An inflatable roof on the Olympic Stadium in Montreal, Quebec
Can you tell us about Aérolande and its creation?
I founded Aérolande in the early 2000s. It is first and foremost a project creation and management firm. I either work alone or am surrounded by a team, depending on the project. Then, if those projects are selected, I call on subcontractors that I choose based on their ability to bring them to life. Aérolande also has a range of “ready-to-use” products, in particular bubble tents for the live performance sector, which are sold by a network of partners. With our partner Absolutehollywood.com, we pioneered the design and manufacture of large inflatable tents for 360° projections.
In the 1980s, I designed small bubble-shaped houses. One thing led to another, I began taking an interest in pneumatic structures which I thought particularly suited to small and mid-sized projects. Around thirty years ago, I patented an inflatable roof system, and it is an improved version of that first roof that I am proposing to use to cover the Olympic Stadium in Montreal.
Few people in Europe are aware of this, but the issue of the Montreal stadium’s roof is a saga that has been going for decades and which has cost Canadians over a billion dollars. Can you tell us about the various problems encountered?
The roof was originally designed to be suspended from a leaning tower that is part of the stadium’s infrastructure. Therefore, it had to be relatively flexible and stretched fairly tight. The concept itself is original, as the roof does not rest on the perimeter of the aperture. The synthetic textile fibre sheet that it is made of must be flexible and strong enough to hold several hundred tonnes of snow, and yet light enough to be suspended.
Logically, the first roof to be used was that designed by Roger Taillibert, the stadium’s architect. It was a retractable roof able to withstand the Canadian climate. However, changes were made to the original design. It was made of a single Kevlar sheet measuring 20,000 m2 that was attached to and stretched across 17 lateral points and 25 oblique anchor points, all linked to the leaning tower by 25 cables. In order to be stable, the roof had to be pulled very taut. However, strong gusts of wind tore small and larger holes near the anchor points, where strong tensions were created. Over time, it became necessary to lower the roof to the ground in order to make repairs. It cost Montreal’s inhabitants quite a sum. In the end, it needed to be replaced.
A second roof was installed in 1998. It is also a roof made up of several sections, small tents with three flying masts attached to a wired structure suspended from the leaning tower that looked very much like a spiderweb. The textile used was PTFE, a fairly durable polymer able to withstand various constraints: the flow of rainwater or sun, exposure to the sun, pollution, etc. However, that was on paper. Before it was even completed, on a day of extreme weather where frost and rain followed snow, a pocket formed on a slightly-inclined flank of one of the tents. Despite its strength, the fabric split and dropped a few tonnes of snow and ice onto preparations for the car show. The roof was repaired, and certain conditions were imposed on the use of the stadium. For instance, it is prohibited to hold events in the stadium if there are more than 3cm of snow on the roof. That is not really a solution. It makes it impossible to hold any events there in the winter.
This lead you to offer your services. Can you describe your solution?
It uses air as a construction material by trapping it inside a flexible and airtight polymer envelope constructed in a predetermined shape. It could be described as a sort of high-tech inflatable mattress. One of its advantages is that the “mattress” can be manufactured using a lightweight material as it is not taken into account in the load-bearing calculations. Its sole purpose is to ensure watertightness. Rigidity is obtained through the tension of cable threads integrated into both surfaces of the mattress and the tension of the many links (over 15,000) between the two walls. It should be noted that those separator links are calculated to give the roof the desired shape. It is a structure that constantly exerts pressure through expansion which does not need to be pulled using external forces. The semi-rigid roof is always expanding in all directions and can be suspended from its 25 anchor points on the top and attached to 17 different points on its sides. All of those points can be controlled automatically.
And this is enough to keep the whole structure up?
Yes, it is! All of the structure’s sections are linked to various steel cables and are attached to and rest on a system of Kevlar or Dyneema nets. This is the same fibre used to make bulletproof vests, which is to say that it is very strong. This 64-mesh, double-decked net (for increased safety) is linked to the different lateral anchor points and to the rigging lines. The meshes are closed by double nets with a tighter mesh (1/20). The bottom-most nets take on the shape of a hammock and do not bend further downwards, thus forcing the envelope to take on the desired shape when it is being inflated.
We also gave a lot of thought to how best to install the roof. Its concept enables it to be assembled entirely on the ground and then hoisted up using the drum pullers integrated into the rigging points. It can be lifted slightly to be partially inflated, inspected and tested. The internal winches can then be activated to lift it further up, while deflating it just enough to let it through the aperture of the stadium’s roof. It will then be fully inflated and secured to the lateral anchor points.
What role does air play?
It plays a crucial role! Without going too far into technical details, it is the part of the system which gives it its shape and maintains it in position. The main retaining system is a stretched double-decked wired dome, supported from the top and the sides, and subdivided into 64 sections. Once inflated, the structure will be sufficiently rigid thanks to slight overpressure maintained and powered by simple centrifugal fans like those used for inflatable structures for children. Without that overpressure, the roof would simply be a fairly large, but sagging, structure. Once filled with air, it takes on its predetermined volume and stiffens, ensuring the whole structure’s stability. The structure does not need to be supported from below by adding cables attached to the infrastructure. This avoids having to use additional complex and heavy fittings. The stiffness provided by the air prevents any ripples under heavy gusts of wind and guarantees that the structure can withstand the weight of 3m of snow, i.e. one hundred times more than the current roof.
How is your project better than others?
First of all, the concept itself gives a pure and smooth aspect that is particularly aesthetically pleasing. Although it is not retractable, it respects the stadium’s creator’s work by using the same support concept. However, this roof can be quickly moved to accommodate large-scale events. In addition to the technical aspects that give it many advantages, this roof can be entirely prefabricated in a workshop. The mattress of the Montreal roof is made up of 64 sections, each made up of nets and two membranes. It can be easily assembled on the ground. In addition, LED lights can be inserted inside the envelope to create lighting effects and make the stadium more attractive for major events such as concerts of international stars, for instance.
In the same vein, dual winches that are invisible from the ground but constant and stationary are inserted inside the structure so that the roof can be adjusted if necessary. Various structures can be attached to it, such as lighting racks or speakers for concerts and various other events. A section of the roof could also be opened to the public by converting a secure section in a sort of giant playground. Finally, this solution is also the most affordable at the current time. This is normal because since air is used to stiffen the structure, it is possible to use less costly materials that are lighter than PTFE. Finally, assembly and elevation do not require cranes or spider-men of any kind, further reducing costs. In the long-term, maintenance costs will be insignificant for such a durable roof. The mattress sections are also covered by another membrane that can withstand weather, which can also be replaced in individual sections. This provides the other components with an unlimited lifetime.
What are your future projects?
While awaiting the authorities’ decision as regards replacing the roof, which has been ongoing for the past decade, I am returning to my first love, creating habitable bubbles and mobile inflatable structures with positive internal pressure in various sizes, ranging up to 60m in diameter for mobile theatres, live performances, festivals, etc.
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