Kama City, Fukuoka Prefecture was created through the merger of one city and three towns in 2006.
Kama City is located in the Kaho Basin surrounded by mountains and forests, and is home to the headwaters of the Onga River, the largest river in northern Kyushu. The site is located in a rich natural environment along the Onga River.
Another distinctive feature of this project was that it was undertaken in the context of both natural (the Kumamoto Earthquake) and social (the merger bonds) factors, as the design process started after immediately after the Kumamoto Earthquake in April 2016 and the construction was due to be completed in March 2020, which was the deadline for the use of the special merger bonds.
Based on the above background, we pursued a rational approach to creating a building that would maximize natural blessings of the Onga River and the unique environment of the basin, while at the same time guaranteeing safety and security and reduce initial costs.
In order to avoid the risk of suspending functions of the disaster prevention center due to a ceiling collapse in the office space in the event of an earthquake, it was necessary to completely eliminate suspended ceilings. For this reason, the office space is equipped with an exposed ceiling made of fair-faced concrete, a raised floor, and an under-floor air conditioning system.
The flat exposed ceiling in the open office space facing the Onga River reflects lush greenery in the surrounding area in a fantastic way.
From the viewpoint of reducing the initial cost, we came up with a compact square plan to minimize the exterior surface area, a single-volume building without a base to minimize the seismic isolation layer, and an "out-frame" (exposed structural frame) facade design to reduce exterior materials. As a result, we made a concrete "rectangular cuboid " that was stripped down to the bare minimum.
The compact concrete "rectangular cuboid", placed like a sculpture on the shore of the Onga River, created a new landscape without obstructing the beautiful landscape of Kama City.
In the basin's unique wind environment, the wind direction is not consistent, but changes with the season and time.
Through environmental simulations, we found that flat columns distributed evenly around the perimeter of the building act as "wind catchers" that direct wind from all directions into the interior. As a result, we conceived a facade design that integrates the structure and the environment in a way that is unique to the basin. In addition, we made an atrium coined the "eco-void" , which acts as a vertical ventilation path to promote gravity ventilation by utilizing the wind brought in through the facade. The "eco-void" serves both to direct the wind and to guide citizens to the counter in a friendly manner, because once you enter this space, you can have a clear view of the counter numbers on each floor. In this way, the floor plan and the sectional plan were determined in a way that reflects the basin environment.
A square volume of the assembly hall wrapped in wood floats in the center of the compact city hall building, with a communication space below it.
By visually highlighting the assembly hall as a "wooden volume" and designing it as a "symbol of the merger", we aimed to strengthen the relationship among the previous municipalities, which had been tenuous before the merger. By imprinting the names of citizens (donors) on the wooden louvers, we hope to foster citizens' attachment to the building and connect them to the heart of the city hall that is deeply rooted in the community. In the past, they had been selling the thinned logs from the municipal forests, but in this project, we decided to use some of the thinned logs as interior materials. Our goal is to form a new tree cycle in Kama City and provide a guideline for the use of wood in public facilities in the city, and also expand the use of wood to community planning in the future.