Making the most of glass
This study, carried out under an Shepherd/aj award, expresses the potential of glass structures through maximisation of the material properties and exploitation of the best fabrication technology available. Bearing connections are proven by project experience where the joining together of glass with glass is via a loadbearing pin. The method of jointing is shown for glass as a pure elastic and brittle material, but the principles apply equally to all brittle materials, including stone and ceramics.
Production of flat glass by the float process limits the module size for the construction of glass structures, and has done since the introduction of industrialised processes of production. In Europe, a typical width of glass from the primary float-glass production process is 3210mm, while the Asian market is geared towards a wider panel of 4000mm. The sheet is commonly cut to a length of 6000mm, but can be as long as 8000mm.
The secondary processes of tempering and lamination offer enhanced safety, strength and thermal performance, but they further reduce the available size, with typical sizes of tempering in Europe being 4000mm long by 2100mm wide. Maximum dimensions available are up to 5000mm x 2400mm or 7200mm x 1700mm. To go beyond the production limits of a single sheet of flat glass, for a span of more than, say, 5000mm, a joint or connection which links a series of elements is necessary. This joint is key in the design of a glass structure.
The designer's choice of connection type is based on various criteria: strength requirements, buildability, cost, method of fabrication and installation, and aesthetics. The transfer of load through bearing makes use of the high strength capacity of glass and minimises the depth and complexity of the connection. Moreover, the high level of machining precision possible in the fabrication of metal and glass elements enables a bearing-type connection to be used. A number of small but technically sophisticated factories are able to fabricate glass elements to a high level of precision. For example, each glass beam for the Yurakucho canopy (shown overleaf) was cut on a computer numeric controlled machine with a maximum length of beam of 5000mm and an apex of 750mm with a tolerance of +/- 0.5mm prior to tempering and lamination.
Early bearing methods ranged from lead putty used for window frames to setting blocks to support the weight of large glass panels. Early projects by Dewhurst Macfarlane and Partners, including a glass extension to a private house in Hampstead, London, and a glass pavilion at Broadfield House Glass Museum, both of which used triple-laminated annealed glass beams, demonstrate how all loads can be transferred through edge bearings at stresses within the parameters of established practice.
The presence of a hole in the glass gives rise to high concentrations of stress, and further drilling inaccuracies not registered during fabrication can lead to fracture of the glass. The design and detailing of the bearing connection thus becomes a primary factor of the overall capacity of the structural system. Given the brittle nature of glass, failure due to local high stresses can be avoided by ensuring that the structural transfer of load to the glass lies within acceptable strength parameters for any loading configuration. Reference must be made to the elastic analysis of glass, as the material cannot yield, to offer a true understanding of performance.
The local stress concentrations around holes in plate material under various loading conditions have been documented by many authors on classical plate theory. General discussion and studies on stress concentrations for holes can be found in Peterson (1974), Timoshenko and Woinowsky-Krieger (1959) and Timoshenko and Goodier (1970). It is important to differentiate between stress concentrations which arise when load is applied to a plate with a hole and when load is applied to a pin in a hole.
It is simplest to consider a very large plate where the width of the hole remains small compared with the overall dimensions of the plate. For such an infinite plate with a circular hole and with load applied to the plate in direct tension, a solution was derived by Kirsch in 1898, with a stress concentration factor of 3. The effect of the hole is localised. The result has been confirmed by strain measurements and the photo-elastic method. Further studies analysed the case of a circular hole near the straight boundary of a semi-infinite plate under tension parallel to this boundary, and a plate subjected to pure bending.
Of more relevance to common connections is the case where two overlapping plates are connected by a transverse pin and the load is applied to a pin in the hole.
Project-based research and experience
Three key projects in the formation of an understanding of bearing connections are: the Yurakucho Canopy at the Tokyo International Forum, completed 1996; a glass ramp for the Daewoo headquarters, Seoul; and a glass facade for the Samsung Jong-Ro Building, Seoul, to be completed in 1999.
The Yurakucho Canopy
A series of glass beams transfers load through inserts called bezels to carefully prepared chamfered holes in the glass. The metallic bezel is isolated with an acetal liner from the glass and the holes drilled on a cnc machine. A 50mm-diameter pin was used to connect the individual blades of glass using best-practice glass technology. The bezels were designed with reference to a testing programme, beginning with direct tension tests on 600mm x 200mm samples, to full-scale tests on strain- gauged glass beams over 5000mm long and 750mm deep with applied simulated loading.
Tensile tests were carried out to assess the strength of 19mm-thick tempered glass panels when loaded via the bearing detail. For the chamfered hole with a 48mm inner diameter and 53mm outer diameter, the mean failure load was 77kN. The predicted value by the elastic analysis in Peterson is 79kN, assuming a panel of reduced bearing thickness to take account of a chamfered hole. To predict stress concentrations for certain hole sizes, a testing rig was fabricated to test the bending failure of glass samples. The test enabled a measurement of the failure tensile stress of glass, with the stress-concentration factor around the hole known to be less than the maximum stress at the glass edge due to bending action.
Glass ramp, Daewoo hq, Seoul, South Korea
The glass ramp comprises five rows of laminated tempered glass beams interconnected with stainless-steel connections of edge-bearing plates, and the bezel and pin elements. The beams span 23m between steel element supports connected to the base building and support surface-treated glass treads.
The project led to the study and testing of edge bearings as an alternative to transferring load through a hole. Initial photo-elastic studies were executed on Araldite scale models of a beam, using polarised light. Local stress concentrations were assessed qualitatively and the level of stress defined by strain-gauge measurement and by testing of full-scale glass beams. The scheme is an example of a long-span solution of multiple beams which support a high level of moment for imposed loading and self weight.
Glass facade, Samsung Jong-Ro Building, Seoul, Korea
The glass-panel facade of the 26-storey building is divided into approximately 12m-long bays. Wind loading to the upper-storey panels is resisted by a horizontal series of five laminated tempered glass beams interconnected between the bay length. A daylit lower-level atrium space is created by a supporting structure of four vertical glass beams. Tests on the glass canopy and ramp led to the development of a bush connection with a square- edged hole and plain bearing.
The testing programme included full-scale tests of both horizontal and vertical beams. The tests were carried out on 15mm-thick tempered glass samples, 200mm wide and loaded through a 60mm-diameter hole with a mean strength of 65kN. The theoretical value of 90kN from Peterson, with a calculated stress concentration of 1.6, assumes a close-fitting pin. The discrepancy between the actual and theoretical value can be explained by considering the effect of the degree of fit. As accurate measurements taken of the hole diameters about two axes showed a wide variance from the specified hole diameter, reference was made to a paper by Cox and Brown (1964) in which tests were carried out on the influence of degree of fit for clearances of 0.7 per cent - 2.7 per cent of the hole area for loads applied via a pin to a hole in the glass. The results show an increase in stress-concentration factor as the clearance increases, leading to a reduced load capacity.
For the final design and production, fabrication of a cam assembly offered a 2mm positional tolerance for location of the pin during installation. The fit of the outer bush to the hole in the glass is determined by the close-tolerance fabrication of the parts. This applies to the drilling and finish of the hole in the glass panel and the precision in manufacture of the bush assembly.
The load-bearing capacity of a single laminated panel of glass may be used for a staircase by considering the balustrade as the primary structure, with each tread distributing load to the balustrade through two pins bearing in a hole in the glass. The balustrade is composed of two 12mm-thick tempered and laminated glass panels. The panels are supported at the lower and upper floors through two holes connected to a main support. The concept can be further extended to include the connection of multiple beams to construct, for instance, a glass bridge.
Double-glazed wall panels 3000mm x 2000mm are hung from an upper fixing of two holes through a sealed double-glazed bearing connection and restrained only laterally, with a sliding connection, at the lower fixing. The development of holes in bearing for single sheets led to a version for the fixing of a double-glazed unit, and proposals for a testing programme have been prepared. The concept could be made more efficient by exploiting the capacity of the aluminium spacer within the cavity to give a structural frame inside the double-glazed panel. The spacer stiffens the unit against deflection under wind loading as the glass panel spans two directions. The unit may comprise single 10mm tempered outer and inner sheets. All joints between panels are sealed with structural silicone. The panels can each be treated with environmental films to achieve a high level of transparency with a high level of thermal performance.
For the concept of a 12m-span glass roof over an atrium space, a series of glass beams are connected at their supports to a steel plate and to each other through holes in bearing. Roof panels are fixed to the beams in a similar way to the Yurakucho canopy. Both the number of beams connected at a junction and the depth reflect the structural action of the span.
An area of future research into brittle materials is the investigation of liner material and thickness. Reinforcement rings are a common method of relieving stress concentrations in access holes in aeroplane wings and fuselages. This liner may be made of materials of varying stiffness (Young's modulus) with elastic constants different from those of the glass sheet.
We have learned in designing connection details in glass structures that stress fields at every point in the element need to be defined. Local stress fields around a bearing connection are the result of a complex interaction between the local and global stress fields in the element. Historical papers on elastic analysis and experimental testing describe how stress concentrations under various single-loading conditions are modelled. The design process should therefore begin with an elastic analysis to define stress-concentration states (although confirmation by experimental testing is required). It is important to note that, despite close prediction of the elastic stress concentrations, glass can fail due to its inherent variability, from flaws in the glass, or from impact. Measures to offer sufficient redundancy in the structure, such as lamination, must be used to ensure adequate safety.
The study has highlighted precision of fabrication and refinement of design as prerequisites for bearing connections. A thorough analysis of joints and connections must be a starting point in the design of a structure, and, at the same time, the production processes must be investigated. Only by fusing these and other relevant data obtained, for example, from research into materials, can the fundamental principles lead to innovative future fabrication processes and concepts.
Dewhurst Macfarlane and Partners: www.dewmac.com
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A paper of similar content and titled 'Bearing Technology in Glass' will be presented at Glass in Buildings, 31 March - 1 April, University of Bath. For more information on the conference contact the Centre for Window & Cladding Technology on 01225 826541, or visit www.cwct.co.uk/conf