Renovation of Otemachi Building /100-Year Building facade Edition

The Otemachi Building, an icon of postwar Japan that has been visited by many businessmen for over 60 years, has continued to exist as the face of the Otemachi streetscape. Rather than rebuilding this aging building, a method of regeneration called "Renovation" was adopted, taking advantage of the strengths of the existing building and updating it for the future. The building's characteristic is its horizontal length. The site block continues for 200m, bordering three parallel streets. By switching the long exterior wall so that it reflects the characteristics of each street, a continuous facade changes its appearance one after another, creating a landscape that changes one after another. The building, which was once a single mass, is reborn as a new streetscape.

The Otemachi Building was completed in 1958. It is one of the buildings constructed during Japan's high economic growth period, the so-called building boom. In accordance with the Building Standards Act of the time, the building was limited to 31m in height. It is a huge building built on a long and narrow site measuring 50m x 200m with a floor area ratio of 1,000%. The central corridor that runs through the building in the longitudinal direction is like a street. While the modern skyscrapers that stand around it are based on vertical movement, this building looks like one of them has been turned sideways.

Due to its aging, the choice was forced to be made between rebuilding and redevelopment, or Renovation, and the latter was chosen here. Although the site shape makes it difficult to build a skyscraper, the floors can be divided into small strips, making it possible to meet today's demand for small office space, such as for start-up companies. Choosing to utilize existing stock rather than rebuilding is an example of the "100-year building" concept being promoted by Mitsubishi Estate Group to realize a sustainable society.

In this Renovation, which makes use of the existing stock, the choice was made to preserve the building's appearance and inherit the view, but rather than conserving the original appearance, the building was redesigned to incorporate the streetscape into its structural components.

The original exterior wall was a 200m long aluminum panel facade with a uniform grid structure along the pillars and beams. This facade borders three main streets, each with a different character. Daimyo-koji in the east was a street facing Western-style architecture including Tokyo Station, lined with brick buildings. Some of these buildings, such as Mitsubishi Ichigokan and the Japan Industrial Club, have been restored. Hibiya-dori in the west faces the stone walls of the outer moat of the Imperial Palace. The buildings facing it use stone motifs. Therefore, the exterior walls of the new Otemachi Building were finished in a brick tile style on the east side and a stone wall style on the west side. In the center where Naka-dori meets the building, high-transmittance glass was used to express a sense of openness through transparency. The design is switched in parts, but they are continuous as one facade.

A new aluminum grid "layered frame" has been added to the exterior wall, which retains the appearance of the existing grid wall. The frame ensures a sense of unity throughout, even as the design is changed. The frame is fitted with brick tile and stone wall style GRC panels, which are designed to create a sense of shadow and depth in order to express the profound history of the building. Louvers have been installed above the openings to reduce solar radiation load.

In many aging buildings, the window sashes no longer meet the performance required today, and this was no different. The replacement of the existing steel sashes with new aluminum sashes had to be carried out while the tenants were still using the floors. For this reason, the new sashes were installed from the outside along with a "layered frame" integrated with the exterior wall panels, and then the old sashes were removed from the inside of the building. The interior work can be scheduled separately according to the convenience of each tenant. There was only about 20cm of space that could be extended from the original wall surface to the outside. In order to fit the frame within that width, detailed studies were repeated.