All you need to know about tensile structures

By that time the Olympics at Munich was on, and I remember distinctly when I saw the stadium on TV that whatever happened I would be an architect.

 

The Munich Olympic Games was a powerful example of what could be done with this structures. The 1972 Olympics, was hosted in Munich and subsequently in 1974 the World Cup. The author of the stadium, with its characteristic textile cover of PMMA, was the architect Frei Otto.

 

This type of structure was my secret love, kept clandestinely in my mine and waiting to be presented to the world at any time in a future project. The Gods had other idea about my profession, time have past and unfortunately I have been unable to be granted a project where I could introduce a tensile structure as part of the solution. Nonetheless, although I haven’t had the chance to use it, I thought that maybe I would share with you readers the techniques, so thus this article.

Historically inspired by tents one of the first shelters conceived by man the tensile structures offer a series of benefits when compared with other structural models.

 

Tensile structure is the term usually used to denominate the structures that mix membranes and steel cables to build large roofs, whose main characteristics are tensile strength, prefabrication, and formal malleability. This type of structure requires very little material, thanks to the use of thin canvas that, when stretched, create surfaces capable of overcoming the forces imposed on them.

 

Predominantly used to cover sports centers, stadiums and industrial and agro-industrial constructions, the tense structures are inspired by ancient systems, used during the Roman Empire. However, from Roman times until the middle of the 20th century, due to low demand and the lack of cable, canvas and connection manufacturers able to withstand the forces generated, there were few technological advances. It was only after the Industrial Revolution, and the unleashing of the era Ford, that the new developments were able to satisfy the intrinsic needs of this construction system. The low cost of mass production and the demand for systems capable of adapting to the most varied terrains through large openings, such as circus tents, for example, encouraged the development of the technique.

 

Steel cables and waterproof membranes

The instability and structural deficiencies of some previous models, due to the application of interwoven cables and very light covers, was resolved in the middle of the last century, thanks to the application of steel cables and waterproof fibre membranes, with a high degree of resistance. These not only provide greater protection against ultraviolet rays, fungi and fire, but also allow a greater or lesser translucency and reflectivity.

 

Such progress was only possible thanks to the physical-structural studies initiated by the German architect and engineer Frei Otto, who from the 1950s made the first scientific studies and designed the first covers with tensioned steel cables, combined with membranes.

 

As a student, Otto visited Fred Severud's office, getting to know the Raleigh Arena in North Carolina and being impressed by the bold aesthetics and comfort of the project. Back in Germany, he began to explore physical models on a small scale, empirically generating several surfaces, using chains, pulled cables and elastic membranes. Very much the method used by Antonio Gaudi at Barcelona. The idea is to use chains to cover a space, invert those chains (upside down) and gravity provides you with the ideal and most economical form for the arches to span from. More simply, if you take both ends of a chain with your hands and let the chain form an arch and if you could freeze that arch, and then keep adding more chains to cover a space that would be the most economical membrane to cover a space.

 

Frei Otto, the master architect

Convinced of the usefulness of stretch ceilings, he developed the first large scale project using the system that would later allow to cover the Olympic stadiums, clubs, zoos and pavilions. In 1957, he founded the Centre for the Development of Light Construction in Berlin. Seven years later, in 1964, he created the Institute of Light Structures (The Institut fur Leichte Flachentragwerke) at the University of Stuttgart, Germany.

 

Author of notable projects, such as the German Pavilion for the 1967 Expo in Montreal and the Olympic Stadium in Munich in 1972, Frei Otto is famous for his intense research work, for which he was honored with the RIBA Gold Medal in 2006 and the Pritzker Prize in 2015. Otto is also the author of the first complete book on tensile structures, "Das Hangende Dach" (1958), intensifying the idea of reinventing material rationality, prefabrication, flexibility and luminosity in the interior space, and even sustainability, when the term was not yet used in architecture.

 

Last week I did confess my secret love for this type of structure as well as my frustration for not being able to make use of its potential in any project, since I had not been lucky enough to be commissioned a scheme where this type of structure was required.

 

We also commented the historic back ground of tensile structures historically inspired by tents one of the first shelters conceived by man, the tensile structures offer a series of benefits when compared with other structural models.

We also remarked that tensile structure is the term usually used to denominate the structures that mix membranes and steel cables to build large roofs, whose main characteristics are tensile strength, prefabrication, and formal malleability.

As we stated last week this type of structure requires very little material, thanks to the use of thin canvas that, when stretched, create surfaces capable of overcoming the forces imposed on them.

To finish off this topic we will simply classify them and explain their main mean of support and with that we will conclude the subject on tensile structures.

 

 

 

Main classification

There are three main classifications in the field of tensile structures: membrane tensioned structures, tensioned meshes and pneumatic structures. The first refers to structures in which the membrane is held by cables, which allows the distribution of tensile stresses through its own shape. The second case corresponds to structures in which a mesh of cables transports the intrinsic forces, transmitting them to independent elements, for example, sheets of glass or wood. In the third case, a protective membrane is supported by means of air pressure.

 

Structurally, the system is formalized by combining three elements: membranes, rigid structures such as poles and masts, and cables.

 

The membranes of polyester fibres, coated with PVC, are easier to be produced and assemble in the factory, a lower cost, and an average durability of around 10 years.

 

The fiberglass membranes, coated with PTFE, have a superior durability, around 30 years, and greater resistance to sun, rain and wind. However, skilled labour is required for its installation.

 

Types of support

In this system, there are two types of support: direct and indirect. The direct supports are those in which the cover is arranged directly on the rest of the structure of the building, while the second case, the cover is deployed from a high point, like a mast.

 

The cables, responsible for the distribution of tensile stresses and the 'hardening' of the tarpaulins, are classified according to the action they perform: loading and stabilization. Both types of cables cross orthogonally, which guarantees their resistance in two directions and thus avoiding deformations. The cables of load are those that receive directly the external loads, fixed in the highest points. On the other hand, the stabilization cables are responsible for strengthening the load cables and crossing the load cables orthogonally. It is possible to avoid connecting the stabilization cables to the ground, using a peripheral fixing cable.

 

The nomenclatures for the different cables are generated according to their position: the upper cable refers to the highest cable; while the 'valley' cables are fixed, under all other cables. Radial cables are ring-shaped stabilizing cables. The upper cables support gravitational loads while the valley cables withstand the wind loads.