Tubular members are well adapted for use as the supporting or supported members in tension structures because:
• they possess good compressive resistance as masts, etc.
• they are more slender than other sections for the same load resistance
• the end details can be designed for cable attachments or saddles
• pinned or articulated end-details can be made, e.g. attachments to the foundations
• in some cases, the tubular members may also act as the tensile elements (particularly where load reversal may cause the same elements to be loaded in compression)
• tubular members possess good torsional properties and can resist torsional effects when curved on plan or in elevation
• cable attachments may be made along the length of the members (with suitable local reinforcement).
Tubular members are often used in external and 'expressed' structures. This is both from a desire to celebrate the structure, and because they provide the slenderness required without an excessive increase in the structural complexity.
Built in the early 1980s, the Inmos factory in Newport provided one of the first and well-known examples of the use of tubular steel trusses and masts in tension-tie structures. The 'wing-like' trusses are supported from the central spine of the building to create long-span flexible space internally. Figure 7.38 shows the form of the building
7.38 Inmos Factory, Newport (architect: Richard Rogers Partnership)
7.39 Tubular arch used to support a cable structure in the Hong Kong Aviary (architect: Hong Kong Architectural Office)
and the multiple ties that were used. The Schlumberger building in Cambridge demonstrated the use of tubular masts to support the fabric roof, as shown in Figure 7.1.
Tubular structures can be designed to support cable-formed roofs by multiple attachment points. Inclined tubular arches are particularly successful, as indicated by the Hong Kong Aviary shown in Figure 7.39. The local detail of the attachment of the stainless steel cables is shown in Figure 7.40.
The cantilevered arms of the masts in the Renault Parts Distribution Centre are tapered and perforated to accentuate their lightness whilst serving to transfer compression forces to the tubular masts (Figure 7.41).
One of the most common forms of structural system employing smaller tubular sections is the 'space grid' or 'space frame', which are three-dimensional structural forms made using standard components and nodes.
The generic term 'space frame' is often used to describe two structural types: space trusses, with inclined 'web' elements, and space frames, comprising three-dimensional modular units. They both rely primarily upon full triangulation of the structure, provided the primary loads are applied directly at the node joints.
Most 'space grids' are based on the manufacture and assembly of standard elements and pre-formed node joint connections which can be easily transported and erected rapidly. Generally, tubular (normally circular hollow sections (CHSs)) sections are preferred for use in space frames because of their good compressive and local bending resistance. The elements are bolted together on site. A recent example of a space grid developed on a series of polygonal shapes is shown in Colour Plate 5.
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