Abstract
This
paper explores
the
architectural
and historic
preservation
issues raised by
seismic repair
and retrofit
work on masonry
buildings. The
first part
examines the
sources of
meaning in
historical
buildings as
cultural
artifacts, using
examples to
illustrate how
buildings can
have
significance
beyond their
visual image or
architectural
style. For this
reason, if
preservation is
to be
successful, the
actual material
fabric of an
artifact must be
preserved. This
fact must be
recognized by
seismic design
engineers so
that seismic
retrofit work
can be carried
out with the
least possible
irrevocable
alteration to
the historic
structural
system as well
as the historic
architectural
finishes. The
paper goes on to
explore some of
the opposing
differences
which have
existed between
contemporary
conservation
technology and
seismic retrofit
practice,
analyzing how
seismic retrofit
work may be able
to benefit from
knowledge and
research
developed in the
conservation
field. In
conclusion, the
paper suggests
four areas where
further work is
needed to
improve seismic
retrofit
practice with
historic masonry
buildings:
research on
mortars and
post-elastic
behavior of the
masonry, the
development of
existing
building-type
specific
building codes,
finding ways to
limit liability
for design
professionals
dealing with
existing
buildings, and
further analysis
on what is an
acceptable life
safety goal for
historic
buildings.
Introduction
Our
approach to the structure of
buildings has gone through a
transformation in modern
times. Traditionally, most
major buildings were solid
walled structures with the
walls bearing directly on
the ground. With the current
predominance of steel and
reinforced concrete as the
materials of choice for
larger buildings, we are now
used to the erection of
frames, onto which the
enclosure cladding system is
attached.
With
the "postmodern"
fascination with historical
forms and details, the
contrast between the old and
new systems has only
recently become particularly
noticeable. This style shift
has brought back a desire to
design buildings which have
the solid walls of their
historic counterparts, but
which, unlike them, have to
be constructed as a series
of light, jointed panels
attached to the underlying
frame. Often the results
simply fail to capture the
kind of texture and meaning
which is found in the older
buildings. Architects
continue to struggle for
solutions, only to find that
the source of the feeling
they are trying to capture
is simply not accessible in
Dryvit, GFRC, Fiberglas, or
panelized veneer brick, with
their frequent need for
expansion joints cutting
across the architectural
details. As engineers work
hard to convert the highly
indeterminate, ambiguous and
nonlinear behavior of
historic masonry
construction into something
which can be understood with
mathematical certainty,
architects struggle to wrest
control of the seemingly
rigid and unyielding
materials of modern day
conventional building
systems, trying to breathe
the kind of subtle life into
them that they find at the
root of the aesthetic
quality of historic
structures.
This
transformation in
construction technology
parallels a similar change
in engineering practice
which now relies to a great
extent on frame analysis for
the design of building
structures. Traditional
heavy wall masonry buildings
tend to defy analysis by the
usual present day methods,
forcing many practicing
professionals to do what
they do not like to do -
designing in part by guess
work. Research in the area
of unreinforced masonry is
so important because without
it, historical buildings
will be lost simply because
engineers and architects
will be loath to touch them
because they cannot be made
to fit their mathematical
design models. This may be
true even though the same
structures have withstood
major past earthquakes, and
the damage record is known.
For example, a number of
historic buildings in
California which survived
the 1906 San Francisco
earthquake are threatened
now more by hazard
mitigation legislation than
by future earthquakes.
The
cultural significance of
historic building fabric
Modern
engineering science, new
materials and current codes
have gone a long way towards
reducing the fear of
catastrophe and death from
earthquakes. This has been
true despite the spectacular
failures which each major
earthquake seems to leave in
its wake. Earthquake design
is an evolving and
constantly changing practice
largely because the actual
events are so rare, and when
they do occur, the
earthquake forces can be so
large that some structural
damage is expected even in
new structures. As a result,
the line between acceptable
and unacceptable risk and
performance is vague and
fluid.
In
the field of historic
preservation, the problem of
seismic risk cannot be
solved by stricter design
codes, better new materials,
or a more stringent
engineering design. It is
exactly these things which
heighten the dilemma with
older structures,
threatening the very
historical qualities which
we seek to save.
It
has become a familiar sight
in many parts of the world
to see the stone exterior
walls of gutted buildings
held up by shoring while
they await the construction
of new interior floors and
roof. Fine old masonry
buildings are often stripped
of their interior finishes,
with the steel reinforcing
rods being erected against
the inside of the exterior
walls in preparation for a
sheet of concrete. Roofs of
ancient tiles or slate are
torn off to be replaced by
new tiles and slate after
the obligatory concrete or
plywood diaphragm is
installed.
One
might ask "what's the
fuss - the exterior walls
have been preserved, have
they not? The interior will
be rebuilt and the new work
will be hidden - the view
will be just the same when
it is all completed".
Many architects, not just
engineers, fail to
understand the meaning of
what is lost along the way
when this kind of work is
carried out. Donald
Appleyard observed:
The
professional and
scientific view of the
environment usually
suppresses its
meaning....
Environmental
professionals have not
been aware of the
symbolic content of
the environment, or of
the symbolic nature of
their own plans and
projects....
Professionals see the
environment as a
physical entity, a
functional
container,...a setting
for social action or
programs, a pattern of
land uses, a sensuous
experience - but
seldom as a social or
political symbol (Appleyard
1978).
Historic
buildings do not just carry
their cultural significance
as relics by image alone.
While understanding the
architectural style and
decorative form of historic
structures is important, the
cultural meaning of many of
the most significant
buildings is resident within
the reality of the artifact
itself. An historic
structure is important
because it is exactly that -
it is old, and thus has been
a part of human lives. As
the English critic John
Ruskin eloquently stated:
Indeed
the greatest glory of
a building is not in
its stones, or in its
gold. Its glory is in
its Age, and in that
deep sense of
voicefulness, of stern
watching, of
mysterious sympathy,
nay, even of approval
or condemnation, which
we feel in walls that
have long been washed
by the passing waves
of humanity....
It
is in that golden
stain of time that we
are to look for the
real light, and color,
and preciousness of
architecture; and it
is not until a
building has assumed
this character, till
it has been entrusted
with the fame, and
hallowed by the deeds
of men, till its walls
have been witnesses of
suffering, and its
pillars rise out of
the shadows of death,
that its existence,
more lasting as it is
than that of the
natural objects of the
world around it, can
be gifted with even so
much as these possess
of language and of
life (Ruskin
n.d.).
Seismic
protection and strengthening
forces us to confront one of
the central dilemmas of
historic preservation - the
fact that preservation is
forced to encompass change
and renewal. Unlike
maintenance and
rehabilitation from decay, a
seismic project may tear
apart a building which was
otherwise in good repair and
make it almost entirely new.
In such an instance, only
the image, rather than the
substance, of much of the
historic fabric is
preserved. Masonry buildings
are particularly vulnerable
to this approach.
Sometimes
seismic projects are
promoted as opportunities to
"restore" the
original appearance of a
buildings, stripping away
the later alterations in
order to return them to
their original appearance.
In Sacramento, California,
the State Capitol is such an
example. The Capitol was
completely gutted in 1976,
leaving only the exterior
walls and the central drum
and dome. All of the
interior floors and walls
were removed and replaced in
reinforced concrete. The
remaining masonry was
covered with an internal
skin of shotcrete and the
floors were replaced in
reinforced concrete. As a
result, while the interior
of this building is now
genuinely spectacular, with
impressive museum rooms,
excellent craftsmanship,
rich materials, stunning
colours and textures, none
of it is genuine. A
"heart transplant"
was authorised when an
"ace bandage" may
have been all that was
needed. The Capitol needed
to be strengthened and
repaired, but one should ask
whether the risk identified
in 1971 could not have been
satisfactorily alleviated by
less drastic, destructive,
and expensive measures.
The
quest for
authenticity, and the
search for
"real"
meaning through
"honesty" of
form, often leads to
the destruction of
that which it seeks by
inducing
fakery....Authenticity
is not a property of
environmental form,
but of process and
relationship....Authentic
meaning cannot be
created through the
manipulation or
purification of form,
since authenticity is
the very source from
which form gains
meaning (Seamon
& Mugerauer 1985).
This
gutting of structures for
seismic strengthening is not
limited to the United
States. For example,
following the 1979
earthquake in Montenegro,
Yugoslavia, many structures
in the historic city of
Kotor have been
reconstructed with
reinforced concrete floors,
replacing the original heavy
timber. In some of these
structures, reinforced
concrete columns have been
cut into the masonry,
forming completely new
reinforced concrete
structures, with the
historic masonry reduced to
a veneer.
Another
example, in Portugal, is the
little mountain village of
Piódau. The Portuguese
government recently listed
this picturesque mountain
village of stone buildings
as an historical site.
Located in earthquake-prone
country, many of the stone
houses are being
strengthened. The typical
seismic strengthening
consists of replacing their
timber floor and roof
structures with reinforced
concrete. Some of the walls,
which had been laid with
very little mortar, are
being re-laid in strong
cement mortar. While
undoubtedly safer, the
visual effect of this work
is the loss of the texture
and feel of the traditional
surfaces. The patina and
sense of the country masons'
and plasterers' handiwork is
erased. If the approach had
been to repair and augment
the timber interior
structures and tie them to
the existing walls, rather
than replace them, the
historical quality of the
buildings would have
survived the life safety
improvements.
The
debate over such
alternatives always turns to
the question of how much
life safety protection is
enough. When existing
archaic construction remains
in use, even if improved,
can it be relied on to
perform adequately? However,
at the core of this issue is
the fact that, unless the
architects, planners and
engineers identify and
understand the importance of
the original structural and
interior fabric of the
historical buildings, and
bring this understanding
into their designs, such
destruction will continue
because they will do what
they are used to doing with
new structures. This
consideration must include
the evidence of the original
handiwork, rather than just
the appearance of a building
from a distance.
Another
striking example is South
Hall at the University of
California, Berkeley.
Constructed of brick with
timber floors in the 1870s,
South Hall is the oldest
surviving building on the
campus. In the 1980s, it was
gutted to undergo seismic
strengthening under the
University's campus-wide
program. The retrofit plans
included the reinforced
concrete
"shotcrete"
jacketing of the inside
surface of many of the
exterior walls, and the
demolition and replacing of
the timber floors with steel
and concrete. In the process
of carving channels into the
walls, it was discovered
that the original builders
had installed bond-iron in
the masonry - continuous
bars of wrought iron which
extended from corner to
corner above and below the
windows in all of the
building's walls. Dog
anchors, which secured the
floors to the walls, were
also discovered hidden in
the walls. At the corners,
the bond iron bars were
secured by large bosses on
gigantic cast iron plates
which formed part of the
architecture of the
building.
Because
the designers had never
thought to investigate the
structural history of the
building, including whether
these great cast iron
ornamental plates on the
corners of the building
served a structural purpose,
the existence of the bond
iron was not known until the
demolition for the retrofit.
All of these bond iron bars
were cut as a result. In
addition, as historically
significant and advanced as
this original system was, no
recordation of its design
was ever conducted. The
irony was that one of the
engineers said that, had
they known of the existence
of the bond iron and the dog
anchors, their designs may
have been different and less
extensive. When it was
discovered, however, it was
too late to change the
designs, and the early
seismic technology was
destroyed.
One
may ask, "why is it
important to preserve what
had been hidden in the
historic walls - nobody
could see it anyway?"
Perhaps documenting it,
which was not done, would
have been sufficient, but
this example also
illustrates one of the
important points about
seismic design - that is
that many engineers and
architects have the false
belief that the today's
engineering design is, not
only better than anything
which has been done in the
past, but is the ultimate
solution which will require
no further interventions.
They believe that their work
will make the building
strong and complete, and
that no-one will have to do
anything other than
maintenance and superficial
remodeling ever again. Here,
at South Hall, the designers
failed to know what had been
put into the walls to resist
earthquakes a mere 100 years
ago, despite the fact that
great cast iron plates to
which the bond iron straps
were attached, were fully
exposed on the outside of
the building. What is there
to make certain that our
successors will be any
better informed about the
work done today?
In
addition, with the
irreversible conversion of
the masonry walls of South
Hall into a veneer of
masonry on reinforced
concrete, the integrity of
those walls as masonry walls
was destroyed. One of the
principal advantages of
masonry is that it can be
repaired by being dismantled
and re-laid. Now it has been
fused together into one
solid mass of unyielding
concrete. Years later, it
will not be possible to
repair the brickwork or
replace the concrete jacket
because of rusting of the
re-bars or for any other
reason. The present-day
seismic work will indeed
last the life of the
building simply because the
building's life is now
forced to be limited to that
of the new work.
This
point may seem far-fetched,
but historical buildings are
worthy of such long-term
consideration. It should be
remembered that the 19th
Century restorers of the
Parthenon introduced iron
cramps which, when they
rusted in the 20th Century,
destroyed some of the
original marbles. Should
anyone wonder whether the
state-of-the-art at the time
of the 19th Century
restoration represented
progress from earlier times,
they should consider the
fact that the ancient
builders had used a less
rust-prone iron, which, when
protected by a lead jacket,
survived over 2,000 years to
this day without distress.
Learning
from the past
Many
people make the mistake of
thinking that it is only our
generation which has
discovered ways of resisting
the threat of earthquakes in
structural design. They come
to believe that older forms
of construction practice
must be more dangerous
simply because they were
designed before current
seismic codes were
promulgated, or before
current engineering
knowledge about earthquakes
had been developed.
Certainly, the introduction
of steel provides ductility
where masonry could not, and
yet the recent discoveries
of the failures of the welds
in over 100 of the 400 steel
buildings affected by the
Northridge Earthquake should
provide some humility in the
face of this awesome force.
While many masonry buildings
have tumbled in earthquakes,
they have not always
tumbled. As was witnessed in
Armenia recently, it was the
modern reinforced concrete
buildings which collapsed,
killing thousands, while the
older masonry buildings
nearby remained mostly
intact, providing refuge for
the displaced occupants of
the newer buildings.
In
places as diverse as Turkey,
Yugoslavia, Kashmir and
Nicaragua, indigenous forms
of construction were
developed or adapted to
respond to the earthquake
threat where available
resources demanded that
masonry continue to be used.
In Kashmir, an elaborate
system of interlocking
horizontal timber runner
beams was used, without
vertical wood columns, to
hold the rubble masonry and
soft mud mortar buildings
together on the silty soil.
Historical reports confirm
that these buildings
withstood earthquakes better
than the nearby unreinforced
brick palace and
British-built government
buildings.
Today,
many of these vernacular
structures are falling in
favor of reinforced concrete
structures, which, because
of poor local construction
practices, may actually
prove to be less resistant
than their "low
tech", unengineered
historic predecessors.
Restoration
professionals sometimes fail
to understand the subtleties
of seismic resistance in
older structures. Believing
that strength and stiffness
is necessary, they destroy
original construction
systems to gain sheer
strength at the expense of
earlier solutions which may
still be valid. In
Dubrovnick, before the
recent civil war, restorers
of the historic palace
uncovered an interior wall
they had thought was solid
masonry to find a
basket-weave of small timber
studs, with brick or stone
masonry loosely fitted
together between the studs.
The restoration engineer
stated at a conference that
this "poorly
constructed wall was
immediately removed and
replaced during the
restoration of the
building". Instead of
being "poorly
constructed", this wall
deliberately may have been
constructed in this fashion
to resist earthquakes. The
wall, which was similar to
Bahareque construction found
in Central America, may have
represented a far greater
understanding of seismic
engineering than pre-modern
builders are given credit
for today.
Building
conservation practice versus
seismic strengthening
While
it is impossible to ignore
present-day advances and
advocate a return to
traditional construction
practice, the narrow
assumption that "new is
always better" can
blind us to the potential
gains which an understanding
of the earlier forms of
construction may provide us
in the present. This is
particularly true for the
advancement of building
conservation and seismic
rehabilitation practice. For
years, these have been seen
as separate and opposing
fields of practice, with
solutions which seem in
basic conflict with each
other. For example, for
years, conservation
professionals have specified
that restoration mortar
consist of a high lime mix
which is weaker than the
masonry units. Code
requirements have
established that mortar must
consist of a high cement
mixture and meet high
strength standards which
have proven to be anathema
to proper conservation of
older masonry walls. The
discovery of the importance
of reducing or eliminating
Portland cement from masonry
mortars in restoration is
one of the cornerstones of
recent conservation
practice:
The
use of lime-sand
mortar ... furnishes a
plastic cushion that
allows bricks or
stones some movement
relative to each
other. The entire
structural system
depends upon some
flexibility in the
masonry components of
a building. A cushion
of soft mortar
furnishes sufficient
flexibility to
compensate for uneven
settlement of
foundations, walls,
piers and arches:
gradual adjustment
over a period of
months or years is
possible. In a
structure that lacks
flexibility, stones
and bricks break,
mortar joints open and
serious damage results
(McKee
1980 - tense
changed for clarity).
This
was not meant to refer to
masonry in earthquakes, but
in light of the Kashmiri
experience it is intriguing
to ask, whether the notion
of a "plastic
cushion" might be an
appropriate concept for
walls subjected to
earthquake forces. It is
worth noting the conflict
between the Historic
Preservation documents which
recommend using the weakest
and most lime-rich ASTM
formula (K) 1 unit cement to
2.25-4 units lime for
restoration work, and the
Uniform Building Code, which
prohibits the use of mortar
weaker than the three
strongest categories, known
as ASTM types M, S & N:
1 unit cement to 0.25-1.25
units lime) for any mortar
used in structural masonry
(which includes, of course,
most historic masonry
walls).
One
reason for this conflict is
that while the Code is
founded upon the performance
of the wall under load at
its design strength at the
point of construction, the
preservation documents are
aimed towards maximizing the
long-term durability of
walls with relatively weak
masonry units in response to
all environmental
conditions. One only needs
to compare the long-term
performance of ancient
masonry and modern masonry
to see the merits in the
softer, high lime mortars,
and yet, the codes now make
beneficial use of this
knowledge difficult. Other
examples abound where modern
uses of masonry has proven
short-lived because of
environmental degradation of
the system. Seismic design
must fit into a larger
performance picture, where
other environmental assaults
are considered as well as
the occasional earthquake.
A
crisis of cost
Concerns
over the impact of seismic
strengthening policies is
more than just one of
potential loss of original
fabric; it is also one of
economics. As long as
politicians and the public
believe that historic
masonry buildings are
enormously risky unless
great sums of money are
spent to convert their
structural systems into
steel or concrete, vast
numbers of important
cultural monuments are at
risk. This issue has
expanded recently in the
United States to include
large-scale 20th Century
masonry buildings
constructed with steel or
concrete frames. It is
exactly the current crisis
with these types of
buildings which confirm the
importance of engineering
research and the development
of specialized codes for
masonry buildings and
historic buildings in
general.
The
crisis can be illustrated by
an example in Oakland,
California, where one brick-
and terracotta-clad steel
frame historical building,
the City Hall, is being
repaired from Loma Prieta
Earthquake damage and
seismically upgraded at the
extraordinary cost of $530
per square foot
($5,700/square meter), which
is more than 3 times the
cost of a new building of
comparable quality. Six
blocks away, another office
building, the Oakland
Medical Building, was just
repaired and seismically
upgraded to the same codes
for a cost of only $11 per
square foot ($118/square
meter). The City Hall design
uses the now popular newly
developed base isolation
technology, while the
Medical Building is a fixed
base design, but both
schemes were promoted as
"cost-effective"
designs to meet the
requirements of the building
code (see Figure 1).
With
a difference between two
projects, both promoted as
necessary and expedient, of
over 35 times, it is evident
that there is little
consensus in this particular
field over what is required
and beneficial to meet the
seismic threat. While
certainly the expected
performance of the base
isolated design is greater
than the fixed base design,
and even though part of the
difference is for interior
remodeling of the City Hall,
it is questionable whether
this justifies 35 times the
cost. While many celebrated
the repair and upgrade
solution for City Hall
because it preserved the
building, historic
preservation suffers in the
long run from such
gargantuan projects as that
of the Oakland City Hall
because the public begins to
believe that such costly
solutions are the only way
to make such buildings safe.
The
situation with bearing wall
masonry buildings in
California is no longer as
distorted. The reason for
this is that recent research
has resulted in the
development of a new code
specific to this building
type. While public
perception on the safety of
masonry buildings is still
unduly negative, and price
spreads between different
engineers' designs can still
be large, the existence of
this new code has helped to
narrow the spread, and make
economical solutions
possible.
Figure
1. Cost comparison between
two retrofit projects.
The
code for masonry buildings,
which has now been adopted
as a model code in
California is Appendix,
Chapter 1 of the Uniform
Code for Building
Conservation. This appendix
contains the engineering
provisions for bearing wall
masonry structures. These
provisions were derived from
the "ABK
Methodology," an
engineering design
methodology for unreinforced
masonry bearing wall
buildings developed by a
team of engineers in Los
Angeles under a research
grant from the National
Science Foundation.
One
of the principal features of
this methodology is the
provisions which anticipate
and exploit the post-elastic
behavior of the wood and
plaster interior partitions
and floor diaphragms, thus
computing a consequential
reduction in the forces on
the masonry walls. Another
result of the ABK research
is the finding that masonry
buildings actually respond
differently from the way the
traditional codes and
engineering approaches
assumed. Rather than
amplifying the forces of the
earthquake, the heavy
masonry-walled building has
the effect of dampening the
shaking by acting as a
"rigid rocking block on
a soft soil base". This
is to be compared with the
common code analysis of
seismic force on a building
which models the building as
a "single degree of
freedom, 5% damped elastic
oscillator with a fixed
base" (Karitois
et al. 1984;
Kariotis, J. 1989, pers.
comm., 3 June).
Using
the ABK method of analysis,
the computed force levels in
an unreinforced masonry
building are lower than
found under conventional
code analysis. The results
of this methodology on the
design of retrofit
strategies for individual
masonry buildings is that
the amount of strengthening
work which is computed to be
required is less than that
shown as needed when
conventional strength based
linear elastic analysis is
used. This approach thus
reduces the retrofit
intervention and costs.
An
even more significant step
for historical buildings in
general has been taken in
California with the adoption
of the State Historical
Building Code. This Code,
which applies to all
historical buildings,
including even those which
are only on local lists,
allows much greater design
and engineering flexibility
than is possible under the
conventional prevailing code
which is primarily meant for
new buildings. Instead of
prescriptive requirements,
the State Historical
Building Code describes
general performance
objectives which must be
met. The specific solutions
are left up to the
designers. The code also
encourages the use of
archaic materials and
systems as part of the
structural system, providing
some minimum values for
these systems where they are
available.
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