Mokuzai Kaikan Office by Tomohiko Yamanashi and Takeyuki Katsuya, Nikken Sekkei, Shinkiba, Tokyo, Japan

By Rob Gregory

The Japanese Association of Wood Wholesalers’ fitting exemplar for the use of wood

There can be no better advert for a construction material than to see it expertly deployed in built form at full scale. In Tokyo, timber is relatively rare, especially on large-scale commercial, cultural or institutional buildings. Kengo Kuma has of course been the most notable advocate for use of this material, and has achieved much on his mission to bring natural materials into the city, as seen in his fine One Omotesando on the city’s glamorous boutique boulevard (AR May 2004). Now the city has another exemplar, fittingly built for the nation’s Association of Wood Wholesalers.

The purpose of this project was to relocate the association in Tokyo, in order to better display the various possibilities of wood in the hope of reviving its popularity as an urban construction material. The building also revives and adapts another of Japan’s architectural traditions through the use of the engawa, a space prevalent in traditional homes that allows a natural breeze to enter the shelter while shutting out strong sunlight, achieving a comfortable indoor environment in the midst of harsh heat and humidity.

In accordance with strict earthquake regulations, the 7582m² seven-storey building employs reinforced concrete for its structural frame. Beyond this, however, timber was specified wherever possible, and the architects paid close attention to detail in order to establish an equivalence between the scale and prominence of the principal concrete frame with that of the secondary timber elements.

As a result, concrete was cast in slender cedar formwork, maintaining the scale and grain of the timber.

In terms of the timber elements themselves everything that can be seen is formed in 115 x 115 mm sections of Japanese cypress; a standard off-the-shelf product. These sections are used in composite panels to create the distinctive cubic engawas, and also to form the remarkable 1,600mm beams that span the full length ofthe 25m rooftop assembly hall.

By using a computer numerical controlled (CNC) cutting machine, a process that would traditionally rely on expert craftsmanship can now be used on larger scale fast-track projects such as this, achieving a high degree of accuracy on complicated abstract designs. With all machine cuts complete, horizontal joints employ a traditional Japanese interlocking profile known as the tsugite technique, while in the vertical orientation oak plugs are inserted to maintain load transfer between cypress sections. Secret steel bolts complete the vertical connection, maintaining a smooth timber surface on the face of the composite structural panels. The tsugite technique was modified to provide the optimal form for milling purposes, but its adaptation is critical to the success of this project, demonstrating how it is possible to use 4m lengths of timber to span almost 30m.

The designers overcame complicated fire safety issues by providing adequate ceiling height to allow smoke to accumulate; a move that not only improved the proportion of the spaces, but also made such an extensive use of timber possible.