Structures as light as air
Inflatable constructions are efficient, lightweight, strong, ecological and safe — introducing a high-tech yet playful, rounded and sensual architectural language
Air is a light and impalpable element able to support heavy bodies such as aeroplanes, means of transport that make part of our everyday lives. Building with air may seem paradoxical, but let us see why this isn’t the case.
The safety of a road vehicle rests on the certainty of having well-inflated tyres. By the same reasoning, air helps provide support, stability and resistance to pneumatic structures. And as with road tyres, the difference between the inner pressure and the outside atmospheric pressure creates the “balloon” effect.
These are simple single-membrane structures, like the ones we are accustomed to seeing over tennis courts. Access is provided along a corridor with a dual or revolving door to avoid losing the slight overpressure inside, which is imperceptible to even the most sensitive people. Of all the structures in existence, this is the lightest. With no size limits, it behaves like a soap bubble, with identical tension at all points inside the membrane. To ensure efficiency, air must be blown into it and it is warm in winter. Very little energy is needed to maintain the over pressure, although it must be modulated to withstand the external stress of the wind.
Simple construction also guarantees safety. The pressurisation system compensates for small losses of pressure, while in the event of a large tear, the lightweight shell (weighing approximately three to four kilograms per square metre) is rendered virtually inoffensive, thanks to its softness and a slow-collapse mechanism, eliminating any source of serious danger to the occupants.
The most evolved systems have double walls and are also known as “pillows” on account of their quilted appearance. A good example is the Allianz Stadium in Munich designed by Herzog & de Meuron for the 2006 FIFA World Cup.
The forerunner, however, was David H. Geiger, who designed the roof of the Hubert H. Humphrey Metrodome football stadium in Minneapolis, Minnesota, in 1982. The open-air stadium had to be protected with an almost weightless structure that would not require reinforcements from the stands or work on the foundations. The roof needed a few repairs in the first five years but then collapsed in 2010 beneath an excessive build-up of wet snow. In 2011, it was reconstructed similar to the previous one but with more modern and reliable materials, using 40,000 square metres of membrane. The stability of this typology stems from the overpressure of the air trapped between two fine walls: an inner and an outer membrane. The space below the pneumatic structure does not need to be pressurised, meaning that no sealed accesses are necessary and it can also cover spaces with no walls. It does not require air compressors and the pressure inside the “pillow” simply has to be monitored.
To understand how these structures function we must explain the concept of restraint. The principle entails creating prior stress, and in pneumostatic structures the air pressure maintains the membrane taut. The membrane tension is then modified within certain limits, without ever being annulled as a result of external loads. Prestressed concrete offers a good example of a structure in a state of restraint, along with toughened glass and taut cables.
The fine polymer membranes employed for these structures have no capacity to withstand compression and are highly resistant to traction. The air blown inside can reach high intensities of a few bars, but humans will never be in contact with this. The internal pressure creates traction in the membrane and this tension is reduced or increased by external stress — snow, wind, heat changes, suspension, etc. — but the traction tension must always be maintained.
To be efficient and ensure a stable form, i.e the desired geometry, low-stretch membranes are adopted, generally made from PTFE (polytetrafluoroethylene, also known as Teflon) and reinforced with fibreglass. Excellent performance is also achieved with
PVC-coated polyester fabric, or the latest fibreglass textiles with silicone coating that allow up to 40 per cent translucence. A transparency of 95 per cent, meanwhile, can be achieved with ETFE, ethylene tetrafluoroethylene.
These dual-layer structures consist of two parallel membrane layers, which are linked to each other by walls or connection points to prevent the internal pressure inflating them like a balloon. Hence the structures display the typical “quilted pillow” pattern. It is also significant that the dry air blown between the two membranes provides good thermal insulation. Furthermore, if the membranes are transparent, they can be anti-UV treated and provide excellent natural light distribution inside the space.
Some years ago, I was working on Renzo Piano’s project to upgrade the Baia di Sistiana, near Trieste, where a large water space was to be protected with a lightweight roof, thus creating an “indoor sea”. I was inspired by the parafoil, a parachute with a wing cross-section. With its multi-cellular structure, the so-called “flying mattress” fills with air when opened and remains rigid throughout the descent to ground, where forward velocity is annulled and it deflates. The lightweight structure designed for Sistiana adopted the same principle: many cells combined to form an immense “pillow” approximately 50,000 square metres in size and with a double transparent Tedlar membrane.
In self-erecting structures, air pressure is used to lend stability to a pressostatic frame
Further development in the sustainability of this construction technique brought the introduction of a fine intermediary layer containing photovoltaic cells between the two membranes, of which at least the upper one is always transparent. This system was adopted for the roof over the courtyard of the new Lombardy Regional Council building in Milan.
Another type, which we can call intermediary, uses air pressure to lend rigidity and stability to a pressostatic frame. This is the construction
In the event of natural disasters, inflatable structures have extraordinary properties due to their lightness
form predominantly adopted for selferecting structures. The configuration is similar to the standard structure for traditional buildings, but the beams and pillars are tubular and inflated by a single compressor. They are interconnected by a pneumatic circuit with safety valves to preventa local tear causing the whole building to collapse. The walls and the ceiling are membranes stretched between the frame.
A space can be created simply and in just a few minutes to serve as emergency shelters, civil protection accommodation, first-aid areas, operating theatres or simply shelters. When they are no longer needed, they are deflated and packed up ready to be deployed somewhere else.
The diverse requirements dictated by the use of these structures do not necessarily call for the standard “soap bubble” form that best expresses the characteristic membrane shape, as represented by Rem Koolhaas in his Serpentine Gallery pavilion. The functional needs, peculiarity of the materials, construction capacity and design flair, accompanied by the technology and structural-analysis instruments that simulate conduct in all conditions of use, allow creations that distance themselves from typical membrane geometries. The restraint created by air pressure offers designers ample freedom, even for more demanding forms such as the parallelepiped which have little in common with the traditional membrane configuration.
As a further note on the subject of safety and, more importantly, in the event of natural disasters such as earthquakes, these structures have extraordinary properties generated by their remarkable lightness. With their small mass, the force applied by earthquake acceleration is extremely limited and, given their great suppleness, even strong tremors will never trigger a structural crisis or situation of danger as occurs in more customary rigid structures. Any danger of collapse can be excluded.
Maurizio Milan, structural engineer, is the founder of Milan Ingegneria. His principal activity involves supporting architects, with whom he has completed over 1000 projects, including complex commissions using unconventional materials and alternative technologies.
Above: In parachutes with a wing cross-section, the adjoining cells fill with air and remain rigid during descent
Above and next page: the biomes represent the second phase of Nicholas Grimshaw’s Eden Project, forming a sequence of eight interconnected transparent geodesic domes. The cladding panels consist of triple-layered high-performance ETFE film pillows; left: inflatable structure in the shape of a parallelepiped