/32" nylon cord
12. 8 x 9-foot lengths of 5/32" nylon cord
13. 4 x 24-foot lengths of 1/2" high strength rope
14. 1 x 200-foot length of 1/2" high strength rope
15. 60 yards of ripstop nylon

These tools help:

1. pipe cutter or pipe saw
2. pipe deburring tool
3. two adjustible vice-grip pliers (for tightening eyebolts in nodes)
4. bolt cutters (for cutting aluminum rods)
5. tape measure
6. scissors (for cutting rope)

Weight:

beams	48 lbs.
nodes	16 lbs.
cables	8 lbs.
couch	5 lbs.
covering	10 lbs.
suspension	15 lbs.
total	92 lbs.

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Nodes

Physical nodes allow for fast construction, keeps the skeleton together long enough to attach the tension cables that keep the whole thing together, provide rigidity so the beams do not flop around, and keep beams from grating at the ends.

Early on, I experimented with making a POD skeleton with only beams and no physical nodes. This involved stringing rope through pipes to chain them together and rely on hanging or an internal spine to keep the structure up. This proved to be nearly impossible to assemble as a free-standing structure. Furthermore, the shell was extremely floppy and in spite of complex cross-bracing.

So for this mini-pod, I had to make 24 nodes with the following requirements:

• Hold beams together long enough to assemble the skeleton with tension cables.
• Resist flexing and hinging.
• Transmit compressive forces between beams without letting the ends get close enough to grate.
• Be easy to produce in quantity with simple tools (e.g. no welding).
• Fit into the ends of 1" pipes.

After much experimentation with everything from rubber balls to metal discs, I perfected the following recipe.

1. Fabricate aluminum rods.

Cut 1/4" aluminum rod into eight 12" pieces. These will be bent at the center to different angles: 3 at 90 degrees, 1 at 60 degrees, and 4 at 30 degrees, like below.

2. Make cross assembly.

Put the ends of the three 90-degree rods and the 60-degree rod together so they approximate a cross shape like below. Use tape on the ends to hold them together.

3. Add inner stabilizers.

A pair of 30-degree rods should rest snug inside (the concave side) the cupped cross. Use tape on the ends to hold them in place.

4. Add outer stabilizers.

A pair of 30-degree rods should rest snug outside (convex side) the cupped cross. They should be perpendicular to the rods added in the previous step. Use tape on the ends to hold them in place. Each limb of the cross should now consist of 4 rod ends.

5. Insert eyebolt.

Wrap extra tape around ends to keep them tight; the eyebolt will create a lot of force to try to pull the rods apart. Put washers (1" wide, 1/4" hole) on the inside and outside of the cross assembly. Push an eyebolt (1/4" width, 2" length) through the washers and the center of the cross. Affix with a nut and make it as tight as possible. The inner washer will actually warp a bit from the force, but this is okay.

This is how the eyebolt, washers, and nut look from the side:

6. Prepare the foam sheet.

Cut a 6" square from a sheet of foam (I used a pilates mat). Cut a 2" slit into each side, starting from the middle of the edge and going toward the center of the square.

7. Attach the foam.

First, lay the cross assembly on the foam square so the eye of the bolt is up and the bottom is on the sheet. The cross limbs should rest on the corners of the sheet:

For each limb (1), peel up a tab of foam from a slit on one side (2), and then the other, so they overlap (3), and wrap tape around this foam cuff. This cuff keeps the nodes' limbs snug inside the beams, so they don't rattle around or slip out before you can attach the tension cables.

The picture below shows a finished node. Notice the addition of a snap ring and 2" steel ring. This ring will be the attachment point for tension cables.

Final weight: 2/3 lb. each, total: 16lbs.

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Beams

My first attempt at making beams was using 1" schedule 40 aluminum. This proved to be way too heavy so I switched to PVC with the same dimensions. I am aware of the environmental problems of PVC, but this is just a test. Bamboo would be an excellent alternative, but I am not currently prepared to work with it, so I will leave that to a future project. PVC is easy to obtain and work with for now.

Dimensions of a beam:

length:	36 inches
external width:	1.315 inches
internal width:	1.029 inches
thickness:	0.133 inches

At these dimensions, the weight is 0.333 pounds per foot. So the weight of a beam is 1 pound, and the combined weight of all beams will be about 48 pounds.

The parts are easily made, simply cutting into 3-foot sections and smoothing the ends with a file. A saw is fastest, but a special pipe-cutting tool eliminates the roughness at the edges. After cutting, use a deburring tool or file to smooth out the inside. A slight degree of beveling inside would be ideal. This will make it easier to slide beams onto nodes and reduce bunching of the node padding.

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Tension Cables

Tension cables keep the beams secured to each other. Without them, the nodes would pop out with a little bit of pressure. They prevent skewing, where square faces squash into diamonds. Finally, they allow for a hanging mode in which weight is transmitted through cables instead of beams.

The challenge is making a large number of cables that fit well enough to maintain equal tension everywhere. There are 18 square faces and 8 triangular faces. Each triangular face only requires one cable around the perimeter to hold it together. But each square face needs two diagnal cables for a total of 36.

I am using5/32" diamond braid nylon cord (50 pound load limit). This is a fairly cheap and probably not very durable rope, but for the first test model, I figure it's okay. In theory, it should be waterproof, but it could be too stretchy for long-term use.

The given measurements are approximate, with an allowable error of plus or minus an inch or so. Cables will stretch anyway, so some future adjustment may be necessary, by tying knots in the middle of the cable for example.

Delta-Cable

The delta-cable is used on triangular faces. It consists of a single rope, 10 feet long, with a steel snap-hook on each end. Simply clip it to one node ring, run it through the other two node rings, and clip back on the original ring. This will keep beams from slipping off the three nodes in the trianfular face.

X-Cable

An X-cable keeps beams locked on nodes and prevents skewing (collapsing into a distended rhombus). There is a 1.5" steel ring in the center, with four cables branching out, terminating in 3" steel snap-hooks. The length of the diagonal of a square face from node to node (measuring from the rings) is 48". The length of each cable, minus ring and snap-hook, is about 20".

For each X-cable, you will need 20 feet of rope, a steel ring, and four steel snap-hooks. The following steps must be performed four times:

1. Prepare a length of rope.

Cut off a 5 foot length. For each end, hold it in a small flame for a few seconds until it turns black. Blow on it until it solidifies. This will keep the ends from fraying, which is a problem with nylon rope.

2. Tie rope to ring.

Tie a couple of "two half hitch" knots, as per the diagram below. (I am not sure why they don't just call it a full hitch.) As rope types go nylon is a bit slippery compared to, say jute, but four half-hitches are very unlikely to slip as long as you make sure they are very tight. Also, try to do it such that an extra inch of rope is hanging off, which will ensure that it will not unravel.

3. Tie on the snap-hook.

Measure 21 inches from the ring down the rope. At that point, tie a single overhand knot to the ring on the snap-hook. This is just to keep the snap-hook from sliding around which will be a nuisance later.

4. Tie remaining end of rope to ring.

Measure another 21 inches from the remaining end of the rope. At that point, tie it to the ring using the same 4 half-hitches in step #2. Again, make sure it is very tight.

5. Tie off any remaining rope.

If you have any rope left, tie it in half-hitches around the double strand of rope. You don't want long dangly bits of extra rope as it will be unsightly and could get tangled.

This picture shows the result:

PICTURE HERE

Knots can weaken rope by almost half. So let's assume a cable will hold only about 25 pounds. With two cords per X-cable branch, and two branches above and below the ring, that gives the equivalent of four cables per face, or 16 cables around the the top and bottom slices of the POD. The total load capacity of these cables would then be 16 x 25 = 400 pounds of weight. That is not enough of a safety margin. Furthermore, as we approach a full load, the cord will start to stretch, making for a very loose POD. Clearly, we cannot rely solely on these X-cables to bear the full load of a hanging POD. When I get to the suspension system, I will add stronger load-bearing cables from top to bottom.

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The Couch

The couch, situated at tier 2, is the part on which you sit or sleep when inside the MiniPOD.

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Covering

Covering keeps the weather out, which is the whole point of an outdoor shelter.

Measurement: 40 inches to a face.

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Suspension

The purpose of the suspension systemn is to keep the MiniPOD upright in freestanding mode and to hoist it up in hanging mode.

The simplest mode is freestanding. Cables keep the POD from falling over and rolling away. Fix the cables into the ground with stakes or tie them to convenient nearby objects:

More complex is the hanging mode. Here the POD hangs from a cable running between two trees:

Four high-load cables run inside the POD through the node rings:

To spread load amongst as many nodes as possible, each cable follows a zig-zag pattern through triangular faces:

Each of these "S-cables" will be 24 feet in length, with a high-strength steel snap-hook at each end.

I am still trying to work out how the POD will be hoisted up. At around one hundred pounds in weight, it will not be easy just pulling on a cable, and it risks damaging the tree. I am considering a block and tackle as a means of hoisting the POD after the cable has been planted in the trees.

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Utility System

The MiniPOD supports a small electrical system inside.

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Construction

Here are the 12 steps for erecting a MiniPOD.

1. Make the bottom square.

2. Assemble the beams in the bottom slice.

3. Add tension cables on bottom slice.

4. Install the couch.

5. Assemble beams in the middle slice.

6. Add tension cables for middle slice.

7. Assemble beams in the top slice.

8. Add tenstion cables for top slice.

9. Install covering.

10. Attach suspension system.

11. Install utility system.

12. Get inside!

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Pictures of the MiniPOD in the Wild

Places it has been used.

Acknowledgements

Thanks to:

• Omni Software's Omnigraffle for diagramming.
• GraphicConverter
• Home Depot