Shimonoseki-shi Kawatana Onsen Koryu Center (Kawatana no Mori) 2

下関市川棚温泉交流センター(川棚の杜) 2

I have long been interested in coexistence of different construction methods, such as “A Small Bath House in Izu” (1988), so I was quite thrilled when mixed methods of polygon-like frame in steel and concrete structure became likely for Kawatana during the process of the study. Kuma san points out in KUMA – G1 that a kind of mediocrity is engendered if you try to segment the space by walls. I thought however, it would likewise be banal if we went for a dome-type architecture in steel or wooden frame, which might also be out of touch with the geographic features of its surroundings. The design we reached this time, therefore, was a revelation to me as it was like a part of the ground linking with its environment, while standing firm-boned.

To elaborate on this mixed structure, the most challenging part was the detail of the steel-frame. The polygon you see on the computer can turn into any shape, as it has no thickness or width (?), but the real steel material has 200mm thickness at minimum. If all this come together at one joint, the detail of that point would be an odd geometry. The polygonal or dome architecture you generally see is in fact limited to certain angles, which is after all a composition of the same detail. For Kawatana, our designing progressed inductively in two parameters – one was to study whether the volume and unevenness of the architecture could be harmonized with its environs, and the other was to confirm whether the shape could be structurally viable. As a result, the patterns of angles became varied according to the number of joints. What we did was to first transform KKAA’s 3D CAD data by a numerical formula into the coordinates of the steel baseline, and then, Nippon Steel, the fabricator, drew the details of all of the joints in 3 dimensions based on the coordinates. We used iron bulbs for the joining of the structural material, considering their flexibility for the mixed angles, but the actual size of the ball became bigger in some parts, depending on the angles.

Works on the concrete were measured by the precision of the steel-frame. If you try to coincide the central line of the steel with the baseline of the shape in the drawing, the ridgeline would naturally not be in uniform in the finish. It would have been easier if the ridgeline was widened or the element of ‘clearance’ was applied, but we wanted to create a line of tense right angle in one way or another. Again, if you want to align the ridgeline in the finished concrete with the baseline for the polygon design, the baseline of the steel would form a different polygon from the one in the drawing. Since this formation had both mountain folds and valley folds, the relations among details would be lost because of these differences. The potential failure of the theory (according to this logic) was overcome by the high plasticity of concrete. Some slightly thicker ridgeline was made where necessary and under the supervision by Kuma san, the concrete was cut and adjusted on site in some parts after it was poured in. Going back and forth between the theory of the design and the actual life-sized joints demanded quite a complicated, three-dimensional way of thinking and resulted in a huge amount of work.

Whenever I visit the site, however, I find the sight where a dune-like volume suddenly emerges from the hot springs town really unique and tense. If you set eyes on it for a while, you will notice that the architecture being harmonized with the neighboring topography of the mountains. Here, sense of tension and harmony exist in concurrence. The building interior also offers two contrary feelings – a centripetal one as if you are wrapped up in a cave, and a spreading one that comes from the composition of various vectors intertwined with the steel frame. The building is made free enough for people to enter from anywhere you like, and I am hoping that the locals will get used to it a bit more and freely make full use of that place. Rather than doing activities taking place anywhere else, I suppose there are many things that can be done only in Kawatana.

Teppei Fujiwara
Kengo Kuma & Associates