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Suspended Assemblies (Bolt & Plate Fixings) One method of providing a frameless glazing facade is to fix together a matrix of toughened glass panes, hung from the building structure. A system of this type, commonly referred to as a suspended glass assembly, was designed and developed in the 1960's by Pilkington Glass Limited. It allowed designers to glaze large openings in buildings, without the use of metal frames or mullions, providing the creation of light and space in buildings with a minimum of visual barriers.
The system comprises a series of specially processed and toughened glass panes, bolted together at their corners by means of small metal patch fittings. Pane to pane joints are sealed with a silicone building sealant, and toughened glass stabilizers (fins) are used at each vertical joint to provide lateral stiffness against wind loading.
The assembly so produced is suspended from the building structure by hangers bolted to its top edge, and is sealed to the building in peripheral channels by means of neoprene strips or non-setting mastic. The concept of the design ensures that the facade is, at all times, "floating" in the peripheral channelling, and problems which might arise due to differential movement between component parts are eliminated. Assemblies can, therefore, be used to advantage when vibratory or seismic forces must be taken into account in the design.
	Weather sealing is carried out at all joints in the facade using a structural silicone building sealant. In the design calculations the structural properties of the sealant, in providing greater stiffness to the facade, are not recognized. However, from extensive laboratory and on-site testing it is known that the sealant does improve the load bearing capabilities of the facade, and its use is therefore an added safety factor in design. The principle behind the design of the fittings for a suspended glass assembly is that all in-plane forces transferred between components are restricted by friction developed at the metal/gasket/glass interfaces, arising from the tension developed in the fixing bolts. The principle behind the design of the fittings for a suspended glass assembly is that all in-plane forces transferred between components are restricted by friction developed at the metal/gasket/glass interfaces, arising from the tension developed in the fixing bolts.
The use of friction to transfer forces between glass and fittings makes it essential that bolts of the correct size and quality, tightened to the specified torque, are used. Although the holes in the glass can resist considerable bearing forces from the bolt, through the hard bush, this is only taken into account in the height limitation for assembly constructions. The friction grip is of particular importance in the design of the splice joints and root support of the stabilizers when an assembly is subjected to wind forces. If required, the coefficient of friction at the metal/gasket/glass interface can be enhanced by the application of a suitable adhesive. The facade panes resist lateral wind forces through the small metal patch plates, 165mm square, supporting the four corners of adjacent panes off the stabilizers. These metal patch plates clamp the glass at the corners of each pane, developing significant stress concentrations at the edges of the patch plate and around the bolt hole. In order to safely design panes supported in this way, it is essential to have a detailed knowledge of the stresses generated around and under the patch plates, for various shapes of pane subjected to different levels of laterial load. Equally essential is knowledge of the strength of the toughened glass at and around the fixing holes. The size of suspended assembly facade panes is rarely limited by deflections. The clamping effect of the patch plates, which reduces deflections, together with the relatively high stresses generated, dictates that most assembly facade panes are stress limited rather than deflection limited in design. Extensive research into both stresses and strengths enables Pilkington to be confident in the successful design of large facades built using this system. Single assemblies can be designed up to 20 metres in height on a 1.5 metre module, and up to 23 metres on a 1.2 metre module. Assuming an adequate main building structure, any height can be specified using multiple assembly design. Any length is possible, and curved facades are not unusual, as exemplified in the Willis Faber Dumas Building by Sir Norman Foster Associates (1975).