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Technical Resources Structural Glass Architecture Opens Up Possibilities Technical developments allow new design potentials By Hugh Dutton Structural use of glass has seen considerable popularity during the last decade, in particular thanks to the use of more sophisticated point-fixed glazing systems. The exploitation of these systems has reached extensive possibilities through the greater understanding of the structural potential of glass itself and the analytical techniques made possible with computers. Our own phobias regarding the fragileness of glass are gradually being replaced by an acceptance of its structural potential. This liberation of glass as an architectural component has led to a greater potential in the articulation of wind bracing structures in cable, steel, and glass itself. The understanding of the symbiotic relationship between these bracing structures and the glass leads to a change of attitude toward these two traditionally distinct domains-glass belonged to cladding and curtainwall trades, while structure belonged to a separate steel trade. The building industry's professional designers and contractors are adapting their working methods to the new potentials of glass and its bracing structure in architecture and a different relationship within the industry. Point-fixed glass first appeared with the Pilkington point-fixed glass system used on the Farnborough office building by Arup Associates and Sir Norman Foster's Swindon Renault factory in the early 1980s. Following these projects, the monumental greenhouses at the La Villette Science museum in Paris, France, were a significant development in the structural use of the glass itself in conjunction with tensile cable and steel structures as wind bracing. The detailing of the glass connections developed on the project were based on the concept of protecting the glass from its own brittle nature while fully exploiting its structural qualities. The La Villette project initiated research and development of the point supported glazing system with spherical bearings incorporated into the bolts in response to this design concept. It is based on a clear analytical approach that allows the designer to predict exactly how the assembly functions and thereby evaluate only the specific load paths that the detail is designed to withstand. The detailing work on the glass connections led to the invention of an articulated bolt which involved the incorporation of a spherical bearing into the glass plane itself as a way of predicting that the glass could only be loaded in its plane for deadloads while local windloads are resisted around the hole perpendicular to the plane. Any local twisting effects at the supporting hole or other "parasitic" loads incurred by supporting structure movement are excluded. Because the point-fixed systems remove mullions and framing from the plane of the facade, the architectural expression depends now as much on the geometrical configuration and structural performance of the wind bracing systems as it does on the glass support system itself. The symbiotic aspect of these two parts of the facade is critical then in the architectural composition. The La Villette articulated bolt detail was developed after the decision was made to use a cable truss to resist windloads on the glass of the greenhouses. Non-linear computer methods were applied to this particularly light structural type to demonstrate its feasibility as a glass support structure. The truss is composed of purely tension components without the typical compression or bending members from near to the glass plane where they would have compromised the transparency. The truss deflects when the load is applied as the cables adapt to a different loading, and the new pivoting bolt detail allows the glass plane to move as the truss adapts to a different configuration without affecting the glass itself. The Future Structure and skin can be considered a building envelope's fundamental technical components. The structure supports it and the skin keeps the weather out. Lighter, more compact and efficient structures allow the skin to play a larger and more responsive role to light, the critical ingredient in the perception of architecture. The skin can become a filter of light-either as a transparent, translucent, or even a variable chameleon-type surface that can change its nature. Technology has permitted a slow and gradual transformation of buildings from solid structural walls with only small holes for light to penetrate through, to thin envelopes which modulate and control light and architectural transparency as well as energy gains and losses. Structure Technological advances in the building industry and developments in structural engineering have removed structure from walls. High performance materials such as reinforced concrete, steel, etc., have allowed efficient structures, reducing the quantity of material. Engineering developments have, through a greater understanding of how buildings behave, further contributed to lightweightness. In particular, the use of computers enables more interactive structures to be conceived and analyzed. Non-linear analysis methods exploit computers by doing multitudes of reiterative calculations for a structure simulating its change in shape as it reacts to load. This closer analysis of how structures behave allows designers to better understand the nature of structures and to further exploit their potential than would be the case with more primitive analysis methods. These building technology developments lead to a concentration of structure. It gets thinner and becomes more efficient. Columns get smaller, bearing walls disappear, and generally, structure becomes lighter weight. Skin The removal of structure from the walls has allowed the skin to become more ephemeral-transparent or translucent-because it no longer has to hold the building up. The skin lets in more light and allows it to play a larger part in the architectural composition. Developments in glass technology have created new possibilities for the skin beyond letting in more light. Some of the recent light surface technologies give a potential to the skin's surface. They can allow the designer better responses to solar energy considerations and a larger scope of architectural expression. The glass surface can become translucent or semi-transparent-in other words, somewhere between opaque and fully transparent. In this gray area, the surface lets light in or filters its passage, becoming a luminous light diffusing surface. In the case of transparent surfaces, an image can be seen through the glass because the light rays are largely unaltered as they pass through the surface. Several treatments that are readily available commercially alter the transparent nature of glass but still allow an image to be perceived. These treatments are largely aimed at improving the solar reflective or absorption characteristics of the glass. These treatments are metallic reflective films, body tinting, or light selective such as dichroic films. In semi-transparent cases, the light surfaces are partially obscured or filtered using a mesh pattern or texture. Coatings or treatments are applied to the surface of the glass which partially affect the transparency. These treatments are fritted glass or vitrified pigments, patterned or perforated sheet interlayers sandwiched between two sheets of glass, or printed solar cell glasses and also translucent stone laminate. Some of the most exciting recent developments in glass technology are what I call chameleon glasses or glasses that can change their transparent or translucent nature. Most of these types of glass are still undergoing research and development and are not available on the commercial market. They are liquid crystal interlayers, thermochromic interlayers, photochromic coatings and interlayers, or more recent holographic diffractive or angle selective films. Though some of these glass treatments, coatings, and interlayers are not yet commercially available or applicable to point-fixed glazing, they offer interesting architectural potential and an intelligent technical response to current concerns about energy performance of buildings. Hugh Dutton is a principal in HDA, a UK design consultant firm which specializes in glazing and structure. Its work involves exploring the architectural potential of steel structures and glass skins conceived as a whole. The material in this article is adapted from a presentation he made at the last Glass Processing Days conference in Tampere, Finland. Source: The Glass Guide for Architects and Specifiers, Glass Magazine, June 2001. |
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