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SEISMIC RETROFIT OF HISTO RIC BUILDING STRUCTURES
Post: #1

SEISMIC RETROFIT OF HISTO RIC BUILDING STRUCTURES

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ABSTRACT
Buildings with historic values are regional cultural assets worth preserving. The
design technologies and building materials and methods that went into the
original construction of these buildings are often drastically different from their
contemporary counterparts, their structural renovation or retrofit brings forth
many technical challenges to the design professional. This paper provides a
general survey of the technical issues pertaining to the seismic retrofit of historic
buildings, and explores various design procedures and construction methods for
that purpose, including innovative technologies such as post tensioning, seismic
isolation, composite wraps, etc. Special attention is given to the typical structural
attributes of historic structures in terms of their structural stiffness, strength and
ductility, how these parameters changed over the years, reliable methodologies
for evaluating these primary structural attributes, and associated design
implications for structural retrofit or hazard mitigations. Much of the discussion
is based on a combination of the perspective provisions in building codes and
alternative performance based approaches to meet the equilibrium, strain
compatibility, and energy dissipation criteria, while a considerable weight is
given to factors that influence preserving non-structural elements of historic
value. A brief summary on cost implications is also provided.
Overview
Buildings with historic value are regional cultural assets worth preserving. At times, they
also represent a potential source of revenue and stimulus for the economical revitalization of
their neighborhoods. The factors used to classify a building as historic may vary in different
countries and cultures, so obviously not every aged building falls into historical or monumental
category. In United States, a building is historic if it is at least 50 years old, and is listed in or
potentially eligible for the National Register of Historic Places and/or a state or local register as
an individual structure, or as a contributing structure in a district. In prevailing practice, older
structures are demolished and replaced by modern buildings due to economical and performance
reasons, unless they can be claimed historic.
The retrofit process is a general term that may consist of a variety of treatments,
including: preservation, rehabilitation, restoration and reconstruction (Kelly, 1996). Preservation
is defined as the process of applying measures to sustain the existing form, integrity, and
materials of a historic property. Rehabilitation refers to the process of creating new application
Paper No. 1565
for a property through repair, alterations and additions while preserving those features which
convey its historical, cultural, or architectural values. Restoration is the process of accurately
restoring a property as it existed at a particular period of time. Reconstruction is described as the
act of replicating a property at a specific period of time. Selecting the appropriate treatment
strategy is a great challenge involved in the retrofit process and must be determined individually
for each project.
Depending on project objectives, preservation and renovation of historic buildings may
involve an array of diverse technical considerations, such as fire life safety, geotechnical hazards
and remedies, weathering and water infiltration, structural performance under earthquake and
wind loads, etc. Since the design methodology and building materials and methods that went into
the original construction of these buildings are often drastically different from their
contemporary counterparts, their structural renovation or retrofit brings forth many technical
challenges to A/E design professionals.
Evolution of building materials
Building materials have evolved gradually throughout the construction history, and the pace of
the evolution is accelerated throughout the past century. Advancements in material engineering
and metallurgy, invention of plastics and fiber reinforced composites, and innovations in
production and treatment of existing building materials are some of the major causes of old and
contemporary building material differences. Improvements in conventional building materials
used both in historic and contemporary structures are described as:
Masonry, stone, and adobe buildings
Bearing wall buildings were the dominant type of structures till late years of nineteenth
century, when they were replaced by steel frame skeleton as the typical structural form in large
buildings. In modern construction, masonry buildings are limited to certain building types and
special locations. Natural stone has not changed, while adobe or bricks have slightly evolved to
stronger, more durable building materials with consistent shapes and sizes. Design and
construction techniques for masonry buildings are improved by using stronger mortar, and
reinforcements to provide more resistance and continuity. Application of concrete filled blocks is
also a major improvement in building masonry structures.
Wood and timber
Wood, as a natural building material, has not been subjected to any major change, but
modern technology provides strength grading methods, wooden panel products, preservation
treatment process and wood protection.
Concrete
Concrete has been subjected to significant evolution during twentieth century. Improved
ingredients, quality control, preparing, and casting process offered stronger and more durable
concretes. Improvements in concrete technology, application of additives, plasticizers, and
improved cements provide light weight, high strength, high workability, shrinkagecompensation,
low porosity, and fiber reinforced types of concrete.
Hot-rolled reinforcing steel
Reinforcing steel has evolved considerably regarding the material properties and shape.
Reinforcement bars initially had square cross-sections, high carbon content, and smooth surface,
where new ribbed, reinforcement bars with limited carbon content provide more ductility and
stronger bond between the steel reinforcement and concrete.
Structural steel
Overall strength of structural steel was improved within past century (See Table 1).
Section dimensions and properties of steel shapes have also been changed and a number of
shapes are considered obsolete and they are no longer produced. Difference in strength, ductility
and weldability must be considered in the retrofit design process.
Table 1: ASTM steel specifications (Handbook of Steel Construction, 7th Edition, CISC, 2000).
ksi MPa ksi MPa
1914* ½ Fu ½ Fu 55 - 65 380 - 450
A7 (bridges) 1924 ½ Fu ≥ 30 ½ Fu ≥ 210 55 - 65 380 - 450
A9 (buildings) 1934 ½ Fu ≥ 33 ½ Fu ≥ 230 60 - 72 410 - 500
A373 1954 32 220 58 - 75 400 - 520
A242 1955 50 350 70 480
A36 1960 36 250 60 - 80 410 - 550
A440 1959 50 350 70 480
A441 1960 50 350 70 480
A572 grade 50 1966 50 345 65 450
A588 1968 50 345 70 485
50 min. to 345 min. to
65 max. 450 max.
A992 1998 65 450
Designation Date Published
Yield Strength Tensile Strength (Fu)
Practice and design concepts
Building codes have been constantly updated in past decades on the basis of various
lessons learned from previous failures (especially earthquake related failures). Advances in
computer programs and hardware have drastically changed the way we do structural analysis and
design. As a rule, newer provisions tend to prescribe better continuity for seismic loadings,
provide more redundancy in structural system, and they exploit inelastic structural capacities to
absorb and dissipate earthquake loads.
Such contemporary code requirements and engineering knowledge base were not
available to designers and builders at the time historic buildings were typically designed and
constructed without detailed assessment of the probabilistic magnitude of loading (especially
load cases related to wind or earthquake) or clear knowledge on structural behavior. Design
methodologies were also quite limited in past days, when engineers were required to perform
hand calculations with numerous estimations in the process. Older design concepts required that
working stresses remain within elastic limits. Higher engineering approximations accompanied
by older design concepts, resulted in over-designed structural members which do not necessarily
improve seismic behavior, but they usually add to dead loads.
Older design concepts mostly focused on the effects of gravity loads and they did not
dedicate enough attention to provide adequate lateral resistance and ductility. Most of historic
buildings provide limited ductility and continuity, especially when subjected to seismic loading.
Unreinforced bearing walls provide limited resistance against lateral loading and a high potential
of discontinuity at corners or connection to the roof. It is very common to notice historic
reinforced concrete building with discontinued flexural reinforcements, no transverse
reinforcement in beam-column joint zones and minimal confinement in columns.
Retrofit process requires local modification of components, minimizing structural
irregularities (in mass and stiffness), structural stiffening, structural strengthening, mass
reduction and seismic isolation to improve the structural performance and comply with current
building codes (i.e. FEMA356, IBC2003, UBC1997). Performance objectives used for historic
retrofit are similar to general objectives used in the performance based engineering context, but
with extra constraints to preserving the historic fabric along with the structure itself.
In most cases, the façade and fixtures are of historic value and preserving them requires
limiting deformation imposed by seismic loads. Limiting deformations is in contrast with the
newer design philosophies that exploit the structural ductility to reduce the required strength. In
seismic retrofit of historic buildings both the global strength and stiffness must be increased to
minimize the deformation and damage to the historic fabric.
Challenges of retrofitting historic fabric
Minimizing noise, disturbance, and damage to the surrounding buildings and providing
temporary shoring and support are typical challenges involved in most retrofit projects.
Depending on the extends of retrofitting, assessed risk, technical limitations, structural historic
value, and economical constraints, the preferred retrofit strategies are studied and prioritized to
preserve the authenticity of historic fabrication and minimize removal of architectural material:
No penetration of building envelope
The process does not require any destructive procedure so the historic fabrication remains
untouched (e.g. composite wraps or chemical treatment). This approach is only applicable to
very limited cases since structural components are mostly either embedded in or covered by the
finishing.
Penetration without breakage
The structural component subjected to retrofitting is accessible, and the retrofit process
only requires drilling holes (e.g. micro piles, epoxy injection, post tensioning).
Breakage with repair
In many cases, some destructive procedures are required to access the structural
component or to perform retrofit process (e.g. fixing and improving welded connections or
installation of base-isolators).
Replace
In cases structural components can not be improved to meet retrofitting objectives or the
damage or deterioration could not be repaired, components are replaced. Replacement process
requires special attention to providing support for the rest of the building, isolating the
component, and maintaining continuity.
Rebuild
In cases a feasible retrofitting solution can not be found, the historic building is
reconstructed, partially or as a whole. This option imposes greater economical burden and the
loss of authenticity may have impacts on historic and cultural values. Typically rehabilitation of
historic buildings requires new structural members and preservation of historic fabric is
accomplished by hiding the new structural members or by exposing them as admittedly new
elements in the building’s history. Often, the exposure of new structural members is preferred
because alterations of this kind are reversible and they could conceivably be undone at a future
time with no loss of historic fabric to the building.
Innovative technologies for historic preservation
Modern materials and equipment provide many retrofit options to improve the behavior
of structural system, global strength, stiffness or mitigate the seismic hazards. Some of the
commonly used techniques in retrofitting are listed below:
Post tensioning
Post tensioning is considered one of the potentially efficient retrofit options for reinforced
concrete or masonry buildings, providing strength and ductility to the overall structure with
minimal intrusion. Masonry has a relatively large compressive strength but only a low tensile
strength. Hence, it is most effective in carrying gravity loads. However, in-plane shear and outof-
plane lateral loads induce high levels of tensile stress also. Commonly, these induced tensile
stresses exceed the compressive stresses and reinforcing (commonly with steel members) must
be added to provide the necessary strength and ductility. The level of compressive stresses can be
significantly raised by post-tensioning the reinforcing steel and the more brittle tensile failures
avoided. Basically, a core hole is placed down through the masonry wall and a high-strength
steel rod (or tendon) is inserted. The bottom of the rod is anchored in the floor or foundation. A
jack is then used at the top of the wall to place high levels of tensile force in the rod.
Base isolation
Base isolators are used to decouple the building response from the ground motion and in
the event of a major earthquake, base isolation will greatly reduce structural and architectural
damage, mostly by shifting the structure natural period (Figure 1). The two basic types of
isolation systems that have been employed are elastomeric bearings (using natural rubber or
neoprene) and the sliders (Teflon and stainless steel).
Post: #2

Buildings with historical values ​​are regional cultural assets worth preserving. The design technologies and building materials and methods that went into the original construction of these buildings are often drastically different from their contemporary counterparts, their structural renovation or retrofit brings many technical challenges to the design professional. This document conducts a general study of technical issues related to the seismic rehabilitation of historic buildings and examines various design procedures and construction methods for that purpose, including innovative technologies such as post-tensioning, seismic isolation, attributes structural characteristics typical of historical structures in terms of their structural rigidity, strength and ductility, how these parameters changed over the years, reliable methodologies to evaluate these primary structural attributes and associated design implications for structural adaptation or risk mitigation. Much of the discussion is based on a combination of perspective provisions in building codes and alternative performance-based approaches to meet the criteria of equilibrium, strain compatibility and energy dissipation, while giving considerable weight to the factors that influence the preservation of non-structural elements of historical value. A brief summary of the consequences of costs is also provided. Buildings with historical value are regional cultural assets worth preserving. Sometimes they also represent a potential source of income and stimulus for the economic revitalization of their neighborhoods. Factors used to classify a building as historic may vary in different countries and cultures, so obviously not all aging buildings fall into historical or monumental category. In the United States, a building is historic if it is at least 50 years old and is inscribed or potentially eligible for the National Register of Historic Places and / or a state or local registry as an individual structure or as a contributory structure in a district. In prevailing practice, the oldest structures are demolished and replaced by modern buildings for economic and performance reasons, unless they can be claimed historical. The adaptation process is a general term that can consist of a variety of treatments, including: preservation, rehabilitation, restoration and reconstruction (Kelly, 1996). Preservation is defined as the process of applying measures to sustain the existing form, integrity, and materials of a historic property.
 

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