Step-By-Step Tensegrity Tower part 4

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This post describes the building of the full-scale tensegrity tower. Ross Esselen and I built this in April 2004 at Wits University as a diversion from our studies.

Like the model in the previous post, the ratio of length between the ropes and the poles was 1:1.414. Materials used: 16 cane sticks (harvested from a campsite) and a 20m roll of lacing cord (nylon hawser-laid twine). Also cut yourself a spacer that is the length of your cord members, so that you can use this to measure your structure as you build.

The staves need to be prepared by cutting all to similar length and marking out the points where the ropes will be attached using a marker (or a knife). The point where the ropes will be attached needs to be prepared by fitting a 'loop' that the 3 ropes can easily be tied to. The loop is attached to the pole using a constrictor, and the loop itself is made by joining the two ends with a sheetbend.

Once the poles are prepared for use, lay them out in the grid, ready to tie the rope on, like the image on the left. If you can work on a flat, clean surface like the rooftop I have used here, that will make construction easiest. At Kontiki, building on long grass made this task much more difficult.



Remember that the the very bottom and very top layers of horizontal cables will only be tied to 4 poles, each 1 unit away from the next. All other layers will be tied to 8 poles, which are spaced 1/2 unit apart. Remember to cut lengths of cord slightly longer than one unit, so that you can tie them on to the loops of each pole.Work systematically, from bottom to top, starting at the same end for every level, and work through tying the horizontal cables together. At this point remember also that you should be leaving one cord tied to only one pole at the edge of the structure so that when you 'roll it up' later you have the cord ready to tie.

The diagonal cables need to be attached now. Remember that a diagonal links the bottom of one pole to the top of the next pole IN THE SAME LAYER. Also, remember that the layers lie in alternate directions (poles will lean one way in one layer and the opposite way in the next one, see the layout photos above or review this post). The 1-unit long spacer is now used to check that the length is right.

We are aiming for a length of one unit here, the same as the horizontal cables, BUT we want to be able to adjust these so that we can level the structure out. Use a trucker's hitch as pictured on the left, to be able to adjust these later. Click through on the link to see how it is tied. Again, one end is tied to the top of a pole, and the other end to the bottom of the next one- I found it easiest to anchor the cord to the top end with a round turn and two half-hitches and tie the trucker's hitch at the bottom.


All of the ropes that can be tied 'in the flat' have been tied, and your tower should like the one in the photo above. You are now ready to start 'rolling up' the tower. As before, the key here is to start at one end and work systematically to the other.

Tie all of the 'horizontal' cords first, remembering to measure that they are the correct length. Once they are tied, you will have the structure in the stage it is in on the left. Now work throught from bottom to top again, tying the final diagonal for each level. This is when the tower will start popping into shape, and strange things will happen- at times the structure looks almost as if it's alive- twisting and jumping as you tighten these chords. Measure them approximately and lock all the diagonal off with a slipped half-hitch so that you can easily adjust them later. Your 'tower' is now almost ready, before you stand it up for the first time, check that it looks roughly like this:




Next post: adjusting, levelling and placing the tensegrity tower.

Step-By-Step Tensegrity Tower part 3

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Having looked at the drawings, the next step is to build a model from skewers and elastic bands, to better understand how the structure works and to get precise measurements. We will estimate dimensions and use the model to adjust our measurements, then scale them up to build the full size tower.

Before you can start building, you will need some skewers and some elastic bands cut to make long strips of elastic. You will also need superglue (cyanoacrylate adhesive)- this is the fastest and easiest way to glue the elastic bands to the skewers. Set aside an hour to do this from start to finish the first time you do it. Once you have practiced, it should take about half an hour to build a model like this.


The idea with this type of tensegrity is to always make the structure lying flat or unrolled like the drawing at left, and then to attach all the pieces together to raise it into shape at the end.

This photograph shows two levels of the structure, attached together and ready to be 'rolled up' into shape as a tower. There are two things to notice about this structure: the two levels are sloping in opposite directions, and the two levels are joined without the skewers touching. The upper deck's skewers all touch the middle layer of elastics halfway between the bottom deck's skewers. One last thing here is the proportion of the structure: if the elastic is 1 unit long, then the skewer is roughly 1.5 (actually 1.414...-the square root of two) units long. I chose this dimension for the sake of simplicity, and it seems to work fairly well. You might be thinking "how is 1.414 simple?" If you look at the image, you will see that the elastic bands lie roughly at right angles to each other. That is because of the proportions of 1: 1.414ish. The elastics were all cut to the exact same lenght as each other, and the skewers were then marked with a permanent marker at the 2 points where the elastics were to be attached. 1 drop of superglue on the end of the elastic, place the skewer on, and in a few seconds the glue has cured (as always with cyanoacrylate, be careful about sticking your fingers to the model).

Once you are finished with the grid and are ready to roll your model up, seperate out the elastics that are going to run horizontally (ie. the blue,turquoise or green ones from this drawing) from the diagonal (or red) ones. Glue the horizontal elastics first, starting at the bottom and working your way up to the top. When you are finished with this, you will bave a very dis-ordered looking bundle of elastic and skewers. The most confusing step is coming. You need to work methodically from the bottom up, attaching the diagonal elastics. The thing to watch out for here is that you work within one level all the time- don't attach an elastic from the first level to a skewer on the second level. Diagonal elastics should only be joining the bottom of a skewer to the top of a skewer on the same level.

The completed model,squashed between two cupboards. The handy thing about using elastic bands is that they will just stretch to adjust the structure to the best shape. So instead of a tower, this is more of a bridge, and we can simply neasure off with a ruler and scale the dimensions up for a larger structure.

A final shot, looking down into the model, showing how the structure is arranged. The next post will look at the actual full-scale structure, built with bamboo and nylon, which was set afloat on the pond in front of the architecture department at Wits university for 2 weeks.

Step-By-Step Tensegrity Tower part 2

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The type of tensegrity we are building is a tower that is made from a series of simple tensegrity units stacked on top of each other. Unlike the flagpole and the Needle Point Tower in this post which have 3 compression members per level, the tower I have photographed for the tutorial has 4 compression members on each level. The drawing above shows the unit for each tower.

Let's look at the unit for now to try and understand a little better how the system works. There are 3 sets of cables- connecting the bottoms of poles together (green), connecting the tops of poles together (blue), and connecting bottoms to tops (red). Every end of every pole has 3 cables tied to it- one that is connecting it to the opposite end of another pole and 2 that are both connecting it to the same end of other poles. When you start building the tower, you need to remember this property otherwise things can get pretty confusing.

To build one of these units, the simplest thing to do is to lay the 4 poles out flat, and then tie 3 of them together, measuring carefully (more about measurements in the next post), leaving the 4th set of connections to be tied later. This gives you the flat layout on the left of this drawing. Once you start tying the fourth set of cables (purple in this photo), your unit will start standing up and taking shape.

The actual tower is made by connecting a whole series of these units together vertically, so that the ring of cables at the bottom of one unit is also the ring of cables connecting the top of the unit below it (ie, so that the blue cables for one unit are also the green cables for another). You can see that we now have a green base, a series of turquoise base/top layers, and a blue top. The red cables continue as they would normally. It's also important to notice that the direction of 'twist' in each layer changes- one goes clockwise, the next anti-clockwise, the next clockwise again etc. This is so that the seam of the tower runs in straight line up the side, which makes life MUCH easier when it comes to assembling and putting up the full size tower. The next post will look at building an elastic band model, before moving on to the full-size tower.

Step-By-Step Tensegrity Tower part 1

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Tensegrities are a type of structural system where all of the compression members (poles/columns) are supported and connected to each other with tensions members (ropes). This means that the poles look as if they are floating in a mass of cables.

As promised, I am going to explain step-by-step how to build a tensegrity tower similar to the flagpole I made at Kontiki this year (shown here with the flag removed so that the structure is visible).

Tensegrity structures (the name means tension-integrity, because the tension cables are what makes the structure stand up) were invented by an American artist called Kenneth Snelson, who has made many tensegrity sculptures. R. Buckminster Fuller, an American inventor and engineer, named the structures after seeing sculptures Snelson had made.

Although they might look confusing, these structures are really very simple, and all that is needed is to be methodical when building them. I recommend building a model first, and this step by step guide will follow that format: a model, then a full-scale tower, and lastly links to some books and websites (and one or two other things).

Lastly, here are some photos of Kenneth Snelson's 1968 Needle Point Tower tensegrity sculpture at the Hirshorn Museum in Washington, D.C.

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Kontiki 2006: team 7 photographs

Gauteng Kontiki took place over the weekend- around 3000 scouts, guides and supporters camped at Murray Park and 66 rafts participated in the weekend. Congratulations to this years winners, 3rd Krugersdorp Guides, who are the first Guide group to win Kontiki. I have put together photographs of my troop, and will post other photographs over the next few days. The theme for 2006 was 'an enchanted journey'. Our troop (as well as quite a few other) chose a Harry Potter slant on this, which will be apparent in the photographs below.

Kontiki is divided into 2 competitions: raft and supporters. Supporters consists of the campsite, competiton meal, play acting and spare time activities (ie. everything except for the raft). Raft competition includes technical and safety aspects of the raft as well as decorations appropriate to a theme.

This is a photograph of our raft under construction on Friday night. The A-frame construction is a descendant of the pontoon ferry raft which we have been using for some time- the structure is trinagulated, and because of this is very stable. Decking has been attached, barrels tied on and you can see how a secondary superstructure frame has been added to hang the decorations. During the construction of the raft, it rained fairly hard, but our scouts had a tarpaulin under which all our ropes were stored, which made construction much easier as all ropes were dry.

The raft was decorated as a Quidditch pitch, complete with corner towers, a quidditch player flying on a broom (visible on the top rope in this photograph), hoops (tied to the running lights and not visible in this photograph) and a golden snitch. The raft crew are shown here modelling the t-shirts with our team number and a quidditch player. Rafts launch at 12 on Saturday (rafts are launched by picking them up and carrying them into the water (this takes about 20 people) and come off the water 24 hours later.


The gateway to our campsite was a portrait as in the Harry Potter books- with a password needed to open the gate. The entire portrait opens like a door, using frapping mallets as hinges. Above the door you can see badges of the four houses of Hogwarts School, modified to match our four patrols. The idea for this gateway came from a troop brainstorming session, and the design was resolved by Alastair Cooper, who also designed the porcullis gateway.

Our camp couch this year was upgraded- to include 2 seating levels, and was reinforced as well- using cargo nets instead of sack cloth for the seats. This was a favourite of our scouts, with the upper level offering a view of the waterfront and some of the rafts, and the lower level facing in towards our camp site. The ladder leading to the upper level is visible behind the couch in this photograph.

Our dining table is a fairly straighforward table, with a tripod at either end supporting the deck of the table and a bench running along either side. There is also a shelter (2006 did not disappoint, with a storm on Friday night- making 20 out of 22 Kontiki weekends that have had rain) as well as a wire chandelier made by a craftsman working on the side of the road (who worked from a photograph of a crystal chandelier).

The only campsite gadget not designed and built by the scouts themselves was the flagpole, where I tried out a rather unusual structure. This type of tower is called a tensgrity system, and none of the brooms in this model are touching each other- they are all supported by a network of cables. In this photograph I am still adjusting the cables to level the tower out. In the next few days I will post a step-by-step guide to building a tower like this, with photographs of every stage and links to other tensegrity structures on the net.


Outside of my normal subject matter of pioneering, are a few other aspects of the supporter's competition:

Firstly, the Beaver Challenge. This is a challenge activity for Cubs and younger Scouts (Brownies and younger Guides as well, of course), which involves moving between a number of bases completing activities, and having a card stamped, with the aim of filling the card with stamps and earning a badge. Our pack scouters run our Beaver Challenge base, and this year consisted of choosing a 'bank vault'to open (one of a grid of matchboxes) from which the participant took the money and moved across to Ollivander's to buy a magic wand based on the 'money' they had found in the bank vault. Roughly 300 magic wands were distributed.

The competition meal was cooked by two of our scouts, and was a 'banquet' complete with a bacon,orange and cheese starter, sausages or chicken as the main course and a chocolate dessert. Menu, budget, table setting, grace, hygiene as well as the 3 courses themselves are judged in this competition. The meal was carried on a table suspended from a 'flying broomstick' frame, and our meal placed 2nd overall.


The Spare Time Activity has become our main parent involvement item at Kontiki- 4 or 5 families as well as all of the scouts on the supporters crew were involved in putting it together. The STA this year consisted of 3 seperate activities (a 'magical circle' made of clothes, as large as possible; a magical creature; and an awareness poster for the Troll Anti-Taunting League). Here is a photograph of our magical creature, which was a working, walking marionette, with a trunk that squirts yellow paralysing poising (also operated from the puppet controls) via a drip spliced into the trunk. Our STA placed 2nd.

The fancy dress was planned and co-ordinated by our scouts, and consisted of a series of portraits talking to each other, 3 wizards and an appearance by Hagrid (extreme right of photograph).

Other photographs of our troop and our sister troop are available here - thanks to Marc for these photographs. Look out for updates over the coming days, including gateways, rafts and the tensegrity flagpole I mentioned above.

Moving your raft around

As part of the lead-up to Kontiki, I'll be posting a few tips about rafts. View all of my raft tips and designs here.
When it comes to moving your raft around, there are 4 realistic options available to you: rowing/paddling, paddlewheels, sailing and punting. I'll take a look at all 4 options briefly.

1. Rowing and Paddling. Paddling uses either a single or double-bladed paddle, and the paddler faces in the direction of travel. Rowing is the use of a single-bladed oar, which is normally held in a crutch and the rower faces towards the back of the raft. Paddles are probably the simplest way of moving your raft around in deep water. The down side is that you can get tired fairly quickly. Rowing with oars is a lot more efficient and potentially pretty fast. The complications here is that you need to sort out decent crutches for this to work. If yo uare a Sea Scout troop, you probably have access to oars and the experience to use them effectively.

2.Paddlewheels. Inspired by Mississippi steamboats,paddlewheels are a very efficient way of moving around, but need a lot of preparation and design beforehand. Paddlewheels are the only piece of the raft that may be pre-constructed for the Gauteng Kontiki competition. Numerous designs are possible, using metals construction or pure pioneering. The paddlewheel shown here is the design of Graham and Gregory Witt and is described here. A pair of paddlewheels placed side-by-side makes a raft easy to steer- it's even possible to turn on the spot. Learn from my mistakes: fasten your paddlewheels AFTER you have launched your raft, it saves you destroying the wheel as you push your raft into the water

Rowing or paddling and paddlewheeling are primary locomotion methods- they can be used at all times to move a raft around. The last two methods I discuss are only useful in certain situations.

3. Sailing. Basically, I am only going to describe the use of a sail when the wind is directly behind you- what sailors call a run. ANY kind of sail you can improvise will help you here: a sheet, a tent (which is what the above drawing uses), a powerkite. Simply raise your sail whenever the wind is travelling in the same direction as you, and lower it at all other times. To sail with the wind coming from any other quarter needs a centreboard of some kind, as well as a rudder.I have only ever seen this attempted once (and I'm unsure whether it was successful). If you are an experienced sailor, by all means try this, please let me know what you come up with.

4. Punting. The final method is for moving in shallow water. Punting is the use of a pole to move the raft by pushing off the bed of the body of water. A comfortable length for a punting pole is 3.5-4m (10-12 feet), and the idea is to push the pole in at a slight backwards angle, as shown in the drawing, then push it backwards and lift it out before the angle gets to large- otherwise it tends to stick. Punting can be very fast, in fact it is probably the fastest method of moving around over short distances. So long as you are in shallow water, two scouts should be punting your raft- one on either side. Punting originated in England on the Thames, and there are a few good tips on punting traditional Thames punts here.

These are the most common methods for propelling a raft, there are others that are less efficient: pushing it while swimming behind, towing it with a canoe or gig, 'inchworming' with an anchor by casting far and then pulling yourself across to the anchor position, to repeat the same action, or using an outboard motor. A word of caution: outboard motors are pretty dangerous pieces of equipment, I recommend you only try using one under the supervision of someone who is experienced in their use, paying special attention to securely mounting it.

Kontiki starts on Friday. I will photograph what I can and report back over the next couple of days about the things I've seen.