KVPT’s Patan Darbar Earthquake Response Campaign - Work to Date - September 2016

Either approach would need a more robust and homogeneous
foundation able to hold everything in place. The
decision of how to strengthen the superstructure is independent
of the need to strengthen the foundation, and
either system could be fastened down to the foundation
to strengthen the column-to-base connections.
Importance of foundations
Foundations are the integral link between structure and
the earth that is creating the seismic action. For this reason,
foundations play a vital role in the seismic behavior
of any building. A more general discussion here of the
importance of foundations will precede the discussion of
project-specific foundation issues.
Buildings experience high stresses at the connection
between the superstructure and the foundation during
an earthquake, partly because of the drastic change of
medium in which the structure is vibrating. Superstructure
is typically unrestrained by outside elements,
because the surrounding air does not resist horizontal
movement and cannot restrain a freestanding building
from displacement. Foundations, or substructure, are set
within the earth where the restraint is largely determined
by soil and bedrock conditions and the types of seismic
waves travelling through the ground during an earthquake.
This connection is vital, and is often a contributing
factor in the failure of historic structures during
earthquakes.
One of the main points of seismic strengthening is to tie
a structure together so that its connections do not burst.
The act of tying the structure together creates a more
robust load path which allows the building to transfer
more of the energy from seismic movement up through
the structure, then back down again to resolve itself in
the foundations. This inherent strengthening of connections
means that the structure may absorb more loads
than previously, and thus stronger foundations will be
needed to accept those loads and transfer them back out
to the ground. The foundations in historic Newar architecture
may have originally seen lesser stresses during an
earthquake because much of the energy transferred to
the building by the earthquake was dissipated in various
ways, from the sliding of bricks against one another in
the mud mortar (which has little cohesive or true bonding
ability), to the shaking of loose timber joints, the
inherent ductility of timber elements, or the partial collapse
of the building.
While we naturally pay more attention to strengthening
parts of the superstructure that we can see, that are
above our heads, to prevent them from collapsing, we
must also remember that the system and cycle of seismic
loads travelling through these buildings ultimately
starts (and ends) underground. If we’re strengthening
the superstructure, it is especially crucial to strengthen
the foundations, as they will likely be seeing more stress
than before.
Homogeneity of foundations is critical in providing stability
to the structure. In materials such as mud mortar
brick masonry, if a heavy load is applied in one area, the
connection between the bricks established by the mortar
is often not strong enough to spread that load out across
a large area. This lack of homogeneity can often result in
local failures at loaded areas, even if the building has a
massive brick masonry foundation. The inherent weaknesses
of the medium and its inability to distribute forces
throughout can result in failure mechanisms that could
be easily avoided otherwise. Brick masonry using lime
mortars as bonding agents slightly increases homogeneity
as compared to mud mortars. But lime mortars chemically
react with water and break down, losing that bond
in foundation applications. There is still a significant
heterogeneity between the brick and mortar elements
(having different strength properties), and under high
stresses, the materials fail at the interface between the
two. Brick masonry with cement mortar is slightly better
suited for foundation applications because of its reduced
water solubility, but the heterogeneity of the material
remains.
87