Download Advanced Retrofitting Methods and Techniques for RC

Advanced Retrofitting Methods and Techniques
for RC Building: State of an Art
Miss. Swati Sanjay Nibhorkar *
M. E (Structure) Scholar,
Civil Engineering Department,
G. H. Raisoni College of Engg and
Management,
Amravati, Maharashtra, India.
[email protected]
Prof. B. H. Shinde
Assistant Professor,
Civil Engineering Department,
G. H. Raisoni College of Engg and
Management,
Amravati, Maharashtra, India.
Abstract: A higher degree of damage in a building is expected during an earthquake if the seismic
resistance of the building is inadequate. The decision to strengthen it before an earthquake
occurs depends on the building’s seismic resistance. The structural system of deficient building
should be adequately strengthened in order to attain the desired level of seismic resistance.The
seismic retrofitting of reinforced concrete buildings not designed to withstand seismic action is
considered. After briefly introducing how seismic action is described for design purposes,
methods for assessing the seismic vulnerability of existing buildings are presented based on the
literature survey carried out. The existing building can be retrofitted using various techniques like
Jacketing existing beams, columns, or joints, Use of Fibre Reinforced Cement, confinement of
column by embedded composite grid etc. So, in this paper, efforts are made to describe
thedifferent retrofitting techniques available and its suitability for particular conditions. The
traditional methods of seismic retrofitting are reviewed and their weak points are identified.
Modern methods and philosophies of seismic retrofittingare also reviewed.
Keywords: retrofitting, RC Building, Seismic zone, Earthquake, Strengthening.
1.0 INTRODUCTION
A
higher degree of damage in a building is
experienced during an earthquake if the
seismic resistance of the building is inadequate.
The decision to strengthen it before an earthquake
occurs depends on the building‟s seismic
resistance. The structural system of deficient
building should be adequately strengthened in
order to attain the desired level of seismic
resistance. The recent earthquakes in Turkey and
India have highlighted the structural inadequacy of
those building stocks with respect to seismic loads
and design. Building owners and occupiers are
now aware of how vulnerable their buildings are
and may wish to undertake strengthening work. In
India there are thousands of apartment buildings
vulnerable to severe damage in a moderate or
larger earthquake [1]. The seismic hazard in the
areas, where those earthquakes have occurred,
has been known for a long time because of similar
events that occurred in the past. It is therefore
legitimate to ask why constructions vulnerable to
earthquakes exist if people and institutions knew
* Corresponding Author
of the seismic hazard. Several causes may have
contributed to the creation of such a situation.
Thus the solution on this hazard is necessary.
Retrofitting of any existing building is a
complex task and requires skill, retrofitting of
Reinforced Concrete (RC) buildings is particularly
challenging due to complex behavior of the RC
composite material. The behavior of the buildings
during earthquake depends not only on the size of
the members and amount of reinforcement, but to
a great extent on the placing and detailing of the
reinforcement. The construction practices in India
result in severe construction defects, which make
the task of retrofitting even more difficult. It is not
surprising that in areas long known to be subject
to the seismic hazard it is not infrequent to find
constructions
vulnerable
to
earthquakes.
Theseconstructions need to be retrofitted to allow
them to withstand the effects of the earthquake
groundmotion expected at the site considered. In
the following sections some advancedmethods
and techniques used for the evaluationof the
seismic resistance and vulnerability of reinforced
concrete buildings will be described together
withtraditional and innovative techniques of
seismic retrofitting of the same structures. This
paper presents a brief review of the available
methods and techniques for retrofitting of RC
building. In this paper, efforts are also made to
describe the different retrofitting techniques
available and its suitability for particular conditions.
There are three sources of deficiencies [2] in
a building, which have to be accounted for by the
retrofitting engineer:
(i) Inadequate design and detailing
(ii) Degradation of material with time and use,
and
(iii) Damage due
catastrophe.
to
earthquake
or
other
The retrofit engineer is expected to estimate
the deficiency resulting from all the three sources,
suggest a retrofit scheme to make up for the
deficiencies and demonstrate that the retrofitted
structure will be able to safely resist the future
earthquake forces expected during the lifetime of
the structure.
2.0 DOCUMENTS DEVOTED TO ASSESSMENT
AND RETROFIT OF EXISTING STRUCTURES
The Indian Standard [3] focuses on providing
guidance on reinstating damaged or weak
elements by rebuilding or strengthening. General
principles as well as some common strengthening
techniques are discussed. For example, details of
encasing reinforced concrete members to improve
strength, and methods of improving floor and roof
diaphragm action are provided. The Indian
Standard appears to concentrate on reinstating or
upgrading gravity load paths of reinforced
concrete members, rather than improving seismic
resistance.
Its
value
therefore
lies
in
implementationof rapid repairs, probably in most
cases to non-engineered structures. While the lack
of emphasis on the need for engineering
evaluation, analysis and design of seismically
deficient structures may be addressed in a future
revision.It might be argued that much repair and
retrofitting will be undertaken without professional
engineering advice. This highlights the value of
practical and detailed documentation in the form of
manuals for contractors who can then retrofit a
limited range of common building types. Such a
document has been recently produced by a NGO
for the Gujarat reconstruction [4].
The document [11] highlights a higher degree
of damage in a building is expected during an
earthquake, if the seismic resistance of the
building is inadequate. The decision to strengthen
it before an earthquake occurs depends on the
building‟s seismic résistance. The structural
system of deficient building should be adequately
strengthened, in order to attain the desired level of
seismic resistance.The guidelines are intended to
provide a systematic procedure for the seismic
evaluation of buildings, which can be applied
consistently to a rather wide range of buildings.
This document also discusses some cost effective
strengthening schemes for existing older buildings
where identified as seismically deficient during the
evaluation process.
The detailed study on United Nations
Industrial Development Organization (UNIDO)
manual, Federal Emergency Management Agency
(FEMA 310), Structural Engineering Research
Center (SERC) and New Zealand Draft Code is
carried out by the author. Author recommended
that seismic evaluation procedures for buildings
are a combination of configuration-related and
strength-related checks. Though there have been
no significant differences in which the
configuration related assessments are carried out,
there is considerable degree of non-uniformity in
the manner strength–related assessments are
carried out. All the documents except UNIDO
manual provide explicit checking criteria for the
configurationally induced irregularities. Strength
checks are performed either at global (structure)
or local (element) level or at both levels as in
FEMA 310. At the global level, there is no shear
stress check given by UNIDO manual and New
Zealand Draft Code and no component level
analysis is performed in UNIDO manual and
SERC report. The review of various evaluation
procedures indicates clearly that FEMA 310 and
New Zealand Draft Code are more suitable for use
in buildings of developing countries, which are not
only difficult to classify in certain „type buildings‟
but also their capacities cannot be estimated with
significant confidence. FEMA 310 provides a more
generalized approach to seismic evaluation, which
is thorough and provides several levels of
assessment with varying degree of complexity
suitable for a large class of structures.
FEMA172, 1992 [5]handbook describes
techniques that engineers can use to solve a
variety of seismic rehabilitation problems in
existing buildings, a broad spectrum of building
types and building components of both structural
and nonstructural. Techniques are illustrated with
sketches, and the relative merits of the techniques
are also discussed. FEMA172,1992 handbook and
its supporting report FEMA173, 1989 [6] and
FEMA174, 1989 [7] documents information need
to develop a seismic rehabilitation program and
establish priorities for rehabilitation. Through the
presentation of nationally applicable guidelines, it
helps local jurisdictions to make informed
decisions
about
rehabilitating
seismically
hazardous existing buildings, and it includes a
review of the relevant technical and societal issues
and a procedure to resolve the issues.
FEMA308, 1999 [8] consists practical
guidance for the repair, upgrade of earthquake
damaged concrete and masonry wall buildings.
Target audiences are design engineers, building
owners, officials, insurance adjusters, and
government agencies. The publication contains
sections on most important aspect of performance
based design, repair technologies, categories of
repair, and nonstructural considerations. The last
section includes repair guides, which provides
outline specifications for typical repair procedures.
In this publication FEMA315, 1998 [9]
discusses the mission, history, and results of
FEMA is Existing Building Program (EBP) which
provides four objectives and 25 tasks to be carried
out through the EBP. The fourobjectives are:
1. Promote seismic rehabilitation and advance
the implementation of previously developed
materials.
2. Monitor the use of and refine existing
materials
3. Develop new seismic rehabilitation tools, and
4. Consider new program directions for the EBP.
Estimated costs for the next ten to fifteen
years and guidelines for plan implementation
are also included.
The publication also describe administrators
with information to assess the seismic vulnerability
of school buildings, hospital building, office
building, apartment building, retail building and to
implement a program of incremental seismic
rehabilitation. Increase in the number of
seismically resistant buildings in all areas of
identified earthquake risk.
Also FEMA 356, 2000 [10] can be used as an
evaluation tool, the more traditional evaluation
resultingfrom the application of FEMA 310 may be
more appropriate for a developing country. Both
documents take a rigorous approach to
determining existing structural conditions by
specifying the as-built information required,
including exposure of primary reinforced concrete
connections to ascertain the standard of
reinforcement detailing. Uncertainties associated
with minimum data collection are accounted for in
the analysis by application of a Knowledge Factor.
This approach might be redundant for buildings
whose existing structures are discounted
completely, due to serious constructional
deficiencies, and in whom additional structures are
inserted to resist lateral loading. The documents
also outline potential geological hazards and
provide guidance onassessment and mitigation.
3.0 IDENTIFICATION OF SEISMICALLY
DEFICIENT BUILDING
Existing buildings [1] can become seismically
deficient when (a) seismic design code
requirements are upgraded since the design of
these buildings with an older version of the code;
(b) seismic design codes are deficient, (c)
engineering
knowledge
makes
advances
rendering insufficient the previous understanding
used in their design, and (d) designers lack
understanding of the seismic behavior of
structures. Indian buildings built over the past two
decades are deficient because of the reasons
stated in (b), (c), and (d) above.
Concept of pushover analysis [12] that is
becoming a popular tool in the profession for
design of new buildings, seismic evaluation of
existing buildings and developing appropriate
strategy for seismic retrofitting of buildings. It is
shown how this analytical technique can be useful
in deciding seismic retrofitting strategy and
techniques.
Pushover analysis of the structures
(Lakshmanan D, 2006) using SAP 2000 evaluating
the various repair strategies for use in the
improvement of the seismic performance of RC
structures are highlighted. The behaviors of
repaired beams of beam column joints are
discussed in detailed. It is observed that inherent
deficiencies in the detailing of the beam-column
joints get reflect even after repair, though the
performance
factors
indicate
significant
improvement. Two of the logical extensions show
that the repair would not be as effective in these
cases.
The nonlinear static analysis of RC building
[13] is performed using pushover approach before
and after retrofitting. The comparison of strength
parameters and pushover curve indicates that
there is increase in ductility. As regards to stiffness
of the building, it is seen that it remains more or
less same up to linear stage, while in nonlinear
stage every point increase after retrofitting. The
strength of the building is correlated with base
shear and the net enhancement in strength after
retrofitting.
This paper [14] introduces a method of
design of structural upgradation using FRC and
discusses the design of enhancement of RCC
elements with FRC, a strategy of upgradation of
RCC frames and use of the developed strategy of
upgradation for retrofitting of RCC frames based
on Capacity Spectrum Method.
4.0 IMPROVEMENT OF EXISTING RC
BUILDINGS
Three levels of improvement of existing RC
frame buildings [1] are possible, namely (a) repair,
(b) restore, and (c) strengthen. The consequence
of any prescribed method of the retrofitting are (a)
adding brick masonry walls in all possible bays in
ground storey, (b) jacketing of all RC columns in
ground storey only, (c) adding steel diagonal
braces in some bays in ground storey, and (d)
infilling existing RC frame with RC structural walls
in some bays in ground storey only. In all cases,
foundation strengthening may be essential. On the
seismic capacity of the building should be
quantitatively evaluated its effectiveness from the
points of view of strength, stiffness and ductility
are shown in Fig 1 Sometimes, a retrofit scheme
may have better performance than the damaged
structure, but still may be poor; the retrofit scheme
that assures at least a basic ductility is preferable
to the others.
This paper [16] concludes that to identify an
efficient retrofitting method for existing open
ground storey RC frame buildings. A two
dimensional RC frame is designed with nonductile. Detailing is subjected to nonlinear static
pushover analysis. The RC frame is retrofitted by
three methods. a) Concrete jacketing of columns
in the ground storey, b)Brick masonry infill in the
ground storey, and c) RC structural wall in the
ground storey panel of all the methods studied the
use of structural wall in the ground storey panel
give the maximum strength and ductility.
The objective of this study [17] is to identify
an efficient retrofitting method for reinforced
concrete buildings. Two buildings: One open
ground storey with infills and the other partial open
ground storey with infills, which are damaged in
the January 2001 Bhuj earthquake, are subjected
to static pushover analysis with code specified
design shear distribution. The observed failure
modes conform to the actual structural damages
sustain by the buildings during that earthquake.
The selectedmethods of retrofitting are a)
Jacketing of columns in the ground storey, b)
Structuralwalls in the ground storey of some
selective panels, and c) Structural walls for all
thestories in some panels. These three basic
schemes are used in combination forascertaining
an economical method giving the maximum
strength and ductility. Of all themethods studied,
the combination of column jacketing in ground
storey and shear wallthroughout the height of the
building with selective strengthening of upper
storey framemembers, give the most economic
and desirable performance.
Fig.1: Relative performance of different retrofit strategies [1]
The present paper [18] deals with the seismic
retrofitting of an existing fourteen storied RC
building frame located in seismic zone IV. The
study includes seismic evaluation and retrofitting
of RC framed building, by using steel bracing and
infill masonry walls. The seismic performance of
two retrofitting techniques such as steel bracing
(V, diamond and cross pattern) and infill walls are
relatively compared. Among three patterns of steel
bracing, cross pattern shows better performance
than V and diamond bracing patterns.
The structural analysis [19] is performed
using a suitable computer analysis program. The
steps involve developing a computational model of
the building, applying the external forces,
calculating the internal forces in the members of
the building, calculating the deformations of the
members and building and finally interpreting the
results. First deficiencies observed in existing RC
buildings for resisting earthquake identified. It is
essential to identify the deficiencies in a building
before undertaking retrofit. Identification of the
deficiencies is also expected to create awareness
for future construction.
5.0 CONCLUSION
A number of experimental and analytical
studies focused on seismic retrofitting techniques
and extensive seismic damage control activities in
practice have contributed to the present state of
development. Further research should be
conducted to improve the selection of appropriate
retrofit techniques using criteria based on
performance, economy and constructability. There
are many seismic retrofit techniques available,
depending upon the varioustypes and conditions
of structures. Therefore, the selection of the type
of intervention is a complex process, and is
governed by technical as well as financial and
sociologicalconsiderations. The following methods
are carried out by most of the researchers which
are concrete jacketing of columns of ground floor,
brick masonry infill in the ground floor, X and V
bracing, shear wall, FRP of beams and columns.
All these topics require further research, and it is
essential for seismic retrofitting of reinforced
concrete structures. Also from the literature it may
be suggested that the Nonlinear static analysis
must be performed so as to obtain the actual
results after performing the analysis of structures.
The following factors such as; Cost versus
importance
of
the
structure,
Available
workmanship, Fulfillment of the performance goals
of the owner, Level of quality control, Irregularity of
stiffness, strength and ductility, Sufficient capacity
of foundation system and Repair materials and
technology available should also be considered
while selecting the seismic retrofitting technique.
References
[1] Murty CVR, (2002), quantitative approach to
seismic strengthening Of RC frame building,
Seminar on seismic assessment and retrofitting
buildings, pp 19-27.
[2] Yogendra Singh, (2003), “Challenges in
retrofitting of RC buildings”, Workshop on
retrofitting of structures, IIT Roorkee, pp 29-44.
[3] Indian Standard, 1993, Repair and Seismic
Strengthening of Buildings – Guidelines, IS
13835:1993, Bureau of Indian Standards, New
Delhi
[4] D. D. Ayala and A. W. Charleson, “REVIEW OF
SEISMIC STRENGTHENING GUIDELINES
FOR R. C. BUILDINGS IN DEVELOPING
COUNTRIES”, Published by Elsevier Science
Ltd. All rights reserved 12th European
Conference on Earthquake Engineering Paper
Reference 820.
[5] FEMA172. NEHRP handbook of techniques for
the seismic rehabilitation of existing buildings.
Building seismic safety council. Washington
D.C, (1992).
[6] FEMA173. Establishing programs and priorities
for the seismic rehabilitation of buildings:
Supporting
report.
Building
systems
development Inc. Washington, D.C, (1989).
[7] FEMA174. Establishing programs and priorities
for the seismic rehabilitation of buildings:
Handbook.Building systems development Inc.
Washington.D.C, 989.
[8] FEMA308. The repair of earthquake damaged
concrete and masonry wall buildings. Applied
Technology Council. Redwood city CA, (1999).
[9] FEMA315. Seismic rehabilitation of buildings:
Strategic plan 2005. Earthquake engineering
research institute. Washington, D.C, (1998).
[10] FEMA 356, 2000, Prestandard and
Commentary for the Seismic Rehabilitation of
Buildings, Federal Emergency Management
Agency, Washington DC
[11] DrDurgesh C Rai, (2005), guidelines for
seismic evaluation and strengthening of
existing building, Provision with commentary
and explanatory examples, Indian Institute of
Technology Kanpur, Document
GSDMA-Earthquake 6, vol. 4.
no-
IITK-
International conference on
engineering and seismology.
earthquake
[12] Sudhir K. Jain, Srikant T, (2002), analysis for
seismic retrofitting of buildings, The Indian
concrete journal, pp 479-484.
[18] Amar Prakash, Thakkar SK, (2003), “A
comparative study of retrofitting of RC
building”, Workshop on retrofitting of
structures, IIT Roorkee, pp 159- 173.
[13] Shailesh Kr. Agrawal, Ajay Chourasia, (2003),
“Nonlinear static analysis for seismic
evaluation and retrofit of RC buildings”,
Workshop on rertofitting of structures, pp 116124.
[19] Dr. Meher Prasad A. Structural analysis for
seismic retrofit, Handbook on seismic retrofit
of building, IIT Madras, pp 8.1-8.27.
Author’s Biography
[14] Abhijit Mukherjee, Amit R. Kalyani, (2004),
“Seismic retrofitting of reinforced concrete
frames with fiber reinforced composites”,
Workshop on seismic evaluation and
retrofitting of building, pp 74-82.
[15] Richard N. White, Khalid Mosalam. Seismic
evaluation and rehabilitation of concrete
buildings, Cornell University, Ithaca NY 14853
USA, pp 177-186.
Miss.S.S.Nibhorkar appeared PG
from Amravati University. Her area
of specialization is Structural
Engineering She is working with
G.H.Raisoni
College
of
Engineering & Mang. Amravati
institution. She has an experience
of 1 year.
Prof. B. H. Shinde obtained PG
from Aurangabad University. His
area of specialization is Structural
Engineering. He is working with
G.H.Raisoni
College
of
Engineering & Mang. Amravati
Institution. He has experience of 5
[16] KaustubhDasgupta, Murty CVR. Quantitative
seismic retrofitting of open ground storey RC
frame buildings.
[17] Ashutosh V Mahashabde, KaustubhDasgupta,
Murty CVR, (2003), “Seismic strengthening of
gravity load designed RC frame buildings”, 4th
years.