Study of Seismic Retrofitting Techniques

Abstract

Many parts of the country have suffered earthquake in last three decades. Seismic protection of buildings is a need-based concept aimed to improve the performance of any structure under future earthquakes. Earthquakes of varying magnitude have occurred in the recent past in India, causing extensive damage to life and property. Some recently developed materials and techniques can play vital role in structural repairs, seismic strengthening and retrofitting of existing buildings, whether damaged or undamaged. The primary concern of a structural engineer is to successfully restore the structures as quickly as possible. Selection of right materials, techniques and procedures to be employed for the repair of a given structures have been a major challenges. Innovative techniques of the structural repairs have many advantages over the conventional techniques. The benefits of retrofitting include the reduction in the loss of lives and damage of the essential facilities, and functional continuity of the life line structures. For an existing structure of good condition, the cost of retrofitting tends to be smaller than the replacement cost. Thus, the retrofitting of structures is an essential component of long term disaster mitigation. In present study, global and local retrofitting techniques are discussed. Conventional techniques (Local and global) of retrofitting are compared with modern technique (Fiber Reinforced Polymers).

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Study of Seismic Retrofitting Techniques
Chetan Jaiprakash Chitte
Department of Civil Engineering, R. C. Patel Institute of Technology, Shirpur-425405 Dist. Dhule, Maharashtra,
India,
---------------------------------------------------------------------***----------------------------------------------------------------------
Abstract - Many parts of the country have suffered
earthquake in last three decades. Seismic protection of
buildings is a need-based concept aimed to improve the
performance of any structure under future earthquakes.
Earthquakes of varying magnitude have occurred in the recent
past in India, causing extensive damage to life and property.
Some recently developed materials and techniques can play
vital role in structural repairs, seismic strengthening and
retrofitting of existing buildings, whether damaged or
undamaged. The primary concern of a structural engineer is to
successfully restore the structures as quickly as possible.
Selection of right materials, techniques and procedures to be
employed for the repair of a given structures have been a
major challenges. Innovative techniques of the structural
repairs have many advantages over the conventional
techniques. The benefits of retrofitting include the reduction in
the loss of lives and damage of the essential facilities, and
functional continuity of the life line structures. For an existing
structure of good condition, the cost of retrofitting tends to be
smaller than the replacement cost. Thus, the retrofitting of
structures is an essential component of long term disaster
mitigation. In present study, global and local retrofitting
techniques are discussed. Conventional techniques (Local and
global) of retrofitting are compared with modern technique
(Fiber Reinforced Polymers).
Key Words: Retrofitting, seismic, FRP ,inertia force,
1. INTRODUCTION
Damages caused by recent earthquakes have exposed the
vulnerability of buildings in India. Many of the non-
engineered and semi-engineered constructions lack the basic
features required for seismic resistance. Many of the so-
called ‘engineered’ constructions, such as multi-storeyed
apartments, are not adequately designed, detailed and
constructed to provide the desired resistance against seismic
forces. This may be attributed largely to a lack of awareness
of seismic resistant design and code requirements. In recent
years, particularly after the devastating Gujarat earthquake
in 2001, there has been a concerted effort nation-wide to
provide for increased awareness, in education and practice,
with regard to seismic resistant design of buildings. There is
now a greater awareness and insistence on adherence to
design code requirements, with regard to new buildings,
especially those constructed by major organizations in the
public and private sectors. It is hoped that this healthy
practice becomes mandatory and adopted by all builders.
Mechanisms need to be evolved by local approving bodies
(corporations, municipalities, development authorities) to
ensure that the buildings conform to the National Building
Code. In particular, the structural design has to be proof-
checked by a competent third party for code compliance.
Ordinary people, investing their life savings in buildings such
as apartment complexes, should also insist on this, in their
own interest. All this is possible with buildings to be built in
the future. But, what is to be done about existing buildings?
Many of these will be found to lack compliance with the
current codes of practice, especially in terms of earthquake
resistance. This is partly attributable to the increased
seismic demand and up-gradation of some seismic zones in
the country, as reflected in the recently revised code of
practice for seismic analysis (IS 1893 Part 1: 2016. Even
‘engineered’ buildings built in the past are likely to lack the
seismic strength and detailing requirements of the current
design codes, such as IS 1893: 2016 and IS 13920: 2016,
because they were built prior to the implementation of these
codes.
2. SEISMIC EFFECTS ON STRUCTURES
2.1 Inertia forces in Structure
Earthquake causes shaking of the ground. So a building
resting on it will experience motion at its base. From
Newton’s First Law of Motion, even though the base of the
building moves with the ground, the roof has a tendency to
stay in its original position. But since the walls and columns
are connected to it, they drag the roof along with them. This
is much like the situation that you are faced with when the
bus you are standing in suddenly starts; your feet move with
the bus, but your upper body tends to stay back making you
fall backwards!! This tendency to continue to remain in the
previous position is known as inertia. In the building, since
the walls or columns are flexible, the motion of the roof is
different from that of the ground. (Fig.1)
Fig. 1 Effect of Inertia in a building when shaken at its base

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Fig. 2 Inertia force and relative motion within a building
Consider a building whose roof is supported on columns
(Fig. 2). Coming back to the analogy of yourself on the bus,
when the bus suddenly starts, you are thrown backwards as
if someone has applied a force on the upper body. Similarly,
when the ground moves, even the building is thrown
backwards, and the roof experiences a force, called inertia
force. If the roof has a mass M and experiences an
acceleration a, then from Newton’s Second Law of Motion,
the inertia force FI is mass M times acceleration a, and its
direction is opposite to that of the acceleration. Clearly, more
mass means higher inertia force. Therefore, lighter buildings
sustain the earthquake shaking better.
3. SEISMIC RETROFITTING
3.1 Earthquake Risk of Housing in India
The projected aggregate effect of expected number of lives
likely to be lost, persons injured, property damaged and
economic activity disrupted due to an expected strong
earthquake in an area, is the earthquake risk of that area.
India has experienced several major earthquakes in the past
few decades and according to IS 1893 (Part I:2016), around
56% (12% in Zone V, 18% in Zone IV, 26% in Zone III) and
44% in Zone II of its landmass is prone to moderate to
severe earthquake shaking intensity. Especially, in the last
25 years, several large to moderate earthquakes have
occurred in the country (Table 1) (Bihar-Nepal border
(M6.4) in 1988, Uttarkashi (M6.6) in 1991, Killari (M6.3) in
1993, Jabalpur (M6.0) in 1997, Chamoli (M6.8) in 1999, Bhuj
(M6.9) in 2001, and Kashmir (M7.6) in 2005, which have
caused more than 25,000 fatalities due to collapse of
buildings.
3.2 Need for Seismic Evaluation of Existing
Buildings
On a priority basis, seismic evaluation and retrofit are
undertaken for the life-line buildings, such as hospitals,
police stations, fire stations, telephone exchanges, broad
casting stations, television stations, railway stations, bus
stations, airports (including control towers), major
administrative buildings, relief co-ordination centres and
other buildings for emergency operations. The next set of
important buildings includes schools, educational
institutions, places of worship, stadia, auditoria, shopping
complexes and any other place of mass congregation. High
rise multistoreyed buildings, major industrial and
commercial buildings, historical and heritage buildings are
also among the important buildings.
Seismic vulnerability of an existing building is indicated
under the following situations:
i. The building may not have been designed and
detailed to resist seismic forces.
ii. The building may have been designed for seismic
forces, but before the publication of the current
seismic codes. The lateral strength of the building
does not satisfy the seismic forces as per the revised
seismic zones or the increased design base shear.
The detailing does not satisfy the requirements of
the current codes to ensure ductility and integral
action of the components.
iii. The construction is apparently of poor quality.
iv. The condition of the building has visibly
deteriorated with time.
v. There have been additions or modifications or
change of use of the building, which has increased
the vulnerability. For example, additional storeys
have been built.
vi. The soil has a high liquefaction potential.
3.3 Goals of Seismic Retrofit
The goals of seismic retrofit refer to the actions to be taken
with reference to the attributes for seismic design, in
qualitative terms. They can be summarized as follows:
i. To increase the lateral strength and stiffness of the
building
ii. To increase the ductility in the behaviour of the
building, this aims to avoid the brittle modes of
failure.
iii. To increase the integral action and continuity of the
members in a building
iv. To eliminate or reduce the effects of irregularities

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v. To enhance redundancy in the lateral load resisting
system, this aims to eliminate the possibility of
progressive collapse.
vi. To ensure adequate stability against overturning
and sliding
3.4 Objectives of Seismic Retrofit
The objectives of seismic retrofit are quantitative
expressions to achieve the goals of retrofit. Of course for a
non-engineered building, the objective may not be
quantifiable. The implicit objective is to provide adequate
lateral strength by strategies that have been tested or proved
to be effective in past earthquakes. The retrofitted building
should not collapse during a severe earthquake.
Objectives of seismic retrofit can be summarized as follows:
i. Public safety only: The goal is to protect human life,
ensuring that the structure will not collapse upon its
occupants or passersby, and that the structure can be
safely exited. Under severe seismic conditions the
structure may be a total economic write- off,
requiring tear-down and replacement.
ii. Structure survivability: The goal is that the structure,
while remaining safe for exit, may require extensive
repair (but not replacement) before it is generally
useful or considered safe for occupation. This is
typically the lowest level of retrofit applied to
bridges.
iii. Structure functionality: Primary structure
undamaged and the structure is undiminished in
utility for its primary application.
iv. Structure unaffected: This level of retrofit is
preferred for historic structures of high cultural
significance.
3.5 Steps of Seismic Retrofit
A retrofit steps for a building refers to the complete process
of retrofitting. For a systematic approach, it is necessary to
be aware of the steps of a retrofitting before undertaking the
retrofit project. The implementation of each step requires a
certain time schedule and finance. All the listed steps may
not be applicable for all projects. Similarly, there may be
detailed sub-divisions of one step for a particular project.
The steps of a retrofit are shown as a flow chart in Fig. 3
4. SEISMIC RETROFIT STRATEGIES/TECHNIQUES
This section presents an overview of the process used to
develop a retrofit strategy once deficiencies of the existing
buildings have been detected and performance objectives
have been apparently determined. The retrofit strategies can
be grouped under global and local strategies. A global
retrofit strategy targets the seismic resistance of the
building. A local retrofit strategy targets the seismic
resistance of a member, without significantly affecting the
overall resistance of the building. It may be necessary to
combine both local and global retrofit strategies under a
feasible and economical retrofit scheme.
Fig. 3 Flow chart showing Steps of Seismic Retrofit
4.1 Global Retrofit Strategies
4.1.1 Adding Shear Wall
Shear walls can be introduced in buildings with frames or in
buildings with flat slabs or flat plates. In the latter type of
buildings, since there are no conventional frames, the lateral
strength and stiffness can be substantially inadequate. A new
shear wall should be provided with an adequate foundation.
The reinforcing bars of the wall should be properly anchored
to the bounding frame (Fig.4).

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Shear walls resist two types of forces: shear forces and uplift
forces. Connections to the structure above transfer
horizontal forces to the shear wall. This transfer creates
shear forces throughout the height of the wall between the
top and bottom shear wall connections. The strength of the
lumber, sheathing and fasteners must resist these shear
forces or the wall will tear or “shear” apart.
Fig. 4 Addition of a shear wall
4.1.2 Adding Infill Wall
Addition of infill walls in the ground storey is a viable option
to retrofit buildings with open ground storeys. In absence of
plinth beam, the new foundation of the infill wall should be
tied to the existing footings of the adjacent columns (Fig. 5).
Else, a plinth beam can be introduced to support the wall.
The lateral load resistance and the energy dissipation
capability of a frame increase with infill. This is a viable
option for the building (with open ground storey) addressed.
Infill walls with reinforced concrete masonry units can act as
shear walls.
Fig. 5 Addition of a masonry infill wall
4.1.3 Adding Bracing
Steel braces can be inserted in a frame to provide lateral
stiffness, strength, ductility, energy dissipation, or any
combination of these (Fig.6). The braces can be added at the
exterior frames with least disruption of the building use. For
an open ground storey, the braces can be placed in
appropriate bays to retain the functional use. The connection
between the braces and the existing frame is an important
consideration. One technique of installing braces is to
provide a steel frame within the designated bay. Else, the
braces can be connected directly to the frame with plates
and bolts.
Fig. 6 Addition of a Steel braces
4.1.4 Base Isolation
Base isolation is a collection structural elements of building
that should substantially decouple the buildings‟ structure
from the shaking ground thus protecting the buildings
integrity and enhancing its seismic performance. The base
isolation tends to restrict the transmission of the ground
motion to the building; it also keeps the building positioned
properly over the foundation.
Base isolation systems have become a significant element to
enhance reliability during an earthquake. Seismic isolation
can be an effective tool for the earthquake resistant design of
structures that can be used in both new construction and
retrofit. One type of base isolation system is Friction
Pendulum Bearing in which the superstructure is isolated
from the foundation using specially designed concave
surfaces and bearings to allow sway under its own natural

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period during the seismic events. Friction Pendulum
Bearings are seismic isolation systems that have been as a
kind of bridge, and building retrofit in numerous cases
around the world. To assess their impact on structure
performance, models are needed to capture the behavior of
these highly nonlinear elements.
Fig. 7 Behavior of building structure with base isolation
system
Base isolation is generally suitable for low to medium rise
buildings, usually up to 10- 12 stories high, which have their
fundamental frequencies in the range of expected dominant
frequencies of earthquakes. Superstructure characteristics
such as height, width, aspect ratio, and stiffness are
important in determining the applicability and effectiveness
of seismic isolation. The seismic base isolation technology
involves placing flexible isolation systems between the
foundation and the superstructure.
A comparative evaluation of the different global retrofit
strategies is provided in Table 1
Table 1 Comparative evaluation of the global retrofit
strategies
Global
Retrofit
strategy
Merits
Demerits
Comments
Addition
of infill
walls
(i) Increases
lateral
stiffness of a
storey
(ii) Can
support
vertical load
if adjacent
column fails
(i) May have
premature
failure due
to crushing
of corners or
dislodging
(ii) Does not
increase
ductility
(iii)
Increases
weight
(i) Low cost
(ii) Low
disruption
(iii) Easy to
implement
Addition
of shear
walls
(i) Increases
lateral
strength and
stiffness
of the
building
substantially
(ii) May
increase
ductility
(i) May
increase
design
base shear
(ii) Increase
in lateral
resistance is
concentrated
near the
walls
(iii) Needs
adequate
foundation
(i) Needs
integration
of
the walls to
the
building
(ii) High
disruption
based on
location,
involves
drilling of
holes in the
existing
members
Addition
of
braces
(i) Increases
lateral
strength and
stiffness
of a storey
substantially
(ii)
Increases
ductility
(i)
Connection
of
braces to an
existing
frame can be
difficult
(i) Passive
energy
dissipation
devices
can be
incorporated
to
increase
damping /
stiffness or
both
5. LOCAL RETROFIT STRATEGIES
Local retrofit strategies refer to retrofitting of columns,
beams, joints, slabs, walls and foundations. The local retrofit
strategies fall under three types: concrete jacketing, steel
jacketing (or use of steel plates) and fibre-reinforced
polymer (FRP) sheet wrapping.
Table 2 Comparative evaluation of the local retrofit
strategies
Local
Retrofit
strategy
Merits
Demerits
Comments
Concrete
jacketing
•Increases
flexural and
shear
strengths
and ductility
of the
member
• Easy to
analyse
•Compatible
with
original
substrate
•Size of
member
increases
•
Anchoring
of bars
for flexural
strength;
involves
drilling of
holes in the
existing
concrete
•Needs
preparation
• Low cost
•High
disruption
•Experience
of
traditional RC
construction
is
adequate

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of the
surface of
existing
member
Steel
jacketing
of
columns
• Increases
shear
strength and
ductility
•Minimal
increase in
size
•Cannot be
used for
increasing
the flexural
strength
•Needs
protection
against
corrosion
and
fire
•Can be used
as a
temporary
measure
after an
earthquake
• Cost can be
high
•Low
disruption
• Needs
skilled labour
Bonding
steel
plates
to beams
•Increases
either
flexural or
shear
strengths
•Minimal
increase in
size
• Use of
bolts
involves
drilling in
the existing
concrete
•Needs
protection
against
corrosion
and
fire
•More
suitable for
strengthening
against
gravity loads
• Cost can be
high
•Low
disruption
• Needs
skilled labour
Fibre
Reinforced
Polymer
wrapping
•Increases
ductility
• May
increase
flexural or
shear
strengths
•Minimal
increase in
size
•Rapid
installation
•Needs
protection
against fire
• Cost can be
high
•Low
disruption
• Needs
skilled labour
5.1 Retrofit Using Fibre Reinforced Polymer
Composites (FRP)
A comparative evaluation of the conventional
techniques with FRP is provided in Table 3
Table 3 Comparison of FRP system with Conventional
Technique
Description
Concrete
Jacketing
Steel
Jacketing
FRP
Wrapping
Remarks
Mode of
strengtheni
ng
Increase
in
concrete
and steel
area
Confineme
nt
Confineme
nt
Preparatio
n
of column
for
strengtheni
ng
Significan
t
dismantli
ng of
cover
concrete.
At least
40 mm
cover
concrete
to be
removed.
Epoxy
primer to
be
applied
on
exposed
surface.
Not major
dismantlin
g work
involved.
Mainly
plaster to
be
removed
and epoxy
primer to
be applied
on
exposed
surface
Only
plaster to
be
removed
and epoxy
primer to
be applied
on
exposed
surface.
For
rectangula
r
columns,
corners to
be
rounded
off.
FRP
involves
minimum
surface
preparati
on.
Drilling of
holes
Large
amount
of
drilling is
required
Large
amount of
drilling is
required
Large
amount of
Drilling is
required
FRP
involves
minimum
work
since
no drilling
is
required.
Additional
weight
Extremel
y high
Very high
Negligible.
No
increase in
weight at
all.
FRP does
not
increase
the dead
weight of
the
structure.
6. CONCLUSIONS
Conventional techniques (Local and global) of retrofitting
are compared with modern technique (Fiber Reinforced
Polymers). Following observations are concluded from the
study:
1. Seismic resistant design of new buildings and
seismic retrofit of existing buildings are essential to
reduce the vulnerability of the buildings during an
earthquake.

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2. Before undertaking seismic retrofit, it is essential to
determine the condition and diagnose the
deficiencies in a building.
3. Seismic evaluation helps to identify the deficiencies
of the building with respect to resistance to seismic
forces.
4. Based on the condition and deficiencies, repair and
retrofit strategies are selected.
5. When a building is severely deficient for the design
seismic forces, it is preferred to select a global
retrofit strategy to strengthen and stiffen the
structure.
6. If deficiencies still exist in the members, local
retrofit strategies are to be selected.
7. A retrofit strategy is to be selected after careful
considerations of the cost and constructability.
Proper design of a retrofit strategy is essential.
8. The failure mode in a member after retrofitting
should not become brittle.
9. A global retrofit strategy that involves a shift in
either of the centre of mass or centre of rigidity
should be checked for torsional irregularity.
10. FRP involves minimum surface preparation,
minimum work since no drilling is required.
11. FRP does not increase the dead weight of the
structure.
REFERENCES
[1]. Handbook on Seismic Retrofit Of Buildings, (April 2007),
By Central Public Works Department & Indian Building
Congress In Association With Indian Institute of
Technology – Madras
[2]. National Disaster Management Guidelines, Seismic
Retrofitting of Deficient Buildings and Structures, June
2014 by National Disaster Management Guidelines –
Seismic Retrofitting of Deficient Buildings and
Structures
[3]. Guidelines For Retrofit Of Concrete Structures - Draft,
(Translation From The Concrete Library No.95
Published By Jsce, September 1999)
[4]. Bhavar Dadasaheb O. , Dhake Pravinchandra D. , Ogale
ramesh A. “Retrofitting of existing R.C.C building by
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in modern engineering and emerging technology vol 1,
issue:5-2013
[5]. Different Strengthening Techniques for RC Columns by
Dr. Gopal L. Rai R&M International Pvt. Ltd.
[6]. Minakshi V. VaghaniȦ, Sandip A. VasanwalaȦ and Atul K.
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2006, pp. 31-48
BIOGRAPHIES
Chetan J. Chitte obtained his
M.Tech. in Structural Dynamics &
Earthquake Engineering from
VNIT, Nagpur and B.E. Civil from
Sardar Patel College of
Engineering, Mumbai. He has 4.5
Years experience in Structural
Designing and 9 Years experience
in area of teaching.