Tensegrity Coffee Table

A year ago I made a living room furniture for a friend of mine with an industrial style; since I am an engineer I wanted to add structural concepts to the design and therefore I made it by inserting tensegrity elements.
The top was made with the "shou sugi ban" technique which is very nice together with the steel frame. Another friend of mine saw this first job and he was amazed and asked for one for himself; I also offered him some finishing variations but he was in love with what I had already done and he wanted it identical.

Safety must come first! For this project I mainly used these power tools:
Welding machine
Angle grinder
Drill, better a drill press
Circular saw

Each of them needs a specific safety equipment that must be worn correctly according to what is written in the user manuals.

To make this 80x50xh.60cm table I used:

• about 7m of 30x2mm square box
• about 2.6m of 30x3mm plate
• 4 feet
• 4 cables of 65cm each
• 4 cables clamps (not needed if you use brake cables with already mounted end caps)
• 4 M10x40 bolts with washer and nut
• wood screw
• 4 L to fix the tabletop
• tabletop

I have spent about 40 €. For the tabletop I used leftover strips from my pitch pine parquet as it has a really beautiful grain; other wood species or a laminated glass sheet can also be used as desired.

The word tensegrity derives from "tensional integrity" and refers to structures whose stability is guaranteed by the coupling:
- tense elements, generally composed of cables;
- compressed elements, more squat, with adequate flexural stiffness.

Tensegrity structures are based on the combination of a few simple design patterns:

• members loaded in either pure compression or pure tension, which means that the structure will only fail if the cables yield or the rods buckle. This enables the material properties and cross-sectional geometry of each member to be optimized to the particular load it carries.
• preload or tensional prestress allows cables to always be in tension, to maintain structural integrity.
• mechanical stability, which allows the members to remain in tension/compression as stress on the structure increases. The structure also becomes stiffer as cable tension increases.

see https://en.wikipedia.org/wiki/Tensegrity

The concept behind this piece of furniture is simple: the upper frame is hung from the lower one through a chain and the 4 lateral cables have the purpose of stabilizing the structure, which otherwise would be in a situation of incipient motion.
The two frames are perfectly identical in shape and size, the only variations are represented: by the feet present in the lower frame only and by the tensioning devices in the upper frame only, in which they will be hardly visible as they are hidden by the wooden top. The frames are made up of a square box of 30mm and 2mm thickness, shaped as per the attached sketch.

The 4 side tie rods are ø1.5mm bicycle brake cables, guaranteed for a load of 40kg each and will need to be pretensioned for about 20kg. To make sure that all cables have the same voltage just pinch the cable and listen to the note produced, you can also help yourself with an app on your mobile phone, which indicates the frequency (Hz) of vibration. For your curiosity you can also evaluate the traction on the cable: https://www.spaceagecontrol.com/calcfunm.htm?MA%5...

The central chain, when the cabinet is unloaded, will therefore be subjected to a traction of 80kg (20kg x4 cables) and therefore cannot be identical to the side tie rods. When the piece of furniture is loaded, the traction on the central chain remains unchanged while that on the tie rods decreases.

The Shou Sugi Ban technique consists in carbonizing the surface of the wood with a flame. Carbonization decreases the moisture contained between the fibers and closes the pores of the wood, making the surface compact and hard. At this point the boards are much better than normal resistant to sources of heat, water and humidity, parasites and insects and UV rays.
This technique was born in Japan and has been widespread on the island since ancient times but has been supplanted by the use of chemical coatings, dyes, paints. Recently this technique has begun to spread worldwide, also thanks to design choices that involve giving color to the tables and at the same time maintaining the contrast with the burnt veins, which remain black.

The procedure I followed consists of charring the upper floor using a simple barbecue, if you have a flame torch it is easier; then I removed the coarser part of the charred wood with a scraper. In this way it is possible to remove only the softer part of the wood as the latewood, reacting to the heat, has become very hard and will keep its black color. Continuing with an orbital sander, I took up the grooves of the earlywoods in order to enhance the contrast between the dark rings and the non-charred light part that I gradually reached.

The wooden boards will also take on a three-dimensional trend, due to the different smoothing of the rings, which is really pleasant to the touch. At this point the board must be finished and you can proceed with a protective oil or varnish for wood, necessary as the carbonization has been partially removed, which can also be preceded by a coloring if desired. I took pictures of some color proofs. The top was then finished with a steel border that reflects the style of the frame and allows you to mask the edges that I did not heat-treated as they would have rounded and I don't like the effect.

The assembly is not particularly complex, it took me about 6 hours to cut, drill, weld and sand the steel frame. To weld perfectly in the square I used a kit of magnets and I always checked the diagonals with a tape measure.

An interesting solution that I adopted concerns the wire tensioners: the cables are fixed to the base frame with a clamp while at the top I inserted the tensioners with M10 bolts. I previously made 2 through holes on the bolts (about 2cm from the head), arranged perpendicularly to each other; in the first hole ø3 we will insert the other head of the cable while in the second hole ø2.5 threaded M3 we will screw a M3x8 bolt that will serve to lock the cable. To tension the wire, simply rotate the bolt counterclockwise and once the right tension is reached, the nut must be tightened inside. The force on the cable, which will tend to turn the bolt clockwise, acts in the same direction as the tightening and therefore the bolt will not loosen. I made the hole for the tensioner in the upper face of the frame which will be covered by the wooden top.

After a first assembly test, in which I also sat on it without problems, I disassembled everything and painted the steel frame with a ferromicaceous paint that gives it a nice anthracite color.

Finally I mounted the top and fixed it with 4 small brackets and self-tapping screws to the steel frame.

I am very satisfied with the result, I hope you like it too!

I also add two photographs of the coffee table I made earlier, with a different shape of the central upright.