Saturday, June 23, 2007

Airplane hangars Orvieto, Italy

The Italian engineer and architect Pier Luigi Nervi (1891–1979) was among the most innovative builders of the twentieth century and a pioneer in the application

of reinforced concrete. In 1932 he produced some unrealized designs for circular aircraft hangars in steel and reinforced concrete that heralded the remarkable hangars he built for the Italian Air Force at Orvieto. None have survived but they are well documented: more than enough to demonstrate that they were a tour de force, both as engineering and architecture.

Nervi had graduated from the University of Bologna in 1913. Following World War I service in the Italian Engineers Corps he established an engineering practice in Florence and Bologna before moving to Rome, where he formed a partnership with one Nebbiosi. Nervi’s first major work, the 30,000-seat Giovanni Berta Stadium at Florence (1930–1932), was internationally acclaimed for its graceful, daring cantilevered concrete roof and stairs. The revolutionary hangars followed soon after.

There were three types, all with parabolic arches and elegant vaulted roofs that paradoxically conveyed a sense of both strength and lightness. The first type, of which two were built at Orvieto in 1935, had a reinforced concrete roof made up of a lattice of diagonal bow beams, 6 inches (15 centimeters) thick and 3.7 feet (1.1 meters) deep, intersecting at about 17-foot (5-meter) centers. They supported a deck of reinforced, hollow terra-cotta blocks covered with corrugated asbestos-cement. The single-span roof measured 133 by 333 feet (40 by 100 meters), and its weight was carried to the ground through concrete equivalents of medieval flying buttresses. The 30-foot-high (9-meter) doors that accounted for half of one of the long sides of the hangar were carried on a continuous reinforced concrete frame.

In the other types Nervi’s fondness for structural economy led to the prefabrication of parts, saving time and money. Type two was his first experiment with parallel bow trusses assembled from open-web load-bearing elements, spanning the 150-foot (45-meter) width of the hangar. A reinforced-concrete roof covering provided stiffening. The third type combined the diagonal configuration of the first and the prefabrication techniques of the second. He built examples of it six times between 1939 and 1941 for air bases at Orvieto, Orbetello, and Torre del Lago. The massive roofs, covered with corrugated asbestos cement on a prefabricated concrete deck, were supported on only six sloping columns—at each corner and the midpoints of the long sides—that carried the weight and thrust beyond the perimeter of the hangars. All the components were cast on-site in simple wooden forms.

The Germans bombed these amazing structures as they retreated from Italy toward the end of World War II. Nervi was delighted to learn that, even in the face of such a tragedy, the prefabricated joints had held together despite the destruction of his hangars. He later included them amongst his most “interesting” works, observing that their innovative forms would have been impossible to achieve by the conventional concrete technology of the day. In the early 1940s Nervi extended his experiments to ferrocimento—a very thin membrane of dense concrete reinforced with a steel grid—which be used to build a number of boats.

He next combined that material with the prefabrication techniques he had developed for the hangars. For Salone B at the Turin Exhibition of 1949–1950, he designed a 309-by-240-foot (93-by-72-meter) vaulted rectangular hall with a 132-foot-diameter (40-meter) semicircular apse at one end. The main hall roof and the hemidome over the apse consisted of corrugated, precast ferro-cimento units less than 2 inches (5 centimeters) thick, supported on in situ buttresses, creating one of the most wonderful interior spaces of the twentieth century.

Nervi’s designs were too complex to be calculated by orthodox mathematical analysis, and he developed a design methodology that used polarized light to identify the stress patterns in transparent acrylic models. A few unbuilt projects were followed by three structures for the 1960 Rome Olympic Games. He built the Palazzo dello Sport (1959, with Marcello Piacentini), the Flaminio Stadium (1959, with Antonio Nervi), and the Palazzetto dello Sport (1957, with Annibale Vitellozzi). The last is a gem of a building whose rational structure is so transparently expressed that the observer can almost see the loads being shèpherded to the ground in a way redolent of late English Gothic fan vaulting.

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