Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanDAMAGE INVESTIGATION AND SEISMIC RETROFITOF BRIDGES IN TAIWAN AFTER 921 CHI-CHI
EARTHQUAKEYu-Chi SUNG1, Kuo-Chun CHANG2, Dyi-Wei CHANG3,
Hsiao-Hui HUNG4 and Kuang-Yen LIU5
1

Professor, Department of Civil Engineering, National Taipei University of Technology
Taipei, Taiwan, sungyc@ntut.edu.tw
2
Professor, Department of Civil Engineering, National Taiwan University
Taipei, Taiwan, ciekuo@ntu.edu.tw
3
General Manager, CECI engineering consultants, Inc.
Taipei, Taiwan, dwchang@ceci.com.tw
4
Associate research fellow, National Center for Research on Earthquake Engineering
Taipei, Taiwan, hhung@ncree.narl.org.tw
5
Associate research fellow, National Center for Research on Earthquake Engineering
Taipei, Taiwan, kyliu@ncree.narl.org.tw

ABSTRACT: The devastating Chi-Chi earthquake on September 21, 1999 broughtcatastrophic disaster to the central part of Taiwan. Some of the bridges located in thisarea suffered severe damage. Damage to these bridges includes fault rupturing, collapsed
spans, landslides, soil settlement, slope failures, flexural and/or shear failures, andliquefaction. These damages made it necessary for part of these bridges to be retrofitted
or rebuilt and also lead special attention to the upgrade of seismic design and retrofit in
Taiwan. The Chi-Chi earthquake brought an intense impact on the seismic design andtechnical development of bridge engineering. Based on the investigations of existingbridges damaged in Chi-Chi earthquake and the significant amount of retrofit research
and actual implementations of seismic evaluation and retrofitting on existing bridges, theseismic design code in Taiwan was revised and a manual related to seismic evaluation
and retrofit for highway bridges was proposed. This paper begins with a brief description
on the damage investigation of bridges located in the catastrophic region after Chi-Chi
earthquake, followed by a general depiction of the urgent recovery, repair andreconstruction of the damaged bridges. In the end, the Seismic Retrofitting Manual for
Highway Bridges proposed by NCREE (National Center for Research on Earthquake
Engineering) was also introduced. This manual includes current advances in earthquake
engineering, the performance-based seismic evaluation technique, and retrofitting design
and measures.
Key Words: Chi-Chi earthquake, highway bridge, seismic assessment, seismic retrofit

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INTRODUCTION
Chi-chi earthquake with the magnitude of ML = 7.3 struck the central region of Taiwan in theearly morning on September 21, 1999. Approximately 1,000 highway bridges, on the provincial orcounty routes in Taichung, Nantou, Changhua, and Yunlin counties, escaped from serious damages,
while approximately 20% of them suffered minor-to-major damage due to fault rupturing, collapsed
spans, landslides, soil settlement, slope failures, flexural and/or shear failures, and liquefaction. This
paper begins with a general depiction of the performance of highway bridges located in the four
counties during the earthquake. Several major damaged bridges with typical damage modes areillustrated and explored. Lessons learned from the field observations are also discussed.
Most of the severely damaged bridges serve as the vital route in these areas and the traffic flowneed to be restored in a short time, so the immediate decision for the urgent recovery plan and thestrategies for seismic retrofit/reconstruction are crucial. At the time shortly after Chi-Chi earthquake,several adverse conditions were against the reconstruction design project. For instance, the design
schedule was tight, causes of failures were not well documented, local design code related to new
devices, such as isolation devices, were not available, and a need to revise “Seismic Resistant Code forHighway Bridges” according to the newly found characteristics of Chi-Chi earthquake was not yet
fulfilled, too. In order to achieve the goal of recovering the traffic flow as soon as possible, the top
priority for the reconstruction design was the upgrade of seismic safety and the reduction of
construction time. In this paper, a brief description on the seismic retrofit strategies, which include
urgent recovery plan, repair and reconstruction strategies is provided. The reconstruction experiences,including several main points of reconstruction design, were also briefly presented.
The extensive damages of bridges due to the Chi Chi earthquake lead special attention to theupgrade of seismic design and retrofit of bridges in Taiwan. Based on the investigations of existingbridges damaged in Chi-Chi earthquake, a significant amount of retrofit and seismic evaluation
research was performed during the last decade. According to these researches, the seismic design code
in Taiwan was revised and a manual related to seismic evaluation and retrofit for highway bridges was
proposed. In addition, several actual applications of seismic evaluation and retrofitting on existingbridges were completed. In this paper, the manual related to seismic evaluation and retrofit for
highway bridges will also be briefly introduced and the retrofitting project performed by Taiwan’s
National Freeway Bureau and Directorate General of Highway based on this manual will also be
summarized.
DAMAGE INVESTIGATION
The primary disaster area in the 921 Chi-Chi earthquake is composed of four counties, Taichung,
Nantou, Changhua and Yunlin. There are approximately 1,000 highway bridges spread on the main
provincial and county routes in the disaster area. The construction completion dates of those bridges
range from 1960 through 1999, and hence are involved in the evolutionary history of seismic designcodes. Table 1 summarizes the locations and damage levels of the inspected highway bridges. In thistable, 80% of the bridges performed well without damage, 17.3% of them suffered minor to moderate
damage, and 2.7% of them received major damage and even collapse.
Table 1 Summary of damaged highway bridge in 1999 Chi-Chi earthquake.
Numberbridges
Non damaged
Minor-to-moderate damaged
Taichung
196
131
52 (26.5%)
Changhua199
182
17 (8.5%)
Nantou410
315
82 (20.0)
Tyunlin176
158
18 (10.2)
Summary 981 (100%)
786 (80.1%)
169 (17.2%)* the percentage is based on the total inspected bridges in each county.
County1835

Major damaged
13 (6.6%)
0
13 (6.6%)
026 (2.7%)

Most of the highway bridges escaped from severe damage and experienced only minor distresssuch as the settlement of approach fills behind abutment back-walls. Approximately 20% of the bridge
inventory suffered minor-to-major damage. Damage to these bridges include collapse of
superstructures, displaced bearings, unseated girders from bearing supports, shear failures in columns,
pier walls, and caissons, abutment back-wall failure, settlement of approach slab, foundation failuresdue to slope instabilities, joint failures in column-to-girder connections, cable fracture, fault rupture,and liquefaction. However, only Lyu-mei and Wan-lun bridges located in Yuen-lin town were observed
to have liquefied appearance since it was not easy to distinguish when it occurred in a flowing river.
Table 2 shows a summary of comparison among 11 different damage modes. It is seen that in these
195 bridges, the ratios of damage to auxiliary facility, abutment, deck, and approach slab are 35.6%,
32.5%, 26.3%, and 21.1%, respectively, significantly larger than those of other damage modes. From
the field observation, it was found that pounding on abutments due to excessive longitudinal vibrationof the superstructure usually occurred with damage to the approach slabs and/or bridge decks. The
deck damage defined in this paper includes the expansion-joint failure.
Regarding to the classification of damaged bridges based on their completed years, extent ofdamage, and structural types, it is found that over half of those damaged bridges were constructed
before 1989, generally before the issue of “Design Code for Highway Bridges” in 1987. Moreover,
over 95% of those injured bridges before 1989 were simply supported. The decreasing percentage of
injured, simply-supported bridges after 1989 may be attributed to the decreasing of simply-supported
bridges and better seismic design methods as well as construction technologies.
Table 2 Summary of comparison among 11 different damage modes
Mode
1
2
3
4
5
6
7
8
9
10
11

Item
Numbercollapse
9
deck51
girder16
bearing18
pier cap
12
column / pier21
foundation
14
abutment
63
approaching slab
41
land slide
19
Ancillary facilities
69

Percentage (%)
4.6
26.2
8.2
9.2
6.2
10.8
7.2
32.3
21.0
9.7
35.4

Table 3 lists all the major damaged highway bridges in the inventory, the associated routes, andtheir primary damage modes. The collapse of the seven bridges was due to fault ruptures crossing thebridges directly. These collapsed bridges have attracted more attention from the researchers andgovernment agencies, while a large number of bridges which experienced minor-to-moderate damage
have not been well discussed and documented. For example, the Wu-Shi bridge with a recorded peakground acceleration (PGA) of 388 gal is located across the Chelungpu fault, as shown in Fig. 1. The
damage includes the span falling and pier damage at the third span. The first two spans from thenorthern abutment were unseated due to the ground movement. However, the right-side piers did notexperience massive shear failure, but the bearings and concrete shear keys were severely damaged.
The piers on the left side suffered major damage due to the large transverse forces transmitted from thesuperstructure without damaging the bearings and concrete shear keys. A similar failure mode is also
found in the Don-Fong bridge (492 gal PGA). Figure 2 shows the relative movement of the spans andthe sliding mechanism. The movement of the superstructure and the dislocation of the bearings haveresulted in settlement between adjacent deck slabs. Similar examples of the sliding mechanism of the
bearings were also observed for the Yen-Feng bridge (500 gal PGA) and Shin-Yi bridge (545 gal
PGA). Under the strike of the Chi-Chi earthquake with high PGAs (greater than 500 gal), thesuperstructures were moved and some bearings were displaced, but the piers suffered only1836

minor-to-moderate damage. The same damage to bearings was also observed in the Kuan-Lung bridge(439 gal PGA). Although abutment fill settlement occurred and the expansion joints were largely
pulled out, the bridge column performed well during the earthquake.
Table 3 Major damaged highway bridges
Name
Shi-wei
Chang-geng
Dong-feng
Pi-feng
E-jianWu-shi
Mao-loh-shi
Ming-tsu
Ji-lu
Tong-tou
Guang-long
Guan-deBei-keng
Long-anCheng-feng
Yan-feng
Pu-ji
Hsing-shi-nan
Yan-ping
Hsin-yiLong-men
Li-yu
Ping-lin
Mo-keng No.1
Mo-keng No.2
Da-feng

RouteProvincial 3
local
Provincial 3
local
County 129
Provincial 3
Provincial 3
Provincial 3
local
County 149
local
local
County 129
County 129
County 136
Provincial 14
Provincial 16County 127
Provincial 3
Provincial 21
Tou 53
Tou 53Tou 6
Provincial 16
Provincial 16
Chung 105

Year
1994
1987
1962/1988
1991
1972
1981/1983
1999
1990
1999
1980
1986
1977
1959
1986
1986
1984
1979
1994
1986
1981
1982
1988
1969
1996
1996
1992

Span (m)
25
25
26
25
11
34.7
40~70
25
150
40
28
20
5.7
35
25.6
35
35
50
13
29
40
39
25
14.6
40
-

Length (m) Damaged mode
75
collapse300
collapse572
girder/column
300
collapse
264
collapse624
collapse/column
500
column700
collapse300
pylon/bearing160
collapse
56
deck/abutment
60
collapse
5.7
deck/abutment
280
column184
column/abutment
455
column105
pier cap
500
column/bearing
78
abutment
180
column480
collapse546
bearing500
collapse/column
14.6
abutment
40
abutment
deck

Fig. 1 (Left) Collapsed spans and damaged piers of the Wu-Shi bridge at the third span
Fig. 2 (Right) Sliding/friction mechanism of bearing of the Don-Fong bridge
The aforementioned six bridges with sliding mechanisms in the bearing systems suffered only
minor to medium damage to the bridge column during the severe shaking. The damage pattern issignificantly different from the major damage modes to bridges observed in the Loma Prieta andNorthridge earthquakes. Although it is easy to understand that damage to a bridge results resulted fromthe high PGA and large ground distortion, the underlying causes of these phenomena may be1837understood by discussing the structure system and comparing the damaged components of bridges.
Multiple-span simply supported bridges with PCI girders are the most commonly seen bridge type inthe existing bridges of Taiwan constructed before 1990. The PCI girders are supported by the rubber
bearings on the cap beam. According to the service purpose, rubber bearings can be considered as ahinge or a roller depending on the details of the bearing systems. For the hinge situation, one
low-strength steel rod is installed in the center of the rubber bearing pad to limit the movement of thesuperstructure due to thermal effects, traffic load, or seismic force in the longitudinal direction.
Besides, two concrete shear keys are also installed on each cap beam to prevent the girders from
unseating in the transverse direction due to a large earthquake. Therefore, because of the difference ofstrength capacity and rigidity of the bearing system in the longitudinal and transverse directions, it isexpected that the shear forces transferred from the bearing to the column will result in more damage inthe transverse direction than that in the longitudinal direction. Also, it is concluded that the bearing
damage can limit the damage to the columns in most circumstances. More importantly, from the pointof view regarding the post-earthquake functionality, these bridges with slight damage to the columns
can be quickly repaired in a very short time and can be used as emergency routes for rescue work.
SEISMIC RETROFIT/RECONSTRUCTION STRATEGIESChi Chi earthquake caused severe damage on several bridges located in central Taiwan. The traffic
break down due to the damage of bridges resulted from earthquake also interfered with the ongoing of
rescue activity to a large extend, so how to retrofit the damaged bridges and recover the traffic flow ina quickest way became an urgent issue. Under the prerequisite that the traffic must be opened up
within a limit time and the bridge can provide a higher degree of earthquake protection in the future,
the recovery strategies include three levels: (1) immediately recovery strategies; (2) retrofit strategies
and (3) reconstruction strategies.
Immediately recovery strategies
After 921 Chi-Chi earthquake, damaged bridges are immediately recovered depend on thedamaged levels, location and emergency conditions.
For the severe damaged bridge with nearby substitutive roads which still remain function, bridge
was closed and emergent repairs or reconstructions were performed during closing period, such as
Wu-shi bridge, Mao-luo-shi bridge, and Hsing-shi-nan bridge.
For the severe damaged bridge without substitutive roads around, temporary bypass or sidewalk
was constructed by embedding RC or steel culverts, filling gravel and sand into cargo boxes, andbuilding temporary steel bridge. Embed RC culvert is useful for a bridge located at a short span andmiddle-depth river. Short time recovery is the major advantage. Engineers can buy pre-cast RC
culverts easily and embed them in the river, and then pave AC on the upper level to open to the traffic.
However, massive sands and rocks accumulated in the river course are not beneficial for the runoff in
the period of flooding. Shi-wei bridge (Fig. 3) is the typical example. With the same reasons of RC
culverts, embed steel culverts are also used a lot for the immediate recovery. It should be noticed in the
period of flooding. Tonng-tou (Fig. 4) is the typical example. Heap cargo boxes is adopted for bridges
with high columns or bridges located at a short span and deep river. Compared to embed RC/steel
culverts at the same place, saving time and space are its advantage. Engineers can fill the cargo boxes
with gravel and sand first, and then heap them to adjust tilted bridge to the original height. Ming-tsu
bridge (Fig. 5) is the typical example. Temporary steel bridge is adopted for bridges located at middle
span and middle depth river. The benefit is to allow discharge, but the construction price is higher.
E-jian bridge (Fig. 6) is the typical example.
As for the moderate damaged bridge, bridge was supported by steel frame, and truck weight andspeed were limited. It is suggested to support the moderate damaged bridge with steel frames, if nosubstitutive roads can share its daily traffic. To prevent further damages, Engineers should investigate
and repair the bridges, and limit the weight and speed of traveling cars in the emergent supported
period. Tong-fong bridge (Fig. 7) and Hsin-yi bridgeare typical examples.

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Repair strategies
For minor damaged bridges, some necessary measures are applied to request sufficient time for thefurther repairing works. For example, CFRP jacket in Hsing-shi-nan bridge (Fig. 8), steel jacket in
Mao-lou-shi bridge, and RC jacket in Yan-ping bridge and Cheng-feng bridge (Fig. 9) are ultilized.
One special example of expanding cap beam width and adding columns was used in Yan-feng bridge.
It is because longitudinal reinforcements were cutoff on the top surface of the columns. In addition, in
Yan-Feng bridge (Fig. 10), because large dislocations occurred at pot bearings, box girder were
removed to original place and bearings were replaced.

Fig. 3 Shi-wei Bridge

Fig. 4 Tong-tou Bridge

Fig. 5 Ming-tsu Bridge

Fig. 6 E-Jian Bridge

Fig. 7 Tong-Fong Bridge

Fig. 8 Shi-shi-nan Bridge

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Fig. 9 Cheng-feng Bridge

Fig. 10 Yan-feng Bridge

Reconstruction Strategies
For a severe damaged or near collapse bridge reconstruction could be an economical choice thanretrofitting or repairing. The primary concern following the earthquake was the need to reestablish
these transportation routes that are vital to the local and global economy as soon as possible. Therefore,
the reconstruction of all the damaged bridges had to be completed in a short time. At that moment after
the earthquake occurred, the collection of earthquake data and revise of new design code were stillproceeding. Reconstruction design was mostly in accordance with the experience of the otheradvanced countries. In addition, since the priority of this reconstruction work was the speed of
construction and the upgrade of safety, but not the reduction of production cost, the basic
consideration and principle of reconstruction design were slightly different from average cases. The
basic consideration of reconstruction includes the following points:(1) The severe damaged or near collapse bridges were reconstructed as soon as possible. Remaining
parts of the damaged bridge with no obvious damages observed should be preserved temporary.
However, in order to strengthen their seismic resistance and ensure an equal seismic capacity with
the nearby bridges, they are suggested to be retrofitted in the future.
(2) Due to the limited time, the extent and level of bridge damages and the reconstruction method were
determined based on visual inspection. During the construction, if new damages which may
jeopardize the safety of bridges were found, other necessary retrofit or protective measures should
be taken.
(3) In consideration of the feeling of the people living in disaster area, the major consideration of thisreconstruction design was the upgrade of seismic resistance, but not the reduction of production
cost.
The principle of reconstruction design includes the following points(1). A peak ground acceleration of 0.33g was used to determine the minimum design horizontal
seismic forces of these bridges. The value 0.33g is the maximum design ground acceleration
regulated in “Seismic Resistant Design Code for Highway Bridges” of Taiwan at that time.
(2). In order to decrease dead loads of superstructure, thereby to reduce the inertial forces induced byearthquake, the preferred construction type for replacement bridges was the use of composite
decks supported by continuous steel plate girders. By doing so, the construction time can beshorten as well.
(3). To reduce the potential of collapse span, continuous bridges with multi-span were adopted.
(4). To regularize seismic force distribution on different piers and provide an effective way to dissipate
seismic energy conducted on bridges, internationally recognized seismic isolation devices wereadopted.
(5). To avoid the original foundations and reduce the blocking of water flow, bridges with long spanand fewer fundations were adopted.
(6). The design details should conform to the requirement of ductility design, so as to increase theinelastic deformation capacity of bridge system.

1840

(7). Sufficient unseating prevention devices that can mitigate the risk of unseating of bridge span andabsorb the shock induced by seismic forces should be installed.
The main points of reconstruction design
1. Selection of bridge site(1) Due to the fact that most of the bridges which need to be reconstructed serve as importantroutes in central Taiwan, the need for recovering the flow of transportation was urgent andthe time left for design was limit, to reselect bridge site became impractical. As aconsequence, all these bridges were rebuilt on the original site, which is the quickest way toconnect the original route.
(2) Chi-Chi earthquake was a result of the rupture of the Chelungpu Fault, a major reverse fault.
Its magnitude and the damage caused was so tremendous that it’s rarely seen in hundreds
year. Therefore, the probability to induce a major fault rupture (displacement due to the
movement of tectonic plates) at the same location in a short time is not high, even though its
probability to suffer from the ground shaking of earthquake is still high. As such, the bridges
were reconstructed at the original site.
2. Use of seismic isolation devices
In view of the reconstruction experience of United States and Japan in the aftermath ofNorthbridge earthquake and Kobe earthquake, respectively, seismic isolation devices were adopted toreduce the transmission of the earthquake motion to the structure. Among the various types of isolation
system, Lead Rubber Bearing (LRB) and High Damping Rubber Bearing (HDR) were most commonlyand had been matured enough for wide-ranging use in the earthquake protection of bridges.
Consequently, these two types bearing were adopted in this retrofit construction.
3. Sufficient support length and unseating prevention device
The most economical and effective way to prevent the unseating of bridge span is to provide
sufficient support length and install appropriate unseating prevention device. In order ensure that theunseating of bridge span will not happen, in addition to the support length that must be provided, atleast two types of unseating prevention devices, such as restrainer and stopper, must be installed at theends of a superstructure. Restrainer is to connects the adjoining superstructure together to prevent
large relative displacement between two adjoining superstructure. Stopper is installed on the top ofpier cap to prevent large relative displacement between superstructure and substructure.
DEVELOPMENT OF THE SEISMIC RETROFIT MANUAL FOR HIGHWAY BRIDGE
The “Seismic Retrofitting Manual for Highway Bridges” was firstly published by the Ministry ofTransportation and Communications in 1999. The 2009 edition expands upon the previous publication
by including procedures for performance-based seismic evaluation and retrofitting of bridges, as wellas basic principles for evaluating and retrofitting scoured bridges with an exposed foundation structure.
The revised manual maintains the basic format of the retrofitting process described in the 1999 manual.
However, major changes were made to include current advances in earthquake engineering, field
experience with retrofitting highway bridges, and the performance of bridges in recent earthquakes aswell as the 1999 Chi-Chi earthquake. The revised manual is comprised of seven chapters as follows:
Chapter1: Seismic retrofitting of Highway bridges
Chapter2: Seismic ground motion hazard
Chapter3: Seismic simplified assessment methods and prioritization
Chapter4: Seismic detailed assessment methods for existing bridges
Chapter5: Seismic retrofitting by using system approach
Chapter6: Seismic retrofitting by using component approach
Chapter7: Special issue on souring and exposed foundation

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Chapter 1 provides a complete overview of the retrofitting process including the philosophy of
performance-based retrofitting, a characterization of the seismic and geotechnical hazards, andsummaries of recommended simplified and detailed assessment procedures, retrofit strategies, andapproaches and measures based on a system or component -by-component approach perspective.
Performance levels PL0, PL1, PL2 and PL3, (Table 4) are recommended according to bridge
importance (Table 5), safety, serviceability, and reparability (both short and long period), with a more
rigorous performance being required for important, relatively new bridges, and a lower level forstandard bridges (Table 6). A higher level of performance is required for event of design earthquake
than for the frequent earthquake. Table 7 shows the assessment methods according to the performancelevel and regularity.
Chapter 2 characterizes the seismic and geotechnical hazards. Basically, the seismic demand
shown in Fig. 11 is the same as the current “Seismic design specification of bridge structures”
published in 2009, except for disregarding the maximum considerable earthquake level with a 2500
return period. Structural performance, especially nonlinear deformation capability, is mainly verified
according to a design earthquake level with 475 return period, which is a design earthquake level
corresponding to a peak ground acceleration of 0.4SDS; while the frequent earthquake level linearly
reduces the force demand from the previous level by dividing it by 3.25. Retrofitted bridge should
display good ductility under the design earthquake level and simultaneously remain elastic under thefrequent earthquake level.
Table 4 Performance matrix of the retrofitting bridge
Performance
level
PL3

PL2

PL1

PL0

SafetyReparability
Short periodLong period

Serviceability

Structure remains
As same as priorelastic and noto the earthquake
unseating.

Simply repair

Repair in short time
Limited damages Repair in shortbyconventionaland no unseating. time
approach
Limited residual Repair in short
Replace or retrofit
inelastic
time with limited
thedamageddeformation and vehicle
weightmemberno unseating
and speed
Collapse
Close and use Toredownorprevention andalternative routepartially rebuild
no unseating

Regular repair
Repair in long timebyconventional
approach
Close and partiallyrebuild the bridge
Tore
down
partially rebuild

orTable 5 Important factor of the bridge
Bridge type
Highway bridge
Bridge is on the main route, or bridge is above important infrastructure, orbridge is the only one connection between two towns
othersImportant factor
1.2
1.2
1.0

Table 6 Performance level for standard/regular or important bridge
Seismic hazard level

Published year of the Design Specification of highway bridge structures

Frequent earthquake
Design earthquake*Note: for important bridge

1995 and 2000
PL3
PL21960 and 1987
PL3
PL1

1842

Before 1960
PL3
PL0 (PL1*)

Table 7 Seismic assessment methods
Performance level

Regular bridge
Linear elastic/dynamic, orNonlinear elastic/dynamic
Nonlinear elastic/dynamic

PL3
PL0, PL1, and PL2

S aD

Irregular bridge
Linear / Nonlinear dynamic
Nonlinear dynamic

S DS

S D1 / T
0 . 4 S DS

T 0D

0 . 2 T 0D

T

Fig. 11 Elastic acceleration response spectrum as the seismic demand for bridge to be retrofitted
Chapter 3 provides two simplified seismic assessment methods. Engineers are encouraged to carryout field investigations using the tables provided in Method A to examine both the unseating
probability and strength capacity. Method B is an in-house task that using fragility curve to determine
the damage probability of a bridge in terms of collapsed, severe, moderate, slight and none for 12
kinds of bridges. For method A, there is no need to perform a detailed assessment if the score issmaller than 30 but it is mandatory to carry out a detailed assessment as described in Chapter 4 when
the score is larger than 60. For the case whose score is between 30 and 60, engineers are also
encouraged to do the detailed assessment so that to check the structure safety, followed by the seismic
assessment and retrofitting strategy, as shown in Fig. 12. The evaluation results from either method isapplied to calculate the strength-ductility and falling index for prioritizing bridges that need to be
retrofitted.
Soilliquefaction

START

NO

Bearing
strength

END

Component
retrofitting

END

Systemretrofitting

NG

NG
Restrainer /Extend seat width

Ductility
inefficiency

NG

Seismic
assessment

Strengthinefficiency

Foundation
Column/pierCap beam

OK

Unseating

Replacing /strengthening

YES
Groundimprovement

OK

Component
retrofitting

Healthy monitoring
Re-assessment
Partial replacement
Demolish and re-construction

OK
END
END

Retrofitting cost
補強費用
New design cost
重建費用

<50%

>50%

Fig. 12 Seismic assessment and retrofitting strategy

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END

Chapter 4 discusses the modeling of the bridge in detail, for the superstructure, bearing system,
substructure, and the foundation. The soil spring model for spread footings, pile foundations andcaissons are also introduced, since the soil profile is both required and important for any seismic
evaluation. If the bridge foundations are located in the river, it is necessary to consider the scouring
effect and exposed foundations. In order to estimate a reasonable behavior of the structure, the revisedmanual allows engineers to use either a linear static, nonlinear static, nonlinear static or nonlinear
dynamic method, according to the seismic hazard level and the regularity of the bridge. Among thosefour methods, the pushover analysis by the definition of plastic hinges and the modified capacity
spectrum method are proposed as the standard procedure to obtain the yielding and anticipatedultimate peak ground acceleration. It is recommended to apply the Kawashima model and the Mander
model for bridges before and after retrofitting, respectively. The pushover curve is transformed tocapacity spectrum as recommend in the ATC-40, however, in stead of finding a performance point, in
this manual, a revised ATC-40 procedure (Fig. 13), without dealing with iteration and converge
problem, is applied to obtain the peak ground acceleration corresponding to the top displacement of
the column or girder. Since performance level, PL1 for example, has been selected as defined in
Chapter1, if the PGA with respect to PL3 is smaller than SDS/3.25, the bridge need a strength
retrofitting. In addition, if the seismic demand, 0.4 SDS, is larger than the PGA of PL1, it is required tomake ductility retrofitting.
Lateral Force

Spectrum acceleration
Elastic demand spectrum
Inelastic demand spectrum
Performance point
Capacityspectrum

PGA(output)

Spectrum(input)displacement

Target displacement demand

Lateral displacement

PGA

0.4SDS
0.4SDS
3.25

Displacement

Fig. 13 Seismic assessment procedure and qualification requirement
Chapter 5 describes the approach to retrofit a complete bridge by following four methods: (1)equalize the inertial force distribution; (2) optimize the existing bearing system; (3) use isolation
bearings; or (4) add dampers. In addition, it is important to avoid girders from falling. To prevent this,the unseating retrofitting method is introduced by extending the bearing-seat length on the cap beam if
there is sufficient space, or by adding devices to prevent unseating, such as for example, restrainersconnecting adjacent girders. Figure 14 shows an example using viscous damper in a highway bridge.
Chapter 6 presents the component-by-component retrofitting approach. A brief summary of theretrofitting measures is shown in Fig. 15. For the bridge columns, based on the ultimate strain theory,
engineers can apply specific formulas as proposed in the notes of this manual to calculate the thickness

1844

for steel, FRP, and concrete jacketing. As a result, after retrofitting the shape of a rectangular column
becomes an elliptical section to ensure that the confining stress can be developed. In addition, thedesign procedure for strengthening the footing can be found similar to that the design procedurereferenced in the “Seismic Retrofitting Manual for Highway Structures” of the 2006, Federal Highway
Administration publication.
Expansion Joint

Isolation
Bearing

Viscous Damper

Fig. 14 Seismic retrofitting by using system approach: isolation bearing and external damper
Sacrificed shear key

Strengthening of the cap beam
restrainer

Steel/ FRP/ Concrete
Jacketing of the column

Strengthening of
the foundation

Ground Improvement
Adding external pileFig. 15 Seismic retrofitting by using component approach
Chapter 7 deals with scouring-induced seismic safety evaluation and retrofitting measures.
Considering the difficulties due to the variation in river channel and soil profiles, a simplified
procedure using a reduction factor as a nonlinear function of the remaining length of the pile orcaisson is proposed for a quick assessment of the structural performance.
CONCLUSIONSThe Chi-Chi earthquake brought an intense impact on the seismic resistant design and technicaldevelopment of bridge engineering. In this paper, the performance of highway bridges located in thecatastrophic region and the extent of damage to them was summarized, followed by a general
depiction of the seismic retrofitting/reconstruction strategies after the earthquake. The revised “Seismic
Retrofitting Manual for Highway Bridges” proposed recently was also briefly described. This manualrefers to the damage investigation and reconstruction experiences after Chi Chi earthquake and also
includes current advances in earthquake engineering, field experience with retrofitting highway bridges.
REFERENCES
Kuo-Chun Chang, Dyi-Wei Chang, Meng-Hao Tsai, Yu-Chi Sung, (2000) “Seismic Performance ofBridges”, Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1, 55-77.
Kuo-Chun Chang, Yi-Chao Tsai, Dyi-Wei Chang, Yu-Chi Sung, Wen-Yi Liao, Juin-Fu Chai,
Hsiao-Hui Hung, Kuang-Yen Liu, Hung-Min Wu, Su-Ren Qi, Yen-Hao Chen, (2009), “Seismic
assessment and retrofit manual for highway bridges”, Report No: NCREE-09-028, Taipei, Taiwan.

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