Tensegrity for Temporary Structures

Michel Moukarzel

Tensegrity as a Structural System

A tensegrity system is formed of two types of components: rods resistant in compression and strings resistant in tension. The synthesis of these components, depending on their characteristics, creates the structure. The form obtained is synergistic in that its behaviour cannot be predicted by the behaviour of its individual elements. According to Motro, a system is a tensegrity if all the components under compression are inside the structure, thus the points between the extremities of these components are not on the boundary. This distinction allows us to classify a structure as a tensegrity where tension is the basis of support, in contrast to a conventional structure where compression is the basis of support.
In order for each vertex in the structure to be in equilibrium, and thus provide the structure with stability, every node must have one compressive element, and at least three cables. The resultant of these forces is in line with the axis of the strut, in order to remove bending moments from the compressive members. Tension and compression is thus in perfect equilibrium in the structure. Tensegrities are lightweight structures; they have a very high resistance to weight ratio. This resistance can be improved by increasing the pre-stress of the structure, as ‘the degree of tension of the pre-stressed components is proportional to the amount of space that they occupy.’ A self-stress state is thus defined when every element is under tension or compression, and the system is in a state of static equilibrium without external forces.

Adaptibility

The concept of adaptability is the logical consequence of the postulated modularity. Adaptability is understood as the ability of the constructor to cope with the demand of lodging functions, by multiplying the module as needed and by shaping volumes in response to functional demands, in the most optimal way possible. Difficult adaptation situations are encountered on irregular terrains. In most cases, existing platforms are adequate for most of the projected structures. However, some structures should be built in less prepared terrains – sloped, tortuous, or along curved lines. The same case of adaptability arises when a broken bridge has to be replaced. In this case both anchored ends of the destroyed bridge are evened out, presenting a basic levelled and aligned base from which to start. In other cases, however, bridges are to be built in irregular ridges, creating the difficulty of having two flattened and aligned bases on which to build.

Component Development

Different components were developed to cover the needs set for a full-scale rescue operation, providing bridges to establish or restore the transportation system, and structures for a field hospital, community spaces including religious, storage and management offices in addition to many types of tents that could be arranged in various patterns to match the anthropological and spatial characteristics of the distressed community. Four main assemblies were created in response to these requirements: The Arch, The Pavilion, The Bridge, and the Spine. The assembled forms obey the ergonomics required to serve the various functions in an optimal ratio of functional utility to space availability. At the same time, these forms can fit a regular modular display of sheets of fabric cut to a basic module for the sake of flexibility, interchangeability and ease of transportation.

Structural Performance

The structural modules were then analysed when subjected to structural stresses (using Finite Element Analysis software) simulating constraining weight conditions, such as a sudden fall of heavy snow at night, a storm, or a crowded pedestrian bridge. The structures proved to be safely resistant to these conditions with minimum deflections and distortions. Under testing, the modular tensegrity system developed proved to have a high ratio of structural resistance to weight of materials used. A physical model of the Pavilion was then built to study the practicality of the system and its construction sequence.
Weight reduction was a priority that justified the adoption of carbon fibre as a structural material. The management of weight was a basic issue for transportation programming, and thus a global system was conceived as one derived from two single components: the strut (200cm in length, 5cm in diameter, 2.5Kg in weight) and the cable (110 cm in length, 0.5cm in diameter). Additional rolls of cables were also provided to serve as ties. A strut and two cables constitute the basic triangular key module, generating the entire polymorphic structural system. Twelve module components weighing 35 Kg each were assembled in a backpack in order to be carried by a man to the final destination (with no other means available).

Implementation

The system is conceived for temporary settings, taking into account that its long-lasting materials are maintained in their condition for a long period of time, in case the reconstruction of the distressed area is delayed. Then the system is dismounted, repacked and stored for redeployment when necessary. It leaves no trace on the ground, other than buried hinge fixtures and partial levelling, a space that will be recovered again by nature. While having a low ecological footprint, the system is ready to be used again, if necessary. Weight reduction was a priority that justified the adoption of carbon fibre as a structural material. The management of weight was a basic issue for transportation programming, and thus a global system was conceived as one derived from two single components: the strut (200cm in length, 5cm in diameter, 2.5Kg in weight) and the cable (110 cm in length, 0.5cm in diameter). Additional rolls of cables were also provided to serve as ties. A strut and two cables constitute the basic triangular key module, generating the entire polymorphic structural system. Twelve module components weighing 35 Kg each were assembled in a backpack in order to be carried by a man to the final destination (with no other means available).