On Tensegrity

In the image below the higher of the two cardboard structures appears to be floating above the other without an obvious means of support. It is a clever arrangement, because the eye is tricked into perceiving an illusion. This is probably because we are used to seeing solid structures supported on columns, however in this case it is quite obvious that the three strings, which link the two parts are much too slender to behave that way. 

That said, on closer inspection the structural arrangement is perfectly stable and fully satisfies the laws of equilibrium. While it may be unusual on the eye its load paths are perfectly rational and with a little thought can be readily understood.

The arrangement belongs to a class known as tensegrity structures. Tensegrity being an amalgam of ‘Tensile’ and ‘Integrity’, which was apparently coined by the American architect Buckminster Fuller in the 1960’s. An alternative name ‘floating compression’ structures was coined by the artist Kenneth Snelson, who was a pupil of Fuller, but its usage is less common. This may be because Snelson’s term does not have the aesthetic quality of Fullers. Nevertheless, ‘floating compression’ is a helpful name, because it hints at the underlying illusion.

A tensegrity structure is one where the compression members are arranged so that they do not meet. It is this characteristic which creates the illusion and it requires a careful arrangement of tension members to achieve the effect. The greater the load in the tension members the more stable the structure becomes. In many cases the tension members are actually pre-tensioned to ensure that the structure can retain its form when subjected to external relieving forces, for example the wind.

In the case of the structure shown above perhaps the best way to understand it is to begin with the cantilever arm of the lower structure. A tension hanger suspended from the cantilever carries a second cantilever at the base of the upper structure. At this point the upper structure is unstable it wants to topple to the right by pivoting about the hanger connection.

The tendency to topple is counteracted by two tension members connected from the top of the upper section back to the base of the lower one. Thus equilibrium is preserved.

Perhaps the most famous tensegrity structure is the skylon, which was built in 1951 for the festival of Britain. The base of the cigar shaped column was approximately 15 m above ground level and its tip was almost 90m high.

Some say the skylon mirrored the post war British economy, because it had no obvious means of support[1]. Though this was of course an illusion.... at least in the case of Skylon.

Sadly Sklyon was scrapped in 1952 on the instruction of Winston Churchill, who thought it a symbol of the prior labour government. Not Churchill’s finest moment and a sad end for an inspired structure. 

[1] Not my gag; wish I could remember where I heard it.