Thursday, October 2, 2008

Shell concrete

In 1919 Dr. Walter Bauersfeld of the Carl Zeiss optical works in Jena, Germany, proposed a planetarium. Following his 1922 success with a 52-foot-diameter (16-meter) iron-rod dome built on the roof of the company’s building—the first lightweight steel structural framework in the world—Bauersfeld consulted the structural engineers Dyckerhoff and Widmann about a larger version. Then, together with their designers Franz Dischinger and Ulrich Finsterwalder, he built the world’s first lightweight thin-shell concrete dome for Zeiss’s sister company, Schott and Partners. It was 131 feet (40 meters) in diameter and only 2.4 inches (6 centimeters) thick. The new structural technology, honed in later structures, made possible clear spans of lighter weight than had ever been imagined. Because concrete shells depend on configuration rather than mass for their strength, and because they exploit the fact that concrete is essentially a fluid, they have been characterized as the ultimate concrete form.

Some of the most exciting examples have come from Spanish engineer-architects. Eduardo Torroja y Miret (1899–1961) was perhaps the most innovative engineer of the early twentieth century, notable for shell concrete roof designs that employed continuous surfaces and eliminated the need for ribs. Three examples should suffice. Torroja’s first thin-shelled concrete roof was for the Market Hall in Algeciras, Spain (1933–1934), designed in conjunction with the architect Manuel Sanchez. The low-rise dome, supported at six points on its perimeter, spans 156 feet (48 meters); it is only 3.5 inches (9 centimeters) thick. In 1935, working with the architects Carlos Arniches Moltó and Martín Domínguez Esteban, Torroja produced the folded plate roof for the grandstand at the Hipódromo de la Zarzuela, Madrid. The cantilever on the graceful structure is 73 feet (22 meters). In the same year, with architect Secundino Zuazo, he designed a 180-foot-long (55-meter) vaulted roof over an indoor pelota court in Madrid. Its two intersecting parallel vaults, of 40 and 21 feet (12.2 and 6.4 meters) radius, respectively, span a total of 98 feet (30 meters); the general thickness of the vast shell is a mere 3.14 inches (8 centimeters). With José Maria Aguirre, in 1934 Torroja founded the Instituto Técnico de la Construcción y Edificación in Madrid to “develop new uses and theories for reinforced concrete”; after his death in 1961, the institute was renamed the Instituto de Ciencias de la Construcción Eduardo Torroja. In 1952, the editors of Concrete Quarterly had claimed with some accuracy, “Torroja has extracted the utmost from his chosen material, concrete; no other material could have given such structures, no other designer has.”

Another Spanish-born architect, a generation after Torroja, earned the nickname “the Shell Builder.” Felix Candela Outeriño (b. 1910), known best simply as Felix Candela, graduated from Madrid’s Escuela Superior de Arquitectura in 1935 but did not practice until he emigrated to Mexico in 1939. First working for others in Acapulco, he moved to Mexico City and set up his own practice, specializing in the design and construction of thin tensile concrete shells. Because this was a new way of building, Candela acted as architect, structural engineer, and contractor, even training the construction workers himself. He began building beautiful churches such as Medalla de la Virgen Milagrosa (1953–1955), Nuestra Señora de la Soledad Chapel (1955), and San José Obrero (1959), all in Mexico City, and the Open Chapel in Lomas de Cuernavaca (1958). The roofs and sometimes the walls are noteworthy for their seamless concrete construction, often only 1.07 inches (4 centimeters) thick. Candela stated, “It is the shape that matters.” He insisted that “the shell must be stable and of a shape which permits an easy way to work. It should be as symmetrical as possible because this simplifies its behavior” (Faber 1963, 199). To this end, he frequently made use of the hyperbolic paraboloid, a form that made the construction of timber formwork easy because it is generated only from straight lines. The best example can be found in Los Manantiales Restaurant of 1958 in Xochimilco near Mexico City; the thin concrete shell structure that encloses its radial plan is based on eight hyperbolic paraboloid segments. Critics have remarked that in Candela’s work both design and structure have been sharpened to the finest edge, imparting “a new dynamism” to architecture.

Shell concrete was promoted in North America by a single expatriate European engineer, the Viennese Anton Tedesko (1902–1994). From 1930 to 1932 he collaborated with the shell concrete pioneers Dischinger and Finsterwalder, and two years later he moved to the Chicago offices of Roberts and Schaefer, for whom he worked until 1967, designing what have been described as “watershed buildings,” including the Sports Palace (1936) in Hershey, Pennsylvania. Its ribbed-barrel, shell concrete roof spanned 255 feet (78 meters) and covered an area measuring 230 by 340 feet (70 by 104 meters). During World War II he designed seaplane hangars in San Diego for the U.S. Navy and later produced the main terminal building at Lambert International Airport, St. Louis, roofed with four bays of huge groin vaults. It was mainly through Tedesko’s evangelism that shells became respected in the United States.

According to engineering historian David Billington, the other U.S. “thin-shell” visionary engineer was Jack Christiansen of Seattle, who was chief designer of the city’s Kingdome (1976, demolished 2000). The building was not technically a dome; its 9-acre (3.63-hectare) roof was formed of hyperbolic paraboloids—22 rib arches with a 5-inch-thick (13-centimeter) concrete shell spanning between them. Kingdome was the last (and biggest) major thin shell built in the United States; it was preceded by the New Orleans Exhibition Hall of 1968 (on which Christiansen worked) and Denver’s Paraboloid (1969), created by I. M. Pei and Tedesko. After experimentation in the sixties and seventies, such large shells were eclipsed by the newer technologies of air-supported structures and steel-framed retractable roofs.

That is not to say that the concrete shell is passé. Since the early 1960s the structural engineer Heinz Isler (b. 1926) has designed more than 1,000 shells, mostly in his native Switzerland. His elegant structures, inspired by natural forms, challenge those of Candela for elegance. Instead of mathematically calculating the forms, Isler designs by experiment, using catenary models in much the way that Antoni Gaudí did almost a century before. Notable among his earlier works are the Wyss Garden Center at Solothurn (1961), a thin, double-curved geometric shell; the Sicli Company Office building (1969–1970) in Geneva; and the Bürgi Garden Center at Camorino (1973).

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