Curved structures

Wood is extremely suitable for use In curved form, although this has not been widely reflected In architectural practice to date. The manufacturers of boats, cars, tools and furniture make extensive use of curved wood. The methods used to curve wood (especially oak) as used In yacht-building may involve heating or burning, which is not appropriate for facade construction since the removal of the charred layer takes too much time and energy. However, alternative methods exist.They include:

• elastic shaping and external fixation;

• elastic shaping and internal fixation;

• plastic shaping distortion;

• sawing the wood into the desired shape.

I. Elastic shaping and fixation in the desired form. In this method, relatively thin boards are bent across a frame and then screwed or glued and screwd into place. The curvature cannot be too great and depends

8.28 St Benedicts Chapel, Sumritg, Switzerland.

on the wood's natural elasticity. The wood will be permanently under stress, which increases the risk of splitting, especially at the fixing points. Over time, the stress is reduced by the process known as 'creeping'. Richard Rogers applied this traditional construction method in his design for the Law Courts in Bordeaux (Figures 8.29, 8.30), representing something of a

8.29 Law Courts (architect: Sir Richard Rogers), Bordeaux.
8.30 Timber construction of the Law Courts in Bordeaux.

departure for his practice.The pear-shaped courtroom is formed by a structure of laminated wooden trusses and posts.This was first clad with 4 mm plywood, sawn to shape. Because the shape includes double curves, while plywood will only curve in one direction, each piece could be of only limited dimensions (Figure 8.30). Insulation has been applied behind the plywood layer, between the laths. Above the plywood is a waterproof skin, and the top layer is of western red cedar boards, mounted diagonally. Although almost every board has to be not only curved but twisted on its axis, the diagonal mounting ensures that the torsion stress is minimized. The top of the 'pear' is truncated diagonally to allow adequate light and ventilation. When boards are used in this way the thickness can be reduced.

2. A second method of producing curved elements involves gluing together thin, curved boards to form elements of greater thickness (lamination). This method is generally used to produce columns and beams. Such columns and beams can then be used as the supporting structure for the curved planks produced using the method described in the paragraph above. It is also possible to use this method to produce curved veneered sheets.We have been unable to find an example, applied as cladding, from architectural practice.

3. Lignin, one of the constituents of wood cells, becomes elastic at temperatures above I70°C. Accordingly wood can be (slightly) bent when heated. Once cooled, it will retain the given form. In this process, wood is accurately cut to length and placed in a steam bath. It is then transferred to a steel mould, the length of the inner side of which is exactly that of the piece of wood to be bent. During the process of bending, it is essential to ensure that the fibres on the outer side of the wood are not pulled apart, since this would result in splitting. The shaping is achieved by pressure applied to the inner side of the bow. Once the wood has cooled and dried, it is removed from the mould whereupon a degree of springback' occurs. The size of the wooden elements thus produced is restricted by that of the steam baths and the bending machines. The maximum curvature depends on the type of wood and the thickness of the element. Not every type of wood can be bent in this way, but beech, ash, oak, maple and acacia are among those that can. Of these, oak (Quercus rubber) and false acacia (Robinia pseudoacacia) are also durable enough to be used for exterior cladding purposes. Bending wood in this way demands great skill and experience.

4. Curved pieces can be sawn from larger; flat sheets of three- or five-ply plywood or solid wood. This technique can be used to produce curved supporting structures, but is not suitable for the finishing elements of the cladding. Unlike in the bending method described in the foregoing paragraph, in which there is always a degree of springback', the exact form of sawn pieces can be determined beforehand. Bending strength will not be the same across the entire length of the element, as it reduces as the degree between the grain and the axis of the pieces increases. The bending strength of wood is always much greater along the grain than across the grain.

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