Brandonisio 2013 Engineering Failure Analysis

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Damage and performance evaluation of masonry churches in the 2009 L’Aquila earthquake Giuseppe Brandonisio , Giuseppe Lucibello, Elena Mele, Antonello De Luca Department of Structures for Engineering and Architecture, University of Naples ‘‘Federico II’’, Naples, Italy article info Article history: Available online xxxx Keywords: L’Aquila earthquake Masonry churches Seismic damage Non-linear analysis Limit analysis abstract The seismic behaviour of masonry churches damaged during the 2009 L’Aquila earthquake is studied in this paper. Four important basilicas are considered in order to derive general conclusions from the damage assessment and the performance analysis. As a general result of the comparison between the post-earthquake survey activity and the structural analyses the possibility of evaluating the seismic safety of churches, and therefore of avoiding destructive damage by means of the design and application of appropriate retrofit inter- ventions, is confirmed. Comparative numerical analyses on a sample of four churches have highlighted another important aspect: the dynamic excitation due to the seismic ground motion activates many vibration modes of the building structure, though all of them are characterised by small participation factors. This fact leads to the following important consequences: the high spectral values of the registered record of the L’Aquila earthquake do not correspond to equivalent high values of base shear; in particular the results showed that in all the exam- ined case studies, the base shear V ratio ranged between 20% and 30% of the church weight. Therefore the appropriate choice of the force reduction factor to be adopted for these mon- umental buildings is not so large since the real shear force value was significantly smaller than the plateau value of the spectral acceleration provided by Italian Code. Furthermore, the awareness of the activation of many local modes under seismic excitation calls for ret- rofit interventions which have to ‘‘tie up’’ the building, thus avoiding local failures that are often observed. The final conclusion is that the observation of damage and failures under real experimen- tal actions, like real earthquakes, are a precious means for the advancement of knowledge in the field of seismic engineering. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Basilica church buildings, which represent a large portion of the Italian cultural heritage, have demonstrated during the past earthquakes to be particularly prone to experience damage and partial or total collapse. The high seismic vulnerability of church heritage is mainly related to the specific building configuration (open plan, pres- ence of slender walls, lack of effective connections among the structural elements), as well as to the mechanical properties of the masonry material, characterised by highly non-linear behaviour and very low tensile strength. These monumental build- ing were often designed by very skilled and courageous architects, who attempted challenging structural schemes, which, 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.01.021 Corresponding author. Address: Department of Structures for Engineering and Architecture, University of Naples ‘‘Federico II’’, P.le Tecchio 80, 80125 Naples, Italy. Tel.: +39 081 768 2439; fax: +39 081 593 4792. E-mail address: [email protected] (G. Brandonisio). Engineering Failure Analysis xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquila earthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Transcript of Brandonisio 2013 Engineering Failure Analysis

Page 1: Brandonisio 2013 Engineering Failure Analysis

Engineering Failure Analysis xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanal

Damage and performance evaluation of masonry churchesin the 2009 L’Aquila earthquake

1350-6307/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

⇑ Corresponding author. Address: Department of Structures for Engineering and Architecture, University of Naples ‘‘Federico II’’, P.le Tecchio 8Naples, Italy. Tel.: +39 081 768 2439; fax: +39 081 593 4792.

E-mail address: [email protected] (G. Brandonisio).

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 Learthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Giuseppe Brandonisio ⇑, Giuseppe Lucibello, Elena Mele, Antonello De LucaDepartment of Structures for Engineering and Architecture, University of Naples ‘‘Federico II’’, Naples, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxx

Keywords:L’Aquila earthquakeMasonry churchesSeismic damageNon-linear analysisLimit analysis

The seismic behaviour of masonry churches damaged during the 2009 L’Aquila earthquakeis studied in this paper. Four important basilicas are considered in order to derive generalconclusions from the damage assessment and the performance analysis. As a general resultof the comparison between the post-earthquake survey activity and the structural analysesthe possibility of evaluating the seismic safety of churches, and therefore of avoidingdestructive damage by means of the design and application of appropriate retrofit inter-ventions, is confirmed.

Comparative numerical analyses on a sample of four churches have highlighted anotherimportant aspect: the dynamic excitation due to the seismic ground motion activates manyvibration modes of the building structure, though all of them are characterised by smallparticipation factors. This fact leads to the following important consequences: the highspectral values of the registered record of the L’Aquila earthquake do not correspond toequivalent high values of base shear; in particular the results showed that in all the exam-ined case studies, the base shear V ratio ranged between 20% and 30% of the church weight.Therefore the appropriate choice of the force reduction factor to be adopted for these mon-umental buildings is not so large since the real shear force value was significantly smallerthan the plateau value of the spectral acceleration provided by Italian Code. Furthermore,the awareness of the activation of many local modes under seismic excitation calls for ret-rofit interventions which have to ‘‘tie up’’ the building, thus avoiding local failures that areoften observed.

The final conclusion is that the observation of damage and failures under real experimen-tal actions, like real earthquakes, are a precious means for the advancement of knowledgein the field of seismic engineering.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Basilica church buildings, which represent a large portion of the Italian cultural heritage, have demonstrated during thepast earthquakes to be particularly prone to experience damage and partial or total collapse.

The high seismic vulnerability of church heritage is mainly related to the specific building configuration (open plan, pres-ence of slender walls, lack of effective connections among the structural elements), as well as to the mechanical properties ofthe masonry material, characterised by highly non-linear behaviour and very low tensile strength. These monumental build-ing were often designed by very skilled and courageous architects, who attempted challenging structural schemes, which,

0, 80125

’Aquila

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though perfectly adequate to bear gravity loads, were not always adequate to resist horizontal forces deriving from seismicevents.

As stated by Huerta [19], the rigorous assessment of the seismic behaviour of complex masonry buildings, and in par-ticular of churches, shows objective difficulties due to several reasons: firstly, the analysis of the masonry materials,characterised by non-linear behaviour and low tensile strength, requires complex theoretical modelling, usually notstraightforward to be implemented in a finite element model. Secondly, the arrangement of blocks and mortar jointsin the structural elements is frequently uncertain and variable; as a result, the mechanical properties of the masonrymaterial may show significant scatters throughout the building, and the experimental characterisation very often im-plies simplifications, which can mislead the real behaviour. Finally, additional difficulties are related to the highly com-posite geometry and morphology, which drive to three-dimensional (3D) models characterised by a large number ofdegrees of freedom.

The above considerations justify the need for specific modelling and analysis strategies to be developed and establishedfor historic masonry churches. In [30] the main available methods which are used for the analysis of masonry historicalstructures are widely discussed, where the significant difficulties linked to computational effort, possibility of input dataacquisition and limited realism of methods are also underlined.

General aspects related to the modelling and analysis of masonry historical structures were studied by the authors in pre-vious works. In [13] a simplified procedure based on both finite element analysis and limit analysis was proposed for assess-ing the seismic capacity of masonry arches, very common in the masonry churches. In [18] a simple formula for predictingthe horizontal capacity of masonry portal frames, that can be recognised as the basic structural element in historical build-ings, was developed by using the limit analysis approach.

Only more recently studies and researches have been devoted to this specific building types, with the aim of analysing theseismic behaviour, of defining suitable methodologies for the assessment of seismic safety, and of suggesting appropriatestrategies of intervention for structural retrofitting.

Lourenço and Roque [24] proposed a simple, fast, and low cost procedure based on a simplified geometric approach forimmediate screening of the large number of churches at risk. The objective is to evaluate the possibility of adopting simpleindexes related to geometrical data as a first (and very fast) screening technique to define priority for further studies. An-other approach for a fast vulnerability assessment of church buildings was proposed by Lagomarsino and Podestà [23], that,on the basis of statistical analysis of damage observed after the earthquakes of Umbria and Marche (1997) and Molise (2002),established vulnerability models that consist of assigning a vulnerability index to the church, taking into consideration bothits weakest elements and the preventive constructive details, as well as to estimate damage according to expected earth-quake intensity. This model was adopted in the Italian Guide Lines for the assessment and mitigation of seismic risk of cul-tural heritage [25] as a tool for the evaluation and mitigation of seismic risk to cultural heritage as well as to give directionand control to seismic strengthening interventions.

In order to better understanding the seismic behaviour of masonry churches, a systematic collection and analysis ofthe damage experienced by churches in important Italian earthquakes have begun starting from the late 1970s: recon-naissance reports specifically focused to masonry church buildings have been prepared after earthquakes of Friuli(1976), Irpinia (1980), Lunigiana and Garfagnana (1995), Umbria and Marche (1997), Molise (2002), etc. These studieshave highlighted the seismic behaviour of churches can be explained through a substructuring analogy, that is dissectingthe whole structure into its constituting parts, the so-called macro-elements, which are characterised by autonomousstructural behaviour under seismic loads. The first suggestion toward this approach is provided by Doglioni et al.[16]: starting from the structural assessment of masonry churches during the 1976 Friuli earthquake, recurrent damageand collapse modes of typical macro-elements are shown and discussed; the typical macro-elements identified by Dogli-oni et al. [16] and widely recalled through the inherent scientific literature [21,13,18,28,22,1] are: façade, aisles, apse,bell tower, dome, triumphal arches, etc.

Within this framework, a ‘‘two-steps’’ procedure for the seismic analysis of basilica churches was proposed by theauthors in 1999 [26] and already applied to different basilica churches [27,6]. In the ‘‘first step’’ of the procedure, thechurch building is analysed in the linear range with 3D finite element models, in order to determine the static and dy-namic properties, and the seismic demand, i.e.: the distribution of horizontal force acting on each macro-element. In the‘‘second step’’ the complex 3D structure is dissected in the constituting macro-elements, which are separately analysedin the non-linear range up to collapse, in order to evaluate the seismic capacity, i.e. the horizontal strength of each macro-element. The results from the two steps, expressed in terms of seismic demand and seismic capacity, are then comparedwith the purpose of assessing, though in an approximate way, the safety level of each macro-element of the global struc-ture. The procedure is complemented by an additional ‘‘sub-step’’, concerning the evaluation of out-of-plane potentialfailures.

In this paper, some general considerations on the severe damage and partial collapses experienced by churches during theearthquake of 6th April 2009 are firstly presented. Four important basilica churches located in the historical centre of L’Aqui-la are then selected as case studies, and are analysed in order to compare the actual seismic damage and the results comingfrom the proposed assessment procedure. Based on this comparison, the reliability of the ‘‘two-steps’’ procedure as a tool forpredicting the real seismic response of churches is confirmed. In addition, some typical damage and failure modes of basilicachurches are recognised, and the rationale for designing effective retrofit interventions is suggested.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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2. The 2009 L’AQUILA earthquake and damage of churches

The L’Aquila earthquake that occurred on 6th April 2009 confirmed the high seismic vulnerability of historical buildings(Fig. 1). The epicentre was near the city of L’Aquila, capital of the Abruzzo region, in central Italy. The main shock occurred at1:32 UTC (3:32 a.m. local time), at the relative shallow depth of approximately 9.5 km, with magnitude MW = 6.3; the dura-tion was about 15 s.

The maximum value of the horizontal peak ground acceleration (PGA) was measured at a near-field station (AQV) locatedapproximately 5 km NW from L’Aquila, on soil class B, and was equal to 0.66g. Two further stations, called AQU and AQK,both located in the L’Aquila city, on soil class B, recorded PGA values of 0.31g and 0.35g, respectively [15]. Corrected accel-eration traces of the main shock show the largest peak value of 0.63g in the EW component at station AQV, with very shortduration and high frequency content of the recordings. In some cases, the recorded values of vertical acceleration were com-parable to the horizontal acceleration ones [2].

The earthquake damaged about 10,000 buildings located both in the city of L’Aquila and in neighbouring small towns,with several buildings that collapsed, causing 308 deads, over 1500 injured and approximately 65,000 homeless.

L’Aquila city suffered a Mercalli–Cancani–Sieberg (MCS) scale intensity of VIII–IX, while in the neighbouring villages ofOnna and Castelnuovo the maximum intensity was of IX–X MCS. This event is aligned with the historical seismicity ofL’Aquila city, that in the past had been struck by several earthquakes of comparable magnitude, particularly in the years:1315, 1349, 1461, 1703, 1915 (the Avezzano earthquake). These seismic events were all characterised by MW > 6.5 [10,20].

It is worth noticing that the 2009 earthquake was characterised by an intensity comparable to the one prescribed by theItalian Building Code [29], and several modern buildings in the L’Aquila centre have showed a damage level compatible withthe one expected according to the life safety limit state, as prescribed by the same Code.

However, the close proximity of the causative fault to the city of L’Aquila caused total or partial collapses of several an-cient buildings located in the historical centre [2,12]: 80% of the monumental heritage was destroyed or severely damaged[1,22]; about 240 historical buildings were struck by the earthquake sequence, among which 170 churches were severelydamaged or partially collapsed.

From the above discussion, it can be concluded that the 2009 L’Aquila earthquake was fully aligned with the past seismichistory and the present codified hazard of the zone, therefore the high seismic vulnerability of church buildings was the ma-jor cause of the widespread damage.

The activity of damage assessment on historical and monumental buildings (churches, palaces, mansions, castles, mills,etc.) started a week after the main shock, thanks to the contribution of several engineers and researchers cooperating withthe interuniversity consortium ReLUIS (Laboratories University Network of Seismic Engineering), which supports the ItalianDepartment of Civil Protection (DPC) in the field of earthquake engineering. The post-earthquake damage assessment wascarried out on more than 1000 churches with a methodology aimed at recognising the collapse mechanisms in the different

Friuli, 1976Mw = 6.4

L’Aquila, 2009Mw= 6.3

Avezzano, 1915Mw = 6.99

Molise, 2002Mw = 5.9

Irpinia, 1980Mw = 6.89

Messina, 1908Mw = 7.24

Belice, 1968Mw = 6.1

Casamicciola, 1883Mw = 5.8

Umbria-Marche, 1997Mw = 6.1

Emilia Romagna, 2012Mw = 6.3

Fig. 1. Evidence for damage to church buildings in past Italian earthquakes (Mw = moment magnitude).

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Fig. 2. Mechanism of façade: (a) overturning of façade of San Paolo a Peltuinum church in Prata d’Ansidonia (L’Aquila), (b) collapse of the gable of San Biagiochurch in L’Aquila, and (c) shear failure mechanisms in the transept façade of San Domenico Maggiore church in L’Aquila.

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architectonic elements of the church, at singling out the need for provisional interventions, and at estimating the restorationcosts [1]. Approximately 25% of the examined churches was declared suitable for immediate occupancy; 65% of churchesneeded provisional interventions in order to prevent further damage due to replica shocks; the remaining 10% of thechurches was classified in different way, namely: (i) partially suitable for immediate occupancy; (ii) temporarily unsuitablefor occupancy; (iii) unsuitable for occupancy; and (iv) without any provisional interventions [28].

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Fig. 3. Damage mechanism of transversal vibration of the nave of Santa Maria dei Raccomandati church in San Demetrio né Vestini (L’Aquila).

Fig. 4. Collapse of dome of Santa Maria del Suffraggio church in L’Aquila.

Fig. 5. Damage mechanism of triumphal arch of San Pietro a Coppito church in L’Aquila.

G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 5

The analysis of seismic damage suffered by the churches has shown that the façade, the domes (of nave, aisle, transept orapse), the triumphal arches and the projections (domed vaults pinnacles, statues, etc.) were the most vulnerable macro-ele-ments. In detail, the statistical analysis made after the activity of damage assessment [28] has pointed out the following sta-tistical distribution of damage location and mechanism type:

� 50% of churches had experienced the activation of façade mechanisms, which consist of (i) global or partial overturning ofthe façade (Fig. 2a and b); (ii) damage at the top of the façade (i.e. at the gable); and (iii) or in-plane shear mechanisms(Fig. 2c);� in 50% of the churches the damage mechanisms due to transversal vibration of the nave (Fig. 3) and to shear failure of side

walls were also observed;

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Fig. 6. Damage mechanism in the apse of San Flaviano church in L’Aquila.

Fig. 7. Damage mechanism in the plane belfry of Sant’Agostino church in L’Aquila.

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� other damage mechanisms were sparsely observed and their occurrence was related to specific architectural configura-tions of the churches. In particular, about 15% of the examined churches showed collapse mechanisms in vaults (Fig. 3),domes (Fig. 4), triumphal arches (Fig. 5) and apses (Fig. 6);� in 20% of the examined churches, the damage mechanism in the ‘‘projections’’ elements (such as domed vaults and pin-

nacles) was observed (Fig. 7);� in 3% of the L’Aquila churches, the collapse of bell tower and belfry occurred (Fig. 8).

3. The case studies: description of churches and observed damage

In the previous section, an overview of damage on churches subjected to the L’Aquila earthquake was presented. L’Aquilacity is appointed as the ‘‘city of 99 churches’’ due to the richness of its religious building heritage, therefore a comprehensivereconnaissance report of all the churches is not feasible in this paper.

The reconnaissance reports made after earthquakes are usually directed to answer some recurrent questions. The ques-tions arisen with specific reference to the subject of ‘monumental churches that were subjected to the L’Aquila earthquake,are the following: Were these monuments particularly vulnerable to seismic action? Was it possible to predict their behav-

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Fig. 8. Collapse of: (a) bell tower of San Pietro a Coppito church in L’Aquila and (b) belfry of a church located in the historical centre of L’Aquila.

Fig. 9. Linearised plans of churches with indication of global dimensions (B, L, H), weight (Ww = weight of walls, Wr = weight of roof) and of transversal (Ti)and longitudinal (Li) macro-elements.

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iour, and subsequent damage, such as partial or total collapses? Are technical codes adequate to provide analytical tools formaking correct predictions? Was the excitation of the specific earthquake particularly severe? Was it within the provisionsof the seismic codes? Was it demonstrated that churches are more vulnerable than common buildings and more vulnerablethan other monumental buildings? Are there any special outcome from this specific event as derived from the observation ofdamage and from the analytical studies?

These questions can be answered in two possible ways; the first one is to make extensive reconnaissance analysis on theentire sample of the buildings that experienced some damage, in order to derive general conclusions from the observed dam-age, as related to the peculiarity of the single monuments. The other possible way is to derive some conclusions based onextensive analytical studies, which, in turn lead to results to be compared to the observed damage. This procedure requiresa smaller number of churches to be analysed, and the key mechanical parameters can be identified from the results obtainedfrom different selected church case studies.

In this paper the second way was chosen and four typical churches were selected for this purpose. The plans of the fourchurches are reported in Fig. 9. The four churches are: Santa Giusta (SG), Santa Maria di Collemaggio (SMC), San Silvestro (SS)and San Pietro di Coppito (SPC). As shown in Fig. 9, SG is a typical church characterised by a central nave and lateral chapels,

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Lateral chapels overturning

Damagein triumphal arch

Overturning of belfry and external leaf of transept wall

Overturning of apse chapel

Collapse of the lateral

chapel vault

Crushing of triumphal arch columns

Shored façade due to previous

restoration works

Overturning of apse chapel

Fig. 10. Santa Giusta church (SG): damage after the 6th April 2009 earthquake.

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while SMC and SS are typical basilica churches with three naves. The plan of SPC church is non-typical, because it is char-acterised by a short and almost square hall, and a transept of comparable size; this configuration is the result of changes andalterations that occurred on the original fabric during the Baroque age. It is clear that the four churches, three of which arebasilica type, are quite different in terms of global size and dimensions; in fact, ‘despite these differences, this selection ofchurches was made to ensure a certain generality of the obtained results.

A brief description of each case study and of its damage pattern caused by the earthquake of 2009 is reported in the nextsub-sections.

3.1. Santa Giusta church (SG)

The church of Santa Giusta was built in the first decades of the XIV century on the ruins of pre-existing masonry walls,according to a basilica layout, with a central nave and lateral chapels.

The bearing masonry structure is realised by using the sack masonry arrangement, typical of L’Aquila region, with theexternal leaves made of small calcareous stones and the filling core made of calcareous stones and hydraulic mortar. Thetriumphal arch and its pillars are made of calcareous freestone. The roof structure is made by wood trusses. It is likely thatthe church, in the current configuration, experienced the 1703 earthquake.

Severe damage due to the earthquake sequence that occurred on 6th April 2009 (Fig. 10), is mainly concentrated in thetransept and apse region, and consisted of: partial collapses due to the overturning mechanisms of a portion of the apse andof the external leaf of the transept south wall; localised damage, i.e. masonry crushing and disaggregation in the columns ofthe triumphal arch, as a consequence of combined compression force and bending moment; and complete detachment be-tween the lateral chapels and the walls of the central nave, probably because the walls separating the chapels were built in alater phase than the construction of the longitudinal external walls.

3.2. Santa Maria di Collemaggio church (SMC)

Santa Maria di Collemaggio church was built in the XIV century; it was greatly modified in the Baroque age, but presently,following a complete dismantlement of the Baroque decorations that occurred in the 1950s, it appears in its native aspect.The building has a basilica plan with central nave, lateral aisles, transept and apse.

The bearing walls are made of the typical sack masonry of L’Aquila; only the most stressed structural elements, such astriumphal arches and pillars, are realised in freestone. The roof structure is made of wooden trusses for the central nave, andof masonry vaults for the remaining parts.

The seismic damage caused by the 2009 earthquake is mainly concentrated in the zone of the presbytery (Fig. 11); in par-ticular, the total collapse of the transept and a partial collapse of the apse were observed. Furthermore, some pillars of thenave suffered evident damage, namely vertical crushing cracks, due to both the longitudinal response and the significant ver-tical component of the earthquake strong motion.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Crushingof central

nave

Collapse of transept and

triumphal arches

Damagein thebelfry

Shear damageof apse

Shored façade due to previous

restoration works

Fig. 11. Santa Maria di Collemaggio church (SMC): damage after the 6th April 2009 earthquake.

Collapseof bell tower

Partialcollapseof apseroofing

Over turning of façadeupper corner

Fig. 12. San Pietro di Coppito church (SPC): damage after the 6th April 2009 earthquake.

G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 9

3.3. San Pietro di Coppito church (SPC)

The San Pietro di Coppito church was built at the end of the XIV century; it has the most non-typical plan among the fourcase studies: the hall of the church has a large, almost square nave, with a small lateral aisle along one side only; also thetransept is large and almost square in plan, while the apse is constituted by three polygonal chapels. The church has one ofthe rare bell towers in the city of L’Aquila.

Similarly to the other cases, the masonry walls are realised in sack masonry and free stone, while the roof structures arewooden trusses, with the exception of the cross vaults covering the apse chapels.

The severe damage after the earthquake of 6th April 2009 (Fig. 12) consists of the overturning mechanism of the façadeupper corner and in the partial collapse of the bell tower, that triggered the collapse of the apse roof.

3.4. San Silvestro church (SS)

The San Silvestro church was built in the same period as the other case studies, i.e. XIV century. It has a very simple archi-tectural layout, with three naves, and apse characterised by polygonal chapels (like in Santa Giusta and San Pietro di Coppitochurches). The bearing wall structures are realised, as in the previous cases, in sack masonry or free stone, while the roof issupported by wooden trusses, with cross vaults only covering the apse zone.

Unlike the other case studies, the San Silvestro church suffered light damage (Fig. 13), mainly concentrated in the belltower, where cracks caused by the seismic overturning moment were observed. The façade also showed some damage,namely the activation of a partial out-of-plane overturning mechanism, concentrated in an upper corner, and vertical cracksdue to dynamic interaction between the façade and the bell tower.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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Rockingdamage

of bell tower

Overturningof façadeupper corner

Shear damageof apse

Fig. 13. San Silvestro church (SS): damage after the 6th April 2009 earthquake.

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4. Performance evaluation through the ‘‘two-steps’’ procedure

The analysis of the four case studies was carried out through a ‘‘two-steps’’ procedure, developed by the authors in pre-vious studies and already applied to different basilica churches [27,6]. Basically, in the ‘‘first step’’, the linear dynamic anal-ysis of the 3D church model provides the seismic demand on the macro-elements, and in the ‘‘second step’’ the horizontalcapacity of the macro-elements are quantified through non-linear analyses of refined 2D models, and/or limit analysis. Thecomparison between the demand on the macro-element defined in the ‘‘first step’’ and the capacity computed in the ‘‘secondstep’’, provides indications of the susceptibility of the church to seismic damage and partial or total collapse.

Preliminary for the ‘‘first-step’’ analyses, a ‘‘linearisation’’ process was been carried out on the architectural plan of thechurches, in order to identify the church macro-elements, which are labelled in Fig. 9 with letter L for the longitudinal walls,and letter T for the transversal walls. This linearisation process provided the pseudo-3D model, consisting in the assemblageof the 2D macro-elements; the eight classes of marco-elements are collected in Fig. 14, i.e.: (1) Apse; (2) First triumphal arch;

1

2

3

4

5

6

7

SG SMC SPC SS

AB

T8

T7

T2-T6

T1

L1L4

L2-L3

AB ABT4

T3

T2

TRASV

T1

L1

L4

L2-L3

T4

T3

T2

T1

L1

L2

T2

TRASV

T1

L1

L2-L3

L5 L4L3

L4 L5

Fig. 14. Table of macro-elements for the examined L’Aquila churches: (1) apse; (2) first triumphal arch; (3) second triumphal arch; (4) nave section; (5)façade; (6) longitudinal front; and (7) arcade/clerestory.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 11

(3) Second triumphal arch; (4) Nave section; (5) Façade; (6) Longitudinal Front, (7) Arcade/Clerestory and (8) 2nd Arcade/Clerestory (in case of more than three naves).

4.1. ‘‘Two-steps’’ procedure: the demand on the macro-elements

The internal force distribution among the single macro-elements as well as the dynamic properties of the overall buildingstructures were obtained through 3D linear dynamic analysis carried out by using the computer code SAP 2000 [31]. Asshown in Fig. 15, the 3D F.E. models of the churches were obtained by using shell elements; according to the indicationsgiven in [9] on mechanical characteristics of L’Aquila masonry, the following properties for the typical sack masonry materialwere assumed: Young modulus E = 1000 MPa, Poisson modulus m = 0.2, unit weight c = 19 kN/m3.

Modal dynamic analyses were carried out by using both the response spectrum obtained from the acceleration historyrecorded at the AQK station, and the elastic spectrum suggested by the Italian Building Code [29]. Fig. 16 shows the compar-ison between the 5% damped pseudo-acceleration response spectrum of the earthquake ground motion (appointed as ‘‘Mainshock 06/04/09 AQK’’) and the elastic spectrum provided by the Italian Code for L’Aquila city, for a returned period TR = 475 -years and soil type B (appointed as ‘‘NTC’08 EL’’). The spectrum of the recorded acceleration history shows a high frequencycontent for short periods (at 0.16 s the pseudo-acceleration reaches a peak value of 1.10g), and for periods in the range 0.72–2.34 s. In the range of 0.16–0.72 s, the recorded spectrum shows pseudo-acceleration values consistent with those providedby the Italian Building Code, while for long period values (longer than 2.34 s), the recorded pseudo-accelerations show astrong decreasing trend, with spectral values equal to about one third of the Code counterparts.

In Fig. 16 the design inelastic spectrum (appointed as ‘‘NTC’08 D (q = 2.8)’’) is also plotted, obtained from the elastic spec-trum, by considering a reduction factor q which takes into account the dissipative capacity of the structure. Regarding thevalue to be considered for the reduction factor q, it has to be underlined that the design shear force for these ‘‘existing ma-sonry buildings’’ is still the object of scientific debate, and therefore also the codified provisions have not yet reached a gen-eral consensus. The Italian Guide Lines for the assessment and mitigation of seismic risk of cultural heritage [25], a documentwhich the NTC’08 design Code explicitly refers to, suggests that the value for the reduction factor q should be selected be-tween 1.5 and 3, depending on the characteristics of the masonry structure; in this paper q = 2.8 has been assumed, becauseit is between 1.5 and 3.0 and, additionally, it is the value of the q-factor suggested by the Italian Building Code (NTC’08) in thecase of a one-storey unreinforced masonry building.

SS13496 shells

SMC24414 shells

SPC12561 shells

SG15172 shells

Fig. 15. Three-dimensional numerical models of the churches.

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Sa/g

T [s]

06/04/09 AQK

NTC'08 D(q=2.8)

NTC'08 EL

Fig. 16. Comparison between the pseudo-acceleration response spectra (main shock of 6th April 2009 AQK) and the elastic spectra provided by the ItalianTechnical Code [29] for L’Aquila city (returned period TR = 475 years; soil type B).

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Page 12: Brandonisio 2013 Engineering Failure Analysis

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Fig. 17. Distribution of the first 100 modal shapes in SG church for the earthquake acting in transversal direction (a) and in longitudinal direction (b)(Meff = modal participating mass ratios) and comparison with the pseudo-acceleration response spectra (main shock of 6th April 2009 AQK).

12 G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx

The dynamic behaviour of the four churches and, in particular, of SG church is summarised in Figs. 17a and b and 18. Thedynamic behavior of the four churches and, in particular, of SG Church is summarized in Figs. 17a, 17b and 18.

The distribution of vibration modes of the SG church in longitudinal and transversal direction is provided in Figs. 17a, 17b,respectively. In the upper part of the figures, the elastic response spectrum obtained from the AQK station Mainshock accel-erogram is reported, while, in the lower part, the modal participating mass ratios (Meff), of the first 100 modes, are plotted asa function of the vibration period (T) and represented through a bullet point.

Concerning the participating mass ratio of each vibration mode, a small contribution was observed, therefore, in order towell describe the dynamic behavior of the structure, it was necessary to take into account the first 100 modes, obtaining atotal participating mass ratio is greater than 80% of the total mass.

Even if the graph cannot be read easily, because of the great number of bullet points, it is very clear that almost all vibra-tion modes have modal participating mass ratio (Meff) less than 10%. Considering for example the longitudinal direction(Fig. 17a), it can be observed that: 1 vibration mode has Meff =16%; 3 modes have Meff in the range 5%–10%; 12 modes haveMeff in the range 1%–5%; and the remaining 84 vibration modes have Meff less than 1%. This means the dynamic response ofthe church is strongly affected by the local behavior of the macro-elements.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 13

From the comparison between the upper and the lower part of Figs. 17a and b, it can be also observed that the vibrationmodes with higher participating mass ratio correspond to spectral acceleration values equal to about 0.4–0.5 g, therefore,lower values of global base shear, compared with those computed considering the peak spectral acceleration (1.2 g), canbe expected.

In the upper part of Fig. 17a and b the elastic response spectrum obtained from the AQK station Mainshock accelerogramis reported to point out that the vibration periods with the greater participating masses correspond to lower spectral accel-eration thus explaining the rather low values of base shear forces obtained.

For the modes characterised by the largest values of participating mass, a 3D-view of building deformed configuration isalso provided; in particular, Fig. 17a refers to modes mainly involving vibrations in the transversal direction of the church,while Fig. 17b refers to the modes in the longitudinal direction. For the sake of brevity, only the results obtained for SantaGiusta church (SG) are herein organised according to this graphical layout. From the data shown in Fig. 17, it is possible todeduce that the first mode associated with not negligible participating mass is a transversal mode, namely the first out-of-plane mode of the longitudinal internal arcades; then, the following modes correspond to higher (2nd, 4th, . . .) modes of thesame longitudinal elements, also involving deformations of façade and transept (Fig. 17); in the longitudinal direction thevibration modes involve out-of-plane deformation of the triumphal arch and of the façade.

Similar observations can be made for the other three case studies; also the results in terms of participating mass for eachmode are confirmed by the data provided in Fig. 18 for all the churches. As a general conclusion it can be stated that, in con-trast with ordinary buildings, where the modal participating mass is generally greater than 70% and the first three modes

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SMC

SPC SS

SG

T [s]T [s]

T [s] T [s]

Fig. 18. Distribution of the first 100 modal shapes in SCM, SPC and SS churches ( = transversal direction; d = longitudinal direction; Meff = modalparticipating mass ratios) and comparison with the pseudo-acceleration response spectra (main shock of 6th April 2009 AQK).

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V/Wtot Transversal direction 06/04/2009 AQKNTC'08 EL

(a)

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V/Wtot Longitudinal direction 06/04/2009 AQKNTC'08 EL

(b)Fig. 19. Base shear (V) normalised to the total weight (Wtot): comparison between NTC’08 and 6th April 2009 main shock in (a) transversal and (b)longitudinal direction.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Page 14: Brandonisio 2013 Engineering Failure Analysis

14 G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx

give a cumulative mass that is generally greater than 85%, in the church buildings the modal participating mass ratios, whennot fully negligible, are generally less than 10%, and the number of modes needed in order to obtain a significant mass isgenerally very high: for the case studies the cumulative participating mass associated with the first one hundred modesranges from 60% for SMC church in the longitudinal direction, to 80% for SG and SS churches, to 86% for SMC church inthe transversal direction.

Concerning the dispersion of vibration modes that was found in the four case studies, very similar results about the spe-cial dynamic behavior of churches are reported in other papers [3, 4, 17] where the Santa Maria all’Impruneta, Farneta Abbeyand S. Maria di Collemaggio basilica churches are studied.

In the diagrams of Fig. 19 the base shear (V) obtained according to both the AQK and NTC’08 spectra, and normalised tothe total building weight (Wtot), are plotted in both the transversal (Fig. 19a) and longitudinal (Fig. 19b) directions. The baseshear V has been evaluated using the combination of maximum modal responses, by considering the product of the modalparticipating masses and the corresponding spectral accelerations, combined using the Square Root of the Sum of theSquares (SRSS) method.

The histograms provided in Fig. 19 allow for two major considerations:

(1) In the four case studies, the base shear V ranges between 20% and 30% of the church weight Wtot. This result is a directconsequence of the already observed dispersion of the modal shapes, with low values of participating mass (less than10%), and corresponding AQK spectral accelerations Sa approximately equal to 0.4g; in fact, considering that the totalparticipating mass is between 60% and 86%, we have:

Pleaseearthq

V ¼ ð60—86%ÞMtotSa ffi ð60—86%ÞMtot0:4g ¼ ð0:24—0:34ÞWtot: ð1Þ

(2) It follows that the high spectral values of the AQK record do not give rise to similar high values of seismic loads on thechurches; therefore for the assessment of these monumental buildings the choice of the reduction factor value is notso significant as in the case of ordinary buildings.

(3) The V/Wtot ratios obtained by means of the ’’NTC’08 EL’’ elastic spectrum are always greater than the ratios obtainedaccording to the AQK spectrum. Quite trivially, this is because, for the period range of the most significant modes, i.e.0.2–1.0 s, the elastic spectrum of the Italian Code gives values of pseudo-accelerations that are systematically higherthan their AQK (Fig. 16).

For each church, the distribution of total base shear V among the macro-elements has been computed and provided interms of Vi/V ratio in Fig. 20, in both the longitudinal and transversal directions. It can be observed that, when the seismicaction is applied in the transversal direction, the shear forces are mainly concentrated in the perimeter elements (Apse, Faç-ade,), with Vi/V ratio ranging between 10% and 30%, and in the transept elements (first and second triumphal arch), with theVi/V ratio from 5% to 10%; on the contrary, lower shear forces act on the internal elements (section nave), in which the Vi/Vratio is always less than 5%. Similar considerations can be made in the longitudinal direction, even though less dramatic dif-ferences among the macro-elements shear forces can be observed.

The chart provided in Fig. 21 shows the percentage of seismic base shear globally absorbed by the macro-elements lo-cated in the direction orthogonal to the applied seismic action (Vout-of-plane/V). In particular, the white bars represent the glo-bal out-of-plane contribution of the transversal macro-elements when the seismic action is applied along the churchlongitudinal direction, while the grey bars represent the out-of-plane contribution of longitudinal macro-elements for earth-quake acting along the transversal direction. In general terms, it can be observed that a larger contribution of orthogonalelements arises when the seismic action is applied in the transversal direction than in the longitudinal one. Furthermore,it can be observed that the global out-of-plane contribution in the longitudinal direction is quite uniform for the fourchurches, being approximately equal to 10–30%; on the contrary, the contribution of the longitudinal macro-elements undertransversal seismic action is highly variable for the different churches, going from 25% in the case of SPC church, to 75% in thecase of SMC church.

4.2. ‘‘Two-steps’’ procedure: the capacity of the macro-elements

In order to evaluate the horizontal strength capacity and the failure mechanism of each macro-element, in the ‘‘secondstep’’ of the analysis procedure all the church macro-elements were analysed in the non-linear range. The non-linear anal-yses were carried out by applying constant gravity loads and by increasing the horizontal loads, proportional to the masses,up to the structural collapse. For this aim, the finite element computer code ABAQUS [32] was used; the material non-lin-earity and in particular the nearly no-tension characteristics of the masonry were accounted for by means of a smeared crackmodel.

A calibration phase for the F.E. models was carried out through a preliminary extensive sensitivity analysis, aimed atassessing the effect of the mesh size, material model parameters and non-linear solution strategy on the inelastic responseof a single macro-element; the results, widely discussed elsewhere [14] suggest the adoption of a shell-element size equal toabout 50 cm. Concerning the material model, according to Capecchi et al. [9] and Borri [8], compressive strength equal to1.0 MPa and tensile strength equal to 0.1 MPa were adopted; in this way the tensile-to-compressive strength ratio assumesthe classic value 1/10.

cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilauake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Page 15: Brandonisio 2013 Engineering Failure Analysis

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AB L1 L2 L3 L4 T1 T2

Vi/V 06/04/09 AQK-Transv.

NTC'08-Transv.

SS

Fig. 20. Distribution of seismic base shear among macro-elements in longitudinal (Long.) and transversal (Transv.) direction.

G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 15

The results of the non-linear analyses on the single structural macro-elements allow for obtaining push-over curves,stress and plastic strain distributions, deformation and collapse modes, and the ultimate horizontal strength capacity. Inthe following, for the sake of brevity, only the values of ultimate lateral strength (Vu) are discussed, particularly in compar-ison with the strength demand (Vi) obtained for each macro-element in the ‘‘first step’’ of the procedure. In fact, a direct,though approximate, assessment of the seismic safety level of the churches can be made by comparing the base shear(Vi) absorbed by each macro-element to its horizontal capacity (Vu). In Fig. 22 this approximate assessment of the churchesunder the main shock 06/04/09 AQK is presented in a graphical way; in particular, for all the macro-elements of the church

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Page 16: Brandonisio 2013 Engineering Failure Analysis

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60%

80%

SG SMC SPC SS

Long. direction

Transv. direction

Vout-of-plane

V

Fig. 21. Seismic base shear (Vout-of-plane) globally absorbed by elements orthogonal to the applied seismic action (V = total seismic base shear).

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L3L4

T2T1(Vi/Vu)MainshockAQK

(d) SS

Fig. 22. Horizontal seismic demand (Vi) to horizontal strength capacity (Vu) ratio for the macro-elements of examined churches.

16 G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx

case studies, the demand-to-capacity ratios (Vi/Vu) are represented as bullet points in the relevant chart, i.e. in the chart ofFig. 22a for SG church, Fig. 22b for SMC church, Fig. 22c for SPC church and Fig. 22d for SS church.

In each chart of Fig. 22, the horizontal lines Vi/Vu = 1, corresponding to the elastic limit condition and the one at Vi/Vu = 2.8,are also plotted. With reference to the first line at Vi/Vu = 1, quite trivially, the points located under the line Vi/Vu = 1 identifymacro-elements that have shown an elastic behaviour under forces derived from the main shock AQK spectrum; further-more, the horizontal line Vi/Vu = 2.8 is also reported in the diagrams, which corresponds to a level of damage consideredacceptable according to the inelastic design spectrum calculated by using a behaviour factor of q = 2.8. Thus, the points fall-ing in the zone bounded by these two horizontal dashed lines correspond to macro-elements that have predicted from thenumerical analyses to develop limited damage, while the points located above the horizontal line Vi/Vu = 2.8 correspond tovulnerable macro-elements, which have predicted to develop unacceptable damage. For this reason, the three zones in thecharts that are defined by the two horizontal dashed lines have been appointed as ‘‘Elastic’’ (when Vi/Vu 6 1), ‘‘Damage’’(when 1 < Vi/Vu 6 1) and ‘‘Unsafe’’ (when Vi/Vu > 1), respectively.

With reference to the charts in Fig. 22 it can be noted that, with the exception of a few perimeter elements, the macro-elements of the churches usually have a strength capacity smaller than the demand counterpart (Vi/Vu > 1). For example, in

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Page 17: Brandonisio 2013 Engineering Failure Analysis

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T4 T5

T6 T7

T8

UnsafeDamage

L4

T2

T3

T5T4

T6

T7

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Macro-element

Fig. 23. L’Aquila main shock vs. elastic spectra provided by the Italian Technical Code [29]: comparison between the demand-to-capacity ratio (Vi/Vu) ofSanta Giusta church macro-elements.

G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 17

the case of Santa Giusta church (Fig. 22a), the points of the macro-elements L4 (transept façade), T2-T6 (nave sections) andT7 (triumphal arch) are located in the part of diagram above the horizontal line at Vi/Vu = 2.8, i.e. in the ‘‘Unsafe’’ part of thediagram, since the demand-to capacity ratios range between 4 and 5.5. Similar observations can also be made by consideringthe analogous results obtained by using the elastic spectrum suggested by Italian Code [29] instead of the main shock AQKspectrum. For this aim, in Fig. 23 the comparison between the demand-to-capacity ratio (Vi/Vu) evaluated according to theAQK spectrum (on abscissa axis) and to Code spectrum (on ordinate axis) is proposed for the Santa Giusta church. The bisect-ing line reported in the diagram corresponds to the equal values of demand-to-capacity ratios, i.e.: (Vi/Vu)Mainshock AQK = (Vi/Vu)NTC’08. The two couples of dashed lines correspond to scatters between the demand-to-capacity ratios (Vi/Vu)NTC’08 and (Vi/Vu)Mainshock AQK of ±10% and ±20%, respectively. Finally, the two squares reported in the chart with bold dashed lines corre-spond to the conditions Vi/Vu = 1 and Vi/Vu = 2.8, already used in Fig. 22 for defining the regions corresponding to the threevulnerability conditions: i.e. ‘‘Elastic’’, ‘‘Damage’’ and ‘‘Unsafe’’.

An examination of diagram of Fig. 23 allows for observing that there are almost no differences in terms of global judgmenton the seismic vulnerability for Santa Giusta church by adopting either the AQK or the Code spectrum. In fact, the macro-elements that are in the ‘‘Elastic’’, ’’Damage’’ or ‘‘Unsafe’’ region for the main shock AQK spectrum, remain in the same regionalso for the Code elastic spectrum. Furthermore, despite of the high peak value of pseudo-acceleration recorded during themain shock (equal to 1.10g at 0.16 s, see Fig. 16), it can be noted that the results obtained by using the Code elastic spectrumare more conservative from a design point of view: in fact, in the period range of interest for the most significant participat-ing modes (i.e. 0.2–0.6 s, see Fig. 17), the Code spectrum gives values of pseudo-acceleration greater than the AQK counter-part, as can be immediately observed in Fig. 23, where almost all points are located in the zone corresponding to scattersbetween 0% and +20% (with the exception of the macro-element T7 (triumphal arch)).

The above considerations, made for SG church, have been also confirmed by the analysis results obtained for SCM, SPCand SS churches.

4.3. The out-of-plane capacity of the macro-elements

Considering the typical structural damage caused by the past earthquakes on the churches, it can be noted that the checkof out-of-plane collapse mechanisms of macro-elements assumes great importance in the seismic verification of masonrychurches, particularly where box-type behaviour cannot be assumed, due to the lack of rigid diaphragm and/or weak con-nections between orthogonal intersecting walls.

For assessment of the out-of-plane response of masonry structures, limit analysis is among the most straightforward,effective and stable procedures. In fact, thanks to its effectiveness, the limit analysis approach has been also introducedin and is strongly recommended by the Italian Codes and guidelines [25,29,11] for the seismic assessment of existing ma-sonry structures, in particular for masonry church buildings.

The application of limit analysis, through a kinematic approach, requires the identification and the analysis of all collapsemechanisms that can be activated in the building. These mechanisms can be defined on the basis of empirical considerations,i.e.: observation on similar structures damaged in past earthquakes, detection of existing crack patterns, considerations onthe quality of connections between intersecting masonry walls and of the bond pattern, accounting for the presence or ab-sence of tie-rods, etc.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

Page 18: Brandonisio 2013 Engineering Failure Analysis

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SG

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Fig. 24. Out-of-plane collapse: verification of façades.

18 G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx

The out-of-plane behaviour and the possible collapse mechanisms of typical church macro-elements have been consis-tently recognised and studied by Italian researchers after past earthquakes, and the outcomes have been used as a basisfor the classification suggested in the Italian guidelines [25].

For the sake of brevity, only the out-of-plane collapse of the façade is discussed in this section. Considering both the par-ticular geometry of the typical façades of L’Aquila church buildings and the indications given by the Italian guidelines [25],four collapse mechanisms have been identified:

� mechanism I: global overturning of the whole façade;� mechanism II: partial overturning of the façade upper part (gable);� mechanism III: horizontal bending of the gable; and� mechanism IV: overturning of the façade upper corner.

For each considered collapse mechanism, the seismic verification consists in comparing the seismic demand (Da) to theseismic capacity ða�0Þ, where Da is the seismic acceleration acting on the portions of rigid bodies involved in the kinematism,and a�0 is the acceleration value that causes activation of the collapse mechanism. Additional information on the evaluation ofDa and a�0 are provided in [6,5].

Quite trivially, the seismic verification is satisfied when Da 6 a�0, i.e. when the demand-to-capacity ratio is less than 1:D=a�0 6 1.

The demand-to-capacity ratio Da=a�0 has been calculated for each mechanism type with reference to the four churches.The results are plotted in Fig. 24, grouped according to the mechanism type. In this chart, the bullet points provide an imme-diate representation of the façade safety conditions; in fact the horizontal line at the limit condition D=a�0 ¼ 1, divides thechart in two zones: the lower zone, which corresponds to the safe condition (i.e. seismic verification satisfied: D=a�0 < 1);and the upper zone, which corresponds to the damage condition (i.e. the seismic verification not satisfied: Da=a�0 > 1).

With the exception of Santa Maria di Collemaggio church (SMC), it can be noted that the demand-to-capacity ratio(Da=a�0) assumes values always greater than 1 for the collapse mechanism IV, in particular equal to 1.23 for Santa Giusta(SG), 1.76 for San Pietro a Coppito (SPC), 1.17 for San Silvestro (SS). Furthermore, the façade of SPC church seems to be alsovulnerable to mechanism II, although the corresponding demand-to-capacity ratio ðDa=a�0 ¼ 1:54Þ is lower than that ofmechanism IV ðDa=a�0 ¼ 1:76Þ.

Therefore, the results obtained from the out-of-plane kinematic analysis of principal façades suggest that, under the seis-mic action, the façade macro-elements of SG, SPC and SS churches are highly susceptible to the overturning mechanisms ofthe façade upper corner. In fact, this kind of damage and collapse mode was clearly observed in the damage survey carriedout after 2009 earthquake, particularly for the studied church cases, as well as in several other churches.

5. Comparison between ‘‘two-steps’’ procedure results and seismic damage

The results obtained from the ‘‘two-steps’’ procedure reported in the previous section have confirmed the high seismicvulnerability of the examined churches.

A sort of verification of the procedure reliability can be carried out by comparing the damage observed in the survey fieldactivity and the one foreseen according to the analysis results. As reported in the following for each church, a good agree-ment between the numerical procedure results and the observed damage has been found, not only in terms of ultimate baseshear but also in terms of collapse mechanisms and crack patterns.

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Fig. 25. Santa Giusta church (SG): comparison between the seismic damage and the results of ‘‘two-steps’’ analysis for the triumphal arch (macro-elementT7) and arcade (macro-element L3).

Fig. 26. Santa Giusta church (SG): comparison between the seismic damage and the results of ‘‘two-steps’’ analysis for the transept wall (macro-elementL4).

G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 19

5.1. Santa Giusta church (SG)

Starting with Santa Giusta church, by observing the analysis results reported in Figs. 22a and 23, it is evident that themacro-element T7 is highly vulnerable; in fact the demand-to-capacity ratio (Vi/Vu) is greater than 4, thus confirming a sub-stantial agreement between the foreseen damage and the crushing failures actually observed. These results are also con-firmed by the deformed shapes of the macro-element T7 reported in Fig. 25, where the plastic strain distribution isexplicitly provided.

From the application of the numerical procedure, it can also be noticed that the transept lateral wall (L4) is one of themost vulnerable macro-elements of Santa Giusta church, since the demand-to-capacity ratio (Vi/Vu) is approximately 4(Fig. 22a). In fact, the transept wall was severely damaged during the 2009 L’Aquila earthquake, and both out-of-planeand in-plane collapses were observed. Regarding the in-plane collapse mechanism, from the comparison proposed in

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Fig. 27. Santa Maria di Collemaggio church (SMC): comparison between the seismic damage and the results of ‘‘two-steps’’ analysis for the arcades (macro-elements L2 and L3) and triumphal arches (macro-elements T2 and T3).

20 G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx

Fig. 26 between the plastic strain distribution and the suffered damage, it can be noted that the ‘‘two-steps’’ procedure is ableto provide the shear cracking pattern that the transept façade has actually suffered during the earthquake, particularly con-centrated in the masonry panel located on the left side of the window.

5.2. Santa Maria di Collemaggio church (SMC)

Concerning Santa Maria di Collemaggio church (SMC), the application of the numerical procedure (Fig. 22b) suggests thatthe most vulnerable macro-elements are the central nave arcades (macro-elements L2 and L3), with ratios (Vi/Vu) equal to 3and 7 respectively, and the first and second triumphal arches (macro-elements T2 and T3), with demand-to-capacity ratio(Vi/Vu) between 5 and 7; the above macro-elements were actually the most severely damaged ones in the 2009 earthquake.In particular the columns of the arcades L2 and L3 that support the transept dome collapsed due to combined bending andaxial load, triggering the whole collapse of the transept (Fig. 27).

5.3. San Pietro a Coppito church (SPC)

For San Pietro a Coppito church (SPC), the non-linear analyses (Fig. 22 c) showed that the most vulnerable elements arethe arcade (L2), with (Vi/Vu) about 4.5, and the first and second triumphal arches (T3 and T4), with (Vi/Vu) greater than 7. Alsoin this case the damage foreseen from the analysis result was observed during the past-earthquake survey, which revealedlocal crushing of the column stones in the L2 macro-element.

5.4. San Silvestro church (SS)

For San Silvestro church (SS) (Fig. 22d), the application of the ‘‘two-steps’’ procedure suggests that less damage would beexpected than for the other three churches. Also this result is confirmed by the actual observed damage: in fact, the centralnave arcades (L2 and L3 macro-elements) exhibited light damage to the columns (local crushing), as also shown by the plas-

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Fig. 28. Out-of-plane collapse of façade: comparison between the seismic damage of (a) San Pietro a Coppito church (SPC) and (b) San Silvestro church (SS),and the collapse mechanisms provided by kinematic analysis.

G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx 21

tic strain distribution in these macro-elements obtained from the analysis. In addition, from the chart of Fig. 22d, the façade(macro-elements T1) and the triumphal arch (T2 macro-element) seem to be highly vulnerable under in-plane seismic loadsgiven that the (Vi/Vu) ratios greater than 4. Although it was not possible to personally collect direct observation data, fromother research reports [1] this result seems to be confirmed by damage (not so heavy) observed in the presbytery.

5.5. Out-of-plane collapse mechanisms

Concerning the out-of plane collapse mechanisms, the results of kinematic analysis suggest that the facades of Santa Giu-sta (SG), San Pietro a Coppito (SPC) and San Silvestro (SS) churches are susceptible to mechanism IV. This mechanism per-fectly matches the seismic damage experienced by the façades of SPC and SS churches, where overturning of façade uppercorner was activated during the 2009 L’Aquila earthquake (Fig. 28). The same comparison between seismic damage and anal-ysis prediction cannot be made in the case of SG church, because the façade was shored at the time of the earthquake occur-rence, due to previous restoration works (Fig. 10).

6. Conclusions

The seismic behaviour of masonry churches damaged during the L’Aquila 2009 earthquake has been analysed in the pres-ent study. Four important basilicas are considered in order to derive general conclusions from the damage assessment andthe performance analysis. The results of the comparison between the post-earthquake survey activity and the numericalanalysis confirms that, nowadays, it is possible to evaluate the seismic safety level of churches and to identify the potentialfailure modes; this, in turn, allows monumental heritage to be preserved and destructive damage to be avoid by adoptingappropriate retrofit interventions. In particular, the reliability of the ‘‘two-steps’’ analysis suggested and applied by theauthors as an analytical tool capable of predicting real behaviour of churches under seismic actions is confirmed.

Other important results concerning the seismic behaviour of masonry churches have been obtained: the dynamic exci-tation due to the seismic ground motion activates many vibration modes of the building structure, though all of them arecharacterised by small participation factors, generally less than 10%; for this reason the high spectral values of the registeredrecord of the L’Aquila earthquake do not correspond to equivalent high values of base shear on the churches. In particular theresults showed that in all the examined case studies, the base shear V ratio ranged between 20% and 30% of the churchweight Wtot. Therefore, as the global shear force on the buildings was significantly smaller than the plateau value of the spec-tral acceleration provided by Italian Code, the appropriate choice of the force reduction factor to be adopted for these mon-umental buildings is not as significant as in the case of traditional residential buildings characterised by shear typebehaviour. Further, the activation of many local modes also calls for retrofit interventions which should ‘‘tie up’’ the building,thus avoiding the local failure modes that are often observed.

The final conclusion is that use of the ‘‘Natural Laboratory of Earth’’ is the best approach to evaluate the seismic behav-iour, and the performance of structural systems as well as the failures occurring in reality, and is an unsubstitutable step forthe advancement of knowledge in the field of seismic engineering.

Acknowledgements

The authors gratefully acknowledge the precious contribution of Dr. Aldo Giordano, eng. Giuseppe Fappiano, Dr. Rosa deLucia and Dr. Roberta Santaniello in the post-earthquake reconnaissance activity.

The authors are also grateful to the Superintendence for Architectural, Landscape, Historical, Artistic and Ethno-Anthro-pological Heritage of L’Aquila district, and in particular to the Arch. Antonello Garofalo for his valuable support to researchactivities.

Please cite this article in press as: Brandonisio G et al. Damage and performance evaluation of masonry churches in the 2009 L’Aquilaearthquake. Eng Fail Anal (2013), http://dx.doi.org/10.1016/j.engfailanal.2013.01.021

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22 G. Brandonisio et al. / Engineering Failure Analysis xxx (2013) xxx–xxx

This research has been supported by ReLUIS Research Project 2009–2012 ‘‘Rete di laboratori Universitari Ingegneria Sismica’’,in the context of the activities of Tasks AT1-1.1a.3.h and AT2-2.3.2.

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