.. .....-.. '
Copyright
o 1996 Bsevler Science Ltd
paper No. 2115. (quote when c/tlng this art/ele)
Beventh World Canterence on Earthquake Engineering II WCBB
ISBN: O 08 042822 3
STATE-OF-1HE-ART IN TECHNlQUES FOR
REHABILITATION OF BunDINGS
SHUNSUKE SUGANO Senior ChiefResearcher, Research and Development Institute, Takenaka Corporation 1-5 Ohtsuka,Inzai-Machi, Inba-Oun, Chiba 270-13, Japan
ABSTRACT
An overview ofthe state-of-the-art in techniques for seismic rehabilitation of existing buildings was presented in this paper with emphasis on research and practice. First typical techniques used for reinforeed concrete, steel and masonry buildings were summarized in accordance with the aims of rehabilitation. New approaches to utilize seismic isolation and supplemental damping as well as conventional strengthening techniques to infill, to brace and to jacket existing strueture were described. Over twenty years research data were reviewed to discuss the improved behavior of rehabilitated structures and components with various techniques. Examples ofboth postearthquake and preearthquake rehabilitations of existing buildings were deseribed in some detaiL. Finally the observed behavior of several rehabilitated bui1dings during particular earthquakes were introduced to discuss the effect of rehabilitation.
KEYWORDS seismic performance, repair, upgrading, retrofltting, strengthening, infill wall, bmces, jacketing. seismic isola tion, seismic damper
INTRODUCTION
in earthquake engineering, the tenn "rehabilitationll is used as a comprehensive term to inc1ude all the concepts of"repairtl , lI upgradingtl , "retrofitting" and "strengthening" that lead to reduce building earthquake vulnerabil ity. in early years ofthe world conference on earthquake engineering (WCEE), the major topic in the teehnical session of rehabilitation was the development of techniques to repair and to strengthen existing stıuctures. After experiences ofsignificant damage to buildings due to several destructive earthquakes, particularly, those whicb bit highly densed urban areas (for examples, 1978 Miyagiken-oki, 1985 Mexico and 1989 Loma Prieta earthquakes), many cases ofpractice ofrehabilitation as well as development ofrehabilitation techniques were reported in the proceedings ofWCEE. Thus, the number of papers on seismic rehabilitation has been increas ing in the WCEE with the increase of total number oftechnical papers (Fig. 1), and the ratio of the number of rehabilitation papers to the total has been also increasing, reaching at over 4% in the latest conference of 10WCEE in 1992. This indicates that the importance of seismic rehabilitation of existing buildings has been recognized year after year in oor society. The recent two earthquakes, Le., the 1994 Northridge eartJıquake and the ı 995 Kobe earthquake could have strongly pushed OUT society to recognize the importance of eartfıquake
- 1
E
.. Number of papers~ SOO 6WCEE (New Delhi 1971)
611
1000 ~
~
7WCEE (Istanbul 1980) 8WCEE (San Francisco 1984
paperson rehabilitation of buildings
9WCEE (Tokyo, Kyoto 1988 IOWCEE (Madrid 1992)
Fig. i Number of papers on seismic rehabilitation of existing buildings in the proceedings of WCEE countenneasures for existing vulnerable buildings. A large number of existing buildings in earthquake prone areas over the world need seismic rehabilitation due to various reasons and motivations, such as code change or earthquake damage. Earthquake damaged buildings may need strengthening along with repair ofdamaged portion for reuse (postearthquake rehabilitation). Gener ally. they are rehabilitated so that their improved seisınic peıformancemay sa.tisfy the required performance by current code. Seismically inadequate buildings. the design ofwhich do not comply with current code. may also need rehabilitation (preearthquake rehabilitation) so that they meet the requirements ofthe code in foree. Many rehabilitation techniques were investigated recent twenty or more years to apply to both preearthquake and postearthquake rehabilitations. They are techniques to iniill. to brace and to back up ex:isting frames and to jacket existing framing members so that lateral resistance and ductility of a building may be increased. in addition to these conventional seismic resistant type techniques, another approaches to reduce seismic re sponse of a building have been recently adapted. Seismic isolation and supplemental damping are such new techniques for rehabilitation. though they have been investigated for long time to use for new buildings. This paper describes the present state of research and practice of techniques for seismic rehabilitation of exist ing buildings. First typical techniques which have been used for reinforced concrete. steel and masonry build ings are summarized in accordance with the aims ofrehabilitation. Because ofthe large volume ofexisting data available. most of the teehniques deseribed herein are those for reinforeed concrete buildings. Examples of techniques used for both postearthquake and preearthquake rehabilitations are deseribed in some details. Re cent approaches to utilize seismic isolation and supplemental damping as well as conventional infilling. brac ing and jacketing techniques are described. Over twenty years research data have been reviewed to discuss the improved seismic behavior of rehabilitated buildings and components with various techniques. Finally the observed behavior of same rehabilitated buildings during particular earthquakes are introduced to discuss the effect ofrehabilitation.
REHABILITATION STRATEGY AND TECHNIQUES Rehabilitation Strategy As shown in Fig. 2, the aims of seismic rehabilitation are; 1) to recover onginal structural performance,
2) to upgrade anginal strucbıral performance, and
3) to reduce seismic response
so as to reduce building earthquake vulnerability. To recover anginal structural performance, damaged or deteriorated portions of a building may be repaired with adequate material or replaced with newelement or material. To upgrade original structural performance there are several approaches (Fig. 2). General approach to
-2
J
-
repair damage and deterioration
recover original perfomıanee
seismic
upgrade original performanee
rehabilitation
ro-
stiffen exisıing strueture
:-
strengthen existing structure improve irregularity and/or discontinuity
- supplement energy dissipating devices H reduee masses r-
-
reduee seismic response
L....I
isoIate exisıing structure
Fig. 2 Seismic rehabilitatİon strategy and measures upgrade original performance is to strengthen existing strueture by the methods deseribed in the following section. To reduee excessive response displacement, a building must be stiffened. lrreguIarity or discontinuity of stiffness or strength distribution which may result in faHure or large distortion at particular portion of a building must be eliminated by changing stnıctural configuration. It is effective to supplement energy dissipat ing devices in the structure to enhanee the capability to dissipate seismic energy and as a reSUıt to reduee seismic response. Another eoncept to reduee seisroic response is to isoiate existing structu.re from the ground (seismic isolation) as well as to reduce building masses. For important buildings which most be fonctİoned after an earthquake or which must preserve expensive and valuable contents, for example, it is particularly effective approach. Strengthening Techniques Manyapproches and techniques have been studied and practiced for recent twenty or more years to strengthen existing struetures. Some of them include to stiffen existing structure and/or to improve irregolarity or discontinuity in distributin of stiffhes or srength ofa building. The aims of seismic strengthening are to provide 1) increased strength, 2) inereased ductiIity, and 3) a proper combination ofthese two features, so as to satisfy the required seismie perfomıance (Fig. 3). The required perfomıanee is eva1uated in temıs of strength and/or ductility. The combination of mengtb and ductility involves the proper balance between strength and stiffness.
Providing inereased strength is the most promising approach for low- to medium-rise buildings. Even if sufficient ductility is provided. adequate strength is required to reduce inelastic displacement Spandrel waIls may be separated from a column to eliminate "captive column" to increase ductility.
provide back-up structure
(a) increase strength seismic strengthening
(b) increase strength and ductility
infiii existing frames
(c) increase ductility
Required Seismic Performance
brace existing frames place side walls jacket existing membe
Ductility
Fig. 3 Typical strengthening methods
-3
peripheral frames
buttresses
cast-in-situ concrete
precast concrete panel
steel panel
concrete blocks
brickinfill
comp. or tens. braces
tens. & comp. braces
steel or concrete
post-tensioning cables
cast-in-situ concrete
precast concrete panel :
steel encasement steel straps concrete or mortar Lcamon fiber
----'
_
1 Typical strengthening teehniques are summarized in Figs. 3 through Fig. 9. Generaily. new elements are added to existing frames to provide increased strength (Fig. 4(a», or existing framing elements are reinforced with new materials to increase flexura1 capacity (Fig. 5(a» and/or to improve ductility (Fig. 5(b». Infill walls and &ide wal1s are east-m-situ or precastwalI elements to be attached to fraınes or to beams. GeneralIy, walls are of cast-in-situ concrete infil1mg existing bare frame. Steel panel mayalsa be a element to infill existing frame. It is necessary to provide CODDectiOllS alang witJı all the peripheıy when as much strength as that of monolithic wai.l is required. Spandrel walls inside the existing frame may be a part of the infill wall when the opening is ";.
f'--
braees side walls (a) strengthening element
infiJI wall
buttress
L
existing /concret"e ehipping stud balt steeJ frame fr'oôIIilioôiIIiI~"t/,
steel frame ~ ----- b) steel to eoncrete (direct connection)
_' ,
,_
mortar
~~.~}~~,~:
expansion anchor
adhesive anchor d) anchor
(b) Connection _F.ig. 4 Typical fr~e stren8ili:e~~ng tec~q'!.~~
~. ~__
,
r
"C,-
existing
eX!sting
new reinforcement i reınforceme"l. reinforeement / angle at eaeh concrete :f /' ., comer -- concrete hoIetoııPing new hoo ,', .. :
,'" : new longitudinal .....t.'J-ol~'+<,J"1 reinforcement .~
.~;j;
~~~{?z:new existing
eove
' p'late alo~ ; '.' plate • column side a) cOıumn jacket b) Beam jaeket c) Joint confinement steel (a) Jacketing techniques to increase frame flexuraI capacity ties beam
..c..
.g:'fv~~. j,,:\j
steel or eonerete jaeket
Bst
ties. or welqed
eXI'sting 1nereasr "1"':: co umrf ~~e~~~city~~~iı<::~""::~;"";ı:'~_ _---,
ın
"
,
,,"
,/wıre fabries el
en~semen
; '.. mortaror
'conerete
~ng ıd:i gap{
/ e
existing eolumn
steel
raps
i
;ti~gle
~
carbon fiber
i ~Lı ~rarb.
(b) Iacketing techniques to increase column shear capacity Fig. 5 Typical member reinforeing techniques - 4
r~
i_,
infilled with coııerete. An existing StrUetUraI wal1 may be strengthened by placing new coııerete waıı panel along with the existing wat!. Typical details of CODnections to existing coııerete are given in Fig. 4(b). Dowel connections in Fig. 4a) are used for infııı walls and side walls. The expansion and adhesive type anehors for dowel connection are ilIustıated in Fig.4d). Steel elements may be simply attached to the existing conerete through mortar fill, as shown in Fig. 4c), while they may be directly attached to the fraınes by bolting (Fig. 4b). Steel systems of brııces and panels with peripheral framewere studied by Yamamoto (1983, Fig.ll) and Aoki (1992) using the connection in Fig. 4c). The steel elements were welded or bolted to the peripheral frame, and the steei fraıne WLIS attached to existing frame through mortar fiıı. Stud bolts were welded to steel fraıne and adhesive aııdıors were insta1led along the existing frame. These bolts and anehors acted as dowels through mortar fill, though they were not connected each other. This connection tolerates more error in dimension of the steel system lo be .Hacbed to existing frame than the direct eonnection.
F1exural capacity of fraınes may be increased with conerete or steel jaeket shown in Fig. 4(a) providing with new loııgitudina1 and lateral reinforcements. it is importanl to adequately arrange lateral reinforcements to aehieve ductile behavior. Beam-to-column eonnection may need continement with steel element, though the constıuction is not easy (Aleoeer 1995, Hakulo, Park 1995). Column duetility may be improved with jacketing teclıniques shown in Fig. 5(b). An existing column is jaeketed with conerete or steel eneasemenl. in increasing ductility ofcolumns with these tec:hniques, the aim is to inerease their shear capacity providing new conerete and/or reinforcemenl. it is very important to provide a narrow gap at the end of steel or eonerete encaseınentto avoid undesired inerease of shear forces resulting from the increase of flexural eapacity.
Based on the lessons leamed from the damage to steel connections by the 1994 Northridge earthquake, some modification methods for steel connection are proposed (FEMA 1995). The aim of the methods shown in Fig. 6 as examples is to shift the plastic hinge away from the connection using hauneh, cover plale or rib. These
•••
•
•••
•
L
[B]~~i ~ altemate
ceınfiguration
(a) Top and bottom haunch
(b) Cover plate connection (c) Upstanding rib connection, modification modifieation Fig. 6 Examples of post~earthquakem~ficationof steel connection (FE~ 1995)
modifıcation
V
existing slab or girder <>
> . . . arıcltonlge ot
WNmosh
A
welded wire mesh shotcrete
~ ~
O( JI <
~
~ ii
,
bored hale
i
,
steel profile
,
00
hsteel tie
y-
(a) Addilion of RC column (Hidalgo 1995)
(b) Strengthening by (c) Confinement with steel seclion welded wire mesh (Alcocer 1994) Fig. 7 Strengthening ofmasolıry infıll wall -5
,
L
,
• methods can be applied to new construction. Existing masonry infıll walls may be strengthened by the addition of new reinforced concrete coIumn (Hidalgo 1995, Fig. 7(a», by Jacketing with c
',.e
no"
(a) High-damping ruhber
0__
.,','
-mW
••• _ •••
(b)Lcad core rubber (c) Frietion pendulum system Fig. 8 Isolation bearings use
:~~~~~~~-~
,~9.- Di i O __?-.U...ll
i8
(;
//~_,
~~!~<::/
f:s '.!!l j ~ :ır-_.;z:.c=:.:.:>< ..__.__._..~
" .•
[l;
1-
(a) shear yielding damper (akade, Seki 1992)
~A ..~....
center plate
~/ ~::ıst~
\ / )\
.1..'
\-nı-,
D-·IT6uUU~l.LL
:
!
'i
II
at::' ;1 ~
(b) vlscoelastlc damper (c) added damping and stiffness (ADAS) (ScholI1986, 500ng 1993) element (SehoIl1990, Kelly 1993)
Fig. 9 Dampers used fOf seisroic rehabilitatlan isolatian system significantly reduces Ihe
BEHAVıoR OF REHABILITATF..D
sn UCTlJ'RES
ReW'reh Q!!..Rehabilitllted StfUetureS The earliest tests in rehabilitarian research were aiming at repair of damaged S!nleture (Plecnic 1977, Gulkan 1977) and the improvement of column ductility by jacketing with ,teel encasement, steel straps or welded wire fabrics (Sasaki 1975, Higashi and Kokusho 1975, Kahn 1980) They were also aiıning at the boosıing of the strength of frames by the addition of precast or east-m-sim wnııs (Higaslıi and Kolaısha 1975, Kahn 1977, Higashi and Ohkubo 1977) In addition, one-story infilled fraınes with vanaus connection details and bracing systems werc exarnined (Higashi 1980, Sugano and fujimum 1980) Threc-story frames, strengmened by infilling and bracing techniqııes, were aıso tesred (Higaslıi 1984). Further t~ts were those for infıll walls (Aoyama 1984, Ramirez 1992), steel braeing ''}'sterns (Yamamoto 1983, Katsumata 1989, Goel 1992, Aoki
-6
1992) and jacketed columns with steel straps (Arakawa 1980), with carbon fiber (Takeda and Katsumata 1988, 1992), with SteeI encasement (Yoshimura 1992, AboutaJıa 1994) and with concrete (Park. Rodoriguez 1992). Beam~lumnjoints ofjacketed frames were a1so tested by A1cocer 1992, and Pıuk and Hakuto 1995. Shear transfer at the connection between new ans existing elements was another issue in strengthening. The behaviors of fasteners and connections were investigated by Eligehausem 1988, Jirsa 1988, Shimizu 1988, Akiyaına 1992, Hosokawa 1992 and Valluvam 1994. Behavior of Strengthened Frames Eıcamples of the
behavior of strengthened frames with various construction techniques are shown in Fig. i O (Sugano and Fujimura 1980). Infill wal1 behaved similarly to monolithic wall, though the strengtb was s1ight1y less. Concrete blocks alsa extensively increased the strength of original frame. Tension braces provided good ductility properties while compression braces and steel panel did not develop their yield strength due to the failure of existing columns or connections. The behavior of strengthened frames with steel systems are summarized in Fig. 11 (Yamamoto 1983). Both the X and V braces and a panel with opening were capable of significantly improving not only strengtb but a1so duc1ility of original mme. The double K braees (Aoki 1992) a1so indicated significantly improved both strength and ductility of original frame. Note that even a steel peripheral frame a10ne could significant1y improve both the strength and ductility. Another receııt test of V braces system with binge device at the joint to the steel frame (Okada, Seki 1992) indicated significant1y increased energy dissipa1ing capability resulting from yielding of shear paneL. Typical load-displacement relationships of strengthened mmes with various eonstruetion techniques are iIIustrated in Fig. 12. This is only qualitative indication of the order of strengtb and duc1ility that miglıt be attained using ditrerent techniques. The findings from the figure were summarized as follows. i) When adequate connections were provide
-7
_n
ISx25 I70cm
of
f
.
~hoig~·~ht~!92~.5cm~!.Jı:iU:~Ef- -1 il ,2.0 R
-
original frame
il
,r,
p ~ bıd (iOSN) - R : drift (!Ili)
t - .... -S.ll -1.11,
concrete blocks
f'
-5.0 1.0 it
posteast wall
__ _ ___ tension braces
Fig. 10 Hysteretic behavior ofstrengtheııed frames with various techniques (Sugano, Fujimura 1980) X-braces
f--20Ocm
~O
i
8.0
35crrl
X
H - BO x BO x 4.5 x 6
X - braces
~ 1\braces
EI panel Fig.
i
V~
181
panel
M - braces
1''1
......---.-- Gl
,/ M-braces
~
oıiginal+
i!! 4.0
steel frame
~ /,/
K - braces
brace: H-80xBOx4.5x6
panel : F-PL-6xSO
..
(Aolei)!
,l
!
i fra me
)~/
/
./
//
o
1.0
3.0
2.0 story drift index(%)
i Behavior of strengthened frame with steel system.s_(yamamoto ı 983, Aoki ı 992) -"------
---~
lBil ~
Vw
1\-braces orıgına
W-PL-4.5 ı
-------K-braces})
monolithick wall
\
--.-
_._-----
KokuShO (1975), Sugano, Fujimura- (1980), Higashi (1980), Hayashi (1980), Aoyama (1984)
infilı wall
+~ . ~
0.60 - 1.00Vw
~LI y~:~;o(1983) ~ .•.•_." .....•
L......c......c
steel framed bra
~.:
i concrete bloclcs
if.-= .Hi ;\; ;~ ~~~::~~~~ura ]
....• .. multi-story infin wall
(1980)
Higaslıi
Abram (1993), Shuller (1994)
~_ _----""'---l~~
v
~
•
(1984)
J:DIIJL ~~~ .". ' ---- rıi~~; ame
0.5
1.0
1.5
story drift index (%)
Fig. 12 Typical load-displacement relationships of strengthened frames with various techniques
--
- 8
l
t::.:
llftnstrengthened
O
i
'-o __
LJ
----------:;)f.,------/~..
/'
'
-
i
1- ~ ~cfp~
~IE·.....j,!--=:::-=·F-i ~[db ~ ~ d_
'
i
onginal column •
i
/
ol
CE CE-S ii . LE LE-S
'I
8.0
".=
,v"LE-S
k
CE-S
...,...LE CE , drift index(96) ,
~-
n~-in-Situ ~
Higashi, Ohkubo (1977)
>
•
steel
jacket
CIL
side wall
/
Park, Rodriguez (1992) concrete jacket
Yoshimura (1992)
t
Sasaki (1975)
~
o
~
'\
i\
~
CIL
til
II
V
/
steel jacket separation of 5pandrel wall
,. ,
Higashi, Kokusyo (1975) _Arakawa ' \ (1980)
/
ii:
concrete steel carbon jacket straps fiber
Takeda, Katsumata ( 1988) ~
t
Aoyama,lchinose (1984)
,... 1.0
2.0 3.0 O story drift index R ('Ko) Fig. 14 TYl'icalload-displacement relationşhips of eolumns. reinforeed with various techniques
SEISMIC REHABillTATION OF EXlSTING BUllDINGS Postearthquake Rehabilitation Recent over 25 years, many buildings were rehabilitated after the damage by destruetive earthquakes using variOllS types oftechniques. The enteria how to restore adamaged building depends on the level ofits damage and the intensity of the earthquake whieh caused the damage (Table I). GeneralIy they were rehabilitated to upgrade their seismic performance so that they may meet the requirement of code in force. The major aims of the rehabilitation are to recover onginal function of the building and to prepare measures ııgainst possible stronger eartlıquake. Used techniques were in wide variety. in early cases, mostly conerete shear wa1ls were selected because they were capable ofproviding large lateral resistance. The building in Pboto i W8S severely damaged by the eartlıquake of 1968 (Tokachi-oki earthquake) and it was rehabilitaled one year after the earthquake by placing eonerete walls a10ng with repair of damaged columns in shear. It was first experience
- 9
Photo 4 Conclllte jacketing
(after 1985 Meı
Pboto 3 Strengthening witlı hutıresses (after 1980 Livermore earthquake)
Photo 5 Sıeel jacketing
(after i 985 Mexico earthqua.ke)
Photo 6 Rehabilitation with seismic isolation (after 1989 Loma Prieta earthquake)
for Japanese engineers to extensively strengthen eıeist;ng strucıure for furure earthqua.ke. The shear wall sırengtbening, however. lakes disadvanıages due to 1) increased weight of the huilding, and 2) separation of space Diagonal steel bracing is aoother solution to provide large lateral resistance of a building, in addition. possible large displacement capacity. In the huilding in Photo 2 which suffered _'ere damage due to shear [ailure of shon coIumns by the 1978 Miyagikeıı-oki earthquake. steel diagonal hraces were plaeed a1ang both longitudinal exterior fnımes through the stories The inereased lateral resistance and the ductility were veriHed by labaratary tests (Kawamata 1980) The sıee! bracing can provide large lateral res;st8nce ond large opening for lighting without incressed weigbt followed by the rehabililation. Buttre5ses were used for the building in Photo 3 damaged by the Livermore eanbquake of 1980 (Freeland 1984). Sharpe 1990 repoıted another case ofbultress strengtbeııing for anoıher building in the same site. The - 1 0
if
buttress does not disturb interlor building space and function., therefore, it is effective method when outer space is sufficiently provided. Photos 4 and 5 show jaeketing of eıcisting frame elements with conerete and steel straps wbieh were seen in Mexico City after the earthquake of 1985. Because of significantly increased design seismic forces after the code revision, massiye jacketing was necessary to meet the code requirements. The building in Photo 6, consisting of wooden bearing walls, was affected by the Loma Priete eartlıquake of 1989 and &eismic isolation was adopted for rehabilitation. Friction pendulum system was used for isolation bearing. Reııtonııjon ofPamaged Buildings
in Kobe
A large number of reinforced concrete, steel reinforced conerete and steel buildings in Kobe City suffered severe damage by the eart1ıquake of 1995. Maııy damaged buildings needed repair for reuse while collapsed ol" some of severely damaged buildings were demolished. The criterla for restoration depends on damage level and the intensity of the earthquake which damaged the buildings. Table i shows a proposed eriteria for restoration (Japan Building Disaster Prevention Association 1991). in the areas which reported seismic inteıısities VI (in JMA scale) or greater, restoration by only "repair" was underway in most of damaged buildings. The restoration design and construetion for damaged buildings were achieved following existing guidelines. Figure 15 shows examples of recommended repair techniques for reinforced concrete and steel bui1dings (Building Center of Japan 1995).
Table i Criteria for restoration of damaged buildings Dam e Level Li t Minor Medium Seismic lower than 5 L::ı. X 1ntensity 5 O O .6. Scale(JMA) higherthan 5 O O O restoration by : Orepair, L>.repair or strengthening, X strengthening or demolitian
'or X X .6.
CoIla se X X X
JMAScale ~ JMA:JopanMetoorological MMScale~ Agency
Damage rank
Rank ın or less
Repair
method
RankIY
u
Sketch of damage
u
Repair cracks Grout mortar or cast concrete Jacket with conerete Repair cracks and Jacket with welded wire Jacket with steel plate and grout partial loss of concrete fabrics and morter mortar Jacket with steel plate, add vertical Repair partia! lass of Jacket with steel plate and wout morter reinforcementand grout mortar conerete (a) Repair of damaged concrete columns
welding after PlIH I-ifH- gouging
~IJ=====ll,,, ~ ~ " i
..
i
i~
'- f-~
•
.. i
\\
",
,ı---------".
stiffening plate Ruptured base metal
\,\\
tt'
stiffening plate Buckled H-section
Cracked welding portion
(b) Repair of damaged steel members Fig. 15 Examples ofrepair techniques ~r 1995 Kobe Earthquake (Building Center ofJapan 1995) - 1 1
-
,
•
•
preeartbquake Rehabilitation Since the publication of the "Stanılards for Seismic Capacity Evaluation of Existing Reinforced Concrete Buildings" (Sapan Building Disaster Prevention Association 1977) a large number of existing buiidiags have been evaluated in Sapan. PartiCUıarly, several ageııcies in charge ofadrninistration of a number ofpublic build ings have performed the evaluation as a part ofseismic countenneasure program. The cases of such evaluatiOll of the Shizuoka Prefecture, Yokohama City and the Tokyo metropolis are suınmarizedin Table 2 (Hirosawa 1992). The Shizuoka Prefectııre, where a magnitude 8 or more earthquake is presunıed, has evaluated 1896 public buildings including schoals, city offices hospitaıs, ete by the year of 1986. Sixty five or more perceııt buiIdiags were judged to be rebuilt or to need rehabilitation. 465 building were actually rehabilitated during the Table2 Examples of rehıi>ilitationproject ofpublic buildings in Japan Municipa1ity
SeismicEvaluation No ofBldgs Use Shizuoka 2078 ~hool, Prefecture cıtv office. ete. Yokobama 870 school, City city office, etc. apattınent Tokyo 48 Metropolis houses reference: Hirosawa 1992, Usami et al 1988
Sei sınic Retrofit Period No orBldgs Period Construction Technigues '77-'86 465 '82-'87 infill walls. stee! braces and panels, column ıacket '82-'84 30-40"10 '87 steel braces, columnjacket of the total underwav 46 '81-'83 infıll walls, side walls, '79-'81 columnjacket
Table 3 Seismicallylsolated Buildings for Rehabilitation Building(city,counUy,yr.completed) Salt Lake City and County Building (Salt Lake City, Utalı, USA 1989) RockweIl Seal Beach Facility (SeaI Beach, Califomia, USA 1991) Mackay School ofMines (Reno, Nevada, USA 1991) Marina Apartments (San Fmncisco, Califomia, USA ı 994) ClıanningHouseRetirement Home (palo AI to, California, USA 1994) LongBeach Hospital (Long Beach, California, USA 1995) Oa1dand City Hall (Oakland, California, USA 1995) U.S. court of Appeals (SanFrancisco, California, USA 1995) KerckhoffHalI, UCLA (San Francisco, Califomia, USA 1995) San Francisco City Hall (San Francisco, Califomia, USA 1996) Los Angeles City Hall (Los Angeles, California, USA ı 996) ParliamentHouse (Wellington,New Zealand, 1995) ParliamentLibrary (Wellington,NewZealand, 1995) Houtoku-Ninorniya Temple (Odawara, Japan, 1997)
Original Structural System Floo~size (of)
lsolatiOll System
Steel braced frarne
5/1 6000
LRB
RC moment frarne
8128000
LRB
URM bearingwall
3/4700
Wood bearingwall i Steel moment frame RC frarne i RC shear wall
4/1900
lIDR&PTFE
sliders
FPS
11/19600
LRB
RC shear wall
12/33000
LRB
Steel framel URM infill
18/14000
LRB
Steel frarne i URM infill
5/33000
FPS
RC frarne/URM infill
6/9300
LRB
Steel frarnel URM
5156000
LRB
Steel framel RC shear wall IURM infıll URM bearing wall
32/82000
lIDR&PTFE sliders
5/26500
LRB
516500
LRB
URM bearingwall Wood frame
LLL 12
Rubher Bearing
+ V.Damper
RC : Reinforced Concrete URM : Umeinforced Masamy lIDR : High Damping Ruhber Bearings LRB: LeadRubberBearings FPS: Frietion Pendulum System PTEE: Poly-Tetra-Fluoro-Ethylane Reference: Mayes 1995, Soong 1992, KeIly 1992, Sapan Society of Seisrnic Isolation 1996 - 1 2
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period of 1982 to 1987. The major construction technique was to use concrete imin walls so that they may provide very high lateral desistance against the presumed huge earthquake. However, in later part of the projec::t, stee1 systems in Fig. 11 were alsa used to avoid the increased weight ofbuilding associated with the StreDgthetıiııgand to provide large opening for lighting.
in the USA, also many bui1dings have been evaluated, particularly, since the Lama Prieta eartbquake of 1989. The rehabilitatian project for UCLA campus buildings, for example, is reported (ingham 1994). Both dam aged or undaınaged buildings may be rehabilitated using conventional techniques (for exarnple, Amin 1994). New techııiques such as seismic isolation and supplemental damping, however, are used recently (Hart 1994, Mayea 1995, Soong 1992). Particularly, seismic isolation has been selected in more than ten buildings for rehabilitation (Table 3). The motivating factors for selecting seismic isolation are (Soong 1992); 1) Historicaı bui1ding preservation,
2) Functionality,
3) Design economy
4) Investment proteetion, and
5) Content proteetion.
The adapted isolating bearings are l) high damping rubber bearings, 2) lead rubber bearings and 3) friction pendulum system. The tecbniques has been used mainly for historical buildings or important building since it was first applied to the historical Salt Lake City and County Building (Table 3).
BEHAVIOR OF REHABILlTATED BUll.-DING DURING BARmQUAKE Most of extensively rehabilitaled buildings have not yet experienced strong ground motion. Only a couple of cases ofbui1dings which experienced strong ground motion after rehabilitation were reported. The followings are the bui1dings in such CBSes. A twelve-story reinforced concrele frame bui1ding in Mexico City was repaired and strengthened after suffer
ing extensive damage during a moderate earthquake that shook the city in 1979 (photo 7). The building suf fered no damage during the event of 1985 even though the shaking was mueh greater than that in 1979. The results of forced vibration tests and analyticaı studies indicated that the steel braced frarnes that were attaehed to the building for strengthening stiffened the strueture, shifting its natural period away from the predominant ground period of 2.0sec (Del Valle 1988). This is the case of successfu1 rehabilitation and similar successful case with steel bracing was reported by Hjelmstad 1988. A three-stoıy reinforeed eoncrete building in Photo 8, whieh is structurally ideııtical to the building in Photo I, suffered severe damage to first stoıy columns due to the earthquake of 1968. The building was rehabilitated with shear walls at the damaged first Story only. Unstrengthened 2nd story columns suffered severe shear cracks due to the recent earthquake of 1994. While adjacent building in Photo 1 which were rehabilitated up to 2nd stoıy did not suffer any severe damage. Importance ofbalanced configuration of strengthening elements is indicated in this ease (Nakano 1995). An old building in Photo 9 was eonstrueted in 1918 in Kobe and ten month before the event of 1995, construc tion for seismie rehabilitation was eompleted. Existing briek walls were eonfined by new concrete frarnes and existing conerete frames were reinforeed with new steel frames arranged along with the emting frarnes. AB a result of the countermeasure, the building did not suffer any damage while some other historicaı bui1dings suffer severe damage in the same area. This is very encouraging ease to indicate the effeetiveness of preearthquake rehabilitatian (Nikkei Architecture 1995).
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Phoıo
7 A rehabiliwed buildiııg iıı Mexico betbre the emtIıquake of 1985 (nadamage)
Ciıy
Photo 9 An old building in Kobe relıabilitated ten month bclore the earthqualce of 1995 (no damage)
Photo 8 A rehabilitated building v..,th concrele walls after 1968 Tokachi-olci earth quake. Unstrengthened 2nd sl.Ory suf fered severe dam_ge by the earth quake of 1994
CONCLUDL"iG RE.YARKS The present state of techniques for seismic reb_bilitation of existing buildings has been overviewed based on the survey of !iıereturesanddata ofresearch and practice. The reaulıs oft/.e revieware summ8IİT.ed as folJows. ı) Because
of the experience of earılıquake damage and the data availabe, most teehniques described herein are those for reinforced concrele buildings. Many typical teehniques LO strengthen existing stn.ıcıu~ have been well investigated in terms of impmved penormance and they have been utilized A1though a few data lıave been availahle regarding me rehabilitaion of steel struetures, the recent ıwo earthqaııalres, the ı 994 Northiridge earthquake and the ı 99~ Kobe earrhquake would have .trongly pııshed the studies on rehabilitarion of steel struetures, therefore, more data will be available in future. The concepts of strengthening methods deseribed here for brick imll walls to contine ",ith COOcTete frame or with steel elements. or to jacket witb concrete and lateral reinforcement can be commonly used, though m_terials, design and eOllStrution may be different in eaeh region
2) in addilion to convenlional seismic resistaııt type rebabilitalion techniques, another approach to isoIale the exisling stnıeuture from the ground or to supplement energy dissipating devices to reduce seismic response have been adopted. Seismic isolation can he applied to critica! or essential facilities, huildings with expensive and valuable comenıs, and structuTes where superior seisnıic penormance is desired. TheiT applicalions are only in sınll numbers now, however. they wiıı be widely used for seismic rehabilitanon. 3) Rehsbiitalion techniques may be seleete
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