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Go to Home Page Conservationtech.com, Building Conservation Technology
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Structural
Conservation PHILOSOPHICAL RULES OF THUMB Prepared for the Kathmandu Valley Preservation Trust, Kathmandu, Nepal, May, 2000 1) General Guidelines ·
Modify
guideline of
"reversibility" to one
of "avoid new work which will
not allow future intervention.” In
other words, the objective on new
interventions should be to avoid
work that will, in the event of
future degradation, impede or
prevent future restoration of the
extant historical construction. (Example:
"Gunite" or reinforced
concrete directly applied to one
side of traditional masonry as a
structural upgrade can frustrate
later repair work, whereas
reinforced concrete used for a
discrete new wall (or in the case
of The Palace of 55 Windows in
Bhaktapur, a floor diaphragm) does
not stand in the way of future
masonry restoration. ·
Design
interventions for extended return
period on maintenance and repairs,
compared to past traditions
because maintenance and upkeep in
the modern world is likely to be
less frequent than in the past. ·
Design
for moisture intrusion and
deterioration reduction should
take priority over all other
technical objectives, including
earthquake reinforcement. (This
is not to diminish the importance
of earthquake protection, but if
decay sets in quickly, all
conservation work, and earthquake
mitigation work is compromised.) ·
Earthquake
hazard mitigation is an
incremental challenge. Any
thoughtfully produced mitigation
strategy can help reduce the risk,
even if other strategies cannot be
undertaken at the same time,
unless that strategy increases a
load on, or reduces the strength
of, an unmitigated element. Earthquakes
of different sizes are possible. Even
if mitigation against a great
earthquake is difficult to
achieve, some mitigation will help
against smaller ones, and may
avoid total destruction in a great
earthquake. There is no one
correct way to do earthquake
hazard mitigation. The
field is wide open for inventive
solutions. It
is best to first define the
objectives, then to devise the
physical solution. Avoidance of
total collapse is a reasonable
first objective. ·
The
temple pagoda form has proven to
be vulnerable in earthquakes -
particularly large earthquakes. In
modern society, the chances of a
massive post-earthquake
reconstruction of the vast number
of extant historical monuments as
now exist is less likely than in
the past - so protective measures
of contemporary design utilizing
modern materials of steel,
concrete and fiberglass are
justified. The
alternative may be the more easy
and permanent loss of the historic
structures altogether. ·
"Don't
fix what is not broken.” The
best conservation work honors and
retains what has withstood the
test of time, and makes changes to
mitigate against recurring decay
or damage.
The introduction
of weatherability and fungicidal/biocidal
treatments is beneficial to the
long-term conservation of the
wooden elements. The introduction
of strips of copper in areas where
water can enter into the wood
fabric may be beneficial in
reducing the likelihood of the
onset of fungal decay. The
UMA MAHEWOR temple (1992
restoration) shows some roof
leakage at peg locations in the
lower roof. Fungal
decay has broken out. Examination
of these locations will be
instructive so as to avoid similar
early roof failures. Longevity
is achieved if the roof can last
at least 20 years without any
repair of the underlayment, so
long as the tile surface is
maintained in an intact condition
- and shifted or broken tiles are
replaced. Rohit's
observation that the grass has not
grown on the palace large pagoda
“DEGU TALEJU” roof below the
gold clad copper roof is an
important one. Perhaps
the introduction of (1) copper
cladding on the top level roof,
and/or (2) copper edge flashing on
all roofs, and/or (3) copper
flashing under the ridge tiles,
will reduce not only the incidence
of fugal attack, but also on the
growth of grass on the lower
roofs. It may be worth
setting up an experiment at the
University or some research center
- to see what additives could be
placed in the mud setting bed to
eliminate grass growth. For
example, perhaps copper shavings
mixed with the mud would provide a
long lasting biocide. Another possible
fungus mitigation measure would be
to install a series of copper
strips as a series of bands
covering all of the peg holes
prior to the installation of the
waterproofing membrane. This
would serve two purposes. (1)
It would provide a dimensionally
stable cover over the holes and
board edges that would reduce the
possible breakage of the overlying
membrane, and (2) it would
impregnate any leaking water with
copper oxides, and thus reduce the
likelihood of fungal attack in the
event that leaks occur. (It
is fungus which causes wood to
rot, not water alone.) BEAM ENDS IN
MASONRY:
Wood end grain is most vulnerable
to moisture intrusion, and thus
decay. Beam-ends
buried into masonry are most
vulnerable to hidden decay, and
their unseen weakening can be
harmful to the seismic safety of
the structure. A
rule of thumb can be to (1) avoid
termination of beams in masonry
wherever possible by extending the
beams all the way through the
wall, or (2) installing a copper
"shoe" over the beam
ends sufficient to cover the end
grain. In
all cases, avoid physical contact
between the wood end-grain and the
masonry. Allow
an air space between timber ends
and the masonry. The
copper should be in contact with
the end grain. TIMBER COLUMN
BASES: In
the case of columns, an air space
is not possible. Solutions can be the
following - and experiments may be
made to see what works best long
term. (1)
Install a cross grain block at the
foot of each column where it rests
on masonry. This
block will avoid end grain contact
with masonry, and also provide a
sacrificial element that can be
easily replaced if it is decayed
without damaging the column. (2)
Cut a black rubber piece the size
of the column - say from an old
tire - and place it under the
column where it rests on the
masonry. (3)
Cut a piece of copper and place it
under the column. (The
copper may be best done in combo
with the rubber. Install the copper in
contact with the wood, and the
rubber in contact with the stone.) 3) Seismic Retrofit
Measures 55 WINDOWS PALACE
ISSUE:
On the conflict over the concrete
diaphragm, if you encounter a
similarly insoluble conflict
again, there is a wood based
alternative. Utilizing
the "Special Procedures"
of the USA Uniform Code for
Building Conservation, you
could install an engineered wood
diaphragm which might avoid a
conflict such as has been
experienced. In
the case of 55 Windows, I think
that the concrete is a better
replacement for mud than nailed
plywood - and it would involve
fewer nail holes into existing
timbers, but this methodology does
allow you to broaden the debate. In other buildings
with wood floors - it may provide
a better alternative to the
installation of a stiff concrete
diaphragm. If plywood is
used in any structure for
structural strengthening, pressure
treated plywood should be used to
avoid attack by termites. I
have seen evidence that termites
in California particularly like
plywood, and will eat it out from
between other more resistant cut
timbers. If concrete is
used, it is important to install a
new layer of some other material
as a pouring surface - with a bond
breaker between the concrete and
the protected original sub floor. This
layer will help to protect the
historic timbers from condensation
from the differential temperature
in the concrete, and can allow for
future removal of all or a portion
of the concrete should it later be
necessary. WOODEN
TEMPLES: (1)
The
Bernard Fielden approach of using
stainless stranded cable tucked
into mortar joints with stainless
steel plates and clamps on the
corners may provide an excellent
barrel-hoop strengthening of the
brick shafts. The
corner plates would be visible
(unless in the attic area), but
the cable would not be visible. (2)
Some
temples may have such a narrow
base, and tall height as to be
more vulnerable to flexural
failure (tipping over), than shear
failure (collapse of the lower
walls). If
this is the case, the seismic
retrofit response will need to be
different. Lowering
the center of gravity by
lightening the top may be
necessary in such cases. (3)
The
walls are so thick and diaphragms
are so small that diaphragm
strengthening in the temples may
not be particularly important. (4)
The
structural upgrade measures
already taken that we have been
shown all appear to be beneficial. These
include the anchoring of the
brackets, the strapping of the
roof timbers, the bolting of the
roof timbers to the roof plates,
the steel straps holding the terra
cotta corner ornaments, etc. These
measures also are valuable in
reducing the rate of damage from
ordinary time and use, not just
earthquakes. (5)
The
installation of the redundant
interior frame appears to provide
added life safety to the PATUKUA
AGACHE Temple. One
element of caution in the design
of an added frame is the
following: The installation of a
timber frame must avoid any
possibility, either by jacking
during installation, or from
subsequent differential
settlement, that the weight of the
upper story masonry is transferred
onto the timber frame and off of
the underlying masonry wall. If
this happens, vulnerability could
be increased. A
significant component of seismic
resistively is the action of the
overburden weight of masonry. If
this disappears during an
earthquake, an underlying masonry
wall could fall out, then leading
to the collapse of the structure. Installation
of secondary timber frames can be
efficacious if used in the manner
of a "strong back.” The
original masonry must continue to
function structurally as a
continuous masonry bearing wall. (It
is my understanding that the
Patukua Agache Temple design has
not interfered with the bearing of
the masonry.)
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