Building with earth - Gernot MINKE (1)

15.37
15.38
Vaults
15.39
15.41
145
An important rule for the design of plinth
and foundation is that the resulting force at
the bottom of the vault must pass through
the inner third of the surface of the foundation.
This means that eccentricity should be
less than 1 /6. The foundation must have a
reinforced concrete or steel beam, which
can also withstand the additional horizontal
forces created by an earthquake.
Illustration 15.37 shows a section of a building
which was built in an earthquake-prone
area in Bolivia. Its plinth has structurally dangerous
proportions, as the resultant force
from the vault creates a bending moment in
the plinth and does not stay within the inner
third of the wall, as necessary. This structure
will readily collapse when hit by an earthquake.
The cross-section of a vault is very important
for stability. For vaults that carry only their
own dead loads, an inverted catenary is the
optimal section, as no bending moments
will occur within the vault. Pointed vaults, as
shown in 15.39, or ”flat“ vaults as shown in
15.40, typical for Iranian architecture, collapse
very easily when hit by seismic shocks,
whereas the vault in 15.41 withstood the
heavy earthquake in Bam, Iran, in December
2003. Only the front part fell off.
Earthquake-resistant building
15.40
The best solution for the facades of vaults is
to build them to be light and flexible, either
of mats covered with earth plaster, or of
timber planks.
Illustration 15.38 shows a design by the
author for an earthquake-resistant low-cost
housing project in the region of Gujarat,
India.
In 2001, a proposal by the author for stabilising
adobe vaults with bamboo arches,
which guarantee a certain degree of ductility,
was realised in a test structure built in
2001 at the University of Kassel, Germany
(15.42 to 15.45). It was built using special
U-shaped adobes that rest on an arch, itself
built of three layers of split bamboo. The
bamboo sections were soaked in water for
three days in order to render them flexible.
Then they were bent over sticks, which
were pushed into the ground along a catenary
curve (15.43). To maintain the shape of
the arch, the three bamboo sections were