Seismic retrofit for rcc structureslocal global consequences

Seismic retrofit schemes for RC
structures and local–global
consequences
G E Thermou1
and A S Elnashai2
1
Demokritus University of Thrace, Xanthi, Greece
2
University of Illinois at Urbana-Champaign, IL, USA
Summary
A review of repair schemes for reinforced
concrete frame buildings is presented in this
paper, within the context of global objectives of
the intervention process. Local as well as global
intervention measures are discussed and their
technological application details outlined. The
effect of the reviewed repair schemes on
the member, sub-assemblage and system
performance are qualitatively assessed. The
important role of the foundation system in the
rehabilitation process is outlined and measures
that are consistent with the super-structure
intervention methods are given. The paper
concludes with a global assessment of the effect of
repair methods on stiffness, strength and
ductility, the three most important seismic
response parameters, to assist researchers and
practitioners in decision-making to satisfy their
respective intervention objectives. The
framework for the paper complies with the
requirements of consequence-based Engineering,
where the expected damage is addressed only
when consequences are higher than acceptable
consequences, and a cyclical process of
assessment and re-assessment is undertaken until
the community objectives are deemed to be
satisfied.
Key words: retrofit; repair/strengthening; rehabilitation; structural intervention; seismic upgrading
Prog. Struct. Engng Mater. 2006; 8:1–15
Published online 19 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pse.208
Introduction
In recent years, devastating earthquakes worldwide
confirmed the deficiencies of building structures. The
experience gained from field observations and back-
analysis led to improvement of the level of knowledge
and the evolution of seismic codes.
The interest of the research community is focused
on buildings that do not comply with current seismic
codes and exhibit deficiencies such as poor detailing,
discontinuous load paths and lack of capacity design
provisions. Since such buildings comprise the
majority of existing building stock, retrofitting is a
rather critical issue. Rehabilitation schemes that will
provide cost-effective and structurally effective
solutions are necessary. Many intervention methods
used in the past have been revised and developed in
the light of the new seismic code requirements and
new methods often based on new materials (e.g. fiber-
reinforced polymers FRPs) have been proposed.
In this paper, the term ‘rehabilitation’ is used as a
comprehensive term to include all types of repair,
retrofitting and strengthening that lead to reduced
earthquake vulnerability. The term ‘repair’ is defined
as reinstatement of the original characteristics of a
damaged section or element and is confined to
dealing with the as-built system. The term
‘strengthening’ is defined as intervention that lead to
enhancement of one or more seismic response
parameters (stiffness, strength, ductility, etc.),
depending on the desired performance.
Framework of seismic rehabilitation
Performance objectives are set depending on the
structural type, the importance of the building, its role
in post-earthquake emergencies, the economic
consequences of business interruption, its historical or
Earthquake Engineering and Structural Dynamics
Copyright & 2005 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2006; 8:1–15

cultural significance, the construction material and
socio-economic factors. They can be specified as limits
on one or more response parameter such as stresses,
strains, displacements, accelerations, etc. Clearly,
different limit states have to be correlated to the level
of the seismic action, i.e. to the earthquake demand
level.
The selection of the rehabilitation scheme and the
level of intervention is a rather complex procedure,
because many factors of different nature come into
play. A decision has to be taken on the level of
intervention. Some common strategies are the
restriction or change of use of the building, partial
demolition and/or mass reduction, addition of new
lateral load resistance system, member replacement,
transformation of non-structural into structural
components and local or global modification
(stiffness, strength and ductility) of elements and
system. In addition, methods such as base isolation,
provision of supplemental damping and
incorporation of passive and active vibration control
devices may apply. The alternatives of ‘no
intervention’ or ‘demolition’ are more likely the
outcomes of the evaluation if the seismic retrofit of
buildings is quite expensive and disruptive.
Socio-economic issues have to be considered in the
decision of the level and type of intervention.
Surprisingly, there are documented cases where
aesthetic and psychological issues dictate the
rehabilitation strategies. For example, in the Mexico
City earthquake of 19 September 1985, where external
bracing was popular, because it instilled a feeling of
confidence in the occupants that significant and
visible changes have been made to the structure to
make it safer[1]. Cost vs importance of the structure is
a significant factor, especially in the case that the
building is of cultural and/or historical interest. The
available workmanship and the level of quality
control define the feasibility of the proposed
intervention approach. The duration of work/
disruption of use and the disruption to occupants
should also be considered. The functionality and
aesthetical compatibility of the intervention scheme
with the existing building is an additional
engagement. Even the reversibility of the scheme in
case it is not accepted on a long-term basis should be
taken into account.
From a technical point of view the selection of the
type and level of intervention have to be based on
compatibility with the existing structural system and
the repair materials and technology available.
Controlled damage to non-structural components and
sufficient capacity of the foundation system are
essential factors that are often overlooked. Issues such
as irregularities of stiffness, strength and ductility
have to be considered in detail.
A convenient way to discuss the engineering issues
of evaluation and retrofit is to break down the process
into steps. The first step involves the collection of
information for the as-built structure. The
configuration of the structural system, reinforcement
detailing, material strengths, foundation system and
the level of damage are recorded. In addition, data
relevant to the non-structural elements (e.g. infill
walls) which play a significant role and influence the
seismic response of structures are also compiled.
Sources for the above information can become visits to
the site, construction drawings, engineering analyses
and interviews with the original contractor. The
rehabilitation objective is selected from various pairs
of performance targets and earthquake hazard levels
(i.e. supply and demand, or response and input pairs).
The performance target is set according to an
acceptable damage level (performance target).
Building performance can be described qualitatively
in terms of the safety of occupants during and after
the event, the cost and feasibility of restoring the
building to pre-earthquake condition, the length of
time the building is removed from service to effect
repairs, and the economic, architectural or historic
impacts on the larger community. Variations in actual
performance could be associated with unknown
geometry and member sizes in existing buildings,
deterioration of materials, incomplete site data, and
variation of ground motion that can occur within a
small area and incomplete knowledge and
simplifications related to modeling and analysis. In
the next phase, the rehabilitation method is selected
starting with the selection of an analysis procedure.
The development of a preliminary rehabilitation
scheme follows (using one or more rehabilitation
strategies) the analysis of the building (including
rehabilitation measures), and the evaluation of the
analysis results. Further, the performance and
verification of the rehabilitation design are conducted.
The rehabilitation design is verified to meet the
requirements through an analysis of the building,
including rehabilitation measures. A separate
analytical evaluation is performed for each
combination of building performance and seismic
hazard specified in the selected rehabilitation
objective. If the rehabilitation design fails to comply
with the acceptance criteria for the selected objective,
the interventions must be redesigned or an alternative
strategy considered.
Rehabilitation options
LOCAL INTERVENTION METHODS
The local modification of isolated components of the
structural and non-structural system aims to increase
the deformation capacity of deficient components so
that they will not reach their limit state as the building
responds at the required level. Local intervention
techniques are applied to a group of members that
suffer from structural deficiencies and a combination
EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS2

of these techniques may be used in order to obtain the
desired behavior for a seismically designed structure.
Injection of cracks
Crack injection is a versatile and economical method
of repairing reinforced concrete (RC) structures. The
effectiveness of the repair process depends on the
ability of the adhesive material (usually epoxies) to
penetrate, under appropriate pressure, into the fine
cracks of the damaged concrete. Flexural cracks and
shear cracks are mainly continuous and therefore
provide unobstructed passages for the epoxy. On the
other hand, longitudinal cracks, which develop along
reinforcing bars as a result of bond failure, are usually
discontinuous and narrow. Difficulties may occur in
repairing the steel-to-concrete bond by epoxy
injection.
This repair method can be used in minor
(50.1 mm), medium (53 mm) size cracks, and large
crack widths (up to 5–6 mm). In case of larger cracks,
up to 20 mm wide, cement grout, as opposed to epoxy
compounds, is the appropriate material for injection
(Fig. 1). In the first step of the application process,
loose material is removed. For the more usual case of
epoxy injection, the surface trace of cracks is fully
sealed with epoxy paste, leaving only surface-
mounted plastic nozzles for injection. The spacing of
nozzles along the crack should be dictated by the
distance epoxy can travel prior to hardening (this
distance depends on crack width and on the viscosity
of the epoxy at the application temperature). In
members with dimensions larger than hardening
distance, ports at both surfaces should be provided
along penetrating cracks. Injection is deemed
complete for a portion of the crack when epoxy is
expelled from the next higher nozzle. Once the repair
epoxy has set, the nozzles are bent and tied firmly.
They can be cut flush and sealed with an epoxy-
patching compound prior to rendering of the affected
member.
Flexural tests on RC beams and beam–column joints
show that the repair process not only eliminates the
unsightly appearance of wide cracks, but also restores
the flexural strength and stiffness of the damaged
member[2,3]. Push-off tests (both static and dynamic)
further indicate that concrete-to-concrete joints can
regain their shear strength after being repaired by
epoxy resin injection.
Shotcrete (Gunite)
Shotcrete is used as a repair method for RC and
masonry structures. There are two distinct types of
shotcrete; dry-mix and wet-mix. Shotcrete can be
applied to almost any surface; it can also be used in
combination with other retrofit schemes (e.g. RC
jacket). Because of its generally low water–cement
ratio and high-velocity impact, it achieves excellent
bond to most competent surfaces. Deficiencies in
shotcrete applicability usually fall into one of the
following five categories[4]: (i) failure to bond to the
receiving surface, (ii) de-lamination at construction
joints or interfaces of various application layers, (iii)
incomplete filling of the material behind the
reinforcing steel, (iv) slough due to excess mixing
water (which can generate voids) and (v) weak
interface between the concrete and steel. The impact
velocity of the material to the application surface is
dependent upon both the exit velocity and the
distance of the nozzle from the surface. Where bond is
important, equipment must be at the proper impact
angle of about 908 and reasonably close to the
application surface. Further, the surface must be clean,
sound and damp. When the shotcrete strikes the
application surface (or other hard objects such as
reinforcing steel), some of the larger and harder
aggregate particles tend to ricochet. These particles
are referred to as rebound and are composed
primarily of the larger aggregate particles, although
some cement and water are included. Because of the
nature of its composition, rebound is not capable of
obtaining significant strength and should not be
allowed in the final work. Many factors affect the
amount of rebound such as: (i) orientation of the
receiving surface, (ii) shotcrete mix design,
(iii) amount of reinforcing steel embedment,
Fig. 1 Application of the: (a) epoxy resin; (b) cement grout injection in beam–column joints
SEISMIC RETROFIT SCHEMES 3

(iv) thickness of the cross-section, (v) impact velocity,
and (vi) spraying technique.
Steel plate adhesion
Steel plate adhesion is mainly used in the case of
beams. Both shear and flexural strength enhancement
can be achieved. When thick steel plates are needed, it
is advisable to use several thin layers instead, to
minimize interfacial shear stresses. A sound
understanding of both the short- and long-term
behavior of the adhesive used is required. In addition,
reliable information concerning the adhesion to
concrete and steel is required. The execution of the
bonding work is also of great importance to achieve a
composite action between the adherents. Prevention
of premature de-bonding or peeling of externally
bonded plates is a most critical aspect of design[5–7].
Steel jacketing
The steel jacketing option involves the total
encasement of the column with thin steel plates
placed at a small distance from the column surface,
with the ensuing gap filled with non-shrink grout[8,9].
An alternative to a complete jacket (exemplified in
Fig. 2b,c) is the steel cage alternative[10,11]. Steel angles
are placed at the corners of the existing cross-section
and either transversal straps or continuous steel plates
are welded on them. In practice, the straps are often
laterally stressed either by special wrenches or by
preheating to temperatures of about 200–4008C, prior
to welding. Any spaces between the steel cage and the
existing concrete are usually filled with non-shrink
grout. When corrosion or fire protection is required, a
grout concrete or shotcrete cover may be provided.
The corrugated steel jacketing technique can be
applied for the rehabilitation of columns and beam–
column joints[12]. Deficient connections are encased by
the steel jacket and the gap between the concrete and
the steel jacket is filled with non-shrink grout. A gap
is provided between the beam jacket and the column
face to minimize flexural strength enhancement of the
beam; which may cause excessive forces to develop in
the joint and column.
Externally bonded FRPs
The ease of application of FRP composites renders
them attractive for use in structural applications;
especially in cases where dead weight, space or time
restrictions exist. Although FRP composites can have
strength levels significantly higher than those of steel
and can be formed of constituents such as carbon
(CFRP), glass (GFRP), and aramid (AFRP) fibers, it is
important to note that its use is often dictated by
strain limitations[13] (Fig. 3a). They are very sensitive
to transverse actions (i.e. corner or discontinuity
effects) and unable to transfer local shear (i.e.
interfacial failure). Clearly, they carry no compressive
forces. Choosing the type of fibers, their orientation,
their thickness and the number of plies, results in a
great flexibility in selecting the appropriate retrofit
scheme that allows to target the strength hierarchy at
both local (i.e. upgrade of single elements) and global
(i.e. achievement of a desired global mechanism)
levels. In general, FRP composites behave in a linear
elastic fashion to failure without any significant
yielding or plastic deformation. Additionally, it
should be noted that unlike reinforcing steel, some
fibers (such as carbon fibers) are anisotropic. This
anisotropy is also reflected in the coefficient of
A A A A A A
(a)
h
(b)
h
(c)
h
Fig. 2 (a) Steel jacketing; (b) steel cage technique using steel straps or (c) steel plates

thermal expansion in the longitudinal and transverse
directions. The large differences in strength
(transverse strength 5 longitudinal strength) and
coefficients of thermal expansion can result in bond
deterioration and splitting of concrete. Moreover,
these can cause lateral stresses and low cycle fatigue
under repeated thermal cycling[15].
The effectiveness of strengthening depends on the
bond conditions, the available anchorage length and/
or the type of attachment at the FRP ends, the
thickness of the laminates, among other less
important factors. According to experimental data,
failure of the FRP reinforcement may occur either by
peeling off (de-bonding) through the concrete near the
concrete–FRP interface or by tensile fracture at a stress
which may be lower than the tensile strength of the
composite material, because of strength
concentrations (e.g. at rounded corners or at de-
bonded areas). In many cases, the actual failure
mechanism is a combination of FRP de-bonding at
certain areas and fracture at others. The choice of
constituents and details of the process used to
fabricate the composite significantly affect
environmental durability. Exposure to a variety of
environmental conditions can dramatically change
failure modes of the composites, even in cases where
performance levels remain unchanged. In other cases,
exposures can result in the weakening of the interface
between FRP composites and concrete, causing a
change in failure mechanism and sometimes a
dramatic change in performance.
In the case of columns, shear failure, confinement
failure of the flexural plastic hinge region and lap
splice de-bonding can be accommodated by the use of
FRPs[16–18]. At this juncture it is important to stress
that none of these failure modes and associated
retrofits should be viewed separately, since
retrofitting for one deficiency may only shift the
problem to another location and/or failure mode
without necessarily improving the overall
performance. For example, a shear-critical column,
strengthened over the column center region with
carbon wraps, is expected to develop flexural plastic
hinges at column ends which, in turn, need to be
retrofitted for the desired confinement levels.
Furthermore, lap splice regions need not only to be
checked for the required clamping force to develop
the capacity of the longitudinal column
reinforcement, but also for confinement and ductility
of flexural plastic hinge[17]. Shear and flexural
strengthening of beams can be achieved by the
application of either epoxy-bonded laminates or
fabrics extending in the compression zone or epoxy-
bonded FRP fabric wrapped around the beam[19–22]. In
the case of beam–column joints, the jacket is designed
to replace missing transverse reinforcement in the
beam–column joint[23–28]. The FRP technique can be
also used for strengthening walls[29].
Selective intervention methods
Where system-optimal performances dictate
selectively modifying specific response parameters to
pre-defined levels, procedures for affecting single
parameters with no effect on others are called for. The
initial development of ‘selective intervention’
techniques, proposed by Elnashai[30] was first applied
to structural walls under static loading[31]. Further
studies applied the techniques to shaking table-tested
walls[32], and culminated with application to a full-
scale four story RC building[33]. The fundamental
parameters governing structural responses to
transverse actions in the inelastic range are: stiffness,
strength and ductility. Consequently, selective
intervention techniques are referred to as stiffness-,
strength-, and ductility-only.
Stiffness-only intervention approaches may be used
in order to accommodate problems related to irregular
distribution of stiffness or to significant reduction of
stiffness due to cracking of concrete members. In the
latter case, if concrete crushing and buckling of
reinforcement bars do not occur the flexural strength
of the members will not necessarily be adversely
affected.
Altering the sequence of plastic hinge formation to
achieve a predetermined failure mode becomes an
essential objective for seismic safety. This requires an
ε
0.02 0.04
Mild steel
2
6
4
GFRP
AFRP
CFRP
σ (GPa)
(a) (b) (c)
Fig. 3 (a) Material properties[14]; application modes of (b) prefabricated shells; (c) FRP sheets

increase in strength of strategically located members.
Only a selective strength-only intervention can be
effective in addressing such deficiency.
Problems with lack of ductility supply may be
confronted by the application of ductility-only
intervention methods. Lot of effort has been put
together towards the investigation of alternative
ductility-only retrofit schemes. Aboutaha et al.[34]
investigated the effectiveness of rectangular steel
jackets for improving the ductility and strength of
columns with inadequate lap splice in the
longitudinal reinforcement. Several types of steel
jackets were investigated, including rectangular solid
steel jackets with and without adhesive anchor bolts.
A similar set of experiments was conducted by Aviles
et al.[35]. The models were deficient in the level of
concrete confinement at foundation level and thus
retrofitted with steel plate wrapping combined with
anchor bolts. Saadatmanesh et al.[36] carried out
experimental work on the application of high-strength
FRP composite straps to retrofit bridge columns.
Ghobarah et al.[37] investigated the effectiveness of
corrugated steel jacketing for the seismic upgrading of
RC columns.
GLOBAL INTERVENTION TECHNIQUES
In case of systems with high flexibility or when no
uninterrupted transverse load path is available then
global intervention techniques are considered. The
most well known global retrofit schemes are
presented hereafter.
RC jacketing
RC jacketing is one of the most commonly applied
methods for the rehabilitation of concrete members.
Jacketing is considered to be a global intervention
method if the longitudinal reinforcement placed in the
jacket passes through holes drilled in the slab and
new concrete is placed in the beam–column joint
(Fig. 4). However, if the longitudinal reinforcement
stops at the floor level then RC jacketing is considered
as a member intervention technique. The main
advantage of the RC jacketing technique is the fact
that the lateral load capacity is uniformly distributed
throughout the structure of the building thereby
avoiding concentrations of lateral load resistance,
which occur when only a few shear walls are
added[38]. A disadvantage of the method is the
presence of beams which may require most of the new
longitudinal bars in the jacket to be bundled into the
corners of the jacket. Because of the presence of the
existing column, it is difficult to provide cross ties for
the new longitudinal bars, which are not at the
corners of the jacket.
To date, apart from qualitative guidelines provided
in some Codes, no specific design rules exist for
dimensioning and detailing of the jackets to reach a
predefined performance target. The uncertainty with
regard to bond between the jacket and the original
member is another disadvantage. Of the many factors
influencing jacket performance, slip and shear-stress
transfer at the interface between the outside jacket
layer and the original member that serves as the core
of the upgraded element are overriding
considerations[39].
The effectiveness of the method has been studied by
many researchers and supported by experimental
work[38,40–42]. In cases where building are in close
proximity to one another, the method is modified and
one-, two- or three-sided jacketing applies[43,44].
Addition of walls
Addition of new RC walls is one of the most common
methods used for strengthening of existing structures.
This method is efficient in controlling global lateral
drift, thus reducing damage in frame members.
During the design process, attention must be paid to
the distribution of the walls in plan and elevation (to
achieve a regular building configuration), transfer of
inertial forces to the walls through floor diaphragms,
struts and collectors, integration and connection of the
Fig. 4 Reinforced concrete jacketing technique

wall into the existing frame buildings and transfer of
loads to the foundations. Added walls are typically
designed and detailed as in new structures. To this
end, in the plastic hinge zone at the base they are
provided with boundary elements, well-confined and
detailed for flexural ductility. They are also capacity-
designed in shear throughout their height and over-
designed in flexure above the plastic hinge region
(with respect to the flexural strength in the plastic
hinge zone, not the shear strength anywhere), to
ensure that inelasticity or pre-emptive failure will not
take place elsewhere in the wall before plastic hinging
at the base and that the new wall will remain elastic
above the plastic hinge zone.
The most convenient way to introduce new shear
walls is by partial or full infilling of strategically
selected bays of the existing frame[45]. If the wall takes
up the full width of a bay, then it incorporates the
beams and the two columns, the latter acting as its
boundary elements (Fig. 5). In case only the web of the
new wall needs to be added, sometimes by
shotcreting against a light formwork or a partition
wall is performed. In the latter case, shotcrete is
normally used for increased adhesion between
the existing and the added material. An alternative
to the cast-in-place infill wall technique is the addition
of pre-cast panels. The pre-case infill wall system
should be designed to behave monolithically, and the
infill wall should be designed with sufficient shear
strength to develop flexural yielding at the base of the
wall[46].
A major drawback of the addition of walls is the
need for strengthening the foundations to resist the
increased overturning moment and the need for
integrating the wall with the rest of the structure.
Foundation intervention is usually costly and quite
disruptive, thus rendering the application of this
technique unsuitable for buildings without an existing
adequate foundation system.
External buttresses
To reduce or eliminate the disruption to the use of a
building, external buttresses may be constructed to
increase the lateral resistance of the structure as a
whole. Such an intervention scheme, in common with
the construction of RC walls, requires a new
foundation system. The foundation scheme would
possibly be eccentric footings (eccentric with respect
to the axis of the buttress to avoid excavation under
the building). The two most intricate problems in
strengthening by building a set of external buttresses
are: (i) the buttress stability may be critical since it is
not actually loaded vertically downwards in the same
way that the structure is. The vertical action on the
buttress is only its own weight. This increases the
possibility of uplifting of the foundations and may
even cause over-turning, (ii) the connections between
the buttresses on the one hand and the building on the
other is far from straightforward. To insure full
interaction and load sharing when the structure is
subjected to lateral actions, the buttress should be
connected to the floors and columns at all levels. The
connection area will be subjected to unusual levels of
stresses that require special attention.
Steel bracing
Steel bracing can be a very effective method for global
strengthening of buildings. Some of the advantages
are the ability to accommodate openings, the minimal
added weight to the structure and in the case of
external steel systems minimum disruption to the
function of the building and its occupants.
Alternative configurations of bracing systems may
be used in selected bays of a RC frame to provide a
significant increase in horizontal capacity of the
structure. Concentric steel bracing systems have been
investigated for the rehabilitation of non-ductile
buildings by many researchers[47–50]. Using the
eccentric steel bracing in the rehabilitation of RC
Fig. 5 Cast-in-place infill walls

structures has lagged behind concentric steel bracing
applications due to the lack of sufficient research and
information about the design, modeling and behavior
of the combined concrete and steel system. Further
research is needed in several areas such as testing of
the RC beam–steel link connection details and design
as well as the development and implementation of
link elements models in analysis software[51]. Post-
tensioned steel bracing can be used for the seismic
upgrading of infilled non-ductile buildings limited
to low-rise and squat medium-rise buildings[52].
The method was successfully used by
Miranda & Bertero[53] to effectively upgrade
the response of low-rise school buildings in
Mexico.
Base isolation
Seismic isolation is mostly adopted for rehabilitation
of critical or essential facilities, buildings with
expensive and valuable contents and structures
where performance well above performance levels is
required. Seismic isolation system significantly
reduces the seismic impact on the building
structure and assemblies. Generally, the isolation
devices are inserted at the bottom or at the top of
the first floor columns. Retrofitting mostly requires
traditional intervention; in the first case the
addition of a floor in order to connect all the columns
above the isolators while in the second case the
strengthening of the first floor columns (enlarging of
the cross-sections, addition of reinforcing bars or
construction of new resistant elements). Nevertheless,
inserting an isolator within an existing column is not
so simple because of the necessity of cutting the
element, temporarily supporting the weight of the
above structure, putting in place the isolators and
then giving back the load to the column, without
causing damages to persons and to structural and
non-structural elements.
Recently, efforts have been made to extend this
valuable earthquake resistant strategy to inexpensive
housing and public buildings[54]. The results of a joint
research program conducted by the International
Rubber Research and the Development Board
(IRRDB) of United Kingdom show that the method
can be both cost effective and functional for the
protection of small buildings in high seismicity
regions. A comparative study conducted by Bruno &
Valente[55] on conventional and innovative seismic
protection strategies concluded that base isolation
provides higher degrees of safety than energy
dissipation does, regardless of the type of devices
employed. Moreover the comparison between
conventional and innovative devices showed that
shape memory alloys-based devices are far more
effective than rubber isolators in reducing seismic
vibrations.
Effect of retrofit on global response
Development of a complete strategy guiding the
retrofit solution through established objectives or
criteria is an ongoing effort of the earthquake
engineering research community. In general, seismic
rehabilitation may aim to either recover or upgrade
the original performance or reduce the seismic
response[56]. In the first case, the retrofit schemes that
will be chosen have to reinstate the structural
characteristics at member level and have negligible
impact on the global response. The crack injection
(epoxy resin injection or grout injection) technique
and the member replacement (substitute part of the
damaged member) may apply.
When the seismic demand is to be reduced, this can
be achieved by adopting base isolation techniques or
by providing the structure with supplemental
dissipation devices. Reducing the masses at each story
level accommodating irregularities in the mass
distribution along the height of the building is an
effective way of reducing seismic demand. In many
cases (in areas of rapid economic and industrial
development) the functionality of residential
buildings is changed and they are used for either
storage or installation of heavy industrial equipment.
Due to the discontinuity in mass distribution the
particular floors are susceptible to failure. Moreover,
the total or partial demolition of the top stories of
structures can result in the reduction of the period so
as to comply with the seismic demand.
In the case of the seismic upgrading, the aim of the
retrofit strategy as an operational framework is to
balance supply and demand. The supply refers to the
capacity of the structural system, which has to be
assessed in detail before selecting the intervention
scheme. The demand is expressed by either a code
design spectrum or a site-specific set of records as a
function of period and shape of vibration
characteristics of the upgraded system. By modifying
strength, stiffness or ductility of the system alternative
retrofit options are obtained, as shown in Fig. 6.
Ductility enhancement applies to systems with poor
detailing (sparse shear reinforcement, insufficient lap
splicing), stiffness and strength enhancement to
systems with inherently low deformation capacity (so
as to reduce displacement demand), whereas stiffness,
strength and ductility enhancement apply to systems
with low capacity or where seismic demand is high[57].
An effective retrofit scheme for dealing with
ductility deficiencies of the structural system is FRP
jacketing. Assuming that the as-built system has been
designed according to the strong-column weak-beam
mechanism approach, FRP jacketing of the vertical
elements provides additional confinement of the
existing columns. The effectiveness of the method
depends on reassuring that slip of longitudinal bars of
the existing column will not occur and to the bond
conditions between the existing member and the new

material. The behavior of the retrofitted structure is
represented herein, for demonstration purposes, by
the behavior of the retrofitted 2-story, 2-bay RC frame
shown in Fig. 7a. The span length of the frame is 5 m,
while the story height is 2.7 m. The columns have
dimensions 0.40 Â 0.40 m, longitudinal reinforcement
ratio rl ¼ 0:77% and confinement reinforcement
volumetric ratio rsw ¼ 0:22% (#6/0.15 m). The
material strengths of the existing structure are C16
and S300. Using FRP jacketing in order to increase the
confinement factor to a value of, K ¼ 2, and by
performing pushover analysis by ZEUS-NL[58] the top
displacement at ultimate is increased by 122%. If the
seismic upgrading targets the modification of
stiffness, strength and ductility levels, RC jacketing
can be chosen as a retrofit solution. The response of
the retrofitted structure depends on the characteristics
of the jacket such as longitudinal reinforcement,
confinement reinforcement and material strengths. In
this case the effectiveness of the solution scheme
depends on the continuity between the existing and
the new material and the effectiveness of anchorage of
the additional reinforcement of the jacket. The
response of the retrofitted frame is shown in Fig. 7b
for two alternative jacket configurations J1 and J2,
respectively. In both cases the jacket dimensions are
0.50 Â 0.50 m, the material strength characteristics C20
and S400, but in the first case ( J1) the total longitudinal
reinforcement ratio of the jacketed cross-section is rlj
¼ 0:85% and confinement reinforcement volumetric
ratio rswj ¼ 0:93% (#10/0.075 m), while in the second
(J2) the total longitudinal reinforcement ratio of the
jacketed cross-section is rlj ¼ 1:31% and confinement
reinforcement volumetric ratio rswj ¼ 1:40%
(#10/0.050 m). The first jacket configuration ( J1)
increases the strength level (maximum base shear) by
55%, while the second ( J2) by 89% (Fig. 7b). In both
cases the ductility level is increased dramatically.
The response modification of the existing structural
system may be achieved by adopting a combination of
the pre-described local and global intervention
techniques. The strategic use of the retrofit schemes
can accommodate all deficiencies observed at local
and/or global level and result in a cost- and time-
effective solution.
SYSTEM-LEVEL DEFICIENCIES
System-level deficiencies such as eccentricities of
stiffness (or strength) and mass in both plan and
elevation are common in existing structures. This class
of deficiency is a consequence of old construction
practices (poor level of confinement details, negligible
material-property control). Due to lack of specific
guidelines most retrofit strategies adopted in practice
are based mainly on experience and in few cases on
simple analysis (with the exception of major
structures in high seismicity regions, such as
California and parts of Japan). Recent earthquakes
have demonstrated that the rehabilitation measures
taken in the past failed to meet the retrofit
performance objectives. In many cases, misuse of the
retrofit solution schemes was observed. A major issue
seems to be the difficulty in understanding the
interaction between the retrofit scheme and the
existing structural system. A sound understanding of
the response of the existing structural system and a
clear definition of the performance objectives of the
0
100
200
300
400
500
Original-Frame
Retrofitted-J1
Retrofitted-J2
0
50
100
150
200
250
0 100 200 300 400 500
Top Displacement (mm)
0 100 200 300 400 500
Top Displacement (mm)
Baseshear(kN)
Baseshear(kN)
Original-Frame
Retrofitted-FRPs
(a) (b)
Fig. 7 (a) Ductility enhancement}FRP jackets; (b) stiffness, strength and ductility enhancement}RC jackets
Ductility
enhancement
∆u
Roof Displacement Roof Displacement Roof Displacement
Base
Shear
existing
structure
rehabilitated
structure
(a) (c)
Base
Shear
existing
structure
Stiffness, strength &
ductility enhancement
rehabilitated
structure
∆u
∆V∆V
Base
Shear
existing
structure
Stiffness & strength
enhancement
rehabilitated
structure
(b)
Fig. 6 Alternative retrofit strategies for seismic upgrading[57]

retrofit strategy are necessary before embarking on
the design of the retrofit solution.
Vertical irregularities (irregularities along the
vertical axis) are due to either irregular distribution of
mass or stiffness along the height of the building. As
mentioned above, buildings may be used for a
different purpose from their original intended
function. The concentration of mass at a particular
story attracts higher seismic forces and results in the
creation of a soft story.
Vertical irregularities may also be due to irregular
stiffness distribution. A special case is the soft-story
mechanism. A common structural configuration
(typical of the construction practice in Southern
Europe) susceptible to a soft-story failure mechanism
is the pilotis frame. The ground story used for
commercial facilities is an open frame (bare frame),
while the stories above are infilled. Under lateral
loading, the ground-story columns have to resist the
large base shear which leads to large story drift
concentrated in the first story. The large demand
increases progressively due to second-order effects,
often leading to the collapse of the structure in a soft-
story mechanism.
Observation of practical application has shown that
there is lack of clarity with regard to the way soft-
story mechanism is treated. Increasing the stiffness of
the ground level only to reach the stiffness of the
infilled floor above is not the correct approach, since
the stiffness of the floor above depends on the
strength of the masonry infills. In a future earthquake,
as soon as the masonry infills start cracking, or even
shed out-of-plane, the localization of damage is
transferred to the story above. The retrofit strategy
should aim to develop a uniform distribution of
stiffness along the height of the building. RC jacketing
and the addition of RC walls can be effective retrofit
solutions provided they are applied to achieve a target
displaced shape.
Horizontal irregularities (irregularities in the plan
of the structure) are due to the eccentricity between
the centers of mass and stiffness. The uneven
distribution of stiffness may be the result of
architectural (e.g. L-shaped buildings) or functional
(e.g. facade of commercial buildings) features. The
position of the elevator shaft walls plays an important
role in the distribution of stiffness in plan. Walls and
columns have to be placed in strategic positions in
order to accommodate irregularities. The retrofit
strategy should aim to balance the stiffness or
mass irregularities in plan. The addition of new
elements (e.g. RC walls, external buttresses)
may be used to advantage in addressing plan
irregularities.
The effect of the various intervention schemes at
local and global level and some useful comments with
regard to the effectiveness of the method and
parameters that should be taken into account in the
design phase are presented in the appendix (Table A1).
ROLE OF FOUNDATION SYSTEM
Seismic upgrading of the super-structure has a direct
effect on the demand imposed on the existing
foundation system. Structural requirements may
dictate considerable strength enhancement in
locations that are connected directly to the
foundations. Capacity design principles immediately
dictate that foundation strengthening is needed.
Moreover, parameters such as soil conditions
and soil–structure interaction play an important
role in foundation-strengthening
projects.
Old buildings mainly supported by isolated
footings and in fewer cases by combined footings are
weak or flexible compared to the current seismic
design philosophy. In the majority of cases, the
foundation system along with the rest of the structure
are representative of construction practices adopted in
the past and may be susceptible to a number of
different modes of brittle failure.
Retrofit strategies may aim at either strengthening
the existing foundation system and/or adding
supplemental foundation elements (footings or piles).
Larger spread footings can distribute the load and
additional reinforcement can increase their shear and
bending resistance. The incorporation of existing
footings into grade beams or mats, which can spread
load over a larger soil area and activate the gravity
loads in other columns in the resistance of the
overturning moments and uplift forces, is another
option. Projects involving the addition of grade beams
or increased size of spread footings usually require
excavation under difficult circumstances and there are
difficulties in pinning or attaching the existing
footings to the new elements[59]. Moreover, piles may
be added to improve the overturning resistance.
Adding piles along the perimeter of the building can
be an easier task from an economical and
constructional point of view compared to the case
where piles are added under the interior of
the buildings.
The selection of the RC jacketing as the retrofit
solution for the super-structure results in a uniform
distribution of stiffness. The retrofit of the foundation
system can be relatively easily accommodated by
extending the jacketing to the foundation level
(Fig. 8). On the other hand, the addition of new
elements (e.g. RC walls, external bracings) may add
strength and stiffness to the building at critical
locations. In these cases, greater demands on the
foundation system are placed. The shear transmitted
between the soil and the strengthened structure may
be higher because of the increased strength and
stiffness of the structure[59]. Stiff structural
components generate large bending moments at the
base. Large overturning movements may cause large
dynamic axial forces to develop in the columns
of braced frames or at the boundary elements of
shear walls.

A foundation system that allows the development
of hinges in the super-structure is vital for the stability
of structural and non-structural components. Seismic
upgrading of foundations is usually a disruptive
process. The cost varies depending on the type and
the level of intervention. In cases where piles have to
be installed in the existing system the cost may
dominate the total seismic retrofit project.
Conclusions
Numerous retrofit schemes adopted in practice for the
seismic upgrading of old and substandard reinforced
concrete buildings are presented. A multiplicity of
factors influence the selection of the retrofit solution
and therefore no general rules apply. To aid in the
selection, the effectiveness of the retrofit schemes and
their interaction at local and global level is explored.
The main system-level deficiencies (vertical and
horizontal irregularities) are presented and related
modeling issues are clarified. The impact of
strengthening of the super-structure on the
foundation system and the alternative retrofit options
for the foundation system are discussed. The paper
concludes with a table summary of the retrofit
options, motivation for use, local and global effects,
technological and design requirements, intended to
provide a quicklook guide to potential users.
Appendix A
The summary of the effect of retrofit on local and
global response is shown in Table A1.
Fig. 8 Strengthening of footings}RC jacketing

.............................................................................................................................................................................................................................
TableA1Summaryoftheeffectofretrofitonlocalandglobalresponse
MethodDeficiencytypeLocaleffectGlobaleffectTechnologyconsiderationsDesignconsiderations
InjectionofcracksShearorshear-flexural
cracks
Flexuralstrengthandstiffness
restoration.Shearstrengthis
regainedinconcrete-to-
concretejoints
Repairmethod}nomodification
oftheresponseofthe
originalstructure
Thequalityandtheenviron-
mentaldurabilityofthe
materialsused
playanimportantrole.The
adhesivematerialshould
penetrateintothefinecracksof
thedamagedconcreteandinfill
allthevoids
Reductionfactorsforconcrete
strengthmaybeusedtotake
intoaccountanyuncertainty
regardingtheeffectivenessof
themethodandqualityofmate-
rials
Shotcrete(Gunite)Extensivecrackpatterns
atconcretemembersor
masonry;converting
non-structuraltostructural
walls
Reinstatementoftheoriginal
characteristicsoftheelement
forrepair;increaseinforce
demandifappliedasa
retrofittingoption
Minimumeffectwhenapplied
asarepairmethodiflayer
isverythinkandwithwire
meshonly.Completechange
ofresponsewhenapplied
otherwise
Judiciousattentiontosurface
cleanliness.Mixdesignis
critical.Experiencedpersonnel
arenecessary
Theappliedlayerofconcrete
providesadequatestrength.Itis
usedoftenincombinationwith
otherretrofitschemes(e.g.RC
jacketing).Amountofreinforce-
mentandthicknessoflayer
dictateslocalandglobaleffects
Steeljacketing}plate
adhesion
Insufficientshearstrength
andductilityduetooldtype
ofdetailing(sparse
confinementreinforcement,
insufficientlapsplicing)
Jacketing:Deformation
capacityisincreased
Plateadhesion:Shearand
flexuralstrengthenhancement
Deformationcapacityis
enhanced.Strengthcapacity
maybeincreasedorremain
thesamedependingonthe
effectoftheretrofitscheme
atlocallevel
Theeffectivenessofthe
methodisrelatedtothe
typeofgroutsusedfor
infillingthegapbetween
thesteeljacketandthe
existingmember.Thebonding
workisofgreatimportance
toachieveacompositeaction
betweentheadherents
Beforedecidingforsteeljacket-
ingprematurefailuredueto
othermechanisms(e.g.pull-out
ofthelongitudinalreinforcement
oftheexistingmember)should
beprevailed.Steeljacketshould
beconsideredasadditionalcon-
finementreinforcement,while
steelplatesadheredatthebot-
tomflangeofbeamsasadditional
bottomreinforcement
FRPjacketingInsufficientshearstrength
andductilityduetooldtype
ofdetailing(sparse
confinementreinforcement,
insufficientlapsplicing)
Columns:Deformationcapacity
isenhanced
Beams:Shearandflexural
strengthening
Beam–columnjoints:Shearfailure
iseliminatedinconnections
Ductilityandshearstrength
atstructurallevelare
improved
Exposuretoavarietyof
environmentalconditionscan
dramaticallychangefailure
modesofthecomposites,
evenincaseswhere
performancelevelsremain
unchanged.Highquality
controlisrequired.The
bondingworkisveryimportant
Theeffectivenessdependson
theanchorageconditionsofthe
longitudinalreinforcementofthe
existingmember.Limitationsdue
tostressconcentrationsshould
beconsideredinthedesign
phase.FRPlayersareequivalent
toadditionalconfinement
Selectiveintervention
methods
Thedamagepatternvaries
dependingonthedeficient
parameter
Increaseofstiffness,
strengthorductility
Structuralresponsecanbe
tunedtomeetthe
performanceobjectives
Experiencedpersonnelare
requiredintheexecutionphase
Refinedmodelingisrequiredin
ordertotakeintoaccountthe
increaseofthespecificpara-
meter.Specializeddesignexpres-
sionsnecessary
RCjacketingInsufficientlateralstrength,
insufficientdeformation
Ifthejacketisappliedatfloor
level,bothaxialandshear
Ifthejacketcontinues
betweensuccessivefloors,
Theuncertaintywithregard
tobondbetweenthejacket
Theresponseismodifiedto
strong-columnweak-beam

.............................................................................................................................................................................................................................
capacityandstiffness
discontinuitybetween
successivefloors
strengthofthecolumn
areimproved,while
flexuralstrengthandstrength
ofthebeam–columnjoints
remainthesame
stiffness,strengthand
ductilityareenhanced
andtheoriginalmemberis
accommodatedbytheuseof
monolithicfactorsforthe
estimationofthedeformation
andstrengthcapacityofthe
compositemember
mechanismwithdistinctplastic
hingeregions.Theseismicde-
mandisincreasedduetoshiftof
theperiod.Uniformdistribution
ofstrengthanddeformation
capacityisattained.Extension
ofjacketingtofoundationlevel
maybenecessary
Additionof
wallsorexternal
buttresses
Insufficientlateralstiffness
andstrength,torsionally
unbalancedstructures
Deformationdemandat
memberlevelisdecreased,
whilestrengthdemand
maybeincreased.Highdemand
atconnectionbetween
existingstructureand
wallsorbuttressesisgenerated
Globallateraldriftsare
controlled.Considerable
strengthandstiffnessare
addedtotheexisting
structuralsystem.Resulting
systemistotallydifferent
fromtheoriginalstructure
requiringfullreassessment
Inthecaseofinfillwalls,the
interfacebetweentheexisting
andthenewelementshould
bechecked.Anew
foundationsystemis
necessaryifwalls(usually
G-shapedintheperimeter
ofthebuilding)orbuttresses
areadded.Inanycase,the
existingfoundationsystem
needstobestrengthenedto
resisttheincreased
overturningmomentandthe
largerweightofthestructure
Acriticalaspectinthedesign
phaseistoinsurefullinteraction
andloadsharingbetweenthe
existingstructuralsystemand
thenewone(infill,externalwalls
orbuttresses).Connectors
shouldbeplacedatfloorlevel
andbehaveelasticallyforthe
designearthquake.Strengthening
ofexistinghorizontalmembers
mayberequired.Responsemod-
ificationofthesystemfrom
sheartocantilevertypeisat-
tainedwithashiftinperiod.
Strategicdistributionofwalls
mayaccommodateanysystem-
leveldeficiencies
SteelbracingInsufficientlateralstiffness
andstrength
Highlevelsofforcemaybe
introducedatbraceendsand
connectionsbetweenbrace
membersandexistingstructure
Lateralstiffnessandstrength
oftheexistingstructure
areincreased.Additional
energydissipationisprovided
Installationofpost-tensioned
barscansignificantlymodify
thedistributionofinternal
forcesofexistingRC
members.Bracingmembers
shouldbedesignedtobehave
inaductilemanner
Thelateralstrengthoftheexist-
ingmembersmaybeadversely
affectedbythelevelofaxial
forcesinducedbythesteel
braces.Strengtheningofcol-
umns,beamsandbeam–column
jointsofbracedbaysneededfor
theadequateperformanceofthe
bracingsystem.Foundationsys-
temshouldwithstandthein-
creasedstrengthandstiffness
effects
BaseisolationRehabilitationofcriticalor
essentialfacilities
Theseismicimpacton
structuralandnon-structural
componentsisreduced
Theseismicenergyisabsorbed
byisolationdevicesinsertedat
thebottomoratthetopofthe
firstfloorcolumns
Specializedengineering
expertiseisnecessary
Thereisnoneedforretrofitting
theupperpartofthestructure.
Theequipmentshouldbepro-
videdwithcapabilitiestowith-
standtheexpectedlarge
horizontaldisplacementbetween
thefoundationandthesuper-
structure

Acknowledgements
The first author was funded by the European
Research Project SPEAR (Seismic Performance
Assessment and Rehabilitation, Contract Number
G6RD-CT-2001-00525) through Imperial College of
Science, Technology and Medicine, London. The
contribution of the second author was partially
funded by the US National Science Foundation
through the Mid-America Earthquake Center, Award
Number EEC 97-01785.
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Dynamics 2002: 31(5): 1067–1092.
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G E Thermou
Civil Engineer, Reinforced Concrete Laboratory,
Department of Civil Engineering,
Demokritus University of Thrace,
Xanthi 67100, Greece
E-mail: gthermou@civil.duth.gr
A S Elnashai
Willett Professor of Engineering,
Director, Mid-America Earthquake Center,
Department of Civil and Environmental Engineering,
University of Illinois at Urbana-Champaign,
2129e Newmark Civil Engineering Laboratory, MC-250,
205 North Mathews Avenue,
Urbana, IL 61801, USA
E-mail: aelnash@uiuc.edu

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