Saturday, July 26, 2008

Airship hangars - Orly, France

The French dominated the early history of human flight. In September 1783 the Montgolfier brothers launched a hot-air balloon carrying farm animals to show that it was safe to travel in the sky, and a few weeks later Pilatre de Rozier and the Marquis d’Arlandes took to the air for a 5.5-mile (9-kilometer) trip over Paris. In 1852 another Frenchman, the engineer Henri Giffard, built the first successful airship—a steam-powered, 143-foot-long (44-meter), cigar-shaped affair that flew at about 6 mph (10 kph). About thirty years later Charles Renard and Arthur Krebs constructed an electrically powered airship that was maneuverable even in light winds. By 1914 the French military had built a fleet of semirigid airships, but they proved ineffective as weapons in the Great War. On the other hand, nonrigid airships were widely used for aerial observation, coastal patrol, and submarine spotting. Their advent generated a different type of very large building: the airship hangar. The first zeppelin shed at Friedrichshafen, Germany (1908–1909), had been 603.5 feet long, 151 wide, and 66 high (184 by 46 by 20 meters). Like most others built Europe, it was a steel-lattice structure with a light cladding. Much more inventive and spectacular were the parabolic reinforced concrete hangars built in from 1922 to 1923 on a small military airfield among farmlands at Orly, near Paris. They were a major achievement of engineering and architecture.

The French engineer-architect Marie Eugène Léon Freyssinet (1879–1962) studied at the École Polytechnique and the École Nationale des Ponts et Chaussées in Paris. After serving in the army in World War I he became director of the Societé des Enterprises Limousin and later established his own practice. A great innovator, he worked mainly with reinforced concrete, building several bridges. By 1928 he was to patent a new technique, prestressing, that eliminated tension cracking in reinforced concrete and solved many of the problems encountered with curved shapes. Simply, steel reinforcing cables were stretched and the concrete poured around them; when it set the cables were released and (because it was in compression) the structural member acquired an upward deflection. When it was loaded in situ the resulting downward deflection brought it back to the flat position while remaining in compression.

At Orly, Freyssinet was presented with a brief that called for two sheds that could each contain a sphere with a radius of 82 feet (25 meters), to be built at reasonable cost. He responded by designing prestressed reinforced concrete buildings consisting of a series of parallel tapering parabolic arches that formed vaults about 985 feet long, 300 wide, and 195 high (300 by 90 by 60 meters). The internal span was about 266 feet (80 meters), and each arch was assembled from 25-foot-wide (7.5-meter) stacked, profiled sections only 3.5 inches (9 centimeters) thick; those at the base of the arch were 18 feet (5.4 meters) deep and those at the crown 11 feet (3.4 meters). Placed side by side, they formed a very stiff corrugated enclosure. Starting at a height of 65 feet (20 meters), reinforced yellow glass windows were cast in the outer flanges of the arches.

Freyssinet specified an easily compactable concrete to ensure that the hangars would be waterproof. It was reinforced with steel bars and poured into reusable pine formwork that was itself stressed with tension rods to create prestressed concrete. The concrete was also designed to flow into every corner of the complicated molds, and it was fast-setting so that formwork could be quickly stripped and reused. The structure was temporarily supported on timber centering, and a network of cables held the formwork in tension until the concrete developed its full strength. In other structures lateral wind loading could be resisted by cross bracing, but because clear spans were imperative, Freyssinet provided the necessary stiffening by “folding” the concrete on the component arches. The selfweight of the massive structure was accommodated by increasing the cross-sectional area of the arches as they approached the ground, where the foundations consisted of deep horizontal concrete pads laid with an inward slope toward the center of the hangars. Tragically, in 1944, U.S. aircraft bombed these revolutionary and beautiful structures.

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