NOZOMI NAKABAYASHI

OBSERVATION: antiseismic REINFORCEMENT STRAW

Learning from a Japanese Kura (store house) Structure at Permaculture Center Kamimomi (in Okayama, Japan)

Kura is a Japanese vernacular store house built with thick earthen walls. The walls are built to about 30cm thickness, much thicker than the walls of typical vernacular houses. The thermal mass of the thick earthen walls helps maintain a stable indoor temperature and moisture levels and simultaneously offer protection from fire and resist the tremors from potential earthquakes.

Although earthen walls tend to be misunderstood as being crumbly, when combined with the tensile strengths reinforced with straw and anti-seismic timber frame design, earthen wall structure is earthquake resistent. The wall structure of the Kura at Permaculture Center Kamimomi (in Okayama, Japan) is an examplary evidence of the use of straw as an anti-seismic reinforcement material.

The wall build-up consists of the following elements: 

- “Nuki” = Loose secondary timber structure spanning columns (4x5 bay): Nuki (horizontal brace) spans across the primary columns. Mabashira (vertical studs) are lapped and nailed to the horizontal brace while the top and bottom of the vertical studs remain unfixed to the wall/cill plate to allow for movement.

- “Take-komai”= Bamboo laths: the horizontal bamboo laths are used in round, gently tied with hemp rope to a bamboo pin, which is pinned onto the primary column. The loose connection allows for movement.

- “Tombo” = Hemp rope reinforcement: hemp ropes are tied to the horizontal bamboo lath at approximately 30-40cm increments. The extensions of the rope are embedded into each layer of earthen plaster coat to give tensile reinforcement to withstand seismic movements.

Above: Illustration of the wall build up, highlighting the antiseismic design features noted with asterisks *

An internal view of the wall, 4 bays to the right have been restored with new earth plaster. The last bay reveals horisontal struts (“Nuki”) and vertical studs ( “Mabashira”). Note: Mabashira. are not fixed to the beam and the cill at top and bottom. This intentional floating arrangement removes racking action from the wall frame, allowing the whole structure to move and absorb the seismic tremors.

Side view of the existing wall revealed at the entrance door. The horisontal bamboo laths are used in round.