The Engineer as Preservationist
Marie Ennis, P.E.
Old Structures Engineering, PC
90 Broad Street
New York, NY 10004
To design appropriate solutions for the unique problems that aﬀect existing buildings,
bridges and monuments, the structural engineer specializing in historic-preservation
projects must also be part historian and part detective.
Engineers in the U.S. have been involved in historic preservation at least since 1966,
the year ASCE's Committee on the History and Heritage of Civil Engineering desig-
nated its ﬁrst National Historic Landmark, the Bollman Suspension and Trussed
Bridge at Savage, Md. In 1969, the Historic American Engineering Record was estab-
lished, followed by the Society for Industrial Archaeology in 1971.
Most historic-preservation projects require collaboration among many professionals: ar-
chitects, structural engineers, mechanical engineers, materials conservators, ﬁne-arts
conservators, landscape architects, historians and others. For example, the structural
engineer (who is concerned with the load-bearing structure of an existing building that
may be sheathed with decorated carved stone or ornate terra cotta) and the materials
consultant (who is concerned with the skin of the building, its cleaning and repair) must
work together to design re pairs that satisfy the concerns of both and meet project spec-
iﬁcations. A design to strengthen a ﬂoor that has a decorative plaster ceiling below it
must be done in coordination with the plaster conservator's needs.
The main guideline for work on historic buildings is the secretary of the interior's Stan-
dards for Rehabilitation, issued in 1977. To qualify for investment tax credits, owners
and developers must adhere to the standards, which cover issues such as opting for
minimal intervention, avoiding the destruction of historic fabric, researching the appro-
priate historical context for alterations and repairs, selecting replacement materials,
and using reversible solutions. Compliance with the standards may not involve selecting
the most straightforward engineering solution nor the most economical design. The cul-
tural signiﬁcance of the structure will inﬂuence design solutions, especially when the
building, bridge or monument must be treated as an artifact.
It is not always practical to comply fully with the standards, especially when life-safety
issues are involved—for example, the idea that structural interventions should be re-
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versible. Sometimes this can be done, as with the reinforcement of the main stairway at
George Washington's Mount Vernon estate in Virginia, visited by almost 1 million peo-
ple annually. A welded steel frame installed in a closet below the stair picks up loads
from the stair and landing and transfers them to foundation elements in the cellar. No
original framing was aﬀected, and the welded steel frame can be fully removed, if de-
sired. (Engineer on this project, and the others mentioned here, is Robert Silman Asso-
ciates, P.C., New York.)
In other cases, reversible interventions
are not possible. Fort Jeﬀerson National
Historical Monument in Dry Tortugas,
Fla., a mid-19th century fortiﬁcation, has
suﬀered severe masonry deterioration.
Along with lack of maintenance, the prob-
lem is caused by iron shutters built into
the masonry of the lower embrasure open-
ings. These Totten shutters, named for
their inventor, General J.G. Totten, pro-
vided excellent protection for soldiers ﬁr-
ing cannons, but iron embedded in brick
and coral aggregate concrete masonry was
an inappropriate material choice for the
area’s tropical climate.
Even if the exposed iron elements had
been properly cared for, it would have
been impossible to maintain the elements
embedded in the damp masonry. The ex-
panding iron components have destroyed
large areas of the scarp walls. With no
practical way to prevent further corrosion,
the iron had to be removed. Large ex-
panses of the bulging walls beyond the im-
mediate vicinity of the embrasure open-
ings will be stabilized in situ using a pin-
ning and grouting system. Neither the
iron removal nor the masonry pinning are
considered reversible, yet these interven-
tions are necessary steps for the overall stabilization and later restoration of this histor-
ically important site.
Another area where adherence to the interior secretary’s standards may not be practi-
cal is in the substitution of a modern material to replace original fabric. An example is
the exterior restoration of Shepard Hall at City College of the City University of New
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Expansive forces of corroding iron shutters at Fort
Jeﬀerson National Historical Monument, Dry Tor-
tugas, Fla., caused exterior withes of brick to peel
York, an ornate Gothic-style building constructed using stone and terra cotta. Terra
cotta arches over the windows that were designed as load bearing had deteriorated too
much for in situ stabilization, so new thin-shell, glass-ﬁber-reinforced concrete replace-
ment material was fabricated to mimic the original appearance. We also redesigned var-
ious details that had led to the deterioration of the original terra cotta.
At another building, the Brooklyn (N.Y.) Post Oﬃce, deteriorated terra cotta was re-
placed with new terra cotta. The condition and the original design detailing allowed us
to replace deteriorated units in kind, with a new anchorage system added behind them.
In addition to the secretary's standards, local guidelines may also aﬀect project deci-
sions. In many towns and cities across the U.S., “Main Street" organizations have been
formed to develop design guidelines for their downtown districts. In New York City,
for example, the Landmarks Preservation Commission will review the aesthetic impact
of a structural intervention if the building is a designated landmark or in a historic dis-
trict. Understanding the interests of the preservation community is a key element in
providing a design acceptable to all.
Often in historic-preservation projects, there is a conﬂict between the goals of pure
preservation and structural stability. The preservation engineer must determine the op-
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Embedding iron in brick and coral aggregate masonry was an inappropriate materials choice in the trop-
ical climate. Since there is no practical way to prevent further corrosion, the iron had to be removed
many areas of the scarp wall will be stabilized in situ.
timum solution, based on review of various codes, appropriate analysis and engineering
As with all architectural/engineering projects, life-safety code requirements aﬀect the
design approach on preservation projects. A historic building may not need to be made
fully compliant with current codes—unless the building use changes or the occupancy is
increased. This was the case with an 1885 landmark building near Union Square in
New York City. Originally designed as a YWCA, its designated use was for public as-
sembly and oﬃces.
The building’s current owner, SGI-USA, a Buddhist organization, plans the same type
of occupancy as the original use required — but with a new assembly space added by in-
serting a ﬂoor in a previously triple-height space. Increasing the square footage meant
that additional exits had to be provided. This was done by adding two new stair towers
and constructing a new elevator core. These alterations fell into the category of a reha-
bilitation. The facade and clay tile roof of the building were not altered in appearance.
The new stair towers are not visible from the street, so they were acceptable to the
Landmarks Preservation Commission review board.
In another project, the National Park Service sought to upgrade six buildings in
Harpers Ferry National Historic park in Virginia for use by park personnel and to in-
crease public visits. The solution was to limit structural reinforcement to areas where
the public would be allowed. In one building, where the second ﬂoor was inaccessible to
the public because of limited capacity, an elevator was to be installed at the rear of the
building to permit disabled access. The Park Service agreed to reinforce only the hall-
way framing and allow visitors to view the second-ﬂoor rooms but not enter them. This
allowed original ﬂooring and plasterwork, which otherwise would have been partially
destroyed, to be retained.
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A project to upgrade six buildings at Harpers Ferry National Historic Park in Virginia limited structural
reinforcement to areas where the public would be allowed.
Similarly, at the Lower East Side Tenement Museum in New York City, visits are re-
stricted to building areas that have been reinforced or repaired, and the size of tour
groups is limited. As funding for further work becomes available, other portions of the
building will be opened to the public.
Local building codes can provide information to assist in analysis of existing buildings.
The masonry section of the New York City Building Code covers many of the building
types in the city and lists a variety of mortar types, some of which correspond to his-
toric lime-rich mortars that can be identiﬁed through laboratory analysis of samples.
Knowledge of the geological properties of the stone used in load-bearing construction is
also important, since variations from within the same quarry can result in portions of a
building performing diﬀerently from each other. This can sometimes be seen on fa-
cades where diﬀerent exposures result in a variety of weathering eﬀects on individual
Consulting early building codes can provide data for analyzing cast- and wrought-iron
elements, as well as early steel shapes. The building’s date of construction will lead to
the proper references for the design guides used by the original engineers. This helps
to understand the design intent and evaluate the current capacity with regard to pro-
Analysis of older timber-framed construction can be problematic. Many of these build-
ings were designed empirically, based on traditional methods. Often the wood used in
historic buildings is of a higher quality than that available today. Allowable stress val-
ues should be carefully considered. The latest version of the National Design Speciﬁca-
tion for Wood Construction has reduced the allowable stress values for most species.
It is possible to analyze a structure that has been standing without signs of distress for
over 100 years using current codes and conclude that it is seriously overstressed. The
availability of high-quality old-growth timber in the 18th and 19th centuries as well as
diﬀerent factors of safety can account for some disparities. Engineers must exercise
judgment when evaluating the extent of intervention required for many vernacular
Sometimes, however, modern methods of analysis can conﬁrm the conclusions of engi-
neers working in the years before computers. An example is the Assembly Chamber
ceiling of the New York State Capitol Building in Albany. The state Oﬃce of General
Services wanted to determine if the original ceiling could be replicated. Constructed in
the 1870s to the design of the architect Leopold Eidlitz, the ceiling was an ornate
ribbed sandstone vault of immense proportions rising 54 ft above the chamber ﬂoor.
The ceiling was eventually dismantled due to cracking and spalling of the stone ribs and
replaced with a ﬂat coﬀered ceiling hung from trusses in an attic space above. Eidlitz
defended his design, but a number of engineers published articles concluding that it
was faulty because the geometry was incorrect. They used a graphic static method de-
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scribed in engineering handbooks of the day. A three-dimensional ﬁnite element analy-
sis done to determine the feasibility of re-creating the structural vault, based on written
descriptions and historical construction photographs (drawings were not available),
showed that the line of pressure in the ribs did not fall within the kernel and that ten-
sion occurred on the upper ribs with a resulting increase in compression on the bottom
face of the carved ribs. The study concluded that to be visually replicated, the ceiling
would have to be a hung system using pre fabricated panels.
Structural investigation can have an immense impact on historic ﬁnishes, so it is critical
to perform all other methods of investigation before making probes or “opening up.”
Despite potential damage to historic fabric, though, the structural investigation is cru-
cial to data gathering.
The ﬁrst step is archival research. Where the primary focus of the work is structural,
this is usually done by the preservation engineer. Searching for original drawings,
maintenance histories, records from the construction period and repair/alteration docu-
ments as well as interviewing personnel familiar with the building are important in
learning about the existing structure.
Next, the site investigation begins. Reliable structural drawings are frequently not
available for a historic building. The structural condition is ascertained ﬁrst by visual
observation, which often requires a great deal of climbing and crawling throughout the
building, followed by a probe-and-testing program. Testing may include mortar analy-
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The planned project at Dungenes Mansion, at Cumberland National Seashore in Georgia, will stabilize
the existing ruins and create a silhouette of the original 18805 house. The new structure will brace the
existing walls and allow for interpretation of the site.
sis, concrete core samples (chloride ion content, petrographic analysis and strength
tests) and identiﬁcation of wood species and of early steel vs. wrought iron vs. cast iron.
Conventional probes allow the engineer to directly review the condition fo concealed
structural elements and to take measurements for computing section properties. The
actual structure in a historic building is often hidden under valuable ﬁnishes. In some
cases, non-destructive techniques — ranging from such low-tech methods as sounding
with an acrylic mallet or a chain drag test to ultrasonic pulse velocity or ﬁber-optic in-
struments — can signiﬁcantly reduce the conventional probing needed.
Nondestructive techniques may also be used to map hidden structure. For example, the
original structural drawings for the New York State Capitol Building were destroyed in
a ﬁre in 1911. Our client, the Oﬃce of General Services, wanted to re-create the draw-
ings and have a load analysis performed to evaluate options for possible building alter-
Where the impact on ﬁnishes would be minimal, conventional probes were made. In
other areas, we used nondestructive methods to locate structure and determine thick-
nesses of heavy masonry construction, including magnetic detection, radar, impact echo
and ultrasonic techniques. These technologies were developed for other applications
and were adapted speciﬁcally to survey this historic landmark.
Research And Education
European engineers, with their rich heritage of buildings and monuments, have been
involved in historic preservation for many years. They have pioneered methods for in-
vestigation and have developed many creative structural solutions. Much of their data
are directly useful to practitioners in the U.S., but the structures here diﬀer from many
European buildings and preservation engineers must have a special understanding of
the evolution of building design and construction unique to the U.S.
Even where early American builders intended to replicate the appearance of their an-
cestral buildings in Europe, deviations from traditional methods occurred either due to
lack of trained craftspeople or through adaptation to locally available resources. Vernac-
ular structures and early skyscrapers are examples of building types unique to the U.S.
Most research dealing with historic American architecture is being done at universities
with graduate programs in historic preservation. Undergraduate engineering programs
still do not generally include course work related to historic preservation — the history
of technology and architecture, historic building materials, and early analytical methods
— as part of their core curriculum.
This should be remedied. As our stock of historic structures increases, there will be a
growing need for formally trained preservation engineers. Also, the personal rewards
of seeing a once-deteriorated old building become a revitalized architectural monument
are considerable, and the work provides a valuable contribution to our cultural heritage.
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Knowing the Language
The historic preservationist has a specialized vocabulary that may not be familiar to
engineers who work on new structures. These deﬁnitions may be helpful for those not
acquainted with the ﬁeld:
•“Preservation”: sustaining the existing form, integrity and material of a
building or structure and the existing form and vegetative cover of a site. It
may include initial stabilization work, where necessary, as well as ongoing
maintenance of the historic building materials and vegetation.
•“Rehabilitation": returning a property to a state of utility through repair
or alteration, which makes possible an eﬃcient contemporary use while pre-
serving those portions or features of the property signiﬁcant to its historical,
architectural and cultural values.
•“Restoration”: accurately recovering the form and details of a property
and its setting as it appeared at a particular period by removal of later work or
by the replacement of missing earlier work.
•“Stabilization": re-establishing a weather-resistant enclosure and struc-
tural stability while maintaining te essential form as it exists.
These terms are frequently used to describe the scope of work on preservation
projects. A dictionary of historical architectural terms can also be useful in identifying
various parts of a structure such as the intrados (interior curve) of an arch or a vous-
soir (wedge-shaped masonry unit in an arch ring).
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