Saturday, March 21, 2009

Wide-Span Structures

Spaces with a large surface with or without internal columns (supports) and bridges with long spans have been constructed since ancient times. Domes, up to the nineteenth century, had a maximum span of 50 metres and it is only relatively recently that the progress in technology has allowed this restriction to be exceeded to the extent that in the twentieth century space coverings with spans of 300 metres and suspension bridges with a span of 2000–3000 metres were being constructed.

Wide-span hall roofs have some kind of supporting structure, which may bring the loads down into the soil, or be supported by separate supports such as masts, columns, frames. They also have a weather shield, which may be a membrane, panels laid on top of the supporting structure or a unified loadbearing and weather-shielding structure. As a consequence wide-span structures may be classified according to one of the three types of structure. This leads to overlapping classification systems since each of the three types of structure may be combined with various classes of the other two types of structure. A dome, for example, has one single structure with a load-bearing and weathershielding function and may be supported in various ways. A membrane may be self-supporting or suspended from masts.

The last 150 years have not only brought with them a gradual increase in span (and height) but also a considerable number of new structural schemes and architectural forms for covering spaces: shells, vaults, domes, trusses, space grids and membranes (Chilton, 2000). A great variety of domes have been developed: Schwedler, Kievitt, network, geodesic, and lamella folded plate domes.

Type of a Schwedler dome

Steel trusses were developed beginning in the nineteenth century. In the first half of the twentieth century reinforced concrete came on the scene as a competitor to steel for long-span structures, for instance in the form of braced or ribbed reinforced concrete domes and roof structures (designs by Pier Luigi Nervi, Eduardo Torroja and Felix Candela). During the 1920s and 1930s thin reinforced concrete shells were constructed. Shells may be not only domes but also cylindrical and prestressed tensile membrane structures. Then up to the present time, a great variety of new structures were added to the list of wide-span structures: steel, aluminium, timber, membranes, space trusses (with one, two or three layers, polyhedra lattices) and tensile (tensioned) structures (Karni, 2000). Another aspect of categorization is the way in which vertical loads are transmitted to the ground: directly by the structure, as is the case with some domes, or by special supports: pylons, masts or columns. In this second category are the ‘masted structures’ (Harris and Pui-K Li, 1996). A masted building may have one, two or more (four, eight, etc.) masts and these can be placed interior or exterior to the building. A special category is formed by rotational structures, which may have one mast, or several within the building envelope or, alternatively multiple masts may be arranged around the perimeter of the building. Some of such rotational structures may be designed for grandstands. It is obvious that the masts not least due to their conspicuous appearance influence greatly the overall architectural design and its details.

Some of the load-bearing roof structures require an external layer on top for water and heat insulation purposes. Competitors to traditional roofing materials (wood shingle, reed, clay tile, stone slab, lead, copper) made their presence felt: corrugated coilcoated steel or aluminium sheet, plastics, foil or textile, factory-built-up composite panel (Selves, 1999). Some of these are also applied as wall cladding or suspended ceilings.

As mentioned earlier, stadiums are increasingly being constructed with a retractable roof, which makes sports events feasible in any kind of weather: typical are the stadiums designed by the Japanese Fumihiko Maki and others. The year 2002 saw the USA’s first retractable football stadium completed in Houston. Here the travelling mechanism of the roof rides on rails along the tops of exposed structural steel super-trusses and during retraction the two trussed panels part in opposite directions. The wheels of the mechanism ride on a single rail. Each of the two 287 metre-long super-trusses is borne on two reinforced concrete super-columns, which are nearly 210 metres apart, thereby eliminating the need for columns in the seating stands (Engineering News-Record, 2000). Plans have been put forward to rebuild the London Wembley Stadium with a retractable roof and with a capacity of 90 000 seats. Other recent designs of retractable roofs demonstrate a number of innovative solution possibilities (Ramaswamy et al., 1994, Levy, 1994).

Sebestyen, Gyula. 2003. New Architecture and Technology.

0 komentar:

  © Blogger templates The Professional Template by 2008

Back to TOP