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30 STRUCTURE AS ARCHITECTURE▲ 3.17 The Great Glasshouse, Carmarthenshire, Wales, Foster and Partners, 1998.Arched roof.structure a vault-like building form. Varying arch spans respond to anirregularly shaped site. Suspended floors either hang from tension hangersunder the arches, or as on the street frontage, are propped offthem. This is an example of reasonably conventional arch usage wherearches are regularly spaced and aligned vertically. But at the GreatGlasshouse, Carmarthenshire, arches form a toroidal dome (Fig. 3.17).The dome’s two constant orthogonal radii of curvature require that thearches distant from the building’s centreline lean over in response tothe three-dimensional surface curvature. Clarity of the arched structuralform is undiminished by the small diameter tubes that run longitudinallyto tie the arches back at regular intervals to a perimeter ringbeam. Apart from supporting the roof glazing they also prevent thearches from buckling laterally and deflecting from their inclined planes.Framed structuresSynthesis of architectural and structural form extends beyond curvedforms. Consider the intimate relationship between orthogonal skeletalstructural frameworks and rectilinear forms. In his discussion of theformative 1891 Sears Roebuck Store in Chicago, Condit asserts: ‘for thefirst time the steel and wrought-iron skeleton became fully and unambiguouslythe means of architectonic expression . . . The long west elevationis developed directly out of the structural system behind it, muchas the isolated buttresses of the Gothic Cathedral serve as primaryvisual elements in its indissoluble unity of structure and form.’ 8