A Survey of Damages to Bridges in Pakistan - Kashmir EQ.pdf

A Survey of Damages to Bridges in Pakistan after the Major Earthquake of 8 October 2005 Syed M. Ali,a) Akhtar N. K han, han,a) Shahzad Rahman,a) and Andrei M. Reinhorn,b) M.EERI

An earthquake measuring Mw 7.6 struck the Pakistan-administered part of Kashmir on 8 October 2005. The epicenter of the earthquake was located 22 km from the city of Muzaffarabad. The earthquake resulted in the loss of more than 80,000 lives and caused extensive damage to property and infrastructure. A survey of an approximately 400-km road network was carried out, in which 90 bridges were inspected for earthquake-associated damage, out of which 14 bridges (16%) experienced damage of varying degrees, of which nine bridges (10%) either failed or became nonfunctional. The survey revealed some of the deficiencies of the construction practices in Pakistan and also highlighted the need for improvement to the country’s current bridge design practices. This pa per reports the prominent types of failures observed and discusses the deficiencies in current design practices. Based on the findings of the survey, various recom rec omme mend ndat ation ionss ar aree ma made de,, wi with th the ob obje jecti ctive ve of mi mini nimi mizi zing ng ear earth thqu quak akeeassociated damages to new and existing bridges in areas with a high seismic risk. [DOI: 10.1193/1. 10.1193/1.36504 3650477] 77] INTRODUCTION

On 8 October 2005 an earthquake measuring Mw 7.6 struck the Pakistan-administered part of Kashmir. The epicenter of the earthquake was located 22 km from the city of Muzaffarabad. The earthquake was caused by the rupture of the Balakot-Bagh thrust fault. The Pakistan Meteorological Department (PMD) and the Norwegian Seismic Array (NORSAR) estimated the rupture length to be 90 km–100 km (PMD (PMD and NORSAR 2006); 2006); Kaneda et al. (2008) estimated the rupture length to be approximately 70 km. The focal depth of the earthquake was shallow; it was reported to be 26 km (USGS (USGS 2005, 2005, Rao et al. 2006). 2006). The earthquake caused severe damage in the areas close to the fault. Figure 1 shows the locatio location n of the epicenter of the earthquake. According to estimates, more than 80,000 people lost their lives and around 4 million people were left homeless (Rao (Rao et al. 2006). 2006). The fault ruptured ture d dire directly ctly ben beneat eath h the high highly ly pop popula ulated ted citi cities es of Bal Balako akot, t, Muz Muzaff affarab arabad, ad, and Bag Bagh, h, causing extensive damage to their infrastructure. A total of 90 bridges were surveyed in the northern part of Pakistan and Kashmir, out of which approximately 25 bridges were located within a 25-km radius from the epicenter.

a)

University of Engine University Engineering ering & Technology, Technology, Dept. of Civil Engineering Engineering & Earthquake Earthquake Engine Engineering ering Center, Center, Peshawar, Peshawar, Pakistan b) University at Buffalo, Dept. of Civil, Structural, & Environmental Engineering, Buffalo, NY 14260

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Earthquake Spectra, Spectra, Volume 27, No. 4, pages 947–970, November 2011;

C 2011, Earthquake Engineering Research Institute V

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

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; an electronic version of this map is accessible online at Google Maps (Ali (Ali 2008); 2008); the same map can also be opened in Google Earth, and GPS coordinates of the plotted bridges can be read. The fault rupture is also plotted on this map using the GPS coordinates taken from Kaneda et al. (2008). (2008). Pakistan and its adjoining regions have a history of major earthquakes, including the magnitude 8.0 Kangra earthquake of 1905, the magnitude 8.0 Pattan earthquake of 1974, and the magnitude 8.1 Quetta earthquake of 1935 (PMD (PMD and NORSAR 2006). 2006). According to the earthquake catalog prepared by PMD and NORSAR, more than 40 earthquakes of magnitude 7 or larger have occurred between 1900 and 2005 in the Himalayan region that influences the northern part of Pakistan and Kashmir (PMD (PMD and NORSAR 2006). 2006). These facts highlight the potential of future earthquakes in this region and it is estimated that earthquakes of magnitude 8 or larger are likely to occur (Bilham (Bilham and Wallace 2006). 2006). While many large cities in the world are located close to active thrust faults and exposed to serious seismic hazard, the surface ruptures of thrust faults are much less common than other fault types and less is known about them; thus more research on thrust faults is needed (Kaneda et al. 2008). 2008). In the case of Pakistan, cities like Muzaffarabad, Balakot, Mansehra,

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

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and Abbottabad are located close to the Balakot-Bagh thrust fault and may experience large-magnitude earthquakes in the future. These facts pose a serious challenge to the engineering community of Pakistan in particular, and engineers across the world, and highlight the importance of understanding the performance of infrastructure during seismic events like Pakistan’s 2005 earthquake. Some of the in-service bridges in the surveyed region date back to the early 20th century. Modern bridges constructed by various departments are gradually replacing the old, typically single lane bridges. Information collected during discussions with officials of various departments suggests that currently there are approximately 6,000 bridges on the national 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 and Highway Transportation Officials (AASHTO) specifications. During the survey the structural details and GPS coordinates of various important bridges 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 to estimate the level of ground shaking that affected the bridges within the survey area. However, some guidance can be taken from the information presented in the report by Durrani et 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 measuring about four for the 5% damped elastic response spectrum in the range of 0.4–2.0 seconds. Durrani et al. (2005) also gave estimates of PGAs for stiff and soft soils, calculated using various models. According to these estimates the PGA at locations 25 km from the epicenter was in the range of 0.25 g–0.4 g and the PGA at locations 50 km from the epicenter was in the range of 0.15 g–0.231 g. The 0.231 g PGA recorded in Abbottabad falls close to the upper range of the estimated PGA for locations 50 km from the epicenter. This may be attributed to the subsurface conditions in Abbottabad being made up of soft soils, which would have amplified of the ground shaking. The distance of the bridges in the survey area from the epicenter ranges from 19 km–85 km. On the basis of attenuation information presented in Durrani et al. (2005), it is the opinion of the authors that the bridges in the study area were subjected to a PGA ranging from 0.15 g–0.4 g, depending upon their proximity to 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 Table1. In the succeeding section, classification of the surveyed bridges is presented based on their importance, their structural form, and the material used for their construction. This classification is accompanied by information pertaining to the levels of damage observed in these bridges. Following on from this section is a detailed discussion of the prominent types of damage and failures observed following the earthquake. Finally, conclusions and recommendations for improvements in the design and planning of bridges are presented. Data was collected during field visits conducted over a period of two years following the October

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Table 1. Limit state classification of bridges based on structural damage Code ND LD MD

Damage

SD

No damage Slightly damaged Moderately damaged Severely damaged

CO

Collapsed

Reusability Reparability

Restorability

Yes Yes Yes

No need Yes Difficult

No need Complete restoration to original state possible Complete restoration to original state possible

Partial

Difficult

No

No

Restoration of the bridge to original state not possible Restoration not possible=reconstruction required

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 in Table 2. The bridges surveyed were assigned limit states according to Table 1. Tables 3, 4, and 5 categorize the results with respect to superstructure, substructure, and the material of construction, respectively. Out of the 90 bridges inspected during the survey, 14 bridges were found to have experienced some form of damage following the earthquake. These 14 bridges are listed in Table 2. Routes, number of bridges surveyed, and population served Number of bridges* Route=location name Havalian Abbottabad Mansehra Mansehra Battagram Besham Mansehra Garhi Habibullah – Muzaffarabad Grahi Habibullah – Balakot Muzaffarabad – Kohala Muzaffarabad Garhi Dupatta Muzaffarabad Subtotal Total

Single-span

Multispan

13 26 1 3 13 7 3 66

2 11 2 1 4 1 3 24

Population served (in thousands)

>200 >150 >50 >50 >150 >200 >150

90

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


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Table 3. Superstructure of bridges organized by limit state classification

Limit state

I Gir der (PSi) = T (RCii) Girder = Slab

Box girder

Truss

Suspension

Arch

Total

60 1 1 – 1 63

3 2 1 1 – 7

2 2 – – – 4

8 – 0 2 2 12

3 – 1 – – 4

76 5 3 3 3 90

ND LD MD SD CO Total i)

PS ¼ Prestressed RC ¼ Reinforced concrete

ii)

Table 6, which also lists the limit states of the bridges following the earthquake, the construction materials used, distances from the epicenter and the rupture zone, importance levels 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 by the bridges. The importance of a bridge is decided on the basis of the availability of alternate routes to the population served. This means that a bridge serving a population without an alternate route will have highest importance and is classified as critical . Bridges are classified as essential when an alternate route is available but is difficult to access. The category others indicates that alternate routes are easily accessible. Since 13 bridges were close to the rupturing fault (near-field), the distance from the fault is also recorded in Table 6. It is worth mentioning that it can be misleading to only consider the distance of a bridge from the epicenter while making a qualitative assessment of the level of shaking a bridge may have experienced. The distance of a bridge from the fault rupture is also an important factor that 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, contained stone 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 of Pakistan and the relatively high costs associated with transporting bricks or materials for Table 4. Substructure of bridges organized by limit state classification

Limit state ND LD MD SD CO Total

Single column

Multicolumn

Wall

Abutments (single span) = tower base

1 3 – – – 4

4 – – – – 4

– – 2 – – 2

71 2 1 3 3 80

Total 76 5 3 3 3 90

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

Limit state ND LD MD SD CO Total

Reinforced concrete (RC)

Stone masonry & mix of stone masonry & RC (SM&RC)

Othersiii)

Total

13 4 1 1 – 19

7 1 2 2 3 15

56 – – – – 56

76 5 3 3 3 90

iii)

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

reinforced concrete (RC) construction, it is not uncommon to find stone masonry bridges in these areas. Data collected for bridges outside the study area indicates that current practices in Pakistan favor bridges with an RC substructure and post-tensioned I-girders with RC slabs. The extent of damage experienced by the bridges in the survey area depended upon the following factors: whether the bridge was located on the hanging-wall or footwall side of the fault, its distance from the fault, the material used in its construction, the quality of construction, and its geometric features. A thrust fault caused the 2005 earthquake and it was observed by the authors that structures on the hanging-wall side experienced more damage due to severe ground shaking as compared to those located on footwall side; this observation was also reported by Kaneda et al. (2008). Out of the three bridges located on the hanging-wall side, one collapsed, one was severely damaged, and one suffered moderate damage. These levels of damage are in accordance with the accepted phenomenon that structures on the hanging-wall side of a fault suffer more damage than those on the footwall side (Kaneda et al. 2008). The collapsed of the bridge that was destroyed (No. 8 in Table 6) was precipitated by landslides. The bridge that suffered severe damage (No. 2 in Table 6) had a tall stone masonry substructure that could not withstand the lateral forces created by the earthquake. The bridge on the hangingwall side that suffered moderate damage (No. 3 in Table 6) is a three-span continuous RC bridge. The continuity of the superstructure of this bridge helped prevent a drop-down of its superstructure and the thick, short solid wall piers helped it survive the ground shaking forces. The effects of the shear forces created by the earthquake were reduced due to the sliding of the deck over the pier walls. The relatively fair performance of the stone masonry bridges on the footwall side of the fault should not be misconstrued as acceptable performance of this construction type, which is notoriously prone to damage by earthquakes. The level of shaking, geometry of the structure, 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 surveyed, irrespective of their structural form. Tables 5 and 6 show that three stone masonry bridges suffered complete collapse, two were severely damaged, and two suffered moderate

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

No.

Bridge name

Materialiv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Slab Bridge Balakot Suspension Bridge Balakot 3-Span Bridge Balakot Garhi Dupatta Bridge Suspension Bridge Maghoi Chela Bandi Bridge Arch Bridge Garhi Dupatta Road Suspension Bridge Kamsar Road Suspension Bridge Thota Bridge on Garhi Dupatta Road Truss Bridge Garhi Dupatta Road Allama Iqbal Bridge Truss Bridge Muzaffara-bad Kund Bridge Besham

SM SM RC RC SM RC SM SM SM SM SM RC SM RC

iv) v)

SM ¼ Stone masonry, RC ¼ Reinforced concrete FW ¼ Footwall, HW ¼ Hanging wall

Limit state Locationv) MD SD MD SD CO LD MD CO SD CO LD LD LD LD

FW HW HW FW FW FW FW HW FW FW FW FW FW Far field

Fault 70 m 150 m 240 m 560 m 760 m 870 m 980 m 1.0 km 1.0 km 1.4 km 1.9 km 2.0 km 2.0 km NA

Epi-center No. of traffic lanes (km) 26.2 25.9 25.8 28.9 27.3 19.5 23.3 17.5 25.8 21.6 21.0 20.6 20.6 83.5

2 1 2 2 1 2 2 1 1 2 1 2 1 1

AASHTO classification

Population served (in thousands)

Critical Other Critical Critical Essential Essential Critical Essential Essential Critical Essential Essential Other Critical

>200 Mainly pedestrians >300 >250 >5 >50 >300 >5 >5 >300 >5 >75 Pedestrians only >10

A S U R V E Y O F D A M A G E S T O B R I D G E S I N P A K I S T A N A F T E R T H E M A J O R E A R T H Q U A K E O F 8 O C T O B E R 2 0 0 5

9 5 3

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damage, whereas of the RC bridges surveyed, none collapsed, only one experienced severe damage, and one experienced moderate damage. The collapse of the three stone masonry bridges is attributed to their height and relatively poor-quality masonry work. RC bridges constructed using some stone masonry experienced damage due to failure of their stone masonry components. Among stone masonry bridges, those with dressed stones and mortar of a uniform thickness performed relatively well. Examination of Table 6 shows that the four stone masonry bridges that collapsed or were severely damaged were either essential or critical bridges. Pakistan is now divided into five seismic zones (GoP 2007), with Zone 1 having the least seismic hazard and Zone 4 having highest. The observations above, along with other observed damages to the stone masonry components of bridges, suggest that the use of stone masonry in bridges in areas classified as Zone 2A, which corresponds to a PGA of 0.08 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 substructure of the multispan RC bridges to nondestructively estimate their concrete strengths and assess the consistency of the quality of concrete in the bridges. A Schmidt hammer measures the rebound of a spring-loaded mass, impacting against the surface of the concrete to be tested. The concrete strength is estimated by correlating the rebound number to the compressive strength of the concrete. The estimated compressive strength of concrete in the majority 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 low as 2,000 psi or less. On the opposite end of the spectrum, the estimated concrete strength of some of the bridges surveyed was in excess of 5,000 psi. In bridges with a low concrete

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damage, whereas of the RC bridges surveyed, none collapsed, only one experienced severe damage, and one experienced moderate damage. The collapse of the three stone masonry bridges is attributed to their height and relatively poor-quality masonry work. RC bridges constructed using some stone masonry experienced damage due to failure of their stone masonry components. Among stone masonry bridges, those with dressed stones and mortar of a uniform thickness performed relatively well. Examination of Table 6 shows that the four stone masonry bridges that collapsed or were severely damaged were either essential or critical bridges. Pakistan is now divided into five seismic zones (GoP 2007), with Zone 1 having the least seismic hazard and Zone 4 having highest. The observations above, along with other observed damages to the stone masonry components of bridges, suggest that the use of stone masonry in bridges in areas classified as Zone 2A, which corresponds to a PGA of 0.08 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 substructure of the multispan RC bridges to nondestructively estimate their concrete strengths and assess the consistency of the quality of concrete in the bridges. A Schmidt hammer measures the rebound of a spring-loaded mass, impacting against the surface of the concrete to be tested. The concrete strength is estimated by correlating the rebound number to the compressive strength of the concrete. The estimated compressive strength of concrete in the majority 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 low as 2,000 psi or less. On the opposite end of the spectrum, the estimated concrete strength of some of the bridges surveyed was in excess of 5,000 psi. In bridges with a low concrete strength, significant variation in the concrete’s compressive strength was observed in the bridge substructure based on the Schmidt hammer readings, which indicates poor quality control during construction. There was less variation in the strength of the concrete in bridges with a relatively higher compressive strength, indicating a fairly uniform concrete quality throughout the bridge substructure. DETAILS OF DAMAGES OBSERVED

This section describes the damages observed in the surveyed bridges in relation to their limit 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 approximately 8 km from Muzaffarabad on the road from Muzaffarabad to Garhi Dupatta. This two-lane, single-span bridge was 15 m long and 8.5 m wide and was under construction at the 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 one with the concrete deck. The substructure of this bridge utilized stone masonry abutments that were 4 m–5 m high. Dropdown of the bridge can be attributed to failure of the stone masonry abutment. The bridge site was 1.4 km from the fault line on the footwall side, and 21.6 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 from the 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 its exterior girder along its entire length. The bridge is a three-span, two-lane continuous bridge with a total length of 100 m. The roadway width is 8 m and the height of the wall piers is approximately 3 m. The bridge is classified as critical , according to AASHTO (2007) specifications. The superstructure has four variable-depth RC girders that moved approximately 1 m in the transverse direction and about 0.5 m in the longitudinal direction, resulting in the unseating of all the girders, which reduced the bridge capacity from two-lanes to a single lane. This bridge is located 240 m from the fault on the hanging-wall side and probably experienced a vertical force of around 1.0 g (Durrani et al. 2005). Dropdown was prevented because of the continuity of the bridge’s girders. Restoration of this bridge was started three months 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 m and is located 560 m from the fault on the footwall side. The height of the abutments is approximately 7 m. The abutments of this bridge have cantilever extensions upon which the central span is simply supported. The central span comprises an RC two-cell box. Figure 4 shows the Garhi Dupatta Bridge. Out-of-phase movement between the cantilevered parts of the superstructure and central box section resulted in the pounding and crushing of concrete at the expansion joint. The forces developed in the superstructure caused the shear failure of a cold joint in one abutment, which is shown in Figure 5.

Figure 4. Garhi Dupatta Bridge.


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, located within the city of Muzaffarabad (No. 12 in Table 6). The height of the bridge pier is approximately 15 m. The center span is simply supported and rests on cantilevered extensions from the piers. Pounding at the expansion joint in the superstructure probably occurred due to asynchronous motion of the two piers. A two-lane slab bridge in Balakot with a span of 6 m and stone masonry abutments that are 1.5 m–2 m high also suffered moderate damage (No. 1 in Table 6). This bridge is classified as critical and was the closest to the fault line of all the bridges surveyed, located 70 m from the fault on the footwall side. The bridge was 26.2 km from the epicenter. This bridge is shown in Figure 6. A steel truss bridge built in the 1900s during British rule spans the Neelum River within the city of Muzaffarabad (No. 13 in Table 6) and is meant for pedestrian traffic only. This single-lane bridge rests on dressed stone masonry abutments, as shown in Figure 7. The light damage does not seem to be entirely attributable to the October 8 earthquake. It is likely that the earthquake enhanced some pre-existing damage, resulting in the observed damage. The relatively better performance of this bridge can be attributed to its massive abutments, which lend stability, and the good quality of construction, including finely dressed stones and a uniform thickness of mortar in the bedding planes. These are typical characteristics of masonry structures constructed during British rule. The RC bridge near Besham on the River Indus is called the Kund Bridge (No. 14 in Table 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 in Muzaffarabad.


Figure 8. Kund Bridge near the city of Besham.

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

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1.5 km south of Besham. It is a single-lane prestressed girder bridge. The bridge has one pier in the river, from which part of the bridge deck is constructed using a balanced cantilever. Simply supported spans have been employed on each side of the balanced cantilever to complete the bridge span. The bridge is shown in Figure 8. On the left bank of the river the abutment has a cantilever extension toward the river. This provides an unbalanced mass toward the river. Cracks were observed in the abutment on the left bank, as shown in Figure 9. The site inspection suggests that some minor cracks may have been present prior to 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 light vehicles, 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 and wooden decks. The bridge pylons are generally made of either reinforced concrete or stone masonry. In some cases the towers are made of reinforced concrete that rests upon stone masonry abutments, and in some cases the entire structure (i.e., the tower and the supporting substructure) is made of stone masonry. Durrani et al. (2005) did not report any significant incidences of damage to suspension bridges, however it was found in the present survey that 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 components. 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 is classified as essential (No. 8 in Table 6). The bridge was located 1 km from the fault line on the hanging-wall side and 17.5 km from the epicenter of the earthquake. The landslides triggered by the earthquake swept away the anchor blocks of the main cables, resulting in the collapse of the bridge. The approach roads to the bridge were also completely swept away by the landslides. Another suspension bridge in Balakot (No. 2 in Table 6) that was located 150 m from the fault on the hanging-wall side and 25.9 km from the epicenter suffered severe damage due to sliding of the anchor block of the side-sway stabilizing cable. Figure 11 shows the damaged 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 transverse direction due to the pull exerted by the fallen anchor block. The approach road failed in this case as well. It is worth mentioning that both the anchor block of this bridge and the girders of the three-span RC bridge mentioned above (No. 3 in Table 6) slid in the downstream direction. This similarity suggests that the direction of the seismic forces generated in these two 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 the hanging-wall side.

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

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

The stone masonry base of a suspension bridge at Thota (No. 9 in Table 6), classified as an essential bridge, suffered severe damage. The stone masonry foundation below the RC tower is 13 m high and the tower height is 10 m. The tower lost approximately one-third of its foundation when the stones gave way. The base shear caused the bond between the stones 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 in Figure 13. Another suspension bridge on Garhi Dupatta Road (No. 5 in Table 6) collapsed due to failure 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, an incident of fire in a small shop located below the anchor cables near one of the towers melted the downstream cable and caused the collapse of the bridge deck, which is shown in Figure 14. This failure highlights the important role of regulation and enforcement to ensure that 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 singlelane steel truss bridge on Garhi Dupatta Road (No. 13 in Table 6) having a span of approximately 50 m (shown in Figure 15) is another example in which access to the bridge was lost, therefore rendering it completely unfit for service. The main cause of the damage to the


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

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

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

approach roads in most of the bridges surveyed was the collapse of their stone masonry retaining walls. A stone arch bridge (No. 7 in Table 6) on Garhi Dupatta Road was constructed using rounded dry stones filled in below the bridge deck. This bridge had moderate damage due to failure of the stone masonry that supported the dry-fill rocks, and the bridge was left unfit for service for all types of vehicular traffic. The bridge was located 980 m from the fault on the footwall side. An RC bridge in Muzaffarabad (No. 6 in Table 6) that has two lanes and piers that are 12 m high suffered no significant damage due to the earthquake except for a horizontal crack in the pier base that may have been a result of poor cold joint preparation at the time of construction. The bridge did not experience a higher degree of damage because it is located 870 m away from the fault on the footwall side (shaking intensity is relatively less on the footwall side of a causative fault). However, considering the fact that Muzaffarabad is now placed in Seismic Zone 4 (PGA > 0.32 g) by the Building Code of Pakistan (GoP 2007) the performance of this bridge needs further investigation as damage accumulated so far 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 a bridge in an earthquake, then from the information presented in Table 6 it can be seen that nine bridges (10%) of the total 90 bridges in the survey area either collapsed or experienced moderate damage, making them nonfunctional as a result of the earthquake. Of these nine bridges, five are classified as critical , three are classified as essential , and one is classified as other . The incidence of damage to critical and essential bridges as a result of a single


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seismic event is considered unacceptable. There can be many reasons for this unacceptable performance, including poor workmanship, construction materials not meeting specifications, and design flaws, etc. As mentioned earlier, nondestructive tests using a Schmidt hammer indicated significant variability in the strength of the concrete in the bridges surveyed. However, it is a far more serious concern that the design of bridges in Pakistan is not regulated by a comprehensive set of specifications. This is a problem that needs to be addressed to improve the safety standards of the country’s bridge infrastructure. This issue is 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 bridges in 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 Pakistan 1986.

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scale prepared by the Geological Survey of Pakistan on the basis of instrumental macroearthquake data from 1905 to 1979 recorded at the Quetta Station in the province of Baluchistan. According to this map, Pakistan was divided into four zones according to level of damage (Zone 0 was defined as a negligible damage area and Zone 3 as a major damage area). 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 earthquake, the Building Code of Pakistan was revised, and during the process the seismic hazard map was also revised. The new map (GoP 2007) is based on ground motions with a 10% probability of exceedance in 50 years (475-year return period), and provides five hazard zones, as shown in Figure 17. Areas such as Balakot and Muzaffarabad were previously placed in Zone 2 (moderate damage), but according to the new hazard map are now placed in Zone 4 (PGA  0.32 g), the zone with the highest seismic hazard. It is also worth noting that a significant portion of Pakistan is now placed in Zone 2B (PGA 0.16 g–0.24 g) or above. The marked increase in the seismic hazard of areas like this makes it imperative to reassess the seismic vulnerability of bridges in high seismic risk areas to determine which bridges need strengthening. The Code of Practice for Highway Bridges (GoWP 1967), published in 1967, was adapted 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 of Pakistan 2007.


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document =specifications in Pakistan for the design of highway bridges. (Note that in 1973 AASHO became AASHTO, or the American Association of State Highway and Transportation Officials, and their latest publication regarding specifications for highway bridges was published in 2007.) Considering the archaic nature of the Code of Practice for Highway Bridges, supplementing it with the more current AASHTO specifications is common practice among bridge designers. Since the Code of Practice for Highway Bridges preceded the availability of seismic zoning maps, it requires that bridges be designed for a lateral force that is 2%–6% of the dead load of the structure (0.02 g–0.06 g). As mentioned above, the first seismic hazard map for Pakistan became available in the Building Design Code of 1986 (GoP 1986) and was based on the MMI scale that does not give design PGA values that can be used in conjunction with AASHTO specifications for the design of bridges. This situation meant that before 2007 bridge designers had no choice but to either arbitrarily assume PGA values or to follow the recommendations of the Code of Practice for Highway Bridges and use 0.02 g–0.06 g as the lateral pseudostatic load. With the revision of Pakistan’s seismic hazard map in 2007, a significant area of the country was placed in Seismic Zone 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 recommended design values of 0.02 g–0.06 g found in the Code of Practice for Highway Bridges, it is critical 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 Pakistan falls within zones that have a high seismic risk, including such large cities as Karachi, Quetta, Gwadar, Peshawar, Abbottabad, Gujrat, and Islamabad. The prevention of damage to bridge infrastructure in future earthquakes would require a safety evaluation of the existing bridges in high seismic risk areas. Work on the development of bridge design specifications specific to Pakistan needs to be initiated; the revision of the Code of Practice for Highway Bridges in light of the latest AASHTO specifications would be a step in the right direction. CONCLUSIONS AND RECOMMENDATIONS

Based on a survey of bridges carried out after the Mw 7.6 earthquake of 8 October 2005, the following recommendations are made: 1. A comprehensive set of guidelines and specifications for the design and construction of bridges should be developed and put in place for use by the engineering community, as well as a regulatory framework. Revision, and perhaps replacement, of the Code of Practice for Highway Bridges with a bridge design code more in line with modern bridge design codes is in order. 2. A thorough a safety assessment of essential and critical bridges located in high seismic risk areas should be undertaken. The design of these bridges should be checked for compliance with appropriate modern bridge design codes and the bridges found to be noncompliant should be strengthened to meet compliance requirements. 3. A significant number of failures of stone masonry bridges and bridge components were observed in the survey area, which can be attributed to poor quality of

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4.

5.

6.

7.

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construction, the size of bridge components, and the level of ground shaking. It is therefore recommended that stone masonry construction in bridges located in Seismic Zones 2A (PGA 0.08 g–0.16 g) and above be avoided, at least until appropriate guidelines and specifications for stone masonry construction in high seismic risk areas become available. In the case of bridges that are less important (where alternate routes exist) or that are constructed in low seismic risk areas, the quality of the stone masonry construction should be ensured by employing dressed stones, good quality mortar, and reasonable member size to minimize damage. Existing bridges having stone masonry in their components should be considered for strengthening. More damage to bridges was observed on the hanging-wall side than on the footwall side of the causative fault. While all the critical and essential bridges in the vicinity of the fault in the survey area should be considered for strengthening, the bridges on the hanging-wall side deserve special consideration. The design of new critical bridges should account for the location of the bridge in relation to the fault. Unseating was a common damage pattern observed in the surveyed bridges. It can be avoided by increasing the redundancy of the bridges, providing longer seats and longitudinal retainers. A small number of bridges in the survey area experienced a complete loss of serviceability due to damage to their approach roads. Therefore, special attention should be given to preclude such damage by carefully planning and designing bridge 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. For existing bridges that are in the vicinity of potential landslide areas, slope stabilization measures should be considered. Damage caused by pounding was observed in a few bridges. In high seismic risk areas, the design should account for pounding-induced forces at expansion joints. Construction joints should be provided at noncritical locations. Suspension bridges usually require tall pylons for their suspension cables and are therefore susceptible to damage from seismic forces. Accordingly, in the case of suspension bridges located in high seismic risk areas, a thorough seismic analysis should be undertaken before the design stage, and suitable construction materials should be selected. A common damage pattern in the suspension bridges surveyed was the sliding of anchor blocks for side-sway stabilizing cables, which caused damaged to the bridge superstructure. It is recommended that the anchor blocks should also be anchored to avoid sliding. Commercial activities that may potentially damage bridge structures, such as activities requiring the use of flammable products, should be prohibited in the vicinity of 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 Earthquake Engineering Research Institute (EERI), in Oakland, California, and the authors would like to 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 would also like to gratefully acknowledge the National Academies of Sciences for sponsoring a visit to California in December 2006 for meetings with researchers, and to the State University of New York (SUNY) in Buffalo for seven months of specialized training in earthquake engineering 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 in the preparation of the draft of this paper. We also gratefully acknowledge the support of Mr. Faisal-ur-Rehman in the preparation of figures for this paper.

REFERENCES Ali, S. M., 2008. Bridges & Kashmir Earthquake of Oct. 2005, Google map, http://maps.google. com/maps?q=http://bbs.keyhole.com/ubb/download.php?Number=1258144&t=k&om=1, accessed 22 January 2009. Ali, S. M., and Shakal, A., 2007. Response to the Pakistan Earthquake of October 8, 2005, The National Acadmies, http//www.nationalacademies.org/dsc/Quake_Report_2007.pdf , accessed 19 December 2008. American Association of State Highway Officials (AASHO), 1961. AASHO Standard Specifications for Highway Bridges, 8th edition, Washington, D.C. American Association of State Highway and Transportation Offcials (AASHTO), 2007. AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, 4th edition, Washington, D.C. Bilham, R., and Wallace, K., 2006. Future M >8 Earthquakes in the Himalaya: Implications from the 26 Dec 2004 M ¼ 9.0 earthquake on India’s eastern plate margin. University of Colorado Boulder, Cooperative Institute for Research in Environmental Sciences (CIRES) and Geological Sciences, http://cires.colorado.edu/~bilham/HimalayanEarthquakes/Kangra CentenaryFinal.htm, accessed 22 January 2009. Dellow, G. D., Ali, Q., Ali, S. M., Hussain, S., Khazai, B., and Nisar, A., 2006. Preliminary reconnaissance report for the Kashmir earthquake of 8 October 2005, in Proceedings, New Zealand Society for Earthquake Engineering (NZSEE) 2006 Conference, paper 31, Napier, New Zealand. Durrani, A. J., Elnashai, A. S., Hashash, Y. M., Kim, S. J., and Masud, A., 2005. The Kashmir Earthquake of October 8, 2005 — A Quick Look Report , Mid-America Earthquake Center, Urbana-Champaign, IL. Earthquake Engineering Research Institute (EERI), 2006. The Kashmir Earthquake of October 8, 2005: Impacts in Pakistan, Oakland, CA, http://www.eeri.org/lfe/pdf/kashmir_eeri_ 2nd_report.pdf , accessed 15 December 2008. Government of Pakistan (GoP), 2007. Building Code of Pakistan: Seismic Provisions, Ministry of Housing, Islamabad. Government of Pakistan (GoP), 1986. Building Code of Pakistan: Seismic Provisions, Ministry of Housing, Islamabad. w

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Government of West Pakistan (GoWP), 1967. Code of Practice Highway Bridges. Highway Department, Lahore. Kaneda, H., Nakata, T., Tsutsumi, H., Kondo, H., Sugito, N., Awata, Y., Sardar, A. S., Majid, A., Khattak, W., Adnan, A. A., Robert, Y. S., Hussain, A., Ashraf, M., Steven, W. G., Allah, K. B., 2008. Surface rupture of the 2005 Kashmir, Pakistan, earthquake and its active tectonic implications, Bulletin of the Seismological Society of America 98, 521–557. Malhotra, V. M., Carino, N. J., 2003. Handbook on Nondestructive Testing of Concrete, 2 nd edition, CRC Publishers, West Conshohocken, PA, USA. Naeem, A., Scawthorn, C., Ali, S. M., Ali, Q., Javed, M., Ahmed, I., Hussain, Z., Ashraf, M., 2005. First Report on the Kashmir Earthquake of October 8, 2005, Earthquake Engineering Research Institute (EERI), Oakland, CA, http://www.eeri.org/lfe/pdf/kashmir_eeri_1st_re port.pdf , accessed 19 December 2008. Pakistan Metrological Department (PMD), and Norwagian Seismic Array (NORSAR) Norway, 2006. Seismic Hazard Analysis and Zonation of Azad Kashmir and Northern Areas of Paki stan, Islamabad, Pakistan. Rao, N. P., Kumar, P., Kalpna, Tsukuda, T., and Ramesh, D. S., 2006. The devastating Muzaffarabad earthquake of 8 October 2005: New insights into Himalayan seismicity and tectonics, Gondwana Research 9, 365–378. U.S. Geological Survey (USGS), 2005. 8 October 2005 earthquake details Web page, Earthquake Hazards Program, http://earthquake.usgs.gov/eqcenter/eqinthenews/2005/usdyae, accessed 22 January 2009. (Received 30 January 2009; accepted 5 January 2011)

A Survey of Damages to Bridges in Pakistan - Kashmir EQ.pdf

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