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Historic Preservation - Technical Procedures

Spectitle:
Guidelines For Rehabilitating Historic Buildings: Structural Systems

Procedure code:
0109112S

Source:
National Park Service, Preservation Assistance Division

Division:
General Requirements

Section:
Reference Standards

Last Modified:
02/24/2012

Details:
Guidelines For Rehabilitating Historic Buildings: Structural Systems



GUIDELINES FOR REHABILITATING HISTORIC BUILDINGS:  STRUCTURAL
SYSTEMS


U.S. Department of the Interior
National Park Service
Preservation Assistance Division
Washington, D.C.


An illustrated booklet addressing the Secretary's Standards and the
guidelines is available from the U.S. Government Printing Office.
The title is "The Secretary of the Interior's Standards for
Rehabilitation & Illustrated Guidelines for Rehabilitating Historic
Buildings", ISBN 0-16-035979-1.

Each of the guidelines included in the booklet mentioned above have
been separated into individual entries for specific use in HBPP.
This entry represents one of many guidelines included in the
booklet and describes RECOMMENDED and NOT RECOMMENDED applications
of the Secretary of the Interior's Standards as they relate to
Structural Systems.  For a list of the Secretary of the Interior's
Standards for Rehabilitation, see 01091-04-S; For general
information relating to the purpose, organization and content of
the individual guidelines, see 01091-05-S.  Both of these entries
should be referenced along with the information contained in this
document.


BUILDING INTERIOR

STRUCTURAL SYSTEM:

If features of the structural system are exposed such as
loadbearing brick walls, cast iron columns, roof trusses, posts and
beams, vigas, or stone foundation walls, they may be important in
defining the building's overall historic character.  Unexposed
structural features that are not character-defining or an entire
structural system may nonetheless be significant in the history of
building technology; therefore, the structural system should always
be examined and evaluated early in the project planning stage to
determine both its physical condition and its importance to the
building's historic character or historical significance.  See also
Health and Safety Code Requirements.


IDENTIFYING, RETAINING AND PRESERVING

1.   Recommended:

    -    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.

    Not Recommended:

    -    Removing, covering, or radically changing features of
         structural systems which are important in defining the
         overall historic character of the building so that, as a
         result, the character is diminished.

    -    Putting a new use into the building which could overload
         the existing structural system; or installing equipment
         or mechanical systems which could damage the structure.

    -    Demolishing a loadbearing masonry wall that could be
         augmented and retained and replacing it with a new wall
         (i.e., brick or stone), using the historic masonry only
         as an exterior veneer.

    -    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.


PROTECTING AND MAINTAINING

1.   Recommended:

    -    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 assuring that structural
         members are free from insect infestation.

    Not Recommended:

    -    Failing to provide proper building maintenance on a
         cyclical basis so that deterioration of the structural
         system results.

2.   Recommended:

    -    Examining and evaluating the physical condition of the
         structural system and its individual features using non-
         destructive techniques such as X-ray photography.

    Not Recommended:

    -    Utilizing destructive probing techniques that will damage
         or destroy structural material.


REPAIRING

1.   Recommended:

    -    Repairing the structural system by augmenting or
         upgrading individual parts or features.  For example,
         weakened structural members such as floor framing can be
         spliced, braced, or otherwise supplemented and
         reinforced.

    Not Recommended:

    -    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.


REPLACING

1.   Recommended:

    -    Replacing in kind -- or with substitute material -- those
         portions or features of the structural system that are
         either extensively deteriorated or are missing when there
         are surviving prototypes such as cast iron columns, roof
         rafters or trusses, or sections of loadbearing walls.
         Substitute material should convey the same form, design,
         and overall visual appearance as the historic feature;
         and, at a minimum, be equal to its loadbearing
         capabilities.

    Not Recommended:

    -    Installing a replacement feature that does not convey the
         same visual appearance, e.g., replacing an exposed wood
         summer beam with a steel beam.

    -    Using a substitute material that does not equal the
         loadbearing capabilities of the historic material and
         design or is otherwise physically or chemically
         incompatible.


NOTE:  THE FOLLOWING REPRESENTS PARTICULARLY COMPLEX TECHNICAL OR
DESIGN ASPECTS OF REHABILITATION PROJECTS AND SHOULD ONLY BE
CONSIDERED AFTER THE PRESERVATION CONCERNS LISTED ABOVE HAVE BEEN
ADDRESSED.

ALTERATIONS/ADDITIONS FOR THE NEW USE

1.   Recommended:

    -    Limiting any new excavations adjacent to historic
         foundations to avoid undermining the structural stability
         of the building or adjacent historic buildings.

    Not Recommended:

    -    Carrying out excavations or regrading adjacent to or
         within an historic building which could cause the
         historic foundation to settle, shift, or fail; or could
         have a similar effect on adjacent historic buildings.

2.   Recommended:

    -    Correcting structural deficiencies in preparation for the
         new use in a manner that preserves the structural system
         and individual character-defining features.

    Not Recommended:

    -    Radically changing interior spaces or damaging or
         destroying features or finishes that are character-
         defining while trying to correct structural deficiencies
         in preparation for the new use.

3.   Recommended:

    -    Designing and installing new mechanical or electrical
         systems when required for the new use which minimize the
         number of cutouts or holes in structural members.

    Not Recommended:

    -    Installing new mechanical and electrical systems or
         equipment in a manner which results in numerous cuts,
         splices, or alterations to the structural members.

4.   Recommended:

    -    Adding a new floor when required for the new use if such
         an alteration does not damage or destroy the structural
         system or obscure, damage, or destroy character-defining
         spaces, features, or finishes.

    Not Recommended:

    -    Inserting a new floor when such a radical change damages
         a structural system or obscures or destroys interior
         spaces, features, or finishes.

    -    Inserting new floors or furred-down ceilings which cut
         across the glazed areas of windows so that the exterior
         form and appearance of the windows are radically changed.

5.   Recommended:

    -    Creating an atrium or a light well to provide natural
         light when required for the new use in a manner that
         assures the preservation of the structural system as well
         as character-defining interior spaces, features, and
         finishes.

    Not Recommended:

    -    Damaging the structural system or individual features; or
         radically changing, damaging, or destroying character-
         defining interior spaces, features, or finishes in order
         to create an atrium or a light well.

                         END OF SECTION