Modular ETFE air cushions illuminate and protect at a busy transit station near Hamburg, Germany.
By Mark Zeh
Hamburg, Germany’s second-largest city, is currently in the midst of a prolonged economic boom. This means that much of its infrastructure, planned and built in the 1960s for a much-smaller population, needs to be updated to meet the demands of current transportation needs and lifestyles.
As in many other post-industrial urban quarters, Barmbek, an area near the Hamburg city center, once built out as inexpensive housing for workers, is being redeveloped for the needs of another generation of city dwellers.
Currently undergoing renovation is its bus and railway transit station, one of the key hubs in the Hamburg transit network, with approximately 60,000 weekday visitors and 2,000 buses per day passing through the station.
In 2004, ap’plan Architekten mbH of Berlin and Stuttgart, Germany, won the competition to re-design the bus stop portion of the station. Planning for the project began in 2006. The portions of the structure on the south side of the station went into service in December 2012.
The bus transit shelter portion of the project consists of three internally lighted roofs on the north and south sides of the station. The roofs on the south side of the station are 55 and 115 meters (180.45 and 377.3 ft.) long, while the roof on the north side of the station is a 265-meter (869.42 ft.) long, continuous shelter, also covering the historical entrance to the station.
The roofs are composed of modular rectangular translucent white ETFE air cushions, mounted atop a steel framework. They are supported by Y-shaped stanchions, spaced 15 meters (49.21 ft.) apart, which support a steel framework for the cushions. Routing for roof drainage, an air pressure manifold for the ETFE cushions, and an internal lighting system for the roof, is all completely concealed inside the steel structure and not visible to users of the station. The cushions are lighted internally, via fluorescent tubes. These may be accessed for service through external flaps or directly.
“We began with the decision to create a counterpoint to the cinder-block faÃ§ade of the existing train station,” explains Dipl- Ing. Julian Vielmo, managing director of ap’plan Architekten mbH. “We wanted to create a light sculpture, not just a roof to protect against the weather. This vision guided our concept development and selection of materials.”
“We tested quite a lot of different materials, but the light transmission properties of translucent-white ETFE foil material allowed us to create a wing of light, which appears to be lighted from itself at night, rather than from an external source,” elaborates Mr. Vielmo. “Additionally, we wanted to use the natural form that air gives to the foil material as an expressive element in the structure. This natural form also creates self-supporting volumes inside the steel framework. We built many mockups to determine how large these volumes could be and what their spacing should be.”
“ETFE cushions can only be created through welding with hot bars,” explains Gerd Schmid, managing director of formTL. “Within Europe, there are only a few specialist firms who can do this work. It also requires some expertise to create cutting patterns, which result in wrinkle-free cushions. When you see the welded-together cushions, before inflation, it’s hard to imagine how they’ll look when they are pressurized.”
Mr. Vielmo explains further, “We spent a lot of time on the design of the geometry of the cushions, developing the form and getting the expression of the material right for this light sculpture was a major design project in our office. In the end, we built a single-axis, full-scale model in an industrial area in Hamburg to confirm how the cushions would look and that our technical solutions would work. We spent a lot of time developing the cutting patterns for the ETFE cushions, and steel frame holding the cushions, such that the cushions would not exhibit wrinkles.”
“The technical details of getting the cushions to appear evenly-lighted were a particular challenge,” reports Mr. Schmid. “We reached a solution where the upper and lower foil layers were separately attached to the steel framework, with a transparent foil strip between them, for the light to shine through. This allowed the fluorescent lamps to shine into the cushions and completely backlight the milky-white upper and lower layers.”
An important aspect of working with ETFE air cushions is the infrastructure that maintains their pressure.
“In this structure, there are separate air pressure supplies for the roofs on the north and south sides of the station,” explains Mr. Schmid. “Air is conducted through piping from the station to the roofs, then distributed to the cushions through manifolds in the steel frames. Each cushion has two air inlets and pressure is controlled through a series of regulators, allowing continual maintenance of the proper pressure.”
Another aspect that influenced material selection is the self-cleaning nature of ETFE.
“ETFE is very well-suited to this high traffic environment,” Mr. Vielmo says, “Compared to glass, it requires much less cleaning and maintenance. Another favorable factor is its light weight—at 600 grams per sq. meter (17.7 oz per sq. yard), there can’t be many lighter structural materials. This enabled us to also use a very light scale steel structure, 30 to 50 percent lighter than that needed for a steel/glass structure, which has positive cost implications.”
A challenge in designing this roof is the full-season weather in Hamburg. The membrane/steel roof is designed to support a typical snow load of 0.85 KN/sq. meter (0.123 psi) and exceptional snow loads of 1.96 KN/sq. meter (0.284 psi). If the loading is more extreme than this, the cushions are designed to deflate.
Since the loading cases vary seasonally, the cushions are inflated to 500 Pa. (0.07252 psi) in the summer and 750 Pa. (0.109 psi) in the winter.
Renovation of the Hamburg-Barmbek transit station is to be completed in 2015, when the area on the north side of the station, around the main entrance, will go into service.