Brunelleschi's brickwork masonry envelope has an "improved" cross-section and consists of inner and outer skins linked by diaphragms. An ingenious pattern of brickwork bonding was adopted to ensure satisfactory composite action. Given the span involved, and certain other constraints such as that the dome had to sit on an octagonal drum, it is difficult to imagine any other form that would have been feasible structurally. This memorable work of architecture is therefore an example of genuine "high tech." The overall form was determined from structural considerations and not compromised for visual effect.
The moldability of reinforced concrete greatly extended the potential for increasing the efficiency with which a dome or vault can resist bending moment caused by semi-form-active load patterns.
Among the earliest examples of the use of reinforced concrete for vaulting on a large scale are the airship hangars for Orly Airport in Paris by Eugène Freyssinet. A corrugated cross-section was used in these buildings to improve the bending resistance of the vaults.
Other masters of this type of structure in the twentieth century were Pier Luigi Nervi, Eduardo Torroja, and Félix Candela. Nervi's structures are especially interesting because he developed a system of construction which involved the use of precast permanent formwork in ferro-cement, a type of concrete made from very fine aggregate and which could be molded into extremely slender and delicate shapes.
The elimination of much of the temporary formwork and the ease with which the ferro-cement could be molded into "improved" cross-sections of complex geometry allowed long-span structures of great sophistication to be built relatively economically. The final dome or vault consisted of a composite structure of in-situ concrete and ferro-cement formwork.
Other notable examples of 20th century compressive form-active structures are the CNIT building in Paris by Nicolas Esquillan and the roof of the Smithfield Poultry Market in London by R. S. Jenkins of Arup.
Long-Span Metal and Cable
Compressive form-active structures are also produced in metal, usually in the form of lattice arches or vaults, to achieve very long spans. Some of the most spectacular of these are also among the earliest, such as the train shed at St. Pancras Station in London (1868) by William Barlow and R. M. Ordish (span 240 feet, or 73 meters) and the Galerie des Machines for the Paris Exhibition of 1889, by Contamin and Dutert (span 374 feet, or 114 meters).
This tradition continues in the present day, and notable recent examples are the International Rail Terminal at Waterloo Station, London, by Nicholas Grimshaw & Partners with YRM Anthony Hunt Associates, and the Kansai Airport building for Osaka, Japan by Renzo Piano with Arup.
Cable-network structures are another group whose appearance is distinctive because technical considerations have been allocated a very high priority, due to the need to achieve a long span or a very lightweight structure. They are tensile form-active structures in which a very high level of efficiency is achieved.
Their principal application has been as the roof structures for large single-volume buildings such as sports arenas, like the ice hockey arena at Yale by Eero Saarinen and the cable-network structures of Frei Otto.
In these buildings, the roof envelope is an anticlastic double-curved surface: two opposite curvatures exist at every location. The surface is formed by two sets of cables, one conforming to each of the constituent directions of curvature, an arrangement which allows the cables to be prestressed against each other.
The opposing directions of curvature give the structure the ability to tolerate reversals of load (necessary to resist wind loading without gross distortion in shape) and the prestressing enables minimization of the movement which occurs under variations in load (necessary to prevent damage to the roof cladding).
In the 1990s, a new generation of mast-supported synclastic (doubly curved but with both curves acting in the same direction) cable networks was developed. The principal advantage of these over the earlier anticlastic forms was that, due to the greater simplicity of the form, the manufacture of the cladding was made easier.
The Millennium Dome in London, which is not a dome in the structural sense, is perhaps the best known of these. In this building, a dome-shaped cable network is supported on a ring of 24 masts. The overall diameter of the building is 1175 feet (358 meters) but the maximum span is approximately 738 feet (225 meters), which is the diameter of the ring described by the 24 masts. The size of the span makes the use of a complex form-active structure entirely justified.
The cable network to which the cladding is attached consists of a series of radial cables, in pairs, which span 82 feet (25 meters) between nodes supported by hanger cables connecting them to the tops of the masts. The nodes are also connected by circumferential cables which provide stability. The downward curving radial cables are prestressed against the hanger cables and this makes them almost straight and converts the surface of the dome into a series of facetted panels.
This characteristic simplifies the fabrication of the cladding. In fact, being tensile, form-active elements, the radial cables are slightly curved, and this curvature had to be allowed for in the design of the cladding, but the overall geometry is nevertheless considerably less complex than that of an anticlastic surface.
Although this type of structure is truly form-active with a shape that is dependent on the pattern of applied load, the designer can exert considerable influence on the overall form through the choice of support conditions and surface type. The cable network can be supported either on a configuration of semi-form-active arches or on a series of masts; it can also be either synclastic or anticlastic and the configurations which are adopted for these influence the overall appearance of the building.
Most form-active vaulted and cable structures have technical shortcomings. They are difficult to design and build and, due to their low mass, provide poor thermal barriers. In addition, the durability of these structures, especially the cable networks, is lower than that of most conventional building envelopes.
Acceptance of these deficiencies is justified, however, in the interests of achieving the high levels of structural efficiency required to produce large spans. In the cases described here, the compromise which has been reached is satisfactory, given the spans involved and the uses for which the buildings were designed.
Angus J. Macdonald teaches in the Department of Architecture, University of Edinburgh, Scotland.
This article is excerpted from Structure and Architecture, Second Edition, copyright © 2001, available from Architectural Press and at Amazon.com.
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