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

2. Timber Column Base Connections
Description
The sal (local hardwood) column is a prominent building
element in Newar architecture. Sal columns are often
used on the ground floor of temples and pātī structures
to form open arcades, and often support brick masonry
walls above. The columns bear either on base stones at
the threshold level, or wooden perimeter beams (lakansi)
that bear onto lower base stones. The connections
between the columns and their base are usually made
with a shallow (approx. 1 inch) square tenon protruding
from the bottom of the column. This tenon is placed
into a mortise within the base material.
Issues
These columns tend to support very high loads, including
brick masonry walls. At the ground floor, the base
connections are critical for the stability of the building.
With the original undersized tenon joints, there is no
positive connection that resists overturning forces at this
connection. With the high loads of the upper structure
displacing during an earthquake, these tenons often pull
directly out of their fitting. Without the lateral shear resistance
provided by the tenon, the column bases displace
and can no longer support the weight of the structure
above. This results in failure of the column and can
lead to collapse of the structure above.
Options for Seismic Strengthening
To prevent the pullout and failure of these base connections,
a structural connection should be introduced to
connect the columns with the base material. This can be
achieved with stainless steel dowels that extend into each
material a depth of about 1/6th the unbraced length of
the column. Added stiffness and tensile strength can be
achieved by inserting a recessed stainless steel base plate
at the interface between materials and using structural
epoxies to secure the embedded dowels.
Top
South Manimandapa
Columns rotated out of their connections
to the lakansi beam during
the April 25, 2015 earthquake. The
tenons were undersized and had
no direct connection to counteract
the overturning induced by heavy
masonry structure above.
Middle Left
Timber columns salvaged from
collapsed temples. The base tenons
were relatively intact on these columns,
signifying that the columns
had rotated out of their bases and
not sheared off at the tenon.
Middle Right
Strengthening via two 25mm dia.
stainless steel rods inserted into
both the column and the base
stone.
Bottom Left and Right
Various strengthening options
using four stainless steel dowels
provide added strength while leaving
existing tenon intact.
(Bottom Left diagram by Evan Speer)
57