A Survey of Damages to Bridges inPakistan after the Major Earthquakeof 8 October 2005

Syed M. Ali,a) Akhtar N. Khan,a) Shahzad Rahman,a)

and Andrei M. Reinhorn,b)M.EERI

An earthquake measuring Mw 7.6 struck the Pakistan-administered part ofKashmir on 8 October 2005. The epicenter of the earthquake was located 22 kmfrom the city of Muzaffarabad. The earthquake resulted in the loss of more than80,000 lives and caused extensive damage to property and infrastructure. A sur-vey of an approximately 400-km road network was carried out, in which 90bridges were inspected for earthquake-associated damage, out of which 14bridges (16%) experienced damage of varying degrees, of which nine bridges(10%) either failed or became nonfunctional. The survey revealed some of thedeficiencies of the construction practices in Pakistan and also highlighted theneed for improvement to the country’s current bridge design practices. This pa-per reports the prominent types of failures observed and discusses the deficien-cies in current design practices. Based on the findings of the survey, variousrecommendations are made, with the objective of minimizing earthquake-associated damages to new and existing bridges in areas with a high seismicrisk. [DOI: 10.1193/1.3650477]

INTRODUCTION

On 8 October 2005 an earthquake measuring Mw 7.6 struck the Pakistan-administeredpart of Kashmir. The epicenter of the earthquake was located 22 km from the city of Muzaf-farabad. The earthquake was caused by the rupture of the Balakot-Bagh thrust fault. ThePakistan Meteorological Department (PMD) and the Norwegian Seismic Array (NORSAR)estimated the rupture length to be 90 km–100 km (PMD and NORSAR 2006); Kanedaet al. (2008) estimated the rupture length to be approximately 70 km. The focal depth of theearthquake was shallow; it was reported to be 26 km (USGS 2005, Rao et al. 2006). Theearthquake caused severe damage in the areas close to the fault. Figure 1 shows the locationof the epicenter of the earthquake. According to estimates, more than 80,000 people losttheir lives and around 4 million people were left homeless (Rao et al. 2006). The fault rup-tured directly beneath the highly populated cities of Balakot, Muzaffarabad, and Bagh,causing extensive damage to their infrastructure.

A total of 90 bridges were surveyed in the northern part of Pakistan and Kashmir, out ofwhich approximately 25 bridges were located within a 25-km radius from the epicenter.

a) University of Engineering & Technology, Dept. of Civil Engineering & Earthquake Engineering Center, Peshawar,Pakistan

b) University at Buffalo, Dept. of Civil, Structural, & Environmental Engineering, Buffalo, NY 14260

947

Earthquake Spectra, Volume 27, No. 4, pages 947–970, November 2011; VC 2011, Earthquake Engineering Research Institute

Fifty-seven bridges were located within a radius greater than 25 km but less than 50 km,and the remaining eight bridges fell outside a 50-km radius from the epicenter.

The map in Figure 1 shows some of the bridges that were surveyed in this region; anelectronic version of this map is accessible online at Google Maps (Ali 2008); the samemap can also be opened in Google Earth, and GPS coordinates of the plotted bridges can beread. The fault rupture is also plotted on this map using the GPS coordinates taken fromKaneda et al. (2008).

Pakistan and its adjoining regions have a history of major earthquakes, including themagnitude 8.0 Kangra earthquake of 1905, the magnitude 8.0 Pattan earthquake of 1974,and the magnitude 8.1 Quetta earthquake of 1935 (PMD and NORSAR 2006). Accordingto the earthquake catalog prepared by PMD and NORSAR, more than 40 earthquakes ofmagnitude 7 or larger have occurred between 1900 and 2005 in the Himalayan region thatinfluences the northern part of Pakistan and Kashmir (PMD and NORSAR 2006). Thesefacts highlight the potential of future earthquakes in this region and it is estimated that earth-quakes of magnitude 8 or larger are likely to occur (Bilham and Wallace 2006).

While many large cities in the world are located close to active thrust faults and exposedto serious seismic hazard, the surface ruptures of thrust faults are much less common thanother fault types and less is known about them; thus more research on thrust faults is needed(Kaneda et al. 2008). In the case of Pakistan, cities like Muzaffarabad, Balakot, Mansehra,

Figure 1. Survey area, epicenter of 8 October 2005 earthquake, fault rupture, and bridges nearthe fault.

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and Abbottabad are located close to the Balakot-Bagh thrust fault and may experiencelarge-magnitude earthquakes in the future. These facts pose a serious challenge to the engi-neering community of Pakistan in particular, and engineers across the world, and highlightthe importance of understanding the performance of infrastructure during seismic eventslike Pakistan’s 2005 earthquake.

Some of the in-service bridges in the surveyed region date back to the early 20th cen-tury. Modern bridges constructed by various departments are gradually replacing the old,typically single lane bridges. Information collected during discussions with officials of vari-ous departments suggests that currently there are approximately 6,000 bridges on thenational highways of Pakistan, 67% of which were constructed prior to the 1980s. The ma-jority of these bridges were reportedly designed using American Association of State andHighway Transportation Officials (AASHTO) specifications.

During the survey the structural details and GPS coordinates of various importantbridges were recorded. It is worth mentioning that only one time-history record of the Octo-ber 8 earthquake is available for the survey area, which was recorded in the city of Abbotta-bad, approximately 54 km from the epicenter (Durrani et al. 2005). This makes it difficult toestimate the level of ground shaking that affected the bridges within the survey area. How-ever, some guidance can be taken from the information presented in the report by Durraniet al. (2005) in this regard. According to this report the recorded peak ground acceleration(PGA) in Abbottabad was 0.231 g (east–west) with the highest amplification ratio meas-uring about four for the 5% damped elastic response spectrum in the range of 0.4–2.0 sec-onds. Durrani et al. (2005) also gave estimates of PGAs for stiff and soft soils, calculatedusing various models. According to these estimates the PGA at locations 25 km from theepicenter was in the range of 0.25 g–0.4 g and the PGA at locations 50 km from the epicen-ter was in the range of 0.15 g–0.231 g. The 0.231 g PGA recorded in Abbottabad falls closeto the upper range of the estimated PGA for locations 50 km from the epicenter. This maybe attributed to the subsurface conditions in Abbottabad being made up of soft soils, whichwould have amplified of the ground shaking. The distance of the bridges in the survey areafrom the epicenter ranges from 19 km–85 km. On the basis of attenuation information pre-sented in Durrani et al. (2005), it is the opinion of the authors that the bridges in the studyarea were subjected to a PGA ranging from 0.15 g–0.4 g, depending upon their proximityto the epicenter.

DAMAGE LIMIT STATE CATEGORIZATION OF BRIDGES

The structural conditions of the surveyed bridges was categorized based on criteria pro-posed by the authors that makes use of five limit states, as defined in Table 1.

In the succeeding section, classification of the surveyed bridges is presented based ontheir importance, their structural form, and the material used for their construction. Thisclassification is accompanied by information pertaining to the levels of damage observed inthese bridges. Following on from this section is a detailed discussion of the prominent typesof damage and failures observed following the earthquake. Finally, conclusions and recom-mendations for improvements in the design and planning of bridges are presented. Data wascollected during field visits conducted over a period of two years following the October

A SURVEY OF DAMAGES TO BRIDGES IN PAKISTAN AFTER THE MAJOR EARTHQUAKE OF 8 OCTOBER 2005 949

2005 earthquake; some findings from these visits are reported in Naeem et al. (2005), EERI(2006), Dellow et al. (2006), Ali and Shakal (2007).

CLASSIFICATION OF BRIDGES

The 90 bridges surveyed cover a road network of approximately 400 km of main roads.The routes covered and the bridges surveyed are shown in Figure 1. Details of the routes,the number of bridges on each route, and the approximate population served are provided inTable 2.

The bridges surveyed were assigned limit states according to Table 1. Tables 3, 4, and 5categorize the results with respect to superstructure, substructure, and the material of con-struction, respectively.

Out of the 90 bridges inspected during the survey, 14 bridges were found to haveexperienced some form of damage following the earthquake. These 14 bridges are listed in

Table 1. Limit state classification of bridges based on structural damage

Code Damage Reusability Reparability Restorability

ND No damage Yes No need No need

LD Slightly damaged Yes Yes Complete restoration to original state possible

MD Moderatelydamaged

Yes Difficult Complete restoration to original state possible

SD Severely damaged Partial Difficult Restoration of the bridge to originalstate not possible

CO Collapsed No No Restoration not possible=reconstructionrequired

Table 2. Routes, number of bridges surveyed, and population served

Number of bridges*

Route=location name Single-span MultispanPopulation served

(in thousands)

Havalian Abbottabad Mansehra 13 2 >200

Mansehra Battagram Besham 26 11 >150

Mansehra Garhi Habibullah – Muzaffarabad 1 2 >50

Grahi Habibullah – Balakot 3 1 >50

Muzaffarabad – Kohala 13 4 >150

Muzaffarabad Garhi Dupatta 7 1 >200

Muzaffarabad 3 3 >150

Subtotal 66 24

Total 90

* The actual number of bridges on the routes listed may be more than the number presented as some minorbridges were not surveyed.

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Table 6, which also lists the limit states of the bridges following the earthquake, the con-struction materials used, distances from the epicenter and the rupture zone, importance lev-els of the bridges according to AASHTO’s Load and Resistance Factor Design (LRFD)bridge design specifications (AASHTO 2007), and the extent of the population served bythe bridges. The importance of a bridge is decided on the basis of the availability of alter-nate routes to the population served. This means that a bridge serving a population withoutan alternate route will have highest importance and is classified as critical. Bridges are clas-sified as essential when an alternate route is available but is difficult to access. The categoryothers indicates that alternate routes are easily accessible. Since 13 bridges were close tothe rupturing fault (near-field), the distance from the fault is also recorded in Table 6. It isworth mentioning that it can be misleading to only consider the distance of a bridge fromthe epicenter while making a qualitative assessment of the level of shaking a bridge mayhave experienced. The distance of a bridge from the fault rupture is also an important factorthat needs to be considered.

DAMAGES OBSERVED: IN CONTEXT OF MATERIAL OF CONSTRUCTION

Table 6 shows that 15 bridges, or around 17% of the total bridges surveyed, containedstone masonry, which is at odds with the normal practice in Pakistan as a whole. However,due to the abundant availability of stone as construction material in the hilly areas ofPakistan and the relatively high costs associated with transporting bricks or materials for

Table 3. Superstructure of bridges organized by limit state classification

Limit stateI Girder (PSi) =

T (RCii) Girder = Slab Box girder Truss Suspension Arch Total

ND 60 3 2 8 3 76

LD 1 2 2 – – 5

MD 1 1 – 0 1 3

SD – 1 – 2 – 3

CO 1 – – 2 – 3

Total 63 7 4 12 4 90

i) PS¼Prestressedii) RC¼Reinforced concrete

Table 4. Substructure of bridges organized by limit state classification

Limit state Single column Multicolumn WallAbutments (single span) =

tower base Total

ND 1 4 – 71 76

LD 3 – – 2 5

MD – – 2 1 3

SD – – – 3 3

CO – – – 3 3

Total 4 4 2 80 90

A SURVEY OF DAMAGES TO BRIDGES IN PAKISTAN AFTER THE MAJOR EARTHQUAKE OF 8 OCTOBER 2005 951

reinforced concrete (RC) construction, it is not uncommon to find stone masonry bridges inthese areas. Data collected for bridges outside the study area indicates that current practicesin Pakistan favor bridges with an RC substructure and post-tensioned I-girders with RCslabs.

The extent of damage experienced by the bridges in the survey area depended upon thefollowing factors: whether the bridge was located on the hanging-wall or footwall side ofthe fault, its distance from the fault, the material used in its construction, the quality of con-struction, and its geometric features.

A thrust fault caused the 2005 earthquake and it was observed by the authors that struc-tures on the hanging-wall side experienced more damage due to severe ground shaking ascompared to those located on footwall side; this observation was also reported by Kanedaet al. (2008). Out of the three bridges located on the hanging-wall side, one collapsed, onewas severely damaged, and one suffered moderate damage. These levels of damage are inaccordance with the accepted phenomenon that structures on the hanging-wall side of a faultsuffer more damage than those on the footwall side (Kaneda et al. 2008). The collapsed ofthe bridge that was destroyed (No. 8 in Table 6) was precipitated by landslides. The bridgethat suffered severe damage (No. 2 in Table 6) had a tall stone masonry substructure thatcould not withstand the lateral forces created by the earthquake. The bridge on the hanging-wall side that suffered moderate damage (No. 3 in Table 6) is a three-span continuous RCbridge. The continuity of the superstructure of this bridge helped prevent a drop-down of itssuperstructure and the thick, short solid wall piers helped it survive the ground shakingforces. The effects of the shear forces created by the earthquake were reduced due to thesliding of the deck over the pier walls.

The relatively fair performance of the stone masonry bridges on the footwall side of thefault should not be misconstrued as acceptable performance of this construction type, whichis notoriously prone to damage by earthquakes. The level of shaking, geometry of the struc-ture, and the quality of construction also play a role in the earthquake-resistance of a bridge.A high incidence of significant damage was observed in the stone masonry bridges sur-veyed, irrespective of their structural form. Tables 5 and 6 show that three stone masonrybridges suffered complete collapse, two were severely damaged, and two suffered moderate

Table 5. Material of construction of bridges organized by limit state classification

Limit stateReinforced

concrete (RC)Stone masonry & mix of stone

masonry & RC (SM&RC) Othersiii) Total

ND 13 7 56 76

LD 4 1 – 5

MD 1 2 – 3

SD 1 2 – 3

CO – 3 – 3

Total 19 15 56 90

iii) Others includes small, single-span bridges where the material of construction could not be clearly identifiedand categorized either because the structural elements were covered by cement plaster or because their construc-tion used a combination of bricks, stone masonry, steel, or wood.

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Table 6. Summary of the key features of bridges that experienced structural damage

No. Bridge name Materialiv)Limitstate Locationv)

Distance from

No. oftraffic lanes

AASHTOclassification

Population served(in thousands)Fault

Epi-center(km)

1 Slab Bridge Balakot SM MD FW 70 m 26.2 2 Critical >200

2 Suspension Bridge Balakot SM SD HW 150 m 25.9 1 Other Mainly pedestrians

3 3-Span Bridge Balakot RC MD HW 240 m 25.8 2 Critical >300

4 Garhi Dupatta Bridge RC SD FW 560 m 28.9 2 Critical >250

5 Suspension Bridge Maghoi SM CO FW 760 m 27.3 1 Essential >5

6 Chela Bandi Bridge RC LD FW 870 m 19.5 2 Essential >50

7 Arch Bridge Garhi Dupatta Road SM MD FW 980 m 23.3 2 Critical >300

8 Suspension Bridge Kamsar Road SM CO HW 1.0 km 17.5 1 Essential >5

9 Suspension Bridge Thota SM SD FW 1.0 km 25.8 1 Essential >5

10 Bridge on Garhi Dupatta Road SM CO FW 1.4 km 21.6 2 Critical >300

11 Truss Bridge Garhi Dupatta Road SM LD FW 1.9 km 21.0 1 Essential >5

12 Allama Iqbal Bridge RC LD FW 2.0 km 20.6 2 Essential >75

13 Truss Bridge Muzaffara-bad SM LD FW 2.0 km 20.6 1 Other Pedestrians only

14 Kund Bridge Besham RC LD Far field NA 83.5 1 Critical >10

iv) SM¼ Stone masonry, RC¼Reinforced concretev) FW¼ Footwall, HW¼Hanging wall

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damage, whereas of the RC bridges surveyed, none collapsed, only one experienced severedamage, and one experienced moderate damage. The collapse of the three stone masonrybridges is attributed to their height and relatively poor-quality masonry work. RC bridgesconstructed using some stone masonry experienced damage due to failure of their stone ma-sonry components. Among stone masonry bridges, those with dressed stones and mortar ofa uniform thickness performed relatively well. Examination of Table 6 shows that the fourstone masonry bridges that collapsed or were severely damaged were either essential or crit-ical bridges.

Pakistan is now divided into five seismic zones (GoP 2007), with Zone 1 having theleast seismic hazard and Zone 4 having highest. The observations above, along with otherobserved damages to the stone masonry components of bridges, suggest that the use ofstone masonry in bridges in areas classified as Zone 2A, which corresponds to a PGA of0.08 g–0.16 g (GoP 2007), and higher requires careful consideration.

During the survey a Schmidt hammer (Malhotra and Carino 2003) was used on the sub-structure of the multispan RC bridges to nondestructively estimate their concrete strengthsand assess the consistency of the quality of concrete in the bridges. A Schmidt hammermeasures the rebound of a spring-loaded mass, impacting against the surface of the concreteto be tested. The concrete strength is estimated by correlating the rebound number to thecompressive strength of the concrete. The estimated compressive strength of concrete in themajority of the bridges surveyed was in the range of 2,500–3,000 pounds per square inch(psi), but in a few of the bridges surveyed the concrete strength was estimated to be as lowas 2,000 psi or less. On the opposite end of the spectrum, the estimated concrete strength ofsome of the bridges surveyed was in excess of 5,000 psi. In bridges with a low concretestrength, significant variation in the concrete’s compressive strength was observed in thebridge substructure based on the Schmidt hammer readings, which indicates poor qualitycontrol during construction. There was less variation in the strength of the concrete inbridges with a relatively higher compressive strength, indicating a fairly uniform concretequality throughout the bridge substructure.

DETAILS OF DAMAGES OBSERVED

This section describes the damages observed in the surveyed bridges in relation to theirlimit states. These damages are summarized in Table 6.

UNSEATING = DROPDOWN

Figure 2 shows a collapsed RC bridge (No. 10 in Table 6) that was located approxi-mately 8 km from Muzaffarabad on the road from Muzaffarabad to Garhi Dupatta. Thistwo-lane, single-span bridge was 15 m long and 8.5 m wide and was under construction atthe time of the earthquake. The bridge was classified as critical, according to AASHTO(2007) specifications. The superstructure comprised five RC girders that were cast as onewith the concrete deck. The substructure of this bridge utilized stone masonry abutmentsthat were 4 m–5 m high. Dropdown of the bridge can be attributed to failure of the stonemasonry abutment. The bridge site was 1.4 km from the fault line on the footwall side, and21.6 km from the epicenter.

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Figure 2. Drop-down of girder bridge due to failure of stone masonry abutments.

Figure 3. Unseating of a three-span continuous bridge in Balakot which is 240 m away fromthe fault on the footwall side.

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Figure 3 shows a bridge in Balakot (No. 3 in Table 6) that experienced unseating of itsexterior girder along its entire length. The bridge is a three-span, two-lane continuous bridgewith a total length of 100 m. The roadway width is 8 m and the height of the wall piers isapproximately 3 m. The bridge is classified as critical, according to AASHTO (2007) speci-fications. The superstructure has four variable-depth RC girders that moved approximately1 m in the transverse direction and about 0.5 m in the longitudinal direction, resulting in theunseating of all the girders, which reduced the bridge capacity from two-lanes to a singlelane. This bridge is located 240 m from the fault on the hanging-wall side and probablyexperienced a vertical force of around 1.0 g (Durrani et al. 2005). Dropdown was preventedbecause of the continuity of the bridge’s girders. Restoration of this bridge was started threemonths after the earthquake and took approximately four months.

DAMAGE TO ABUTMENTS AND POUNDING

Pounding caused severe damage to the Garhi Dupatta RC Bridge (No. 4 in Table 6),which is 21 km from Muzaffarabad. This two-lane single-span bridge has a span of 120 mand is located 560 m from the fault on the footwall side. The height of the abutments isapproximately 7 m. The abutments of this bridge have cantilever extensions upon which thecentral span is simply supported. The central span comprises an RC two-cell box. Figure 4shows the Garhi Dupatta Bridge. Out-of-phase movement between the cantilevered parts ofthe superstructure and central box section resulted in the pounding and crushing of concreteat the expansion joint. The forces developed in the superstructure caused the shear failure ofa cold joint in one abutment, which is shown in Figure 5.

Figure 4. Garhi Dupatta Bridge.

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Figure 5. Shear failure of abutment at cold joint of Garhi Dupatta Bridge.

Figure 6. Damage to stone masonry abutment of single-span slab bridge in Balakot.

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Damage due to pounding was observed in a two-lane bridge that is 8 m wide, locatedwithin the city of Muzaffarabad (No. 12 in Table 6). The height of the bridge pier is approx-imately 15 m. The center span is simply supported and rests on cantilevered extensionsfrom the piers. Pounding at the expansion joint in the superstructure probably occurred dueto asynchronous motion of the two piers.

A two-lane slab bridge in Balakot with a span of 6 m and stone masonry abutments thatare 1.5 m–2 m high also suffered moderate damage (No. 1 in Table 6). This bridge is classi-fied as critical and was the closest to the fault line of all the bridges surveyed, located 70 mfrom the fault on the footwall side. The bridge was 26.2 km from the epicenter. This bridgeis shown in Figure 6.

A steel truss bridge built in the 1900s during British rule spans the Neelum River withinthe city of Muzaffarabad (No. 13 in Table 6) and is meant for pedestrian traffic only. Thissingle-lane bridge rests on dressed stone masonry abutments, as shown in Figure 7. Thelight damage does not seem to be entirely attributable to the October 8 earthquake. It islikely that the earthquake enhanced some pre-existing damage, resulting in the observeddamage. The relatively better performance of this bridge can be attributed to its massiveabutments, which lend stability, and the good quality of construction, including finelydressed stones and a uniform thickness of mortar in the bedding planes. These are typicalcharacteristics of masonry structures constructed during British rule.

The RC bridge near Besham on the River Indus is called the Kund Bridge (No. 14 inTable 6) and is classified as critical. This bridge is located 140 km from Abbottabad and is

Figure 7. Damage to stone masonry abutment of a steel truss bridge constructed in the 1900s inMuzaffarabad.

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Figure 8. Kund Bridge near the city of Besham.

Figure 9. Cracks in abutment of Kund Bridge near Besham.

A SURVEY OF DAMAGES TO BRIDGES IN PAKISTAN AFTER THE MAJOR EARTHQUAKE OF 8 OCTOBER 2005 959

1.5 km south of Besham. It is a single-lane prestressed girder bridge. The bridge has onepier in the river, from which part of the bridge deck is constructed using a balanced cantile-ver. Simply supported spans have been employed on each side of the balanced cantilever tocomplete the bridge span. The bridge is shown in Figure 8. On the left bank of the river theabutment has a cantilever extension toward the river. This provides an unbalanced masstoward the river. Cracks were observed in the abutment on the left bank, as shown inFigure 9. The site inspection suggests that some minor cracks may have been present priorto the earthquake, which most likely grew worse during the event.

DAMAGES TO SUSPENSION BRIDGES

The hilly parts of northern Pakistan and Azad Kashmir have rivers and deep ravines.Typically, single-lane suspension bridges are provided at these river crossings for lightvehicles, as traffic volumes are low and the cost of constructing such bridges is less com-pared to other types of bridges. These bridges use high-strength steel suspension cables andwooden decks. The bridge pylons are generally made of either reinforced concrete or stonemasonry. In some cases the towers are made of reinforced concrete that rests upon stonemasonry abutments, and in some cases the entire structure (i.e., the tower and the supportingsubstructure) is made of stone masonry. Durrani et al. (2005) did not report any significantincidences of damage to suspension bridges, however it was found in the present surveythat a significant number of suspension bridges experienced damage due to the earthquake.The high incidence of damage in these bridges is attributed to their stone masonry compo-nents. The relatively tall, slender masonry pylons exhibited vulnerability to earthquake-

Figure 10. Suspension bridge on Kamsar Road, Neelum River near Muzaffarabad.

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induced forces. In the suspension bridges with RC pylons placed over masonry abutments,the masonry abutments were damaged due to the earthquake-induced forces in the pylons.

Figure 10 shows a collapsed suspension bridge on Kamsar Road in Muzaffarabad that isclassified as essential (No. 8 in Table 6). The bridge was located 1 km from the fault line onthe hanging-wall side and 17.5 km from the epicenter of the earthquake. The landslides trig-gered by the earthquake swept away the anchor blocks of the main cables, resulting in thecollapse of the bridge. The approach roads to the bridge were also completely swept awayby the landslides.

Another suspension bridge in Balakot (No. 2 in Table 6) that was located 150 m fromthe fault on the hanging-wall side and 25.9 km from the epicenter suffered severe damagedue to sliding of the anchor block of the side-sway stabilizing cable. Figure 11 shows thedamaged bridge and the anchor block that slid. The stone masonry base of the RC tower,which was 6 m high, also suffered damage, and the bridge was displaced in the transversedirection due to the pull exerted by the fallen anchor block. The approach road failed in thiscase as well. It is worth mentioning that both the anchor block of this bridge and the girdersof the three-span RC bridge mentioned above (No. 3 in Table 6) slid in the downstreamdirection. This similarity suggests that the direction of the seismic forces generated in thesetwo bridges was predominately in the downstream direction, which is approximately per-pendicular to the strike of the fault. Both these bridges are within 240 m of the fault on thehanging-wall side.

Figure 11. Sliding of anchor block and failure of approach road to suspension bridge inBalakot.

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The stone masonry base of a suspension bridge at Thota (No. 9 in Table 6), classified asan essential bridge, suffered severe damage. The stone masonry foundation below the RCtower is 13 m high and the tower height is 10 m. The tower lost approximately one-third ofits foundation when the stones gave way. The base shear caused the bond between thestones in the foundation to fail. The damage to the tower foundation is shown in Figure 12.Access to this bridge for vehicles was lost due to failure of the approach road, as shown inFigure 13.

Another suspension bridge on Garhi Dupatta Road (No. 5 in Table 6) collapsed due tofailure of its stone masonry tower, which was 7 m high. The collapse left the two main sus-pension cables intact so the deck could continue to be used by pedestrians. However, anincident of fire in a small shop located below the anchor cables near one of the towersmelted the downstream cable and caused the collapse of the bridge deck, which is shown inFigure 14. This failure highlights the important role of regulation and enforcement to ensurethat no activities that could potentially damage bridges are allowed in their vicinity.

ACCESS TO BRIDGE AND OTHER FAILURES

Many bridges were left unfit for service due to failure of their approach roads. A single-lane steel truss bridge on Garhi Dupatta Road (No. 13 in Table 6) having a span of approxi-mately 50 m (shown in Figure 15) is another example in which access to the bridge waslost, therefore rendering it completely unfit for service. The main cause of the damage to the

Figure 12. Damage to stone masonry base of suspension bridge near Thota Village,Muzaffarabad.

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Figure 14. Failure of stone masonry tower and failure of downstream side suspensions cablesof bridge on Garhi Dupatta Road.

Figure 13. Loss of approach road to suspension bridge near Thota Village, Muzaffarabad.

A SURVEY OF DAMAGES TO BRIDGES IN PAKISTAN AFTER THE MAJOR EARTHQUAKE OF 8 OCTOBER 2005 963

approach roads in most of the bridges surveyed was the collapse of their stone masonryretaining walls.

A stone arch bridge (No. 7 in Table 6) on Garhi Dupatta Road was constructed usingrounded dry stones filled in below the bridge deck. This bridge had moderate damage dueto failure of the stone masonry that supported the dry-fill rocks, and the bridge was left unfitfor service for all types of vehicular traffic. The bridge was located 980 m from the fault onthe footwall side.

An RC bridge in Muzaffarabad (No. 6 in Table 6) that has two lanes and piers that are12 m high suffered no significant damage due to the earthquake except for a horizontalcrack in the pier base that may have been a result of poor cold joint preparation at the timeof construction. The bridge did not experience a higher degree of damage because it islocated 870 m away from the fault on the footwall side (shaking intensity is relatively lesson the footwall side of a causative fault). However, considering the fact that Muzaffarabadis now placed in Seismic Zone 4 (PGA> 0.32 g) by the Building Code of Pakistan (GoP2007) the performance of this bridge needs further investigation as damage accumulated sofar might lead to failure of the bridge pier in future strong ground shaking.

If light damage is taken as the threshold limit defining the acceptable performance of abridge in an earthquake, then from the information presented in Table 6 it can be seen thatnine bridges (10%) of the total 90 bridges in the survey area either collapsed or experiencedmoderate damage, making them nonfunctional as a result of the earthquake. Of these ninebridges, five are classified as critical, three are classified as essential, and one is classifiedas other. The incidence of damage to critical and essential bridges as a result of a single

Figure 15. Loss of approach road to steel truss bridge on Garhi Dupatta Road.

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seismic event is considered unacceptable. There can be many reasons for this unacceptableperformance, including poor workmanship, construction materials not meeting specifica-tions, and design flaws, etc. As mentioned earlier, nondestructive tests using a Schmidthammer indicated significant variability in the strength of the concrete in the bridges sur-veyed. However, it is a far more serious concern that the design of bridges in Pakistan is notregulated by a comprehensive set of specifications. This is a problem that needs to beaddressed to improve the safety standards of the country’s bridge infrastructure. This issueis discussed in the following section.

CODE COMPLIANCE OF EXISTING BRIDGES

It is important to note that the seismic hazard map used for both buildings and bridgesin Pakistan until 2007 was the one specified in the Building Code of Pakistan (GoP 1986).This hazard map, shown in Figure 16, was based on the modified Mercalli intensity (MMI)

Figure 16. Seismic Hazard Map based on MMI scale from the Building Code of Pakistan1986.

A SURVEY OF DAMAGES TO BRIDGES IN PAKISTAN AFTER THE MAJOR EARTHQUAKE OF 8 OCTOBER 2005 965

scale prepared by the Geological Survey of Pakistan on the basis of instrumental macro-earthquake data from 1905 to 1979 recorded at the Quetta Station in the province of Balu-chistan. According to this map, Pakistan was divided into four zones according to level ofdamage (Zone 0 was defined as a negligible damage area and Zone 3 as a major damagearea). The area covered in the present survey was classified as a moderate damage area(Zone 2), corresponding to an MMI intensity of VII. Following the October 2005 earth-quake, the Building Code of Pakistan was revised, and during the process the seismic haz-ard map was also revised. The new map (GoP 2007) is based on ground motions with a10% probability of exceedance in 50 years (475-year return period), and provides five haz-ard zones, as shown in Figure 17. Areas such as Balakot and Muzaffarabad were previouslyplaced in Zone 2 (moderate damage), but according to the new hazard map are now placedin Zone 4 (PGA� 0.32 g), the zone with the highest seismic hazard. It is also worth notingthat a significant portion of Pakistan is now placed in Zone 2B (PGA 0.16 g–0.24 g) orabove. The marked increase in the seismic hazard of areas like this makes it imperative toreassess the seismic vulnerability of bridges in high seismic risk areas to determine whichbridges need strengthening.

The Code of Practice for Highway Bridges (GoWP 1967), published in 1967, wasadapted from the 8th edition of the American Association of State Highway Officials’(AASHO) specifications of 1961 (AASHO 1961) and stands as the sole officially issued

Figure 17. Seismic Hazard Map based on 475-year return period from the Building Code ofPakistan 2007.

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document=specifications in Pakistan for the design of highway bridges. (Note that in 1973AASHO became AASHTO, or the American Association of State Highway and Transporta-tion Officials, and their latest publication regarding specifications for highway bridges waspublished in 2007.) Considering the archaic nature of the Code of Practice for HighwayBridges, supplementing it with the more current AASHTO specifications is common prac-tice among bridge designers. Since the Code of Practice for Highway Bridges preceded theavailability of seismic zoning maps, it requires that bridges be designed for a lateral forcethat is 2%–6% of the dead load of the structure (0.02 g–0.06 g). As mentioned above, thefirst seismic hazard map for Pakistan became available in the Building Design Code of1986 (GoP 1986) and was based on the MMI scale that does not give design PGA valuesthat can be used in conjunction with AASHTO specifications for the design of bridges. Thissituation meant that before 2007 bridge designers had no choice but to either arbitrarilyassume PGA values or to follow the recommendations of the Code of Practice for HighwayBridges and use 0.02 g–0.06 g as the lateral pseudostatic load. With the revision of Paki-stan’s seismic hazard map in 2007, a significant area of the country was placed in SeismicZone 2B or above (PGA> 0.16 g) and the survey area was placed in Seismic Zone 4(PGA> 0.32 g). Since the new PGA values are significantly higher than the recommendeddesign values of 0.02 g–0.06 g found in the Code of Practice for Highway Bridges, it is crit-ical that the engineering community in Pakistan work to ascertain the safety and code com-pliance of bridges constructed before 2007, especially the ones constructed prior to 1986.

It is evident from the seismic hazard map (Figure 17) that a significant area of Pakistanfalls within zones that have a high seismic risk, including such large cities as Karachi,Quetta, Gwadar, Peshawar, Abbottabad, Gujrat, and Islamabad. The prevention of damageto bridge infrastructure in future earthquakes would require a safety evaluation of the exist-ing bridges in high seismic risk areas. Work on the development of bridge design specifica-tions specific to Pakistan needs to be initiated; the revision of the Code of Practice for High-way Bridges in light of the latest AASHTO specifications would be a step in the rightdirection.

CONCLUSIONS AND RECOMMENDATIONS

Based on a survey of bridges carried out after the Mw 7.6 earthquake of 8 October2005, the following recommendations are made:

1. A comprehensive set of guidelines and specifications for the design and construc-tion of bridges should be developed and put in place for use by the engineeringcommunity, as well as a regulatory framework. Revision, and perhaps replace-ment, of the Code of Practice for Highway Bridges with a bridge design codemore in line with modern bridge design codes is in order.

2. A thorough a safety assessment of essential and critical bridges located in highseismic risk areas should be undertaken. The design of these bridges should bechecked for compliance with appropriate modern bridge design codes and thebridges found to be noncompliant should be strengthened to meet compliancerequirements.

3. A significant number of failures of stone masonry bridges and bridge componentswere observed in the survey area, which can be attributed to poor quality of

A SURVEY OF DAMAGES TO BRIDGES IN PAKISTAN AFTER THE MAJOR EARTHQUAKE OF 8 OCTOBER 2005 967

construction, the size of bridge components, and the level of ground shaking. It istherefore recommended that stone masonry construction in bridges located in Seis-mic Zones 2A (PGA 0.08 g–0.16 g) and above be avoided, at least until appropri-ate guidelines and specifications for stone masonry construction in high seismicrisk areas become available. In the case of bridges that are less important (wherealternate routes exist) or that are constructed in low seismic risk areas, the qualityof the stone masonry construction should be ensured by employing dressed stones,good quality mortar, and reasonable member size to minimize damage. Existingbridges having stone masonry in their components should be considered forstrengthening.

4. More damage to bridges was observed on the hanging-wall side than on the footwallside of the causative fault. While all the critical and essential bridges in the vicinityof the fault in the survey area should be considered for strengthening, the bridges onthe hanging-wall side deserve special consideration. The design of new criticalbridges should account for the location of the bridge in relation to the fault.

5. Unseating was a common damage pattern observed in the surveyed bridges. It canbe avoided by increasing the redundancy of the bridges, providing longer seatsand longitudinal retainers.

6. A small number of bridges in the survey area experienced a complete loss of serv-iceability due to damage to their approach roads. Therefore, special attentionshould be given to preclude such damage by carefully planning and designingbridge approaches. Bridges can suffer damage due to landslides or falling debris.For new bridges, attention should be given to this issue when selecting a site. Forexisting bridges that are in the vicinity of potential landslide areas, slope stabiliza-tion measures should be considered.

7. Damage caused by pounding was observed in a few bridges. In high seismic riskareas, the design should account for pounding-induced forces at expansion joints.Construction joints should be provided at noncritical locations.

8. Suspension bridges usually require tall pylons for their suspension cables and aretherefore susceptible to damage from seismic forces. Accordingly, in the case ofsuspension bridges located in high seismic risk areas, a thorough seismic analysisshould be undertaken before the design stage, and suitable construction materialsshould be selected.

9. A common damage pattern in the suspension bridges surveyed was the sliding ofanchor blocks for side-sway stabilizing cables, which caused damaged to thebridge superstructure. It is recommended that the anchor blocks should also beanchored to avoid sliding.

10. Commercial activities that may potentially damage bridge structures, such as activ-ities requiring the use of flammable products, should be prohibited in the vicinityof bridges.

ACKNOWLEDGMENTS

The survey described in this paper was conducted as part of Syed Ali’s PhD research.The authors would like to expresses their gratitude to the Higher Education Commission of

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Pakistan for their sponsorship of this survey. Funding was also provided by the EarthquakeEngineering Research Institute (EERI), in Oakland, California, and the authors would liketo thank EERI for sponsoring a visit to San Francisco to present their research at the 8th

U.S. National Conference on Earthquake Engineering in April 2006. The authors wouldalso like to gratefully acknowledge the National Academies of Sciences for sponsoring avisit to California in December 2006 for meetings with researchers, and to the State Univer-sity of New York (SUNY) in Buffalo for seven months of specialized training in earthquakeengineering at the Multi-Disciplinary Center for Earthquake Engineering Research(MCEER) and the Structural Engineering and Earthquake Simulation Laboratory (SEESL).These trips provided opportunities to interact with fellow researchers, which was helpful inthe preparation of the draft of this paper. We also gratefully acknowledge the support ofMr. Faisal-ur-Rehman in the preparation of figures for this paper.

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(Received 30 January 2009; accepted 5 January 2011)

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