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

Top
Typical strut roof connection
detail, with no direct structural
connection between strut and the
roof structure it supports. The strut
is notched around the purlin, but
is only held in place laterally by
timber pegs (cukul). These timber
pegs often lack proper maintenance
and are either rotten, loose, or
broken, thus providing no lateral
resistance. Lack of proper restraint
of the elements in this connection
can lead to separation of strut from
roof, and ultimately roof collapse.
Middle
Uma Maheshvara Temple, Patan.
Strut roof connection detail after
KVPT preservation in 1992, with
concealed stainless steel bolts to
create a direct structural connection
between the roof elements.
This provides added structural continuity
to resist separation of these
elements during an earthquake.
Bottom
KVPT detail adding steel reinforcing
to top of corner strut to secure
it to roof members. This connection
not only secures strut in place,
but adds tensile strength to the
timber purlin corner connection.
The combination of added tensile
strength at the corner and load distribution
through several fasteners
allows better utilization of wood’s
strength without localizing all lateral
forces through the half-lap joint,
where purlins have reduced section.
5. Roof Strut Connections
Description
In typical Newar architecture, roofs have large overhangs,
preventing rain from splashing on the sensitive
brick masonry walls in mud mortar. The overhangs are
supported by large timber struts, typically made of the
strong sal hardwood. These struts are often very intricately
and deeply carved on tiered temples and many
other structures with religious functions.
Issues
The struts supporting roofs are often simply wedged
into place, with no real connection at either end to resist
sliding from lateral seismic forces. They are primarily
held in place by the roof weight above and the frictional
forces between the struts and the main structure, and
rest mainly on small wooden or brick corbels built into
the lower level of the brick masonry walls of the building.
The struts are connected to purlins along the outer
edge of the roofs via a notched end to set the strut under
the plate. This weak connection often results in struts
shaking loose during earthquakes, which can result in
progressive collapse of the roof. As the struts, especially
corner struts, provide support for the heavy roof, the
quality and consistency of the load path they provide
must also be ample for the high roof loads. The detailed
carvings in the struts often go through the entire depth
of the strut, weakening this load path.
Options for Seismic Strengthening
To improve the load path and stability of struts, it is
important to provide direct structural connections to the
roof structure. On typical struts, this can be achieved
with concealed stainless steel rods connecting strut, purlin,
rafters and planking. On corner struts, a bolted steel
plate connection can also increase stiffness and tensile
strength at the corner of the building.
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