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Handbook for Seismic Rehabilitation of Existing Buildings
by
H.S. Lew
National Institute of Standards and Technology
Gaithersburg, MD 20899, USA
ABSTRRACT safety of existing buildings constructed for
or leased by the federal government which
Advances in earthquake-related technology were designed and constructed without
during the past few decades have led to a adequate seismic design and construction,”
realization that seismic risk to life and President issued Executive order 12941 in
property can be reduced significantly by December 1994 requiring federal
improving seismic performance of existing government agencies to reduce seismic risk
seismically deficient buildings. Detailed of those structures which pose risk in terms
post-earthquake investigations of building of occupancy and potential secondary
failures have provided engineers with impact.
considerable information concerning the
details of building design and construction Since 1984, the Federal Emergency
that enhance earthquake resistance. Since Management Agency (FEMA, now part of
the 1989 Loma Prieta earthquake and the the Department of Homeland Security)
1994 Northridge earthquake, a large number published a series of earthquake-related
of research programs for the development of documents including: FEMA 310 Handbook
seismic rehabilitation methods have been for the Seismic Evaluation of Buildings
carried out in the United States. This paper (FEMA 1998) and FEMA 356 Prestandard
presents the scope of a seismic rehabilitation and Commentary for the Seismic
handbook being developed by the Rehabilitation of Buildings (FEMA 2000).
Interagency Committee on Seismic Safety in These two documents have either gone
Construction for use by U.S. federal through or being processed as Standards of
government agencies. the American Society of Civil Engineers
(ASCE). This document (Handbook), as a
KEYWORDS: buildings, earthquake, companion to these two documents, will
rehabilitation, seismic engineering, guide design professionals seismic
structures rehabilitation techniques for various types of
buildings. The techniques are based on
input from a group of experts involved in
1.0 INTRODUCTION seismic rehabilitation and on a review of
existing literature published through 2003.
One of the objectives of the Earthquake
Hazards Reduction Act of 1977 (P.L. 95-124 The handbook is divided into three parts.
as amended) is ". . . the development of Part 1 provides background on seismic
methods for rehabilitation and utilization of evaluation, categories of seismic
man-made works so as to effectively resist deficiencies, classes of rehabilitation
the hazards imposed by earthquakes . . . ." techniques, and general strategies to develop
In response to Public Law 101-614, which rehabilitation schemes. Part 2 includes
requires the President to adopt “standards detailed description of seismic deficiencies
for assessing and enhancing the seismic that are characteristic of each FEMA model
1
building type (FEMA 1998) and techniques affect the total expected inelastic
commonly used to mitigate seismic displacement and added strength may reduce
deficiencies. Part 3 includes seismic nonlinear demands into acceptable ranges.
rehabilitation techniques not necessarily
related to a specific building type such as A deficiency in global strength is common
those related to diaphragms, foundations, in older buildings either due to a complete
and nonstructural components. Part 3 also lack of seismic design or a design to an early
includes global techniques that seismic building code with inadequate strength
isolation or addition of damping. requirements.
2.0 SEISMIC VULNERABILITY If prescriptive equivalent lateral force
methods or linear static procedures have
The vulnerability of a building subjected to been used for evaluation, inadequate
an earthquake is dependent on seismic strength will directly relate to unacceptable
deficiency of that building relative to a demand-to-capacity ratios within elements
required performance objective. The of the lateral force-resisting system.
seismic deficiency is defined as a condition
that will prevent a building from meeting the 3.2 Global Stiffness
required performance objective. Thus, a
building evaluated to provide full occupancy Global stiffness refers to the stiffness of the
immediately after an event may have entire lateral force-resisting system although
significantly more deficiencies than the the lack of stiffness may not be critical at all
same building evaluated to prevent collapse. levels. For example, in buildings with
narrow walls, critical drift levels occur in the
Depending on the vulnerability assessment, upper floors. Conversely, critical drifts most
a building can be condemned and often occur in the lowest levels in frame
demolished, rehabilitated to increase its buildings. Stiffness must be added in such a
capacity, or modified so that the seismic way that drifts are efficiently reduced in the
demand on the building can be reduced. critical levels.
Thus, structural rehabilitation of a building
can be accomplished in a variety of ways, each Although strength and stiffness are often
with specific merits and limitations related to controlled by the same existing elements or
improving seismic deficiencies. the same retrofit techniques, the two
deficiencies are typically considered
3.0 COMMON SEISMIC DEFICIENCIES separately. Failure to meet evaluation
standards is often the result of a building
Regardless of the evaluation method used, placing excess drift demands on existing
failure to meet the stipulated performance poorly detailed components.
objective implies certain seismic
deficiencies. These deficiencies are 3.3 Configuration
described below:
This deficiency category covers
3.1 Global Strength configuration irregularities that adversely
affect performance. In codes for new
Global strength typically refers to the lateral buildings, these configuration features are
strength of the vertically oriented lateral often divided into plan irregularities and
force-resisting system. For degrading vertical irregularities. Plan irregularities are
structural systems characterized by a features that may place extraordinary
negative post yield slope on the pushover demands on elements due to torsional
curve, a minimum strength requirement may response or the shape of the diaphragm.
apply. In certain cases, the strength will also Vertical irregularities are created by uneven
2
vertical distribution of mass or stiffness Although the primary gravity load design is
between floors that may result in adequate, the post elastic behavior is not,
concentration of force or displacement at most often due to inadequate configuration
certain levels. In older existing buildings, and spacing of ties.
such irregularities are seldom taken into
consideration in the original design and, Identification of detailing deficiencies is
therefore, normally require rehabilitation significant in selection of mitigation
measures to mitigate. strategies because acceptable performance
often may be achieved by local adjustment
3.4 Load Path of detailing rather than by adding new
lateral force-resisting elements. In the case
A discontinuity in the load path, or of gravity concrete columns, acceptable
inadequate strength in the load path, may be performance can be achieved by enhancing
considered overarching because this deformation capacity by adding confinement
deficiency will prevent the positive rather than by reducing global deformation
attributes of the seismic system from being demand by adding lateral force-resisting
effective. The load path is typically elements.
considered to extend from each mass in the
building to the supporting soil. For example, 3.6 Diaphragm Deficiencies
for a panel of cladding, this path would
include its connection to the supporting floor The primary purpose of diaphragms in the
or floors, the diaphragm and collectors that overall seismic system is to act as a
deliver the load to components of the horizontal beam spanning between lateral
primary lateral force resisting system (walls, force-resisting elements. Diaphragm
braces, frames, etc.), continuity of these deficiencies include such factors as
components to the foundation, and finally inadequate shear or bending strength,
the transfer of loads between foundation and stiffness, or reinforcing around openings or
soil. reentrant corners. Inadequate local shear
transfer to lateral force-resisting elements or
Many load path deficiencies are difficult to missing or inadequate collectors are
categorize because the strength deficiency categorized as load path deficiencies.
may be considered to be part of another
element. For example, an inadequate 3.7 Foundation Deficiencies
construction joint in a shear wall could be
considered a load path deficiency or a shear Foundation deficiencies can occur within the
wall deficiency in the category of global foundation element itself, or due to
strength. inadequate transfer mechanisms between
foundation and soil. Element deficiencies
3.5 Inadequate Component Detailing include inadequate bending or shear strength
of spread foundations and grade beams,
Detailing, in this context, refers to design inadequate axial capacity or detailing of
decisions that affect a component’s or piles and piers, and weak and degrading
system’s behavior beyond the strength connections between piles, piers and caps.
determined by nominal demand, often in the Transfer deficiencies include excessive
nonlinear range. An example of a detailing settlement or bearing failure, excessive
deficiency is poor confinement in concrete rotation, inadequate tension capacity of deep
gravity columns. Often in older concrete foundations, or loss of bearing capacity due
buildings, the expected drifts from the to liquefaction.
design event will exceed the deformation
capacity of such columns, potentially 3.8 Other Deficiencies
leading to degradation and collapse.
3
Deficiencies that do not fit into one of the Construction cost is always important and is
categories described above may include: balanced against one or more other
considerations deemed significant.
• Geologic hazards However, sometimes other economic
considerations, such as the cost of disruption
• Adjacent buildings to building users, or the value of contents to
be seismically protected, can be orders-of-
• Deteriorated structural materials magnitude larger than construction costs.
Thus, cost may be the only criterion applied
4.0 STRATEGIES FOR when choosing among equivalent
REHABILITATION SCHEMES rehabilitation options.
4.1 Technical Considerations 4.4 Seismic Performance
Selected techniques must eliminate Prior to the emphasis on performance based
deficiencies, preferably more than one design, perceived qualitative differences
deficiency. First one should consider between the probable performance of
enhancing existing elements such as shear difference schemes would be used to assist
walls, moment frames, and bracing frames. in choosing a scheme. Specific performance
The deformation compatibility between new objectives are often set prior to the
elements and existing elements must be development of schemes. Objectives that
considered. In some cases, the application require a limited amount of damage or
of base isolators or damping devices is the "continued occupancy" will severely limit
most efficient way to eliminate deficiencies. the retrofit methods that can be used and
may control the other issues.
4.2 Non-technical Considerations
4.5 Short-term Disruption of Occupants
The solution chosen for rehabilitation is
almost always dictated by building-user When seismic rehabilitation is done at the
oriented issues rather than by merely time of major building remodeling,
satisfying technical demands. There are five disruption issue is minimized. However, in
basic issues that are of concern to building cases where the building is partially or
owners or users: completely occupied, this parameter
commonly becomes dominant and controls
1. Construction cost, the design.
2. Seismic performance,
3. Short term disruption of occupants, 4.6 Effects on Long-term Functionality of
4. Effects on long-term functionality of Building
building, and
5. Aesthetics, including consideration of This characteristic is often judged less
historic preservation. important than others. The planning
flexibility is only subtlety changed.
All of these characteristics are always However, it can be significant in building
considered, but an importance will occupancies that need open spaces such as
eventually be put on each of them, either retail spaces and parking garages.
consciously or subconsciously, and a
combination of weighting factors will 4.7 Aesthetics
determine the scheme chosen.
In historic buildings, considerations of
4.3 Cost preservation of historic fabric usually
control the design. Performance objectives
4
are controlled by limitations imposed by • Concrete moment frames
preservation. In non-historic buildings, A complete system of concrete
aesthetics is commonly stated as a criterion, beams and columns. Lateral loads
but is often sacrificed, particularly in favor are resisted by cast-in-place
of minimizing cost and disruption to tenants. moment frames.
5.0 REHABILITATION TECHNIQUES • Concrete shear wall buildings
(Bearing wall systems)
Different building types require different Usually all concrete with flat slab
mitigation techniques for a specific seismic or precast plank floors and concrete
deficiency. Depend on building types and bearing walls. Little, if any, of the
associated seismic deficiencies, alternative gravity loads are resisted by beams
recommendations are made to satisfy the and columns. Lateral loads are
performance objective of rehabilitation. resisted by shear walls.
5.1 Building Types • Concrete shear wall buildings
(Gravity frame systems)
Rehabilitation techniques are being Buildings have columns and beams
developed for 17 common building types, or columns and slabs that
which are defined in the FEMA documents. essentially carries all gravity loads.
They are: Lateral loads are resisted by
concrete shear walls surrounding
• Wood light-frames shafts, at the building perimeter, or
One- and two-detached dwellings isolated walls placed specifically
of one or more stories in height. for lateral resistance.
• Multi-story, multi-unit residential • Concrete frames with infill masonry
wood frames shear walls
Large residential buildings with Buildings are normally older
commercial space at the ground buildings that consist of essentially
floor. complete gravity frame assemblies
of concrete columns and floor
• Steel moment frames systems. The floors can be of a
Buildings consist of steel beams variety of concrete systems
and columns, and lateral forces are including flat plates, two way slabs,
resisted by moment frames. and beam and slab. Exterior walls
are constructed of unreinforced
• Steel braced frames masonry, tightly infilling the space
Buildings consist of frame between columns horizontally and
assemblies of steel beams and between floor structural elements
columns. Lateral forces are resisted vertically, such that the infill
by diagonal steel members placed interacts with the frame to form a
in selected bays. lateral force resisting system.
• Steel frames with infill masonry • Tilt-up concrete shear wall
shear walls buildings
Buildings are normally older Buildings are constructed with
buildings that consist of gravity perimeter concrete walls cast on the
frames with unreinforced masonry, site and tilted up to form the
tightly infilling the space between exterior of the building. The
columns. majority of these buildings are one
5
story with wood roof framing; is often used for hotel and motels
however, a good number of multi- and is similar to the concrete
story buildings also exist with bearing wall type.
composite deck floors and a wood
or steel framed roof. • Unreinforced masonry bearing wall
buildings
• Precast concrete frames with shear Buildings consist of unreinforced
walls masonry bearing walls, usually at
Buildings consist of concrete the perimeter and usually brick
columns, girders, beams and/or masonry. The floors are typically
slabs that are precast off the site of wood joists and wood sheathing
and erected to form a complete supported on the walls and on
gravity load system. This building interior post and beam construction.
type has a lateral force-resisting
system of concrete shear walls, 5.2 Rehabilitation techniques
cast-in-place or precast. In
California, precast floor T-beams or For each of 8 categories of deficiencies
hollow core planks are covered by a described in 3.0, rehabilitations
cast-in-place topping slab, methods are recommended for the
reinforced to provide diaphragm following five categories of techniques
action. In other areas, methods of as appropriate:
joining floor sections vary, and
include use of welded insert plates. • Add new elements,
• Reinforced masonry bearing wall • Enhance existing elements,
buildings
(Similar to tilt-up concrete shear • Improve connections between
wall buildings) elements,
Building are constructed with
reinforced masonry (brick cavity • Reduce demand, and
wall or concrete masonry unit)
perimeter walls with a wood or • Removal deficient elements.
metal deck flexible diaphragm.
6.0 REHABILITATION TECHNIQUES
• Reinforced masonry bearing wall NOT RELATED TO SPECIFIC
buildings BUILDING TYPES
(Similar to unreinforced masonry
bearing wall buildings) Seismic rehabilitation techniques (1) not
Buildings are multistory, and necessarily related to a specific building
typically has interior concrete type such as those related to diaphragms,
masonry unit walls and shorter foundations, and nonstructural components,
diaphragm spans. and (2) significant global techniques that
could be applied to any building, such as
• Reinforced masonry bearing wall seismic isolation or addition of damping are
buildings included in this category.
(Similar to concrete shear wall
buildings with bearing walls) 7.0 CONCLUDING REMARKS
Building consist of reinforced
masonry walls and concrete slab A handbook of techniques for seismic
rehabilitation of existing buildings is being
floors that may be either cast-in-
developed by the Interagency Committee on
place or precast. This building type
6
Seismic Safety in Construction. This
handbook will be a companion publication to
related publication on the seismic evaluation
of existing buildings published by the Federal
Emergency Management Agency, namely
FEMA 310 (the NEHRP seismic evaluation
handbook) and FEMA 356 (the NEHRP
seismic rehabilitation handbook).
At the present time, there is a variety of
approaches to seismic rehabilitation, each with
specific merits and limitations. This handbook
will serve as a guidance document for Federal
government agencies involved in seismic
rehabilitation. This handbook will provide:
● A general understanding of the common
deficiencies in the structural and
nonstructural elements that cause seismic
performance problems, and
● Recommended techniques that might be
used to correct deficiencies for various
building types.
8.0 REFERENCES
FEMA 1998, FEMA 310 Handbook for the
Seismic Evaluation of Buildings – A
Prestandard, Federal Emergency
Management Agency, Washington, D.C.
January.
FEMA 2000, FEMA 356 Prestandard and
Commentary for the Seismic Rehabilitation
of Buildings, Federal Emergency
Management Agency, Washington, D.C.
November.
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