A few years ago there was a fad in London and other cities for using airships as mobile advertising hoardings. So when a company in Germany announces plans to start building airships, known as CargoLifters, it all seems familiar.
Think again. Those little airships bobbing in our skies bear the same sort of relationship to the planned CargoLifters as a biplane does to a 747. CargoLifters will be immense and, because they need a home to protect them from the weather and a place for their construction and maintenance, their development involves the construction of an immense hangar.
Arup was appointed engineer for what turned out to be the largest free-standing hangar in the world. Munich practice SIAT Architektur + Design is the architect for the building in Brand, 50km south of Berlin in Germany. The hangar is designed to house two CL160 CargoLifters side by side. They are each 260m long with a maximum diameter of 65m, designed to carry 160 tonnes of cargo up to 10,000 km at a cruising speed of 56 km/h without the need to refuel.
Requirements for the building design were that it:
minimised the building volume lminimised the building surface laccommodated two CL160s next to each other lrelated in shape and choice of material to the airship 'theme' lrelated to and clearly developed technically in construction terms from past airship hangars
guaranteed an economic, functionally efficient and aesthetically pleasing building, in which risk factors were minimised within the available timescale.
As the fifth of these points indicates, the building is not without precedent. Germany was the home of the airship, a vital part of its aviation history - indeed Stuttgart still has a Graf Zeppelin hotel. Although several hangars were built between 1898 and 1938, none survives and documentation is scarce.
But research has uncovered some technical information and, although the new hangar sits firmly in the line of development, it does also include some definite innovations.
The CL160 is a blimp, not a Zeppelin.
This fact is not merely of interest to planespotters, or even airship spotters. Zeppelins were long and thin; blimps are much rounder. This means that, although the new hangar is not that much longer than some of its predecessors (the Saarbrücken hangar of 1936, for example, was 275m long), it is much taller and wider.
Arup designed a pavilion for the 1984 Liverpool Garden Festival which some wag dubbed 'the Hedex capsule', and the CargoLifter terminal, albeit on a much larger scale and with a different structural system, could take the same name. Each end is semicircular with a radius of 100m. The central part, 160m long, is semi-circular in section, with a height of 107m. The doors at either end are in eight sections, two fixed and six moving, which open with a clamshell action, within their own footprint. This is innovative and not inexpensive but, by allowing the building to follow so closely the shape of the blimps, it saves on materials for both superstructure and flooring. As well as being visually pleasing, giving a good indication of what is inside, the shape is aerodynamic, reducing the wind loadings.
As Matthew Teague writes on page 2, there is a tendency to describe large buildings in terms of other architectural icons. So, St Paul's Cathedral would just pierce the top of the hangar, Sydney Opera House would sit comfortably within one half and the statue of Liberty would nestle within the profile of the doors. But far more impressive than any of these comparisons is the volume of the building, at 5,200,000m 3.The central section of the hangar consists of a fabric structure covering five steel arches at 35m centres, springing off concrete plinths that act as covered entrances. The arches have a clear glass roof between their top chords, allowing in daylight. Research has shown that these arches, which are fully moment stiff, are a new structural form.
Earlier hangars were built with hinged arches because they were easier to design and build, but the rigid form uses less steel.
The steel arches have cross bracing between them internally and props on the outside to avoid overall torsional buckling.
This makes them stiff enough to resist the horizontal thrust from the sliding doors.
The arches have a structural height of 8m and span 225m, with top chords at 3.4m centres and bottom chords at 2.0m. The 'curved' arches are in fact made up of 17 straight sections. An 8m deep truss, similar in construction to the arch beams, connects the arches at roof level. A PVC-coated membrane forms the covering.
Each end of the hangar forms a quarter segment of a sphere, made up of eight door elements. The shell-shaped elements are fixed to a hinge at the top of the end arch and guided horizontally by rails at the bottom.
Each element has two motor drives at both ends at ground level.
It was essential to make the doors as light as possible, not only to save on materials but also to minimise the loadings on the steel arches, the size of the drive motors and the foundation requirements.
The designers therefore used a shell principle. The inner part of each door segment is a spherical grid of identical horizontal, vertical and diagonal elements that are rigidly jointed. Corrugated metal sheet cladding spans this grid between the upward-curving side beams. The shell is connected eccentrically to the side beams, to the top flange on one side and the bottom flange on the other side. This allows the segments to slide under each other.
It is almost impossible to get a sense of the massive space enclosed within this structure. But look down near ground level and there are some tiny blocks running along the straight sides. These in fact are two-storey offices, almost entirely dwarfed. When the first prototype airship emerges next year, it will astound spectators. Few are likely to reflect that such massive structures need even more massive homes. If they do think about it, they should be even more amazed.