Curtain walls can be constructed using wood applied in a 'unitised system' as described in 'Metal Box Framing' in Chapter 7 (Figure 7.4). The wooden posts and lintels of the frames are pre-glued in the factory, whereupon they are fixed to the supporting structure on site prior to being sealed. Glass or panels can be placed into the frames in the factory or on site.Where the panels are connected there is always a double post and lintel. In comparison with a stick system this double posts and lintels are less elegant.
When using the 'stick system', the connections between posts and lintels must be made on site. Because wooden connections have to be glued in all exterior structures, and because glued connections
are difficult to get right in this situation, wood has not been used as a material for the frames of curtain wall systems until relatively recently. However; metal connectors have recently become available, manufactured by companies such as Raico of Germany (Figure 8.1 5). Depending on the type used, glass weighing up to 700 kg total can be incorporated.The width of posts and beams is the same as that of most metal systems, at 60 mm. In the heaviest type, it is necessary to make a groove in the frame in order to ensure that any torsion applied by the glass to the horizontal lintel is transferred into the upright. The problem of the effective exclusion of water and the mounting of the rubber seals may be solved by using aluminium profiles with rubber strips.The wood provides the necessary
strength and rigidity. However; affixing the aluminium profiles to the wooden frames can be problematic: wood doesn't expand lengthways when temperature rises, while aluminium expands rather a lot. This difference in expansion causes problems unless slotted holes are used and enough allowance has been taken into account. The combination of wooden load-bearing elements, metal connectors and glass panels has been slow to emerge because the manufacturers tend to focus on one particular material. The manufacturer of a combined system must have expertise and production resources covering two or more materials, which demands significant investment. The use of different materials derived from different manufacturers gives rise to problems of liability and guarantees. However, an advantage of wooden frames compared to aluminium is the extremely low thermal expansion coefficient which means that little allowance has to be made for distortion or movement. Besides exterior walls with aluminium ridge-pieces, we now see those with wooden ridge-pieces. An example is the design of Mahler, Günter Fuchs for the University ofWiesbaden (Germany) (Figure 8.16).This usage calls for extremely durable wood (or wood which can be treated to become extremely durable). In most cases, the frames are made of laminated wood, or Laminated Veneer Lumber (LVL).These materials are available in longer sizes, have high stability and high stress-bearing ability due to the elimination of flaws such as knots and splits, and the constant quality of production.
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