Field investigation on the performance of building structures during the 12 May 2008 Wenchuan...

Engineering Structures 31 (2009) 1707–1723

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Engineering Structures

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Field investigation on the performance of building structures during the 12 May2008 Wenchuan earthquake in ChinaBin Zhao a,∗, Fabio Taucer a, Tiziana Rossetto ba ELSA Unit, Joint Research Centre, European Commission, 21027 Ispra (VA), Italyb Department of Civil and Environmental Engineering, University College London, WC1E6BT London, UK

a r t i c l e i n f o

Article history:Received 6 November 2008Received in revised form29 January 2009Accepted 11 February 2009Available online 27 March 2009

Keywords:Wenchuan earthquakeField investigationSeismic performanceBuilding structure

a b s t r a c t

A devastating earthquake struck the southwestern Chinese province of Sichuan on 12 May 2008, leaving69,227 dead and 374,643 injured, with 17,923 people still missing five months after the main event. Theepicentre of the earthquake was located in Wenchuan County, which triggered a fault rupture lengthof about 300 km, stretching northeast through Beichuan County and reaching Qingchuan County; manytowns on both sides of the fault were severely damaged/destroyed, reaching an earthquake damageintensity of XI. This paper presents the findings of a post-earthquake reconnaissance field mission carriedout by the Earthquake Engineering Field Investigation Team (The Institution of Structural Engineers,UK) and by the European Laboratory for Structural Assessment of the Joint Research Centre of theEuropean Commission, through the description of the damage sustained by three of the towns thatsuffered the largest levels of devastation: Yingxiu Town ofWenchuan County, Beichuan Town of BeichuanCounty, and Hanwang Town of Shifang City. The work focuses on the description of building performanceduring and after the disaster, in particular of reinforced concrete frame, reinforced concrete confinedmasonry, unreinforced and unconfined masonry, industrial, local vernacular and historical buildings.The information and recommendations provided in this paper will be useful for future engineeringapplications in similar earthquake risk regions.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

On 12 May 2008, a devastating earthquake struck at 02:28 pmlocal time (06:28 UTC) the Sichuan Province of the People’sRepublic of China. The China Earthquake Administration [1]estimated the magnitude of the event as Ms 8.0, with a focal depthof 14 km; USGS [2] estimations gave a magnitude of Mw 7.9 and afocal depth of 19 km. The earthquake occurred in the north–southseismic belt of mainland China, along the LongmenMountain faultzone whichmarks the boundary between the LongmenMountainseast of the Qinghai-Tibetan Plateau and the Sichuan Basin. Theepicentre was located at 31.0 N and 103.4 E, approximately 3 kmsouthwest of Yingxiu Town in Wenchuan County. The earthquakefault rupture started from the epicentre, stretching northeast forabout 300 km [3], passing through Beichuan County and reachingQingchuan County; most of the buildings and infrastructureslocated on both sides of the fault rupture line were seriouslydamaged/destroyed.Fig. 1 shows the earthquake intensity distribution, based on

the intensity map officially released by the China Earthquake

∗ Corresponding author. Tel.: +39 0332789247; fax: +39 0332789049.E-mail address: [email protected] (B. Zhao).

0141-0296/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2009.02.039

Administration on 28 August 2008, following a long and detailedfield survey [4]. The damage intensity reported in many of themost severely affected towns reached levels of X and XI, includingWenchuan County, Beichuan County, Mianzhu City, ShifangCity, Qingchuan County, Mao County, An County, DujiangyanCity, Pingwu County and Pengzhou City, for a total area ofapproximately 26,000 km2. The main event and its aftershocksresulted in 69,227 deaths and 374,643 injured, with 17,923 peoplestill missing five months after the main event. At least 15 millionpeoplewere evacuated from their homes,more than5millionwerereported to be homeless and the direct economic loss is estimatedat RMB 845.1 billion (US$125.6 billion) [2,5].China is a country that has been historically exposed to de-

structive earthquakes,with the last event of comparable disastrousproportions being the magnitude 7.9 Tangshan earthquake of 28July 1976, where 255,000 people lost their lives [6]. According tohistorical records, the area affected by the Wenchuan earthquakehad previously experienced a total of eight destructive earth-quakes, all with magnitudes larger than seven, with the largestevent being themagnitude 7.5 Diexi earthquake of 25 August 1933in Mao County, killing more than 9300 people.The scientific work on seismic design of structures started in

China in the 1950s, which led in 1974 to the introduction by theChinese Construction Committee of the first seismic design code

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1708 B. Zhao et al. / Engineering Structures 31 (2009) 1707–1723

Table 1Locations and maximum PGAs of the recorded strong motions of the Wenchuan earthquake.

Observation station Location Direct distances (km) Maximum PGAs (g)Epicentre Fault line Surface rupture E–W N–S Vertical

Wolong Wenchuan County 19 19 23 0.98 0.67 0.97Qingping Mianzhu City 88 10 3 0.84 0.82 0.64Bajiao Shifang City 67 20 10 0.57 0.59 0.65Zengjia Guangyuan City 314 87 86 0.43 0.42 0.19

Fig. 1. Wenchuan earthquake intensity map and visited area (based on theintensity map from the China Earthquake Administration [4]).

for buildings, known as TJ11-74. Four years later, in 1978, a revisedversion was issued as TJ11-78, to include the lessons learnedfrom the Tangshan earthquake. In 1989 a new seismic code wasintroduced, known as the Chinese national standard GBJ11-89. Thecurrent Chinese seismic design code for buildings GB 50011-2001was introduced in 2001. According to the seismic zone map of thecurrent seismic design code GB 50011-2001 [7], the areas seriouslyaffected by the Wenchuan earthquake were classified as zonesof Intensity 7, with the towns of Mao County and Pinwu Countyhaving a basic design peak ground acceleration (PGA) of 0.15g anda plastic design PGA of 0.31g (corresponding to a rare event), andthe towns ofWenchuan County, Dujiangyan City, Beichuan County,An County, Li County, Mianzhu City, Shifang City, Pengzhou Cityand Qingchuan County having a basic design PGA of 0.1g and aplastic design PGA of 0.22g .The Wenchan earthquake is estimated to have induced PGAs

ranging from 0.34g to 1.24g in the epicentral region [2], while theactual recorded Maximum PGA is on the order of 0.98g [8,9]. Thelocations, as well as the distances from the epicentre, fault line andsurface rapture, of the four main earthquake observation stations,Wolong, Qingping, Bajiao and Zengjia station, in the earthquakearea and their recorded maximum PGA values in E–W, N–S andVertical directions are listed in Table 1. The Wolong station is theclosest to the epicentre, at a distance of 19 km, Qingping, andBajiao stations are close to the mid-point of the fault line, whileZengjia station is near the northeast end of the fault line. ThePGAs of the Wenchuan Earthquake are several times larger thanthose predicted by the Chinese code, in addition, the predominantperiods of the recorded ground motions are distributed between0.1 s and 0.5 s [9], which are consistent with the main periods oflow masonry and reinforced concrete buildings in the area. Bothaspects are cited among the causes of the heavy damage, largenumber of casualties and high economic loss associated with thisearthquake.Through a joint activity of the Earthquake Engineering

Field Investigation Team (EEFIT) of the Institution of StructuralEngineers (UK) and of the European Laboratory for Structural

Fig. 2. Cities and Towns visited in the affected area (satellite image fromGoogleEarth).

Assessment of the Joint Research Centre of the European Commis-sion, a post-earthquake field reconnaissance mission [10,11] wascarried out in the affected area; the itinerary of the team includedthe visit to 22 towns and cities, starting at Chengdu City and endingat Jiangyou City (Fig. 2). Based on the findings of the field investi-gation, the seismic performance of buildings in the area affectedby the main earthquake event and its aftershocks is summarisedby selecting a set of three of the towns that suffered the highestlevels of damage, all located in mountain areas close to the earth-quake rupture fault line. The damage to cities located farther awayfrom the fault rupture line, such as Chengdu, Deyang, Mianyangand Guangyuan, is not described in the present work, as build-ing structures there suffered relatively slight or no damage, dueto their better design and construction quality, as well as to theattenuation of the seismic ground motions.Following the description of the damage sustained at the

selected three towns, the seismic performance of the differentbuilding types found in the affected area is described: reinforcedconcrete frame, reinforced concrete confined masonry, unrein-forced and unconfined masonry, industrial, local vernacular andhistorical buildings. The reinforced concrete shear wall construc-tions are not presented, as they are rare in the affected area and noassociated damage was observed during the field investigation.

2. Damage overview of three selected towns near the faultrupture line

In the following, an overview of the damage sustained byYingxiu Town of Wenchuan County, Beichuan Town of BeichuanCounty, and Hanwang Town of Shifang City is described. Thesetowns were selected as they were almost fully destroyed duringthe main earthquake event, with buildings and infrastructuressuffering full or partial collapse with structural damage beyondrepair. Moreover, all these towns received extensive coverageby the news media after the earthquake and were chosen bythe Chinese Government as exhibition sites for establishing

B. Zhao et al. / Engineering Structures 31 (2009) 1707–1723 1709

Fig. 3. Overview of building damage at Yingxiu Town of Wenchuan County.

the Wenchuan Earthquake Museum; many collapsed or heavilydamaged structures at these three townswill be preserved for fieldeducational purposes.

2.1. Yingxiu Town

Yingxiu Town is the closest town to the epicentre, numberingabout 7000 residents before the earthquake. Fig. 3(a) shows thelocation of the old town area, including the Yingxiu Primary Schooland the local hospital, while Fig. 3(b) and (c) show the town areaon the other side of the Min river. These three pictures show thatthe Yingxiu Town had been turned to ruin, with brick and rubblemounds of collapsed buildings next to standing partially collapsedones with severe structural damage; most of these structures hadpractically no efficient seismic design, andmanyof themwere builtbefore or during the 1980s.

In order to collect more useful information for future engineer-ing applications, the discussion below will focus on the XuankouMiddle School. The school was located in the southwestern partof Yingxiu Town and it had been moved there from the neigh-boring Xuankou Town after the construction of the Zipingpu dam.The school, which was built after 2005, was composed of fullynew buildings of either clay brick infill wall reinforced concreteframe construction or of reinforced concrete confined masonryconstruction. Fig. 3(d)–(j) show the damage sustained by theXuankou Middle School at the time of the EEFIT team field visit on14 July 2008. These figures show that all the school buildings col-lapsed or were severely damaged. It is important to keep in mindthat since these pictures were taken several weeks after the mainearthquake event, they show further damage resulting from thepost-earthquake rescue operations. In particular, the right part ofthe building shown in Fig. 3(h) and the whole building shown in


Fig. 4. Overview of building damage at Beichuan County Town.

Fig. 3(j) suffered only partial or ground floor collapse during theearthquake; however, these structures were demolished after therescue operations for safety reasons.

2.2. Beichuan Town

Beichuan Town was practically destroyed due to strong groundshaking and massive landslides, with damage levels similar tothose registered in Yingxiu Town. Beichuan Town is located closeto the main rupture zone of the earthquake fault line. Fig. 4(a)–(c)show general views of the affected area of Beichuan Town at thetime of the field visit. The central part of the town is shown inFig. 4(b), with rubble and debris from collapsed buildings nextto standing structures that suffered severe structural damageassociated with partial and ground floor collapse. Fig. 4(a) and (c)show the landslides on the left and right sides of the valley and the

damages induced, which buried and/or destroyed those buildingsand streets that were closest to the mountain slope.The buildings of the Beichuan Middle School, which are located

in the south of the town, are shown in detail in Fig. 4(d)–(j). Fig. 4(d)shows the debris of two collapsed 5-storey reinforced concretemoment resisting frame classroom buildings; the first buildingwas built in the 1990s, while the second building is of recentconstruction, built in 2003. The older building fully collapsedduring the earthquake, while the newer building pancaked onits two lower floors; both buildings were demolished after theearthquake, which made it difficult tracing back their mode offailure and construction details at the time of the field visit. Thecollapse of these two structures signifies that neither achieved thecollapse prevention limit state, and therefore cannot be deemedto have been adequately designed. Fig. 4(e) shows a much older


Fig. 5. Overview of building damage in the central area of Hanwang Town, Shifang City.

3-storey classroom building built in the 1960s, constructed out ofclay brick columns (retrofitted by steel bands in the 1980s), claybrickwalls, reinforced concrete beams, precast reinforced concretefloors and traditional Chinese timber-frame-tile roofs; the buildingsuffered from damage of the roof and partial collapse of membersin the top storey, while no significant damage was observed atthe lower floors. Fig. 4(f) shows the new canteen building, whichexperienced severe damage at the reinforced concrete columns,especially at the connections. Fig. 4(g) shows one of the two newlybuilt student dormitory buildings, both of reinforced concreteconfined masonry; damage to these two buildings was limited,with cracks developing mainly on the clay brick walls underthe window frames. In Fig. 4(h)–(j) are shown three of five oldbuildings of masonry construction located at the south end of thecampus, which were used as offices and apartments for teachers.In spite of the ground deformation that was observed next to thesebuildings, none of them showed signs of significant damage.

2.3. Hanwang Town

With more than 60,000 residents, Hanwang Town is a largeindustrialized town in Shifang City. The town is located at the edgeof the Sichuan Basin next to the LongmenMountain range, and themain part of the town is relatively flat. As in many other severelyaffected towns, debris from collapsed buildingswas found inmanyareas of the town. In the following, several representative buildingstructures near the central square and along Yingbin Avenue,which is the main street of Hanwang, are presented (Fig. 5).Fig. 5(a) shows the clocks of the Hanwang bell tower located at

the central square, which have become a monumental symbol ofthe Wenchuan earthquake after having stopped at the time of themain earthquake event following an electric power failure; the belltower itself suffered only slight damage, with some small cracks ofthe corner columns and separations between the tower and thesurrounding ground.


Fig. 5(b) and (c) show the damage patterns of two 6-storeyreinforced concrete frame buildings, the local Financial Buildingand the Great Orient Hotel, which are located along the northernYingbin Avenue and very close to the central square. Both buildingssuffered similar levels of performance associated with damageof the infill walls and of the glass facade. Fig. 5(d) shows theeastern side of the central square,with debris froma fully collapsedmasonry building and other three severely damaged buildings.Fig. 5(e) shows the entrance gate of the Dongfang Steam TurbineWorks, which is one of the three largest steam turbine producersin China. The industrial facilities of this company, which wereconstructed between 1966 and 1974, suffered heavy damage [12],and will be relocated in Bajiaojin Town and Tianyuan Town inDeyang City. The moving of these facilities, which employed morethan 20,000 staff and workers, will cause a great impact in theeconomy of Hanwang Town, which had been much dependent onits activities during the past 40 years.Fig. 5(f) shows a newly built apartment building at the south

end of the Dongfang Steam Turbine Works gate, exhibiting onlylimited damage. Fig. 5(g)–(j) show the partial collapse of fourbuildings, close to the central square and along the southern part ofYingbin Avenue. These buildingswere built between the 1970s and1980s, characterized by poor design and quality of constructionand materials. Fig. 5(k) shows damage and partial falling of thebrick walls of a masonry building, due to the irregular plan andabsence of confinement from reinforced concrete members.

3. Reinforced concrete frame buildings

Reinforced concrete moment resisting frame structures are themost common type of construction found in public and commercialbuildings built during and after the 1980’s in the affected area,including schools, hospitals, governmental offices, hotels, financialand business centres.With some exceptions, such as in DujiangyanCity, reinforced concrete moment resisting frames were also usedin the construction of low rise residential buildings. According tothe current Chinese codes for the seismic design of buildings [7]and concrete structures [13], capacity and ductile design is thebasis for the earthquake design of reinforced concrete momentresisting frames, consisting of weak-beam/strong-column systemswith beam–column joints capable of ensuring the formation ofplastic hinges at the beam ends while preventing brittle failureof the columns and the connections. The Chinese code alsoensures adequate detailing of the reinforced concrete membersand connections to allow for the development of ductility andenergy dissipation. However, the data collected in the field showsthat most of the reinforced concrete building frames did notperform as intended by the current Chinese code: most failuresdeveloped at the columns, either through shear failure or throughexcessive deformation demands at the ground floor columns in thepresence of soft-storey mechanisms, leading to the partial of fullcollapse of the building structures.The two classroom buildings of the Xuankou Middle School in

Yingxiu Town shown in Fig. 3(d) and (i) overturned and collapsedduring the earthquake. Fig. 6(a) shows the rear view of theclassroom building partially shown in Fig. 3(i), suggesting that thecollapse started from the ground floor after failure at the top endof the column. Other catastrophic collapses of reinforced concretemoment resisting frame buildings are those associated with theBeichuan Middle School in Beichuan County, represented by thetwo 5-storey buildings, one that was built in the 1990’s (Fig. 4(d),labelled as d in the map) that fully collapsed, and another one builtin 2003 (labelled as d+ in the map of Fig. 4) that pancaked over itstwo lower floors. Although both structures had been demolishedat the time of the survey and no evidence could be traced on thegeometry and detailing of the structure, it is fair to state that noneof the two structures followed capacity design principles.

Severe damage of beam–column joints was also observedduring the field visit, such as that shown as Fig. 6(b) in theYuanshan Housing Complex of Dujiangyan City, composed of 27residential buildings between three and six stories in height;construction began in 2006 and most of the buildings were stillunder construction at the time of the earthquake. Failure of thebeam–column joint was the result of insufficient confinementand shear reinforcement at the joint, as well as of the irregulargeometry in plan and the absence of capacity design principles.Another requirement of adequate seismic detailing is to prevent

short column failures, which may develop due to inappropriatestructural arrangements or to continuous openings at the top ofinfill walls not foreseen in the design of columns. As the lengthof the short column is relatively small, its stiffness increases, thusattracting higher loads with a corresponding increase in the shearforce: the column fails in shear as the shear demand is higher thanthe shear capacity corresponding to the design of a more flexibleand longer column failing in flexure. Where short columns cannotbe avoided, very stringent detailing for transverse reinforcementis required in seismic zones [14]. Short column failures werewidely observed during the field assessment, such as that shown asFig. 6(c) in the housing complex in Dujiangyan City. Other failuresassociated with beam–column joints and short columns may beidentified in the school canteen (Fig. 3(e)) and in the connectionstructure between the two buildings shown in Fig. 3(f), wherethe shear force induced by the stair structure sheared off the firststorey column at mid-height. It is strongly recommend that insimilar structures the stair beam should be recognized as a mainstructural member and be fully taken into account in the seismicdesign.Fig. 7(a) shows a new 4-storey police office building in Yingbin

Avenue, Hanwang Town of Mianzhu City, which was completednot long before the earthquake disaster. A close view of a cornercolumn at the first storey of the building (Fig. 7(b)) shows thatlarge levels of damage developed at the top end of the column,withminimal damage associatedwith the corresponding beamend; thisis another example of a design not in line with capacity designprinciples, notwithstanding that the structure was constructedseven years after the publication of the current Chinese seismicdesign codes.Fig. 8(a), (b) and (c) show a 6-storey building which suffered

soft-storey failure at the ground floor resulting from not consider-ing in the design phase the change in relative stiffness providedby the infill walls between the ground and the upper storeys.Fig. 8(c) shows the large drifts and damage sustained by the groundfloor column, where, relative to the stiffer upper storeys, most ofthe deformation demands from the earthquake occurred. Similarto what has been recorded in past earthquakes in other areas ofthe world [14–16], soft-storey mechanisms, especially at the firststorey, has been the most common mode of failure of mid-risereinforced concrete moment resisting frame buildings. Most of theresidential buildings visited during the field assessment had shopsat the ground floor, each shop occupying one bay in the directionparallel with the street with their front end opened towards thestreet and separated from the other shops by partition walls inthe direction perpendicular to the axis of the street, thus makingthe buildings particularly vulnerable to soft-storey collapse in thedirection parallel with the street.As shown in the above examples, inadequate arrangement and

consideration of the infill walls during the design phase mightcause a partial weakness in the structural system, such as soft-storey, and induce serious consequences during an earthquakeevent. Therefore, where discontinuities of infill walls are present orin areaswhere high deformation demands are expected, infill wallsshould be considered as part of the lateral load resisting systemwhen analysing their interaction with the reinforced concretemoment resisting frame.


Fig. 6. Performance of reinforced concrete moment resisting frame buildings: (a) Xuankou Middle School in Yingxiu Town of Wenchuan County; (b) and (c) beam–columnjoint and short column failures observed in the Yuanshan Housing Complex of Dujiangyan City.

Fig. 7. Structural damage of a 4-storey newly constructed reinforced concrete moment resisting frame building in Hanwang Town of Mianzhu City: (a) overview of thebuilding; (b) close up view of a first storey corner column showing damage at the top end.

Fig. 8. Soft-storey mechanisms: (a) and (b) overview of a 6-storey residential building in the Yuanshan Housing Complex of Dujiangyan City; (c) close up view of a cornercolumn at the first storey.

The damage to infill walls performing as non-structuralmembers is described in the following. The materials which areof most common use for the construction of infill walls in theearthquake affected area: clay brick, hollow clay brick and lighthollow concrete block. Typical damage to infill walls is shownin Figs. 9 and 10. Fig. 9(a) shows an overview of the newShifang People’s Hospital building in the downtown area of ShifangCity, exhibiting horizontal cracks at the boundary between thereinforced concrete beams and the clay brick infill walls, evidencedin the close up view shown in Fig. 9(b); slight diagonal crackson the infill walls were also observed. Fig. 9(c) shows the older10-storey building of the same hospital which was built in the1990s. A close-up view at the first storey (Fig. 9(d)) shows

extensive damage of the hollow concrete block infill walls,resulting from the large drifts experienced at the first storey, due inpart, to the irregular geometry in plan and in height of the building.Fig. 10(a) shows a 6-storey financial building in downtown area ofJiangyou City; close-up views of infill wall damage at the secondstorey of the building are shown in Fig. 10(b) and (c), where bothclay brick and hollow clay blocks were used as infill wall material,even within the same floor of the building. The damage sufferedby the infill walls, which are offset from the column axis to createa corridor of approximately 1.6 m, is clearly non-structural, andresults from the low construction quality of the infill walls andits inability in accommodating the large drifts experienced by thebuilding at the second storey.


Fig. 9. Infill wall damage of the Shifang People’s Hospital in downtown area of Shifang City: (a) and (b) 8-story new hospital building and close-up view of the infill walls;(c) and (d) 10-story old hospital building and close-up view of the infill walls at the first storey.

Fig. 10. Infill wall damage of a 6-story office building in downtown area of Jiangyou City: (a) view of the building from the courtyard; (b) and (c) close-up views of the infillwalls at the second storey.

Fig. 11. Example of a well-designed reinforced concrete frame building observed in Jiangyou City downtown area: (a) overview of the 6-story building; (b) close-up viewof the cracks developed in the plastic hinge region of the second storey beam.

Fig. 11(a) shows a 6-storey reinforced concrete frame buildingwith clay brick infill walls in Jiangyou City downtown area. Theclose up view of Fig. 11(b) shows cracks that had developed at theends of the beam at the interior beam–column joint of the secondstorey, being this one of the few cases that were observed duringthe fieldmission of plastic hinges forming at the beam ends, whichis a good example of adequate seismic design based on capacitydesign principles.

4. Masonry buildings

The majority of the residential buildings in the affected areawere constructed out of masonry. Reinforced concrete confined

masonry was also used in the construction of schools, hospitals,governmental offices and other public buildings, with heights upto 5 storeys. Many residential and public buildings built beforethe 1980s were constructed out of unreinforced or unconfinedmasonry. Masonry buildings, in particular unreinforced andunconfined, were responsible for most of the full and partialcollapses observed in the affected area.

4.1. Unreinforced and unconfined masonry buildings

The structural system of unreinforced and unconfined masonrybuildings consists of precast concrete planks resting on reinforced


Fig. 12. Crack patterns of masonry walls with window openings: (a) cross cracks between window openings in the horizontal direction, Dujiangyan City; (b) cross cracksbelow window openings, Hanwang Town of Mianzhu City; (c) cross cracks resulting as a combination of the first two crack patterns, Yingxiu Town of Wenchuan County.

Fig. 13. Damage and collapse of unconfinedmasonry buildings: (a) cracking of the bearingwall of a 3-storey building inWeicheng Town ofMianyang City; (b) severe damageof a single storey building in Xiushui Town of An County; (c) a collapsedmasonry buildingwith hanging precast concrete planks attached to thewall of a neighbouring higherbuilding at the same town.

concrete beams supported by clay brick walls; timber roofswith light roof tiles often used in one- to two-storey buildings.Unreinforced and unconfined masonry buildings have a historyof very poor performance during strong earthquakes. Fig. 12(a),(b) and (c) show the three types of cross crack patterns found onmasonry walls with window openings. The most common type ofcross crack pattern is the one that occurs between windows in thehorizontal direction (Fig. 12(a)), in one or several stories, affectingthe lateral and vertical stability of the building and generallyresulting in partial or full collapse of the structure. The other twocross crack patterns occur at thewalls below thewindow openings(Fig. 12(b)) or as a combination of the first two crack patterns(Fig. 12(c)).Fig. 13(a) shows a fully cracked masonry wall of a 3-storey

unreinforced masonry building in Weicheng Town of MianyangCity (Fig. 13(a)): in the absence of the confinement that otherwisewould had been provided by reinforced concrete beam and columnelements, a major crack developed at the left corner of the secondstorey and progressed diagonally up to the third storey. Fig. 13 (b)shows the highly damagedmasonrywall of a single storey buildingin Xiushui Town of An County, while Fig. 13(c) shows the remainsof a collapsed unreinforcedmasonry buildingwith hanging precastconcrete planks.Fig. 14(a) shows a residential 4-storey building in Yingbin

Avenue, Hanwang Town of Mianzhu City, exhibiting most of thedamage patterns described in the previous paragraphs. Fig. 14(b)and (c) show damage associated with cracking of the masonrywalls between window openings in the horizontal direction in

the second and third storeys, Fig. 14(d) shows detachment of thefront wall after overturning of the upper part of the building, whileFig. 14(e) shows extensive cracking of the masonry wall normal tothe axis of the street, as well as damage resulting from poundingwith a neighbouring structure.Fig. 15(a) shows a 3-storey masonry school classroom building

in Weicheng Town of Mianyang City, which suffered from thecollapse of its masonry wall parapets at the external corridorsof the third storey (Fig. 15(b)); Fig. 15(c) shows the undamagedparapets at the second storey. The collapse of the parapets maybe attributed to the absence of an efficient connection of themasonry wall with the corridor slab, as well as to the strongershaking experienced at the third storey. Although the collapse ofthe parapets is non-structural and did not compromise the stabilityof the structure, it posed a great threat to the school children inthe court yard as well as to those evacuating the building fromthe third storey. The building also suffered extensive damage atthe masonry walls of the ground floor, revealing the absence ofreinforced concrete columns (Fig. 15 (d)).

4.2. Confined masonry buildings

Most of the masonry buildings built in the affected areaafter 1980 are of reinforced concrete confined brick masonry.The construction of this type of structure consists in raising theclay brick walls while leaving vertical empty slots for placingthe steel reinforcement of the concrete columns; these empty


Fig. 14. A severely damaged 4-storey unconfined masonry building in Hanwang Town of Mianzhu City: (a) overview of the building; (b), (c), (d) and (e) close-up views.

Fig. 15. Damage and local collapse of a 3-storey school building in Weicheng Town of Mianyang City: (a) overview of the building; (b), (c) and (d) close-up views.

slots are then flanked by wooden formwork fixed to the surfaceof the walls to allow pouring and casting of the concrete ofthe column elements, thus forming a solid joint with the claybrick masonry walls [17]. Fig. 16(a) shows a typical confinedmasonry building in Xiaoyudong Town of Pengzhou City, showingthe arrangement of reinforced concrete beams and columnsproviding confinement of the brick masonry infill walls at thefirst two storeys. The good performance of the building showsthat the reinforced concrete confined brick masonry structurewas efficiently designed, especially if one considers that the townsuffered large levels of damage and destruction, and that theneighbouring building had fully collapsed.Fig. 16(b) and (c) show the severe damage and partial collapse

of a 4-storey confinedmasonry building in Reigu Town of BeichuanCounty; the structure was built in the early 1980s, and exhibitedextensive cracking and out-of-plane failure of the masonry walls,due to the large number of window openings and open spacesat the ground floor, as well as to the insufficient confinementprovided by the deficient arrangement (i.e., due to the largeopenings) of the reinforced concrete members.In Fig. 3(g) and Fig. 17(a) are shown the newly built confined

masonry buildings of the Xuankou Middle School in YingxiuTown of Wenchuan County, which were severely damaged.

Fig. 3(g) shows a 5-storeymasonry student dormitory building thatcollapsed at the ground floor and in the middle portion through allfloors. Fig. 17(a) shows three 5-storey teacher apartment buildingsof the school, labelled as I , I1 and I3 in the map of Fig. 3. Allthree buildings suffered severe damage to their masonry wallsand reinforced concrete confinement members, especially at thefirst or lower stories. Fig. 17(b) shows a close-up view of a cornercolumn at the first storey, where the shear crack of the masonrywall migrated into the reinforced concrete column, inducing shearfailure of the column and breaking of the longitudinal rebars.Fig. 17(c) shows the out-of-plane collapse and overturning of themasonry walls in correspondence with the staircase of a schooloffice building, labeled as k in Fig. 3. The collapse of the walls wascaused by the heavy weight of the clay bricks and by the relativeweakness of the reinforced concrete members being unable toprovide any effective confinement.Fig. 18(a) shows two large diagonal shear cracks that deve-

loped across the first two storeys of the masonry walls of a5-storey confinedmasonry building in Dujiangyan, shearing off thereinforced concrete beam atmid span at the first storey (Fig. 18(b))and damaging the reinforced concrete column at the secondstorey (Fig. 18(c)); both reinforced concrete members were lightlyreinforced and of poor concrete construction.


Fig. 16. Damage of confined masonry buildings: (a) a typical confined masonry building in Xiaoyudong Town of Pengzhou City; (b) and (c) severe damage andpartial collapse of a 4-story confined masonry building in Reigu Town of Beichuan County, and close-up view at the rear side.

Fig. 17. Damage of the newly built confined masonry buildings of the Xuankou Middle School, Yingxiu Town of Wenchuan County: (a) and (b) damage of three 5-storyapartment buildings, and close-up view of one confined corner column; (c) partial collapse of a masonry wall in correspondence with the staircase of a school office building.

Fig. 18. Severe damage of a 5-story confined masonry building in Dujiangyan City: (a) shear cracks across the masonry wall at the first and second storeys; (b) close-upview of the reinforced concrete beam at the first storey showing failure at mid-span; (c) close-up view of the damaged reinforced concrete column at the second storey.

In Fig. 19(a) is shown a severely damaged confined masonrybuilding in Weicheng Town of Mianyang City, which sufferedfrom extensive cracking at the first storey. Fig. 19(b) shows ahighly damaged reinforced concrete column of poor constructionmaterials that experienced large drifts in the direction parallelwith the axis of the street, which resulted from the large shopopenings at the ground floor and by the fact that the columns,designed to provide confinement to themasonrywalls, and as suchdetailed with minimum reinforcement, experienced during theearthquake flexural deformation demands corresponding to a highductile reinforced concrete frame element. Fig. 19(c) shows a largediagonal crack that sheared out a concrete column of the same

building in the direction normal to the axis of the street, resultingfrom the transfer of lateral loads, after failure of the masonry wall,to the reinforced concrete columns, which designed to provideonly confinement of the masonry walls, failed due to insufficientshear capacity. The failure mode shown in Fig. 19(c) highlightsthe need of studying in more depth the deformation capacityof confined masonry structures after reaching their maximumstrength, which is especially important when the actual demandsare larger than those assumed in the design.Fig. 20(a) shows the partial collapse of a classroom building in

theWeicheng Primary School inWeicheng Town ofMianyang City.The building itself suffered limited damage during the earthquake,


Fig. 19. Severe damage of a confined masonry building with in Weicheng Town of Mianyang City: (a) overview of the building; (b) close-up view of the ground floorreinforced concrete column; (c) diagonal cracks developing from the masonry wall into the ground floor reinforced concrete column.

Fig. 20. Partial collapse of a school building of the Weicheng Primary School in Weicheng Town of Mianyang City: (a) overview of the building; (b) partial collapse of thebalconies; (c) close-up view of a short column failure mode at the base of the arch facade.

while severe damage and partial collapse occurred in the facade,balconies, external corridors and stairs, as shown as Fig. 20(b).The collapse of these elements was due to an inappropriate designof the facade, which having to support the vertical load from thebalconies, was unable to transfer the lateral loads induced by theearthquake, after a short column mode failure due to insufficientshear capacity at the joint between the arches of the facade and theparapet on the slab of the balcony (Fig. 20(c)).

5. Industrial buildings and facilities

Most of the industrial buildings and facilities in the area nearthe earthquake fault rupture were severely affected. Fig. 21 showsthe Aba Aluminum Plant in Xuankou Town of Wenchuan County,built in 1991 is one of the biggest factories in the County, with aproduction of 14,000 ton per year prior to the earthquake disaster.In Fig. 21(a) is shown the main factory building, a reinforcedconcrete structure next to the Min river, which suffered severedamage at the upper floors and collapse of the roof. Fig. 21(b)shows a reinforced concrete prefabricated shed of the samefactory, located above the main building and next to NationalRoad 213, where the facade collapsed after detaching from themain structure, following large displacements of the columns thatdeveloped plastic hinges at the base after losing their connectionwith the light steel truss structure of the roof.In Fig. 22 is shown the Xiting Package Ltd., a medium size

package factory located in Mianzhu city and composed of three

building units. Fig. 22(a) and (b) show the partial collapse ofthe facade at the top of two of the building units, resultingfrom out-of-plane overturning of the masonry bricks induced bythe large displacements of the prefabricated reinforced concreteshed structure and by the absence of restraints normal to theplane of the facade. Fig. 22(c) shows the debris from the collapseof the roof structure of the central building unit, after failurein the transverse direction of the supporting elements whichcould not accommodate the differential displacements resultingfrom the difference in stiffness between the left, more rigid,and the right, more flexible part of the shed that had largeopenings for logistic purposes. Fig. 22(d) and (e) show damageat the corbel connections between the prefabricated column andbeam elements; the extensive cracking at the supports withoutdifferential displacements shows that some type of steel pinconnection was present and that it was effective in maintainingthe integrity of the connection.The out-of-plane collapse of the clay brick gable shown in

Fig. 22(a) and (b) was a common failure mode observed in thefacade of many precast reinforced concrete industrial sheds of theaffected area; similar failures were observed in the two sheds ofthe Dongfang Steam Turbine Works shown in Fig. 5(e), as well asin the reinforced concrete factory building of Dongqi TransportLtd. (Fig. 23(a)), located south of Hanwang Town, Mianzhu City,and which was able to resume activities shortly after the disaster(Fig. 23(b)) in view of the smaller scale of damage of the gable.Fig. 24 shows a new reinforced concrete precast industrial shedsouth of Mianzhu City, which did not suffer damage to the


Fig. 21. Damage to industrial buildings of the Aba Aluminum Plant in Xuankou Town of Wenchuan County: (a) factory buildings next to the Min river; (b) partial collapseof a prefabricated shed next to National Road 213.

Fig. 22. Damage and partial collapse of the Xiting Package Ltd. factory complex in Mianzhu City: (a) overview of the left unit; (b) overview of the right unit; (c) overview ofthe collapsed mid-portion; (d) and (e) close-up views of two selected beam–column connections.

Fig. 23. Factory building of Dongqi Transport Ltd., located south of Hanwang Town, Mianzhu City: (a) outside view; (b) inside view.


Fig. 24. A new factory building south of Mianzhu City, no damage was observedfrom the outside.

structural precast members nor to its facade that was made ofthin metallic construction and properly connected to the mainstructure. The use of light material for the construction of thefacade of similar industrial buildings constitutes a good practiceof seismic design, as it reduces the inertia forces generated bythe earthquake, both on the precast reinforced concrete membersto which the facade is attached as well as on the facade itself,minimising the probability of out-of-plane collapse and the threatto users of the structure.Fig. 25(a) and (b) show the industrial facilities of a factory in

Xuankou Town ofWenchuan County and of a small manufacturingplant in An County, respectively. The steel structure and thereinforced concrete chimney of the factory in Xuankou Townsuffered no apparent structural damage, while both the masonrychimney and the light steel tower of the smallmanufacturing plantin An County collapsed during the earthquake. Similar to what hasbeen observed in similar past strong earthquake events, a largenumber of masonry chimneys partially or fully collapsed in thearea affected by the Wenchuan earthquake.During the field visit, some old industrial buildings and facilities

which had been abandoned or were out of service before theearthquake event were also visited. In Fig. 26(a) and (b) areshown the buildings of an old abandoned cement processingfacility in the Xiaoyudong Town of Pengzhou City; the buildings,made of unconfinedmasonry, collapsed orwere severely damaged.Another old abandoned industrial facility is shown in Fig. 26(c);the facility, located in Hanwang Town of Mianzhu City, wasconstructed out of reinforced concrete and suffered from roofcollapse and local collapse of its members. It is clear that withthe fast industrial growth of China, maintaining, retrofitting ordemolishing older factories will become an important issue in themanagement of these structures in what concerns seismic safety.

6. Vernacular and historical buildings

Single- and two-storey residential houses built out of woodframe constructionwith lightweight roof tiles, in combinationwithwooden, clay brick, light concrete block or mud infill walls, werefound in villages and small towns, as well as in some large townsand cities of the area affected by the disaster; most of these housesare non-engineered and self-built by locals. In general, this typeof construction showed good performance during the earthquake,owing to their light inertial mass and to energy dissipated byslipping of the unanchored tiles at the roof. In some cases, thewallsof these constructions suffered severe damage, in the case of mudand clay brickwalls, due to their lowquality and large unsupportedlengths, and in the case of wooden walls, due to the absence ofeffective connections. Slipping and falling of the roof tiles was themost common type of damage in the houses visited during the fieldinvestigation, in some cases failure of the roof was also observed,due to the absence of effective connections and bad quality orageing of the roofs. Fig. 27(a) and (b) show three types of vernacularhouses in Reigu Town of Beichuan County, which suffered onlyfrom damage to the roof tiles; these houses performed relativelywell when compared to the masonry and reinforced concretestructures that were heavily damaged in the same affected area.Fig. 27(c) and (d) show two vernacular buildings visited in HeqingTown of An County and Xuankou Town of Wenchuan County,which experienced partial collapse of the roof structure.Historical buildings in the area are of timber frame wall con-

struction, with timber frame roofs and light roof tiles. Comparedto vernacular buildings, historical buildings are generally largerwith stronger timber members, and have ornaments on thefacade and at the top of the roof. Fig. 28(a) shows the exhibitioncentre for nature and culture in Dujiangyan City, where slippingand falling of the tiles can be easily identified. Fig. 28(b) showstwo timber columns of the same building resting on stone bases:the column on the right side of the picture shows a residual dis-placement relative to the stone base, which may had reduced thelateral force demands after sliding of the column with respectto the base during the earthquake. Fig. 28(c) shows a close-upview of a timber beam–column connection, exhibiting no signs ofdamage. Fig. 28(d) shows another historical building, the ErwangTemple in Dujiangyan city; although the same timber frame struc-tural systemwas used, it suffered diffused damage, due to its taller,narrower and more irregular geometry.

7. Conclusions and recommendations

The Wenchuan earthquake caused a major disaster of vastproportions, both human and economical, which will affect theregion for many years to come. The high number of humancasualties and the large economic losses in built-up areas was theresult of the high levels of damage sustained by building structures,

Fig. 25. Industrial facilities in the affected area: (a) a factory in Xuankou Town of Wenchuan County; (b) a small manufacturing plant in An County.


Fig. 26. Severe damage and partial collapse of abandoned industrial buildings and facilities: (a) an old cement processing facility in Xiaoyudong Town of Pengzhou City;(b) an old masonry industrial building in the same town; (c) an industrial building in Hanwang Town of Mianzhu City.

Fig. 27. Damage to vernacular buildings: (a) and (b) vernacular buildings in Reigu Town of Beichuan County; (c) a vernacular building in Heqing Town of An County;(d) a vernacular building in Xuankou Town of Wenchuan County.

especially in the areas close to the fault rupture zone, where manybuildings suffered full or partial collapse with structural damagebeyond repair. From the information gathered during the fieldmission it is possible to conclude that the poor performance ofbuilding structures is the result of the combination of severalfactors:(i) The high levels of ground motion experienced in the affectedarea, which imposed demands as high as five to six timesthose stipulated by the current Chinese seismic design codeGBJ 50011-2001.

(ii) The poor construction quality of most building structures,both in terms of materials and seismic design, even instructures of recent constructionwhich did not conform to thecurrent Chinese seismic design code.

(iii) The large stock of unconfined and unreinforced masonrybuildings, as well as of reinforced concrete confined masonrybuildings, which in the presence of irregular geometries, poormaterials and inadequate detailing, offer lowmargins of safetywith brittle modes of failure.

(iv) The wide use of solid clay bricks for the construction of infillwalls and non-structural elements, which due to their largeweight, result in an increase of the forces induced by theearthquake.

(v) The frequent use of large openings at the ground floorof building structures to accommodate shops, commercialactivities, parking and circulation of people, leading to theformation of soft-storey mechanisms.

The following recommendations are proposed to reduce theseismic vulnerability and risk of the new, as well as of the existing,building constructions of the affected area:

(i) The seismic hazard of the affected area should be revisedin the current Chinese seismic design code, acknowledgingthe higher earthquake ground motions recorded during theWenchuan earthquake.

(ii) The seismic provisions of the current Chinese seismic designcode should be strictly enforced during the reconstructionphase and beyond, by means of adequate checking of


Fig. 28. Historical timber frame buildings: (a) exhibition centre for nature and culture in Dujiangyan City; (b) close-up view of the wooden columns; (c) close-up view ofthe wooden beam column connections; (d) Erwang Temple in the same city.

the design, as well as by proper field inspection duringconstruction.

(iii) Unconfined and unreinforced masonry should no longer beadopted for the construction of building structures in highseismicity areas. Likewise, the use of reinforced concreteconfined masonry structures should be confined to low-rise,regular structures, possibly limiting their use to residentialand commercial use. It is also recommended that theseismic performance of these structures should be furtherinvestigated, examining the complete load–displacementpath until failure and revising the safety factors for design,recognising the high variability in the quality of materials andcraftsmanship, and the potential brittle failure modes of thesestructures.

(iv) In the construction of new reinforced concrete momentresisting frames, capacity design principles should be strictlyenforced, together with adequate detailing, confinement andshear resistance of joints and potential plastic hinge regions;particular attention should be devoted to prevent shortcolumn failure modes and to ensure a proper transmission ofinertial forces to the foundation. An example of the latter isto guarantee that the inertial forces of stairs are adequatelytransferred to the structure, by avoiding stair beams framingat the mid-height of column elements.

(v) Proper action should be taken to avoid future soft-storeycollapses, by acknowledging the actual stiffness contributionof infill walls in reinforced concrete frames and by recognisingthe added demands induced in weak storeys by the in-height irregular distribution of infill walls. Where possible,irregularities in-plan and in-height should be avoided, and useof lightweight infill walls should be encouraged.

(vi) The use of solid brick masonry for non-structural elementsshould be limited as much as possible, and should be replacedby lightermaterials, especially for the construction of parapetsand facades, including those of prefabricated reinforcedconcrete industrial buildings.

(vii) A coordinated programme of evaluating the vulnerabilityand associated risk of the existing building stock should becarefully implemented, adopting up-to-date methodologiesand techniques for upgrading and retrofit.

Acknowledgements

The authors acknowledge the Civil Engineering College of SouthWest Jiaotong University at Chengdu, P. R. China, for hosting andproviding the logistic support for the field mission; the assistanceprovided by Prof. Qiao Li (Dean), Prof. Lingkan Yao, Prof. JianlinMa,Prof. Wei Peng and Prof. Ying Wang is gratefully appreciated. Thefield investigation was organised by the Earthquake EngineeringField Investigation Team (EEFIT, The Institution of StructuralEngineers, UK) under the leadership of Matthew Free (Arup), andby European Laboratory for Structural Assessment (ELSA Unit) ofthe Joint Research Centre of European Commission.

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