In search of a design that would lend the company a more distinct identity, a Japanese wood-processing concern held an architectural competition for the new administration building of its branch works in the prefecture of Nagoya. The brief required an extensive, versatile and innovative use of the company’s own products-timber goods for housing construction-in the design of the new complex. The application of relatively slender cross-sections specially developed for this purpose was a further constraint.
In the course of various trials conducted with different structural systems, it proved possible to exploit these restrictive conditions to create a form of construction of distinctive character. Adhering to the requirements of the spatial brief, the prize-winning design is divided into three sections, each of which reveals an individual treatment of the specified materials.
The conference area is distinguished by its perforated walls, which were constructed with stacked timber sections offset to each other. Above this is the canteen, which has an area of 200m2. Here, a timber-framed construction with relatively small spans was used.
The 500m2 column-free office tract is covered by an upward-curving suspended roof structure. The increasing height of the space, which rises to an apex, helps to generate thermal currents and is also conductive to natural ventilation. Daylight enters vis the two long sides and via a roof light that faces on to the upper-roof division between the two main volumes of the complex. The curving form of the suspended roof is also an attempt to illustrate the innate elasticity of timber.
Most suspension roof structures in timber consist of slab materials that are either supported by steel bearers or are subject to direct tension loads. In the present scheme, in contrast, linear elements were used to demonstrate the versatility of the client’s products.
The curved structure consists of 1,496 three-meter-long laminated timbers with cross-sectional dimensions of 12cm x 15cm. The individual members can be carried by a single worker, which greatly simplifies the preassembly process. A complete element consists of 136 such timbers threaded on to five steel cables that are pre-stressed with a loading of 20 tons. The overall roof area comprises eleven elements of this kind, 3 meters wide and 16.5 meters long, set next to each other. A series of studies was carried out based on various structural systems. This led to the development of a composite system that combines the advantages of unfixed curved beams on bearers (with no transmission of horizontal loads) with those of tensioned cable structures, where slenderly dimensioned members can be used. In the latter case, under ideal loading conditions, the supporting elements are subject solely to vertical loads, and the roof structure can be constructed without stays.
The curved form of this type of structure results from the state of equilibrium between the dead weight and the rigidity of the tensioned timber elements. Horizontal forces caused by thermal and wind action can be resisted by small stay members along the upper edge of the roof and through the triangular timber construction along the lower eaves line.
From trials carried out on a working model, the planners were able to determine the optimum deflection curve, whereby the dead weight remained unchanged and the rigidity was regulated by the degree of pre-tensioning. For this purpose, the lower roof support was designed in a fixed, immovable form, whereas the appropriate position at the upper edge was determined by making horizontal adjustments on a roller bearer.
This method was applied to the full-size situation on site. With the help of temporary scaffolding, one element at a time could be stressed to the requisite curvature before being raised by crane on to its bearings in its final position over the hall. To increase the rigidity and to tie the units together at their abutments, narrow steel strips were fixed to the upper faces of he timber elements.
Through a simple application of the construction timbers that were provided, it was possible to execute the structure as foreseen with a minimum of jointing.