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In Soft Shells, Hans-Joachim Schock draws together ideas on membrane- and-steel shell roof design, as seen through case studies Soft shells - hatching ideas

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Soft Shells* may be just a catchy title; it may also be an attempt to encapsulate an increasingly diverse field. Some of the structures illustrated make the classic synclastic use of membranes - doubly curved shapes like a saddle. Attachment of the fabric to the ground, to other buildings and to masts allows pretensioning of the fabric. Distortion of one curvature of the fabric by wind, snow, etc, leads to an increase in tension in the other curvature - an inherently stable arrangement. Other shapes are synclastic - shells with all radii on the same side of the shell, such as air-supported bubble shapes.

An increasing number of shells use steel-framed arches against which membranes are tensioned, say as a tunnel of arches or a segment of a sphere. Clearly the arches are in compression (as are masts). And what of cable- net structures like the San Diego Convention Centre (or Millennnium Dome, aj 27.11.97), or a space-frame in Italy with infill fabric panels, where the membrane plays no structural function? Is this 'tensile architecture', as in the book's subtitle?

Most of the schemes illustrated use woven fabrics, either pvc-coated polyester or teflon (ptfe)-coated glass fibre. But Schock also illustrates an increasing use of foils - rolled or extruded homogeneous membranes, usually plastic. Foil cushions to provide insulation - two foils held apart by air pressure - are the most common use. Some are used as insulating roof linings, other as the outer roof covering. The foils illustrated in the book look in good condition, though it is worth noting that the fabric and foil canopy built last year at Upper Ground on London's South Bank has seven lozenge-shaped foil cushions of which four have let in rain, and one is about a quarter full.

The book has little to say about why the shell roofs illustrated were built, except that a few are temporary structures and a couple are mobile. Since tension cables and masts can produce high point loads at the ground, the use of shells for mobile structures is not necessarily an ideal solution.

Some shells are getting more environmentally sophisticated. Insulation may be provided by foil cushions or two membranes sandwiching mineral fibre. This can produce good average performance, though there is usually a multiplicity of local cold bridges at cables and other attachment points. A second, outer or inner membrane may be needed to avoid condensation drips. Inner and outer membranes may be separated by anything between 250mm and 2m, providing a ventilation route for cooling in summer or for prewarming ventilation air in winter. Masts are an obvious location for air vents. Shells can be made airtight enough for effective mechanical ventilation/air conditioning. Bright fabric roofs (moderate light transmission) with low glare are particularly favoured for indoor sports.

While regular membrane curvatures can create acoustic problems by focusing sound, this is not insuperable. The Mediadron in Munich, for example, has a hemispherical air-supported auditorium. It is hung with sound-absorbent fabric sails with 6cm of mineral wool and reflectors of acrylic sheet to produce a concert-quality acoustic. The light and sound systems and hvac equipment have to be supported on separate steel framing inside the dome.

The book has a few limitations. Schock is a structural engineer not greatly interested in built form or the architectural quality of details, though there are many drawings and pictures. Some issues of built form are addressed, like dealing with too-flat curvature and the consequent danger of ponding by increasing tension or providing supplementary cable trusses. Schock also tends to present the structural detailing in words rather than drawings, which can be difficult to follow.

Though the book lacks the sophistication of presentation of the recent Jorge Schlaich book (aj 18.12.97), its range of examples is wider. You just have to work a bit harder to pull out the ideas.

* Soft Shells: Design and Technology of Tensile Architecture. Hans-Joachim Schock. Birkhauser. uk distributor - Momenta. 176pp. £44.

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