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TECHNICAL SEMINAR REPORT ON
SEISMIC RETROFITTING TECHNIQUES In Partial Fulfillment of the Academic requirements for the award of
Bachelor of Technology in Civil Engineering Submitted By
D.CHANDANA VARMA !"#$!A"!%& '
MALLA REDD( EN)INEERIN) COLL)E (Afillated to JNT! "yderabad# $aisamma%uda! &hula'ally ( Post ia ia )om'ally# Secunderabad !*++,++
SEISMIC RETRO*ITTIN) TECHNI+,ES
A-/rac/ $any e-istin% structures located in seismic re%ions are inadequate based on the current seismic desi%n codes. In addition! a number of ma/or earthqua0es durin% recent years ha1e underscored the im'ortance of miti%ation to reduce seismic ris0. Seismic retrofittin% of e-istin% structures is one of the most effecti1e methods of reducin% this ris0. In recent years! a si%nificant amount of research h as been de1oted to the study of 1arious stren%thenin% techniques to enhance the seismic 'erformance of 23 structures. "owe1er! the seismic 'erformance of the structure may not be im'ro1ed b y retrofittin% or rehabilitation unless the en%ineer selects an a''ro'riate inter1ention technique based on seismic e1aluation of the structure. Therefore! the basic requirements of rehabilitation and in1esti%ations of 1arious retrofit techniques should be considered b efore selectin% retrofit schemes. The 'ur'ose of this re'ort is to 'resent the 1arious seismic retrofittin% characteristics and technical as'ects of the ma/or inter1ention methods for reinforced con crete (23# buildin% structures.
!. INTROD,CTION
Sei0ic re/rofi//ing is the modification of e-istin% structures to ma0e them more resistant to seismic acti1ity! %round motion! or soil failure due to earthqua0es. 4ith better understandin% of seismic demand on structures and with our recen t e-'eriences with lar%e earthqua0es near urban centers! the need of seismic retrofittin% is well ac0nowled%ed.
The aftermath of an earthqua0e manifests %reat de1astation due to un'redicted or sli%ht. This dama%e to structures in its turn causes irre'arable loss of life with a stri0in% e-tensi1e dama%e to innumerable buildin%s of 1aryin% de%ree i.e. either full or 'art of casualties. As a result fri%htened occu'ants may refuse to enter the buildin% unless assured of the safety of the buildin% from future earthqua0es. It has been obser1ed that ma/ority of such earthqua0e dama%ed buildin%s may be safely reused! if they are con1erted into seismic resistant structures by em'loyin% a few retrofittin% measures. This 'ro1es to be a better o'tion caterin% to the economic considerations and immediate shelter 'roblems rather than re'lacement of buildin%s. $oreo1er it has often been seen that retrofittin% of buildin%s is %enerally more economical as com'ared to demolition and reconstruction e1en in the case of se1ere structural dama%e. Therefore! seismic retrofittin% of buildin% structures is one of the most im'ortant as'ects for miti%atin% seismic ha5ards es'ecially in earthqua0e6'rone countries. arious terms are associated to retrofittin% with a mar%inal difference li0e re'air! stren%thenin%! retrofittin%! remouldin%! rehabilitation! reconstruction etc. but there is no consensus on them. The need of seismic retrofittin% of buildin%s arises under two circumstances7 (i# earthqua0e dama%ed buildin%s and (ii# earthqua0e61ulnerable buildin%s that ha1e not yet e-'erienced se1ere earthqua0es. The 'roblems faced by a structural en%ineer in retrofittin% earthqua0e dama%e buildin%s are7 (a# lac0 of standards for methods of retrofittin%8 (b# effecti1eness of retrofittin% techniques since there is a considerable dearth of e-'erience and data on retrofitted structures7 (c# absence of consensus on a''ro'riate methods for the wide ran%e of 'arameters li0e t y'e of structures! condition of materials! ty'e of dama%e! amount of d ama%e! location of dam9e! si%nificance of dama%e! condition under which a dama%ed element can be retrofitted etc. Buildin%s ha1e been desi%ned accordin% to a seismic code! but the codes ha1e been u'%raded in the later years. :ssential buildin%s must be stren%thened li0e hos'itals! historical monuments and architectural buildin%s. Buildin%s that are e-'anded! reno1ated or rebuilt. The retrofit techniques outlined here are also a''licable for other natural ha5ards such as tro'ical cyclones! tornadoes! and se1ere winds from thunderstorms. 4hilst current 'ractice of seismic retrofittin% is 'redominantly concerned with structural im'ro1ements to reduce the seismic ha5ard of usin% the structures! it is similarly essential to reduce the ha5ards and losses from non6 structural elements. It is also im'ortant to 0ee' in mind that there is no such thin% as an
earthqua0e6'roof structure! althou%h seismic 'erformance can be %reatly enhanced throu%h 'ro'er initial desi%n or subsequent modifications. A lar%e number of e-istin% buildin%s in India are se1erely deficient a%ainst earthqua0e forces and the number of such buildin%s is %rowin% 1ery ra'idly. This has been hi%hli%hted in the 'ast earthqua0e. 2etrofittin% of any e-istin% buildin% is a co m'le- tas0 and requires s0ill! retrofittin% of 23 buildin%s is 'articularly challen%in% due to co m'le- beha1ior of the 23 com'osite material. The beha1ior of the buildin%s durin% earthqua0e de'ends not only on the si5e of the members and amount of reinforcement! but to a %reat e-tent on the 'lacin% and detailin% of the reinforcement. The construction 'ractices in India result in se1ere construction defects! which ma0e the tas0 of retrofittin% e1en more difficult. There are three sources of deficiencies in a buildin%! which ha1e to be accounted for by the retrofittin% en%ineer7 (i# inadequate desi%n and detailin%! (ii# de%radation of material with time and use! and (iii# dama%e due to earthqua0e or other catastro'he. The retrofit en%ineer is e-'ected to estimate the deficienc y resultin% from all the three sources! su%%est a retrofit scheme to ma0e u' for the deficiencies and demonstrate that the retrofitted structure will be able to safety resist the future earthqua0e forces e- 'ected durin% the lifetime of the structure. This 'a'er 'resents a brief re1iew of the a1ailable methods and techniques for retrofittin% of 23 buildin%.
1. PER*ORMANCE OB#ECTIVES
In the 'ast! seismic retrofit was 'rimarily a''lied to achie1e 'ublic safety! with en%ineerin% solutions limited by economic and 'olitical considerations. "owe1er! with the de1elo'ment of Performance based earthqua0e en%ineerin% (PB::#! se1eral le1els of 'erformance ob/ecti1es are %radually reco%nised7 •
Public safety only. The %oal is to 'rotect human life! ensurin% that the structure will not colla'se u'on its occu'ants or 'assersby! and that the structure can be safely e -ited. nder se1ere seismic conditions the structure may be a total e conomic write6off! requirin% tear6 down and re'lacement.
•
Structure sur1i1ability. The %oal is that the structure! while remainin% safe for e-it! may require e-tensi1e re'air (but not re'lacement# before it is %e nerally useful or considered safe for occu'ation. This is ty'ically the lowest le1el of retrofit a''lied to brid%es.
•
Structure functionality. Primary structure undama%ed and the structure is undiminished in utility for its 'rimary a''lication. A hi%h le1el of retrofit! this ensures that any required
re'airs are only ;cosmetic; 6 for e-am'le! minor crac0s in 'laster ! drywall and stucco. This is the minimum acce'table le1el of retrofit for hos'itals. •
Structure unaffected. This le1el of retrofit is 'referred for historic structures of hi%h cultural si%nificance.
2. EARTH+,A3E DAMA)E AND HERITA)E STR,CT,RES Ty'ical earthqua0e dama%e to most older and historic buildin%s results from 'oor ductility66or fle-ibility66of the buildin% and! s'ecifically! 'oor structural connections between walls! floors! and foundations combined with the 1ery hea1y wei%ht and mass of historic materials that are mo1ed by seismic forces and must be resisted. In buildin%s that ha1e not been seismically u'%raded! 'articularly unreinforced masonry buildin%s! 'ara'ets! chimneys! and %able ends may dislod%e and fall to the %round durin% a moderate to se1ere earthqua0e. 4alls! floors! roofs! s0yli%hts! 'orches! and stairs which rely on tied connections may sim'ly fail. Interior structural su''orts may 'artially or totally colla'se. nreinforced masonry walls between o'enin%s often e-hibit shear (or dia%onal# crac0in%.
''er stories may colla'se onto under6reinforced lower floors with lar%e 'erimeter o'enin%s or atriums. nbraced infill material between structural or ri%id frame su''orts may dislod%e. Ad/acent buildin%s with se'arate foundations may mo1e differently in an ea rthqua0e creatin% dama%e between them. Poorly anchored wood frame buildin%s tend to slide off their foundations. Factors influencin% dama%e in an earthqua0e
$. CONDITION O* E4ISTIN) B,ILDIN) $uch of the dama%e that occurs durin% an earthqua0e is directly related to the buildin%
5. SEISMIC RETRO*ITTIN) AND B,ILDIN) CONSERVATION "erita%e structures can be di1ided mainly from earthqua0e 'oint of 1iew in to two main cate%ories7 ,. ndama%ed or before earthqua0e herita%e structures7 The retrofittin% needed for these of structures is mainly to increase their seismic stren%th throu%h reinforcement of structural members. This ty'e of a''roach is 0nown as conser1ation a''roach. =. &ama%ed or after earthqua0e herita%e structures7 For earthqua0e dama%ed herita%e structure retrofittin% techniques are ado'ted to restore and rehabilitate the dama%ed structure. This is done by stren%thenin% walls! columns! and o1erall fabric of the structure without dama%in% the character of the buildin%. In case of 'artial or full colla'se of the 'ortion of the buildin% it is reconstructed by use of as much historic material is a1ailable as 'ossible and after 'ro'er documentation old structure from a1ailable records.
%. RETRO*ITTIN) STRATE)IES *OR RC B,ILDIN)S The need for retrofittin% or stren%thenin% of earthqua0e6dama%ed or earthqua0e61ulnerable buildin%s in India ha1e been tremendously increased durin% recent years after the de1astatin% Bhu/ earthqua0e with an alarmin% awa0enin% for sufficient 're'aredness in antici'ation to face future earthqua0es. $any 'rofessional en%ineers are accustomed to the desi%nin% of new buildin%s but they may find themsel1es not fully equi''ed to face the challen%es 'osed at the time of stren%thenin% the e-istin% buildin%s with a 1iew to im'ro1e their seismic 'erformance. This section 'resents the most common de1ices for retrofittin% of reinforced concrete buildin%s with technical details! constructional details and limitations.
6. METHODS *OR SEISMIC RETRO*ITTIN) O* STR,CT,RES 2etrofittin% of e-istin% structures with insufficient seismic resistance accounts for a ma/or 'ortion of the total cost of ha5ard miti%ation. Thus! it is of critical im'ortance that the structures that need seismic retrofittin% are identified correctly! and an o'timal retrofittin% is conducted in a cost effecti1e fashion. Once the de cision is made! seismic retrofittin% can be 'erformed throu%h se1eral methods with 1arious ob/ecti1es such as increasin% the load! deformation! and>or ener%y dissi'ation ca'acity o f the structure (F:$A! =+++#. 3on1entional as well as emer%in% retrofit methods are briefly 'resented in the followin% subsections.
6.!CLASSI*ICATION O* RETRO*ITTIN) TECHNI+,ES There are two ways to enhance the seismic ca'acity of e-istin% structures.. The second is a member le1el a''roach of retrofittin% or local retrofittin% which deal! with an increase of the ductility of com'onents with adequate ca'acities to satisfy their s'ecific iirnit states. Based On the abo1e conce't the a1ailable techniques of retrofittin% of reinforced concrete buildin%s may be classified as7
Retroftting Techniques
Global
Local
Adding new shear wall
Adding steel
Jacketing o
Jacketing o
Wall thickening
Base isolation
?enerally structural le1el retrofittin%s are a''lied to the entire structural lateral lo9 resistin% system is deemed to be deficient. 3ommon a''roaches in this re%ard are em'loyed increase stiffness and stren%th with limited Achie1in% desired ratio between to additional stiffenin% and stren%thenin% is the art of seismic retrofittin%. The most common modifications include the
addition of structural loads! steel braces! infill walls! base isolate or su''lemental ener%y dissi'ation de1ices. The addition of new reinforced concrete shear wall is the most oftenly 'ractised de1ice which has 'ro1ed to be effecti1e for controllin% %lobal lateral drifts and for reducin% dama%e in frame members. Steel braces are used to ma0e the e-istin% buildin%s stiffen. 3oncentric eccentric bracin% schemes may be used! in the selected bays of an 23 frame contributin% increase the lateral resistance of the structure infill wall $A4 be em'loyed for stren%then of reinforced concrete buildin%s. which has !teen effecti1e in the case of one to three so buildin%s that may be e-tended u' to fi1e stories. The lateral stren%th of e-istin% columns be increased by addin% win% walls @buttresses similar to infillin%. These technique are not so 'o'ular because it may require a 1acant site around the buildin% and enou%h resistance from 'iles or foundation of the buttress 3:B. ,#. At some occasions it mi%ht be easy to achie1e the retrofittin% ob/ecti1es by means of %lobal mass reduction. $ass reduction can be accom'lished by remo1al of u''er stories hea1y claddin%! 'artitions and stored %ood. The conce 't of seismic base isolation is based on decou'lin% of structure by introducin% low ho ri5ontal stiffness bearin% between the structure and the foundation. This is found to be efficient for seismic resistance and inter1ention is required only at foundation le1el. The su''lemental de1ices such as addition of 1iscous dam'er! 1isco6 elastic dam'er! frictional dam'er in dia%onals of bays of frame substantially reduces the earthqua0e res'onse by dissi'ation of ener%y.
Cocal retrofittin%s arc ty'ically used either when the retrofit ob/ecti1es are limited or treatment of the 1ulnerable com'onents is needed. The most 'o'ular and frequently used method in local retrofittin% is /ac0etin% or confinement by the /ac0ets of reinforced concrete! method reinforced 'olymer ($P#! carbon fibre etc. Jac0etin% around the e-istin% members increases lateral load ca'acity of the structure in a uniformly distributed way with a minimal increase in loadin% on any sin%le foundation and with no alternati1e in the basic %eometry of increase the buildin%.
6.1 STR,CT,RAL LEVEL OR )LOBAL' RETRO*IT METHODS Two a''roaches are used for structure6le1el retrofittin%7 ti# con1entional methods based on increasin% the seismic resistance of e-istin% structure! and (ii# non6con1entional methods based on reduction of seismic demands. Conven/ional 0e/ho7
3on1entional methods of retrofittin% are used to enhance the seismic resistance of e-istin% structures by eliminatin% or reducin% the ad1erse effects of desi%n or construction. The methods Include addin% of shear wall! infill walls and steel braces.
6.2 ADDIN) NE8 SHEAR 8ALLS One of the most common methods to increase the lateral stren%th of the reinforced concrete is to ma0e a 'ro1ision for additional shear walls. The technique of infillin%>addin% new shear walls is often ta0en as the best and sim'le solution for im'ro1in% seismic 'erformance. Therefore! it is frequently used for retrofittin% of non6ductile reinforced concrete frame buildin%s. The added elements can be either cast6in6'lace or 're6cast concrete elements. New elements 'referably be 'laced at the e-terior of the buildin%! howe1er it may cause alteration in the a''earance and window layouts. Placin% of shear wal0 in the interior of the structure is not 'referred in order to a1oid interior mouldin%s!
Technical coni7era/ion9
The addition of new shear walls to e-istin% frame has many technical considerations which may be summari5ed as (a# determinin% the adequacy of e-istin% floor and roof slabs to carry the seismic forces8 (b# transfer of dia'hra%m shear into the new shear wal0 with do wels8 (c# addin% new collector and dra% members to the dia'hra%m8 (d# increase in the wei%ht and concentration of shear by the addition of wall which may affect the foundations. Con/r:c/ional coni7era/ion9
The first consideration durin% construction is to find locatir! where walls can be added and well located which may ali%n to the full hei%ht of the buildin% to minimi5e torsion (4ylie! ,D#. It is often desirable to locate walls ad/acent to the bear between columns so that only minimum slab demolition is required with connections male to beam at the sides of columns. The desi%n of the shear wall may be similar to new construction. The lon%itudinal reinforcement must be 'laced at the ends of the wall runnin% continuously throu%h the entire hei%ht. In order to reali5e this end! the reinforcement has to 'ass throu%h holes in slabs and around the beams to a1oid interference. To achie1e both conditions! boundary elements can be used. Althou%h it would also be con1enient to ha1e continuous shear reinforcement but in its absence! the walls must be adequately connected to the beams! slabs and columns ensurin% 'ro'er shear transfer throu%h shear connectors. 4all thic0ness also 1aries from ,* to =* cm (D to ,+ inch# and is normally 'laced e-ternally. This retrofittin% system is only adequate for concrete structures! which brin% forth a bi% increase in the lateral ca'acity and stiffness. A reasonable structural ductility may be achie1ed if the wall is 'ro'erly desi%ned with a %ood detailin%. The connection to the e-istin% structure has to be carefully desi%ned %uarantee shear transfer.
Li0i/a/ion9
The main limitations of this method are7 (i# increase in lateral resistance but it is concentrated at a few 'laces! (ii# increased o1erturnin% moment at found ation causes 1ery hi%h u'liftin% that needs either new foundations or stren%thenin% of the e-istin% foundations! (iii# increased dead load of the structure! (i1# e-cessi1e destruction at each floor le1el result in functional disability of the buildin%s! (1# 'ossibilities of adequate attachment between the new walls and the e-istin%
structure! (1i# closin% of formerly o'en s'aces can ha1e ma/or im'act on the interior of the buildin% or e-terior a''earance.
6.$ ADDIN) STEEL BRACIN)S Another method of stren%thenin% is the use of steel bracin%! which also has similar ad1anta%es. The structural details of connection between bracin% and column are shown I the fi%. The installation of steel bracin% members can be an effecti1e solution when lar%e o'enin%s are required. This scheme of the use of steel bracin% has a 'otential ad1anta%e o1er other schemes followin% reasons7 • • • • •
hi%her stren%th and stiffness can be 'ro1ed o'enin% for natural li%ht can be made easily! amount of wor0 is less since foundation cost may be minimi5ed! the bracin% system adds much less wei%ht to the e-istin% structure! most of the retrofittin% wor0 can be 'erformed with 'refabricated elements and disturbance to the occu'ants may be minimi5ed.
Technical coni7era/ion9
The steel bracin% system can be used for steel structures as well as concrete structures7 se1eral researchers ha1e re'orted successful results while usin% steel bracin% to u'%rade the stren%th and stiffness of reinforced concrete structures. It has 'erformed well linear beha1iour e1en u' to twice the desi%n code force. The effecti1e slenderness ratio should be 0e't relati1ely low so that braces are effecti1e in com'ression as well as tension! su%%ested l>r ratio are E+ to D+ or e1en lower. 3ollector
6.5
BASE ISOLATION
Base isolation! also 0nown as seismic base isolation or base isolation system! is one of the most 'o'ular means of 'rotectin% a structure a%ainst earthqua0e forces. It is a collection of structural elements which should substantially decou'le a su'erstructure from its substructure restin% on a sha0in% %round thus 'rotectin% a buildin% or non6buildin% structure
subsequent modifications. In some cases! a''lication of base isolation can raise both a structure
Isolation units are the basic elements of a base isolation system which are intended to 'ro1ide the aforementioned decou'lin% effect to a buildin% or non6buildin% structure.
=.
Isolation com'onents are the connections between isolation units and their 'arts ha1in% no decou'lin% effect of their own. Si%nificantly Increase the Period of the Structure and the &am'in% so that the 2es'onse is Si%nificantly 2educed
Bae Iola/ion in Buildin%s
Iola/or Co0;onen/ -e/
*ig9 An Iola/ion In/erface i for0e7
Ho< e=ac/ly 7oe Bae Iola/ion 8or>?
$ost ty'es of Isolators e-hibit nonlinear beha1ior
Cen%thenin% of the Structures Period and increased dam'in% that result in a lar%e scale decrease of the Seismic 2es'onse. *orce @ Di;lace0en/ Rela/ionhi; a/ /he Iola/ion In/erface
Re;one of Bae Iola/e7 B:il7ing ver: *i=e7 Bae Re;one
6.% #AC3ETIN) Jac0etin% is the 'rocess whereby a section of an e-istin% structural member is restored to ori%inal dimensions or increased in si5e by encasement usin% suitable materials. A steel reinforcement ca%e or com'osite material wra' can be constructed around the dama%ed section onto which shotcrete or cast6in6 'lace concrete is 'laced.
Jac0etin% is 'articularly used for the re'air of deteriorated columns! 'iers! and 'iles and may easily be em'loyed in underwater a''lications. The method is a''licable for'rotectin% concrete! steel! and timber sections a%ainst further deterioration and for stren%thenin%. Permanent forms are 'referred where 'rotection a%ainst weatherin%! abrasion! and chemical 'ollution is desired. Before a''lyin% /ac0ets! all deteriorated concrete must be remo1ed! crac0s must be re'aired! e-istin% reinforcement must be cleaned! and surfaces must be 're'ared. The surface 're'aration im'ro1es the bond of the newly 'laced materials with the e-istin% structure! which is difficult for underwater re'airs. For underwater conditions! a 'lastic shell may be a''lied at the s'lash 5one to hel' minimi5e abrasion. A drawbac0 of /ac0ets is that they occu' y s'ace that was earlier a1ailable for other uses.
Jac0etin% of colums
/ac0etin% of beams
&. CONCL,SION •
Seismic 2etrofittin% is a suitable technolo%y for 'rotection of a 1ariety of structures.
•
It has matured in the recent years to a hi%hly reliable technolo%y.
•
But! the e-'ertise needed is not a1ailable in the basic le1el.
•
The main challen%e is to achie1e a desired 'erformance le1el at a minimum cost! which can be achie1ed throu%h a detailed nonlinear analysis.
•
O'timi5ation techniques are needed to 0now the most efficient retrofit for a 'articular structure.
•
Pro'er &esi%n 3odes are needed to be 'ublished as code of 'ractice for 'rofessionals related to this field.