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

Manimandpa
Base isolation detail showing two
layers of concrete foundation separated
by elastomeric rubber isolator.
This isolator essentially reduces
the frequency of vibration of the
structure by cutting the direct
connection between the structure
and the ground, allowing seismic
forces to be transferred through the
rubber isolator pads.
Sketch by Evan Speer
sents issues at South Manimandapa because of the relatively
shallow water utility pipes that supply the adjacent
public well. The beams could be set shallower but would
then need additional weight to act as counterbalance,
which would involve using a thicker cementitious mortar
encasement. This cementitious mortar encasement,
being below grade, would likely have to be some sort of
Portland cement. The use of steel below grade was more
likely to gain approval, but the use of a Portland cement
encasement could present issues.
Another concept investigated was that of base isolation,
to reduce the friction of the building atop the foundations.
This approach seeks to reduce the shear forces
transferred into the building structure by reducing the
stiffness of the connection between the the structure and
the ground. This is accomplished by using elastomeric
rubber bearing pads to “isolate” the base of the structure
from the ground. This strategy reduces acceleration of
the structure and thus slows the frequency of its vibration.
The building will still move but at a much slower
rate, which the building elements and their connections
are more likely to withstand. Advantages of this system
include virtual invisibility after implementation, and the
creation of a solid base from which to build up. Introduction
of this system drastically reduces seismic risk on
structures and allows for reduction in seismic strengthening
in above-grade architectural spaces. This method,
however, requires two layers of strong and homogeneous
structure within foundation and can be initially
expensive to implement. The new technology may also
be difficult to obtain or regulate in Kathmandu Valley,
and with the typical long-period of earthquakes in Nepal,
with slower vibrations and longer displacements, the
system did not seem to be worth the added cost and difficulty
in the approval process.
The most current and preferred iteration of foundation
design is to introduce a slab with a shear key. This iteration
developed from an earlier design with a mat slab
foundation, involving a thick concrete slab that uses the
weight of the concrete and plinth to counter the overturning
of the structure. This option allows for the continuity
and stability of a mat slab, but with a thinner and
shallower slab that transfers the base shear of the building
into the ground through a cruciform concrete “key”
that goes deeper into the ground to essentially “lock” the
foundation into the site. This system consolidates the
foundation to make it less invasive than a mat slab or
grillage, does not require as much concrete as a mat slab
or as much overall depth as the grillage, and would still
be hidden after construction. This design ends up being
closer to the traditional configuration of the structure, as
the columns would attach down to a flat surface, but the
new consolidated plinth will stiffen the base, ultimately
cutting down on displacements within the superstructure.
This will also act as a solid base to anchor the plinth
base stones, strengthening the load path coming down
from the columns.
This system still, however, utilizes as the main foundation
system reinforced concrete, for which it has proven
difficult thus far to obtain full permissions. Proper implementation
of this system requires a great deal of care
in detailing and placement of reinforcing steel, especially
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