Identify, Retain and Preserve
Identifying, retaining, and preserving structural
systems--and individual features of systems--that are
important in defining the overall historic character
of the building, such as post and beam systems, trusses,
summer beams, vigas, cast iron columns, above-grade
stone foundation walls, or loadbearing brick or stone
walls.
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The interior of this barn in Shelburne, Vermont,
is a magnificent space that included an overhead
hay loft with tracks that allowed workers to drop
hay to the floor below. Photo: NPS files.
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Altering visible features of historic structural systems
which are important in defining the overall historic
character of the building so that, as a result, the
character is diminished.
Overloading the existing structural system; or installing
equipment or mechanical systems which could damage the
structure.
Replacing a loadbearing masonry wall that could be
augmented and retained.
Leaving known structural problems untreated such as
deflection of beams, cracking and bowing of walls, or
racking of structural members.
Utilizing treatments or products that accelerate the
deterioration of structural material such as introducing
urea-formaldehyde foam insulation into frame walls.
Stabilize
Stabilizing deteriorated or damaged structural systems
as a preliminary measure, when necessary, prior to undertaking
appropriate preservation work.
Failing to stabilize a deteriorated or damaged structural
system until additional work is undertaken, thus allowing
further damage to occur to the historic building.
Protect and Maintain
Protecting and maintaining the structural system
by cleaning the roof gutters and downspouts; replacing
roof flashing; keeping masonry, wood, and architectural
metals in a sound condition; and ensuring that structural
members are free from insect infestation.
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Non-destructive evaluation techniques can be
of significant value in historic preservation
projects. Impulse radar is being used to evaluate both the orientation and depth of the cracks visible on the surface of the column shaft. Photo: Edmund P. Meade, P.E.
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Examining and evaluating the existing condition
of the structural system and its individual features
using non-destructive techniques such as X-ray photography.
Failing to provide proper building maintenance so that
deterioration of the structural system results. Causes
of deterioration include subsurface ground movement,
vegetation growing too close to foundation walls, improper
grading, fungal rot, and poor interior ventilation that
results in condensation.
Utilizing destructive probing techniques that will
damage or destroy structural material.
Repair
Repairing the structural system by augmenting or
upgrading individual parts or features using recognized
preservation methods. For example, weakened structural
members such as floor framing can be paired with a new
member, braced, or otherwise supplemented and reinforced.
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The new base isolator allows the structural
support member at the foundation to move horizontally
as it absorbs the earthquake forces. Photo: ©Jonathan
Farrer.
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Upgrading the building structurally in a manner that
diminishes the historic character of the exterior, such
as installing strapping channels or removing a decorative
cornice; or damages interior features or spaces.
Replacing a structural member or other feature of the
structural system when it could be augmented and retained.
The following work
is highlighted to indicate that it represents
the greatest degree of intervention generally
recommended within the treatment Preservation,
and should only be considered after protection,
stabilization, and repair concerns have been addressed.
Limited Replacement in Kind
Replacing in kind those visible portions or
features of the structural system that are either
extensively deteriorated or missing when there
are surviving prototypes such as cast iron columns
and sections of loadbearing walls. The new work
should match the old in materials, design, color,
and texture; and be unobtrusively dated to guide
future research and treatment.
Considering the use of substitute material
for unexposed structural replacements, such as
roof rafters or trusses. Substitute material should,
at a minimum, have equal loadbearing capabilities,
and be unobtrusively dated to guide future research
and treatment.
Replacing an entire visible feature of the structural
system when limited replacement of deteriorated
and missing portions is appropriate.
Using material for a portion of an exposed structural
feature that does not match the historic feature;
or failing to properly document the new work.
Using substitute material that does not equal
the loadbearing capabilities of the historic material
or design or is otherwise physically or chemically
incompatible.
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