31-01-2013, 03:30 PM
TENSEGRITY STRUCTURES AND THEIR APPLICATION TO ARCHITECTURE
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ABSTRACT
Tensegrity is a relatively new principle (50 years old) based on the use of isolated components in compression inside a net of continuous tension members, in such a way that the compressed members (usually bars or struts) do not touch each other and the pre-stressed tensioned members (usually cables or tendons) delineate the system spatially and in a self-equilibrated state.
Tensegrity structures are 3-D trusses where some members are always in tension while others are in compression. The Tensegrity concept offers a high level of geometrical and structural efficiency and results in modular and lightweight structures. However, the concept of Tensegrity is still not a part of the main stream structural design wing due to various reasons.
The main aim of this work is to prove if it is possible to find some applications for such an atypical kind of structure, in spite of its particular flexibility and relatively high deflections. For that it is essential to understand the structural principles of floating compression or tensegrity, and to define the fundamental forces acting on it.
. INTRODUCTION
Tensegrity structures are 3-D trusses where members are assigned specific functions. Some members remain in tension while others are always in compression. Usually for compressive members, solid sections or bars are used; and string or cable type elements can be used as the tensile members.
Most bar–string configurations will not be in equilibrium. Hence, if constructed they will collapse to a different shape. Only bar–string configurations which are pre-stressed and in a stable equilibrium will be called Tensegrity structures. If well designed, the application of forces to a Tensegrity structure will deform it into a slightly different shape in a way that supports the applied forces.
The word “Tensegrity” is a contraction of the phrase “tensional integrity”. It can be traced back to Buckminster Fuller who first coined the phrase in his 1962 patent application. The construction of the first true Tensegrity structure is however attributed to the artist Kenneth Snelson who created his X- piece sculpture in 1948.
In his patent, Snelson describes Tensegrity as a “….class of structures possessing, what may be termed discontinuous compression, continuous tension characteristics.” This discontinuity was also recognized by Buckminster Fuller in his patent description, when he stated that “….the structure will have the aspect of continuous tension throughout and the compression will be subjugated so that the compression elements will become small islands in a sea of tension.”
Another important aspect is the stability. A Tensegrity system is established when a set of discontinuous compression components interact with a set of continuous tensile components to define a stable volume in space.”
A more mechanical description is given by Hanaor who describes Tensegrity structures as “internally pre-stressed, free standing pin-jointed networks, in which the cables or tendons are tensioned against a system of bars or struts.” This description introduces the fact that the system is pre-stressed and pin-jointed. This implies that there are only axial forces present in the system and there is no torque.
The general definition of a tensegrity structure is stated as:
“The geometry of a material system is in a stable equilibrium if all particles in the material system return to this geometry, as time goes to infinity, starting from any initial position arbitrarily close to this geometry”.
The bars are rigid bodies and the strings are one-dimensional elastic bodies. Hence, a material system is in equilibrium if the nodal points of the bars in the system are in equilibrium.
To summarise, the above descriptions cover most of the aspects of the Tensegrity concept which are listed as follows:
1. Pin-jointed bar frameworks: Tensegrity structures belong to the structural group of pin-jointed three-dimensional trusses.
2. Pure compressive/tensile members: Tensegrity structures contain only pure compression and tension members. And tension elements used are cables which can sustain only tension.
3. Localisation of compression: In classic Tensegrity structures the compressive elements are discontinuous. They seem to be floating in a continuous network of tension elements.
4. Pre-stressed structures: A state of pre-stress or self-stress is required for the stability of the structure since it stabilizes internal mechanisms.
CONCEPT OF TENSEGRITY STRUCTURES
Tensegrity structures are structures based on the combination of a few simple but subtle and deep design patterns:
1. Loading members only in pure compression or pure tension, meaning the structure will only fail if the cables yield or the rods buckle.
2. Preload or tensional pre-stress, which allows cables to be rigid in tension.
3. Mechanical stability, which allows the members to remain in tension/compression as stress on the structure increases.
Because of these patterns, no structural member experiences a bending moment. This can produce exceptionally rigid structures for their mass and for the cross section of the components.
A conceptual building block of tensegrity is seen in the 1951 Skylon tower which follows the typical tensegrity structure concept. But there are variations such as the Needle Tower which involve more than three cables meeting at the end of a rod. These cables define the position of the end of the rod which is considered as a well-defined point in space and the other additional cables are simply attached to this well-defined point.
Eleanor Hartley points out visual transparency as an important aesthetic quality of these structures.Korkmaz put forward that the concept of tensegrity is suitable for adaptive architecture due to its lightweight characteristics.