Optical Glass House / Hiroshi Nakamura & NAP
Text description provided by the architects. This house is located among tall buildings in downtown Hiroshima, overlooking a street with lots of cars and trams. To obtain privacy and tranquility in this environment, we have placed a garden and an optical glass facade on the street side of the house. The garden is visible from all rooms, and the serene and silent landscape of passing cars and trams adds richness to life in the house. The sunlight from the east, refracting through the glass, creates beautiful light patterns. Rain hitting the water basin skylight manifests water patterns on the entrance floor. Light filtered through the trees in the garden shimmers on the living room floor, and a super light curtain of spray coated metal dances in the wind. Although located downtown in a city, the house allows residents to enjoy the changing light and ambiences of the city throughout the day and to live with the seasons.
Optical glass facade
A facade of some 6,000 blocks of pure glass (50mm x 235mm x 50mm) was used. The pure glass blocks, with their large unitary surface, effectively block sound and allow the creation of an open and clearly articulated garden that admits the landscape of the city. To achieve such a facade, glass molding was used to produce extremely high transparency glass from borosilicate, the raw material of optical glass. The casting process was extremely difficult, as it required both slow cooling to remove residual stresses inside the glass and high dimensional accuracy. Even then, however, the glass retained microscopic surface roughness, but we actively welcomed this effect, as it would produce unexpected optical illusions in the interior space.
The 8.6mx 8.6m facade was so large that it could not stand independently if it was built by laying rows of glass blocks barely 50mm deep. So we drilled the glass bricks with holes and threaded them over 75 stainless steel bolts hanging from the beam above the facade. However, such a structure would be vulnerable to lateral stresses. So with the glass blocks we also threaded flat stainless steel bars (40mm x 4mm) at 10cm intervals. The flat bar sits in the 50mm thick glass block to make it invisible, and thus a uniform 6mm seal between the glass blocks has been achieved. The result: a transparent facade seen from the garden or the street. The facade appears as a waterfall descending downwards, diffusing the light and filling the air with freshness.
The glass brick facade weighs around 13 tonnes. The support beam, if constructed in concrete, would therefore be massive in size. Using steel frame reinforced concrete, we prestressed the steel beam and gave it an upward camber. Then, after giving it the load of the facade, we poured concrete around the beam and in this way we minimized its size.