Vulnerability assessment of reinforced concrete bridge structures in Algiers using scenario earthquakes

Bull Earthquake Eng (2014) 12:807–827DOI 10.1007/s10518-013-9523-7

ORIGINAL RESEARCH PAPER

Vulnerability assessment of reinforced concrete bridgestructures in Algiers using scenario earthquakes

Abderrahmane Kibboua · Hakim Bechtoula ·Youcef Mehani · Mounir Naili

Received: 5 April 2013 / Accepted: 16 September 2013 / Published online: 25 September 2013© Springer Science+Business Media Dordrecht 2013

Abstract Algeria has an inheritance of more than 11,000 bridges, with approximately 5,000road bridges of which more than 30 % have a high probability to be exposed to major earth-quakes and serious damages in the future. Therefore, it is of great importance to retrofitthe existing bridges and assess their seismic vulnerability and set a permanent monitoringsurvey to follow the change of their dynamic characteristics such as natural frequency andmodal damping. The assessment of the seismic vulnerability of existing RC bridges wascarried out based on the consistent and complete post earthquake survey after the Boumerdesearthquake. The information on the damaged existing RC bridges was investigated and eval-uated by the authors. This paper presents a simple and efficient inspection method for thepreliminary evaluation of the seismic vulnerability of existing bridge structures. To assessthe seismic damage of 148 existing bridges, two seismic scenarios were carried out using“Khair al Din” and the “Zemmouri” faults that are capable to generate earthquakes with amaximum acceleration of 0.8g. The main findings of this study are summarized in this paper.

Keywords Existing RC bridges · Seismic vulnerability assessment ·Post seismic inspection · Seismic scenarios · Seismic damage

A. Kibboua (B) · H. Bechtoula · Y. Mehani · M. NailiNational Earthquake Engineering Research Center, CGS, 01, Rue Kaddour RAHIM, BP 252,Hussein Dey, Alger, Algeriae-mail: [email protected]

H. Bechtoulae-mail: [email protected]

Y. Mehanie-mail: [email protected]

M. Nailie-mail: [email protected]

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808 Bull Earthquake Eng (2014) 12:807–827

1 Introduction

The previous earthquakes such as the Loma Prieta (USA, 1989), Kobe (Japan, 1995), Izmit(Turkey, 1999), Chi-chi (Taiwan, 1999) confirmed that bridges could be very vulnerablestructures under dynamic (cyclic) loading. Seismic behaviour problems of long structures,such as bridges, are not new and have already been addressed in the past at the time ofgreat earthquakes: Niigata (Japan) in 1964, San Fernando (California) in 1971, El Asnam(Algeria) in 1980. The earthquake of Kobe (January 17th, 1995) confirmed the need tounderstand clearly the behaviour of these structures.

The Boumerdes earthquake struck on 21 May 2003 at 19:44 local time and is con-sidered as one of the most damaging earthquake in Algeria. The shallow earthquakeof magnitude M6.8 was located offshore, 7 km north of the locality of Zemmouriin the wilaya (province) of Boumerdes and about 50 km of the capital city Algiers.The earthquake caused damage in area of 100 km long and 35 km wide. The epicen-ter was located in the city of Boumerdes and generated a tsunami that was observedas far away as the southern coast of Spain, but there was little or no local damage.The affected area is heavily developed and urbanized. There were 2,287 people dead,and about 11,000 injured. Damage was estimated at USD 5 billion. Approximately182,000 apartments and private houses were damaged and 19,000 dwellings were ren-dered uninhabitable. The earthquake left more than 100,000 people homeless (Bendimerad2004).

Thirty four (34) communes (districts) of fifty seven (57), the most populated and affectedby earthquake damage were considered in this study. Utilizing the field observed damagedata and the Japanese Methodology (Kubo and Katayama 1977), based on Engineering judg-ments, the seismic vulnerability was evaluated for those bridges with a large number data inorder to get a statistically significant sample size.

Expert survey teams investigated the field and conducted post-earthquake evaluation onselected bridges that did not suffer from the event. The data collected were analyzed inorder to evaluate the seismic vulnerability of the Reinforced Concrete, RC, bridges that arerepresentative in the capital Algiers. Hereafter, the main results of the study are presentedusing the Japanese methodology.

2 Description of the urban study area

2.1 Localization

The study area covers almost all the Wilaya of Algiers, capital of Algeria. This is oneof the high seismic potential zone with many active faults. Urban development experi-enced a rapid progress in Algiers without the development of proper disaster preven-tion systems against potential earthquakes. After the great 2003 Boumerdes earthquake,it became urgent and necessary to prepare a master earthquake disaster prevention planin order to mitigate possible future seismic damages in Algiers (CGS 2006). The seis-mic micro zoning mapping covers a total area of approximately 225 km2 and a popula-tion of 2,624,428 inhabitants including the surrounding urbanized area. Thirty four (34)communes of fifty seven (57), the most populated and built areas were concerned in thisstudy. Figure 1 shows the map of the concerned study area delimited by the solid bold blackline.

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Fig. 1 Map of the concerned study area (225 km2)

2.2 Seismic hazard assessment

Many seismic hazard investigations and research projects were developed for the northernpart of Algeria by the two research centers CRAAG and CGS using the seismic instrumen-tation network (Déverchère et al. 2005). Based on the observed damage, the post earthquakesurvey (Meslem et al. 2012), and taking into account the consideration of the current situa-tion and the past experience of Algeria from previous earthquakes, possible scenarios weredrawn up. In total, six major active faults have been identified and chosen at the peripheryof the region subject to the survey and used to create a seismic scenario so as to estimatemagnitudes of possible future earthquakes in consideration of recurrence periods. The Sahel,Chenoua, Blida, Thenia and Zemmouri faults, in addition to Khair Eddine offshore fault wereconsidered in this study. Figure 2 shows the location of each active fault. Expected magnitudeat the bedrock and at the soil surface associated with a 475 years return period for each activefault, are summarized in Figs. 3 and 4, respectively.

2.3 Post earthquake survey of existing RC bridges

The number of existing RC bridges in the thirty four (34) communes was estimated inaccordance with the GIS data and the result of inventory survey. A total sampling numberof 148 existing RC bridges were investigated in the whole study area. The bridges inventorywas conducted by experts and engineers of CGS. The post earthquake survey was conductedin order to estimate their damage and to develop in the future the seismic risk planning forordinary bridges in Algiers.

3 Methodology

When bridges suffer heavy damage due to a major earthquake, emergency rescue effortswill be disturbed and various social functions will be frozen for a long time (Priestley et al.

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Fig. 2 Identified active faults around the capital Algiers

1996). Failure of a bridge structure can determine an extensive disruption to the traffic systemeven though each failure is limited to a particular point in the road system (Choi 2002). Forinstance, although the road itself is safe, if some bridges are destroyed, the road networkwill not function and emergency rescuers will be unable to reach sites where assistance isneeded (Hwang et al. 2000). In addition, the road network in terms of reconstruction willbe useless because bridges repair in term of time are extremely long compared to the roadrepair (Légeron 2000). Thus, collapse of bridges should be prevented as much as possible(Moschonas et al. 2009). A significant number of existing reinforced concrete bridges requiremaintenance and a permanent survey (Kibboua et al. 2008). In Algeria, a rapid evaluationprocedure for the diagnosis of existing bridges is needed. The seismic damage of bridges isestimated for two earthquake scenarios: “Khair al Din” and “Zemmouri”.

Here after, some of the vulnerability assessments of existing bridges methodologies avail-able in the literature (Guéguen 2013) are summarized.

3.1 JHPC methodology: Japan

After the 1995 Kobe earthquake the Japan Highway Public Corporation (JHPC), a companydependent on the Japanese government and responsible for approximately 6,500 km of tran-sit charge highways, saw many of its bridges destroyed. Considering the particularly poorbehavior of certain old constructions, repair and reconstruction costs that have proven to beconsiderably high, it was decided that all the bridges should be reinforced. Orders of prioritywere defined as a function of the socio economic impacts incurred by the eventual collapse ofthe bridge, simplified considerations of vulnerability and available budgets (Légeron 2001).

The Japanese Ministry of Public Works has decided to proceed to a complete reinforcementto all existing bridges on freeways and national motorways of a length greater than 15 m.The bridges have to be inspected in agreement with the established methodology to estimatethe seismic vulnerability without complex calculations.

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Chenoua FaultSahel Fault

Khair al Din FaultBlida Fault

Thenia FaultZemmouri Fault

Fig. 3 Acceleration distribution at bedrock

3.2 Caltrans methodology: USA

The California department of Transport (Caltrans) has elaborated a method of hierarchyallowing for the establishment of a program of bridge reinforcement. This method combinedseismic activity, risk, vulnerability and magnitude (Gilbert 1993). According to this approach,the seismic activity and the risk constitute the main criteria and the economic considerationsprevail over the vulnerability. The criteria linked to the seismic activity essentially representsthe activity of the local faults, whereas the risk criterion takes into account the conditionsof the ground, predictable accelerations of the rock along with the average duration of theearthquakes. These data are exploited under the form of maps. The criterion of importanceis based on the economic variations linked to data relative to the traffic and the type of roadinfrastructure. The determination of the vulnerability parameter is based on the completionof the multicriteria.

3.3 OFROU methodology: Switzerland

Despite a moderate context of seismicity comparable to that of the French mainland, Switzer-land is one of the rare countries to dispose of an official methodological document spe-

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Sahel Scenario Earthquake Chenoua Scenario Earthquake

Blida Scenario Earthquake Khair al Din Scenario Earthquake

Zemmouri Scenario Earthquake Thenia Scenario Earthquake

Fig. 4 Peak ground acceleration distribution at ground surface

cific to the diagnostic and seismic reinforcement of the existing bridges (OFROU 2005).According to the methodology the evaluation of the seismic safety of bridge is made intwo steps. The first step of the evaluation consists of an initial classification of the struc-tures according to criteria of importance (construction class, surface of the deck, etc.) orqualitative criteria of vulnerability (construction type, height of abutments, presence ofseismic abutments, particular weak points, etc.). The second step of evaluation consistsof a genuine diagnostic based on a calculation of structure leading to the determinationof the acceptable nominal acceleration for the structure (before damage of the supports orthe fall of the deck). The level of seismic vulnerability of the structure is then expressedby the factor of conformity defined by the ratio of this acceptable acceleration over thestatutory nominal acceleration to be considered for a new structure. Depending on thevalue of the calculated conformity factor, the structure is considered as in need of rein-forcement as a priority and urgent fashion, judged as sufficiently resistant or to be rein-forced.

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Site Investigation

Scoring by Categories

- Ground Type- Liquefaction Potential- Girder Type- Bearing Type- Maximum Height of Abutment / Pier - Number of Spans- Minimum Bridge Seat Width- Seismic Intensity Scale- Foundation Type- Material Abutment / Pier

Inspection of Drawingsand Specifications

PGA

Scenario Earthquake

Evaluation's Method

Fig. 5 Flowchart of seismic vulnerability analysis of bridges

3.4 SISMOA methodology: France

The SISMOA method, a specific tool has been developed in France to estimate the vul-nerability of existing bridges under seismic actions (Marchand et al. 2006). It was inspiredfrom analogue studies developed in other countries like California (Priestley et al. 1996),Japan (Légeron 2001), Italy (PIARC C4.4 2007) and Switzerland (OFROU 2005). Based ongeometrical and typological criteria, this qualitative method resulting from the vulnerabilityassessment of the different parts of bridge structures, was established from the observationand interpretation of damages caused by past earthquakes on bridges. The purpose at thattime was to get a tool able to determinate which bridges should be retrofitted in priority inorder to meet seismic requirements.

3.5 KATAYAMA’s methodology

As discussed previously, various procedures for assessing the seismic vulnerability of existingbridges have been proposed in a wide range from simple to precise inspection (Kawashimaand Unjoh 1990), the Katayama’s methodology is very effective in evaluating bridges fromthe viewpoint of falling-off the girders and is used as a first screening. An outline of theinspection method which is capable to assess the seismic vulnerability taking into accountthe damage features developed in recent earthquakes is shown in Fig. 5. The main purpose ofthe procedure consider five principal factors: The site investigation which consist to conduct abridge inventory survey in order to get a data basis for the analysis, the inspection of drawingsand specifications because seismic design procedures have been reviewed and amended inthe light of lessons learned through past seismic damages and development of new seismicdesign methods. The intensity of earthquake ground motion due to the scenario earthquake isintroduced in terms of PGA and converted to MSK intensity. The liquefaction potential dueto scenario earthquake is provided from geotechnical expert and hazard map. The scoringby categories which considers the type, materials, shape and slope of the superstructure areincluded to represent the properties of superstructure. The properties of substructure arerepresented in terms of type, pier, height of abutment and materials. The type of substructureis taken into account in term of girder type. Vulnerability of bridge depends on propertiesderived from hazard and resistance. Each property is weighted and then summed up. The

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Table 1 Inspection format for seismic vulnerability of bridges in Algiers

Item Category Category score

Ground type Stiff/Hard: slightly/no weathered rock 0.5

Medium: weathered/moderately weathered rock 1.0

Soft: deposited soil/diluvium 1.5

Very soft: deposited soil/alluvium 1.8

Liquefaction potential No liquefaction 1.0

Possible liquefaction: 0 ≤ PL < 15 1.5

Liquefaction: 15 ≤ PL 2.0

Girder type Arch or rigid frame 1.0

Continuous 2.0

Simple 3.0

Bearing type With specific device (prevent girder from falling) 0.6

Bearing (with clear design concept) 1.0

Exist two bearing (it can move axial direction) 1.15

Others (no bearing, etc) 1.1

Max. height of abut./pier Less than 5 m 1.0

5–10 m 1.35

More than 10 m 1.7

Number of spans 1 Span 1.0

2 Spans or more 1.75

Min. bridge seat width Wide: 70 cm or wider 0.8

Narrow: less than 70 cm 1.2

No seat: 0 cm 1.1

Seismic intensity scale (MSK) MSK < 7.885 (JMA: less than 5.0) 1.0

7.885 ≤ MSK < 8.680 (JMA: 5.0 to less than 5.5) 2.1



10.270 ≤ MSK (JMA: 6.5 and more than 6.5) 3.5

Foundation type Pile Bent 1.4

Others 1.0

Material of abutment/pier Plain concrete or masonry 1.4

Reinforced concrete or others 1.0

relative importance of the different variables on the predicted bridge rate of failure is shown inTable 1. These values were determined from an engineering judgments based on the statisticalanalysis as well as the past experience of seismic damages.

Based on the statistical analyses on the factors which are likely to contribute to the seismicsusceptibility of bridges, Katayama’s evaluation method is adopted to assess the seismic vul-nerability of a number of bridges based on a simple inspection without complex calculation.Furthermore, the method is able to detect the bridges which have possibility for sufferingdamages in terms of falling off the girders when subjected to the ground shaking due tothe earthquake. It should be noted that in Algeria 80 % of the existing bridges are multiplesimply supported girders. The major cause of damage to this type of bridges from the past

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earthquakes is attributed to the excessive relative movement between the superstructure andsubstructure. Hence, the choice of this method is suitable for this study.

Although the accuracy of the adopted method is insufficient for assessing the seismic vul-nerability of bridges without complex calculation, it provides a realistic basis for assessing theseismic vulnerability of bridges from which fatal damages such as falling-off superstructurewere most likely developed from excessive relative movement between the superstructureand substructure, and from failure of substructures due to inadequate strength. This methodis based on regression analysis on bridge damage data, and has many positive aspects sinceit was essentially elaborated based on common sense and empirical rules: it is physicallysound, it may be expeditly employed, it allows detect out the variables that are most likelyto affect structural failure.

There are a number of bridge structures which are considered to have lower seismic safetyfrom the view of current design practice (Kawashima 2002). Therefore, it is of considerableimportance to retrofit the existing highway bridges which have high vulnerability for seismicdamages (Padgett and DesRoches 2008). The falling of the girders can determine seriousimpacts to the road system. The methodology adopted in this paper, generally referred to“Katayama’s method” is selected to carry out this study.

This method should allow us to improve the data relative to the considered scenarios ofcrises, and more globally, to improve the knowledge and the prevention of the seismic risk atthe scale of the territories: elaboration of the plans of risk prevention, organization of rescueand planning of the future urbanization such as judicious implantation of strategic buildingsand structures.

4 Factor affecting the seismic vulnerability of bridges

For detecting the factors which make bridges susceptible to seismic damage (Unjoh et al.2000), the degree of seismic damage was evaluated and classified into groups by referring topost earthquake reconnaissance reports and a numerical value was assigned for each sample.A total of eleven (11) items likely to affect the probability of a girder falling are selectedand considered as quantity to be analyzed statistically. They characterize the properties of abridge that are likely to have influenced on the damage degree. These items are shown in thefirst column of Table 1. Each item is subdivided into several categories; a weighting factor isaffected by each of them and shown in the “category score” at the third column of Table 1.The rank of damage degree of i th bridge is denoted here by yi , for the N items (M j numberof categories of j th items) that have likely contributed to develop seismic damage. Eachproperty (item) is weighted and then summed up. Weights are derived from observation ofdamages from past earthquakes. The predicted damage degree rank yi was assumed to havea form of:

yi =N∏

j=1

M j∏

k=1

X δi ( jk)jk (1)

in which X jk represents a waiting factor (category score) for the kth category of the j th item,and δi ( jk) represents a dummy variable that it takes a value of 1 if the characteristics of thei th bridge correspond to the category k in item j , and 0 otherwise. The multiplication signfrom 1 to N th value is denoted

∏Nj=1where N is the total number of items.

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Table 2 Seismic capability of Khair al Din and Zemmouri active faults

Fault Length (Km) Dip angle (◦) Depth (Km) Rupture area (km2) Mw

Khai El Din 100 ± 20 45 20 2,830 ± 565 7.4 ± 0.3

Zemmouri

Fig. 6 Peak ground acceleration distribution: Khair al Din scenario earthquake

5 Earthquake scenarios

Based on the results of damage estimations and according to the ASCE post earthquakeinvestigation report (Curtis 2004), two earthquake scenarios were chosen for the presentpaper: Khair al Din and Zemmouri. The maximum credible earthquake that can be generatedhas been derived from their segmentation and down dip geometry of the fault plane, asimaged by the offshore data from the Maradja cruise (Déverchère et al. 2010). Assuminga 100 ± 20 km long fault dipping 45◦ southward to a depth of 20 km, the seismogeniccapability of their sources is: Mw = 7.4±0.3, which corresponds to a rupture area of 2,830±565 km2.

The maximum credible earthquake for identified active faults in the Algiers regionis summarized in Table 2. The estimated distribution of a maximum acceleration at theground surface is illustrated in Figs. 6 and 7 for Khair al Din and Zemmouri scenarios,respectively.

The relationship between the Japanese Meteorological Agency (JMA) scale and the seis-mic intensity Medvedev–Sponheuer–Karnik scale (MSK) is show in Fig. 8.

The original Katayama’s method is based on the MSK scale that was modified to besuitable to the newly adopted JMA scale. In Algeria the used seismic intensity scale is thepeak ground acceleration (PGA), hence the need of a relationship between the PGA and theMSK scales in order to be able to use the adopted methodology for this study. The relationshipbetween the PGA and the seismic intensity in MSK scale is shown in Fig. 9 (CGS 2006).The obtained intensities derived from this relation were used in this paper. The JMA-MSKrelationship shown at Fig. 8 is given just for illustration and to get a rough idea on the valuesbetween the different intensities.

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Fig. 7 Peak ground acceleration distribution: Zemmouri scenario earthquake

y = 1.6474x + 0.2044

R2 = 1

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

4 4.5 5 5.5 6 6.5 7

JMA

MS

K

Fig. 8 Empirical relation between JMA and seismic intensity scale in MSK

Fig. 9 Empirical relationbetween PGA and seismicintensity scale in MSK

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Table 3 Definition of damage rank of bridges

Class of damage rank Originalthresholdvalue

Modifiedthresholdvalue

A

High probability of girders falling ≥ 30 ≥ 30

Generates huge deformation

Impossible to use for long term and requires reconstruction

B

Moderate probability of girders falling 26 to lessthan 30

22 to lessthan 30

Generates deformation

Impossible to use temporarily and requires repairing / rehabilitation

C

Low probability of girder falling < 26 < 22

Generates small deformation

Possible to basically use after inspection

6 Analysis of bridges

Hundred forty eight (148) bridges that did not suffer from the Boumerdes earthquake (M6.8) of 2003 were analyzed. The original values of the predictor, given by Eq. 1 and shownin the second column of Table 3, to estimate the damage rank of bridges was based on 30damaged bridges selected as samples during the Kanto earthquake of 1923 (M 7.9), the Fukuiearthquake of 1948 (M7.3) and the Niigata earthquake (M7.5) of 1964. These values weremodified; see the third column of Table 3, to fit the Algerian bridge type and the observeddamage after Boumerdes earthquake (Ousalem and Bechtoula 2003). Vulnerability of seismicdamage was then classified into three groups. The definition of the class of damage rank andthe initial value are modified for this study according to damage results from Boumerdesearthquake in one hand and by taking into account the engineering judgement based onexperience in the other hand.

7 Evaluation of seismic vulnerability

Falling-off of bridge girders was not observed in the Boumerdes earthquake; however, somebridges suffered damage such as permanent deformations and cracks. The verification ofthe method and the threshold value is examined through the damage which occurred tothe Sebaou Bridge in the city of Boumerdes and El Harrach Bridge in the city of Algiers.The intensity of earthquake ground motion was introduced in the analysis in term of MSKintensity. Because it is apparent from the past damage surveys that bridge damage was mostlikely developed due to inadequate strength and/or excessive deformation of substructures(Iwasaki et al. 1972). The properties of substructure were represented in terms of pier’stype, height and material (Kibboua et al. 2011; Avsar et al. 2011, 2012). The mechanicaldevice to prevent the falling-off the superstructure, which is one of the unique features ofJapanese seismic design practice for highway bridges, was also considered as an importantitem (ERDCJ 2002). Ground condition was considered in term of ground type, irregularity,

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Fig. 10 View of Sebaou Bridge

Table 4 Analysis results ofSebaou bridge


Girder type Simple 3.0

Bearing type Bearing and system forprevention of girdersfalling

0.6

Max. height of abut./pier 5–10 m 1.35

Number of spans 2 Spans or more 1.75


Foundation type Others 1.0

Material of abut./pier Reinforced concrete orothers

1.0

and soil liquefaction potential. This latter was evaluated in accordance to geological and siteseismic conditions.

8 Illustrative examples

8.1 Bridge on Sebaou river

The Sebaou Bridge is made of successive isostatic reinforced concrete beams as illustratedin Fig. 10. The result of the proposed method for this bridge is summarized in Table 4.

The bridge crosses a river in a region where the soil is alluvial. Liquefaction that occurredat Boumerdes earthquake, as shown in Fig. 11a, was considered in the analysis. The lateralground spreading toward the river was observed at many places around the bridge as illustratedFig. 11b. The liquefaction potential index (PL) is estimated as more than 15.

The MSK intensity scale at the site was reported as IX (9); this value is between the rangesof VIII1/2 (8.5) to IX1/2(9.5). Hence, 3 categories for the seismic intensity scale called Case1, Case2 and Case 3 were selected as shown in Table 5. The total score of each case was: 25.7(Case 1), 29.4 (Case 2) and 36.7 (Case 3). Consequently, the class of damage rank based onKatayama’s method is judged as class “B” (moderate probability) or “A” (high probability).

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Fig. 11 a Observed liquefaction. b Spreading toward the river

Table 5 Geological and seismic condition of Sebaou bridge


Ground type Very soft: deposit soil/alluvium 1.8

Liquefaction potential Liquefaction: 15 ≤ PL 2.0

Seismic intensity scale (MSK) MSK = IX Case 1: VII1/2 < MSK ≤ VIII1/2 2.1

Case 2: VIII1/2 ≤ MSK < IX1/2 2.4

Case 3: IX1/2 ≤ MSK < X 3.0

Fig. 12 Displacement of a girder and lateral movement of a Pier of Sebaou Bridge

The post earthquake field investigation of the Sebaou Bridge showed that some girderswere displaced due to movement of some piers of around 50 cm or less as shown in Fig. 12;however, the girders did not fall off. If no prevention system or less seat width was appliedand/or a major earthquake occurred, the girders might have fallen down.

Hence, the damage rank of the Sebaou Bridge is evaluated as class “B” (moderate proba-bility) that is very close to class “A” based on the above mentioned damage condition.

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Fig. 13 View of El Harrach Bridge

Table 6 Analysis results of ElHarrach bridge


Girder type Simple 3.0

Bearing type Bearing with rubber 0.9

Max. height of abut./pier 5–10 m 1.35

Number of spans 2 Spans or more 1.75


Foundation type Others: pile 1.0

Material of abut./pier Reinforced concrete orothers

1.0

From the field investigation, the authors advise to set a continuous reinforcement in theconcrete pavement across the girder joint that may avoid the falling off the girders andcontribute to the stability during strong earthquakes. Recently, devise of falling-off preventionof superstructures, which is one of the unique features of the Japanese seismic design practicefor highway bridges, was introduced in practice. The device includes stoppers to preventexcessive relative movement between super and substructures, connections either betweensuper and substructures or between adjacent girders, and increase the seat length to preventfalling-off of superstructure from crest and/or abutment.

8.2 Bridge on El Harrach river

Structure of the El Harrach Bridge is composed of reinforced concrete piers and steel girdersas illustrated in Fig. 13. The scores of the items of this bridge are given in Table 6.

The ground at the site is alluvial soil. The bridge crosses a river in a lowland area whereliquefaction did not occur during Boumerdes earthquake. Thus, the liquefaction potential isjudged as null, PL=0. The MSK intensity scale at the site was reported as IX (9), which isbetween the ranges of VIII1/2 (8.5) and IX1/2 (9.5). Three categories for the seismic intensityscale are considered, Case 1, Case2 and Case 3 given in Table 7 with their scores.

The total score of each case was 19.3, 22.1 and 27.6 for case 1, case2 and case 3, respec-tively. Consequently, the class of damage rank based on the methodology is judged as class“C” (low probability) or “B” (moderate probability). The post earthquake survey of the actual

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Table 7 Geological and seismic condition of EL Harrach bridge


Ground type Very soft: deposit soil/alluvium 1.8

Liquefaction potential No liquefaction 2.0

Seismic intensity scale (MSK) MSK = IX Case 1: VII1/2 < MSK ≤ VIII1/2 2.1

Case 2: VIII1/2 ≤ MSK < IX1/2 2.4

Case 3: IX1/2 ≤ MSK < X 3.0

Fig. 14 Displacement of a girder of El Harrach bridge

damage for the El Harrach Bridge showed that some girders were slightly displaced withpropagation of cracks (see Fig. 14); however, the girders did not fall off.

Based on the engineering judgment and the site investigation, the damage level of the ElHarrach Bridge is evaluated as class “B” (moderate probability) that is very close to class“C”.

9 Verification of the method

Table 8 shows the results of the method with modified threshold values and the actual damageto each bridge which matched well with the proposed method. This indicates that Katayama’smethod is suitable for the damage estimation of the Algerian RC bridges.

An evaluation of the seismic vulnerability is made for 148 bridges in Algiers. The assess-ment and the actual damage condition for the selected bridges are described hereafter.

10 Application of the evaluation method

As it was mentioned before, the evaluation method was applied to analyse 148 sample bridges.It should be noted here that the evaluation method was used to predict the seismic vulnerabilityof bridges subjected to two earthquake scenarios: the “Khair al Din” and the “Zemmouri”.

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Table 8 Summary for the Katayama’s method verification

Bridge Case byMSK scale

Totalscore

Class of damage rank Verification

Katayama’smethoda

Actualdamage

Sebaou 123

25.729.436.7

BBA

B Falling of the girders did not occur, butdisplacement was generated.Probability of falling girders isevaluated by the actual damage as class“B” that is very close to class “A”.Hence, the result of the method showsa good match for the actual damage

El Harrach 123

19.322.127.6

CBB

B Falling of the girders did not occur, andslight displacement was generated.Probability of falling girders isevaluated by the actual damage as class“B” that is very close to class “C”.Hence, the result of the method showsa good match for the actual damage

a Threshold value for evaluation of the class applies the modified value

Fig. 15 Seismic intensity distribution on MSK scale: Khair al Din scenario earthquake

Figures 15 and 16 show the seismic intensity distribution of the two seismic scenarios forthe capital, Algiers.

Table 9 shows a summary of the predicted damage rank of the seismic vulnerability ofthe 148 selected bridges. The results show that for the Khair al Din scenario, the class ofdamage rank was 3, 19 and 126 bridges classified in damage rank A, B and C, respectively.For Zemmouri scenario these values were 4, 7 and 137.

It is important to note that, for the two scenarios, the seismic vulnerability rank is quitesmall for rank A and B, but high for rank C. This implies that the seismic vulnerability isreasonable for those bridges of rank C which are quite safe against earthquake. We can alsonote that for damage rank B, the Khair al Din scenario gives a value which is more than twotime grater that the one found using Zemmouri scenario. Figures 17 and 18 show the locationand the damage distribution.

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Fig. 16 Seismic intensity distribution on MSK scale: Zemmouri scenario earthquake

Table 9 Summary of bridgedamage estimation

Class of damage rank Number of bridges [ratio (%)]

Scenario earthquake

Khair al Din Zemmouri

A: high probability 3 (2.0 %) 4 (2.7 %)

B: moderate probability 19 (12.9 %) 7 (4.7 %)

C: low probability 126 (85.1 %) 137 (92.6 %)

Total 148 148

Khair al Dine scenario (3: High, 19: Medium and 126: Low)

Fig. 17 Bridge locations and probability of girders falling-off of: Khair al Din fault

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Zemmour scenario (4: High, 7: Medium and 137: Low)

Fig. 18 Bridge locations and probability of girders falling-off of: Zemmouri fault

11 Conclusions

A screening evaluation method to assess the seismic vulnerability of existing bridge struc-tures, which had likely contributed to assess those of the Algiers region, was presented. Theadopted method was applied to Algiers, which is a typical Mediterranean city, located in amoderate to high seismic hazard area. Thirty four communes (districts), the most populatedof Algiers were considered in this study. The assessment of the seismic vulnerability con-cerned one hundred and forty eight existing RC bridges. The assessment showed that thedamage on the bridges was minor. Three of the bridges were damaged but did not collapsefor Khair al Din earthquake scenario. For the two scenarios, the seismic vulnerability rank isquite small for rank A and B, although it is sufficiently high for Rank C. This implies that theseismic vulnerability is satisfactory for those bridges of Rank C which are quite safe againstearthquake. We also noted that for damage rank B, the Khair al Din scenario gives a valuewhich is more than two times greater that the one found using Zemmouri scenario.

From the field investigation, the authors advise to set a continuous reinforcement in theconcrete pavement across the girder joint that may avoid the falling off the girders andcontribute to the stability during strong earthquakes. Generally, in the study area, with theexception of masonry bridges and colonial bridges built before 1962, there is a low probabilityof the girders falling off, because, in most cases, enough support width has been provided.

Following the damage caused by the Boumerdes earthquake on 21 May 2003 (M 6.8), thePublic Works Ministry of Algeria elaborated a specific seismic regulation for design (RPOA-2008 2010) for the first time. From April 2010, all new constructed bridges must be designedand verified using this new bridge seismic code. The obtained seismic damages constitute anexcellent information sources and tools for risk managements, emergency planning and alsouseful for civil protection, prevention and preparedness for the city of Algiers.

Acknowledgments The authors sincerely thank all the researchers and engineers who have participated tothe success of this project. They are also grateful to people and institutions who provided valuable data toconduct this study.

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