All you need to know about tensile structures Final part
Last week I confessed 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.
I also commented on the historic background of tensile structures, which were originally inspired by tents, one of the first shelters conceived by man, and the fact that 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 describe structures that mix membranes and steel cables to build large roofs, whose main characteristics are tensile strength, prefabrication, and formal malleability.
As stated last week this type of structure requires very little material, thanks to the use of thin canvas that, when stretched, creates surfaces capable of overcoming the forces imposed on them.
To finish off this topic we will simply classify them and explain their main means of support and with that we will conclude the subject of 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 where 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 formalised by combining three elements: membranes, rigid structures such as poles and masts, and cables.
Membranes of polyester fibres, coated with PVC, are easier to produce and assemble in the factory, cost less, and have an average durability of around 10 years.
Fibreglass membranes, coated with PTFE, have a superior durability of around 30 years, and offer 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. Direct supports are those in which the cover is arranged directly on to 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 stabilisation. Both types of cables cross orthogonally, which guarantees their resistance in two directions, thus avoiding deformations. Load cables are those that receive external loads directly and are fixed at the highest points. On the other hand, stabilisation cables are responsible for strengthening load cables and crossing them orthogonally. It is possible to avoid connecting stabilisation 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 ringshaped stabilising cables. The upper cables support gravitational loads while the valley cables withstand the wind loads.