A multidisciplinary insight into the collapse causes of the masonry clock tower and castle in Finale...

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A MULTIDISCIPLINARY INSIGHT INTO THE COLLAPSE CAUSES OF THE MASONRY CLOCK TOWER AND CASTLE IN FINALE

EMILIA, ITALY

Gabriele Milani1, Maurizio Acito1, Claudio Chesi1, Carla Di Francesco2, Giacomo Mar-tines3

1Dept. of Architecture, Built Environment and Construction Engineering, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milan, Italy

e-mail: gabriele.milani, maurizio.acito, [email protected]

2 Direzione Regionale per i Beni culturali e Paesaggistici dell’Emilia-Romagna, MiBAC Strada Maggiore, 80, 40125, Bologna, Italy

e-mail: [email protected]

3 Dept. of Civil and Environmental Engineering and Architecture, Politecnico di Bari Via E. Orabona 4, 70125 Bari, Italy

e-mail: ignazio.carabellese, giacomo.martines, [email protected]

Keywords: Masonry, Case study, Emilia Romagna earthquake, Architectural Heritage Col-lapse, Vulnerability Evaluation. Abstract. The devastating earthquake sequences of the 20-29th May 2012 in Emilia-Romagna, Italy, caused the full collapse of the masonry clock tower and the partial collapse of the masonry castle in Finale Emilia, one of the historical cities most stricken by the seismic event. The present technical note is aimed at presenting some results of a comprehensive and multi-disciplinary analysis conducted by the authors at the Technical Universities of Milan and Ba-ri in cooperation with the Italian Cultural and Architectural Heritage Ministry (MiBAC), within a wider research project aimed at the reconstruction of the structures, after a deep un-derstanding of the causes at the base of the collapses occurred. The two main seismic events, occurred on May 20 and 29th, were characterized by the pres-ence of low frequency ground motions, with relevant vertical components. Consequently, structures having natural periods greater than those typical of ordinary buildings -such as the ones analyzed- have been severely damaged. The analyses conducted comprise a detailed geometric recover of the volumes and materials used before the collapses, basing on post-seismic photogrammetric surveys, historical re-search on existing documentation and repeated in situ investigation. Despite the apparently good texture of the external masonry walls, in situ surveys have proven the particularly low tensile strength of the joints, an impressive deterioration of mortar as well as bed interlocking within external and internal layers.

Gabriele Milani, Maurizio Acito, Claudio Chesi, Carla Di Francesco, Giacomo Martines

Structural analyses are conducted in agreement with the Italian Guidelines for the Architec-tural Heritage, evaluating the safety indexes of the structures assuming as input acceleration those provided by the Italian code and the real ones occurred during the seismic events. Simplified but still effective procedures are employed, basing on some reasonable assump-tions for masonry material, such as the very low tensile strength and the formation at failure of partial collapses with rigid motions of macro-blocks along pre-determined crack patterns. Results of the analyses confirm the largely insufficient performance of the structure under moderate horizontal accelerations and the need of improving mechanical properties and mit-igating vulnerability connected to geometrical issues by means of ad-hoc restorations.

1 INTRODUCTION

The Italian Ministry for the architectural heritage (MiBAC) commissioned to the Technical University of Milan, with scientific responsibility by the authors [1], wide and comprehensive numerical analyses aimed at better understanding the causes of the collapse, occurred during the May 2012 seismic events and subsequent aftershocks, of two monuments in Final Emilia. Finale is one of the towns within the seismic crater and one of those that suffered the most the effects of the earthquake. In parallel the Technical University of Bari was commissioned for a detailed survey of the damaged structures, both using photogrammetric and laser scanner techniques and by means of historical research.

The two monuments under study are the ancient clock tower, represented in Figure 1 and now totally collapsed and the castle, see Figure 2, severely damaged and partially collapsed in the upper part. The clock tower became the symbol of the earthquake and many images of its precarious equilibrium conditions after the main shock are available everywhere on the net, showing one half of the structure still uncollapsed near the other ruined, with a vertical crack passing roughly through the middle of the clock (Figure 3-a, -b). The castle, which is consti-tuted by a main building with a L shape and an independent central tower (Mastio) exhibits important crack patterns and partial collapses mainly on the upper part of the towers (Figure 3-c, -d) located on the corners. The central tower (Mastio), which will be the object of future research and is not investigated in the present work because structurally independent from the castle, is totally collapsed.

The collapsed clock tower and the castle are located at the extremes of the ancient walled town, in correspondence of the river Panaro. During the last centuries, the city underwent sub-stantial changes: the walls were pulled down more than 2 hundred years ago to urbanistically give a development possibility to the city, whereas the river has been deviated for public health reasons.

Only recently the Italian norms for constructions (NTC 2008 [2][3], OPCM 3274 2003 [4]) have classified as seismic zone a relevant part of the Italian territory previously unclassified and this happened also to the portion of the Emilia-Romagna region stricken by the earth-quake, where the structures are located.

In the present paper, the preliminary results of a wide and comprehensive numerical analy-sis conducted by the authors are reported. Numerical analyses conducted are many and most of them are sophisticated studies. They comprise (1) the evaluation of the vulnerability indi-ces according to Italian Guide Lines (GL) on architectural heritage [5], (2) linear static and response spectrum analyses (the latter conducted with both the Italian code spectrum and the real ones occurred during the seismic event), (3) non linear pushover analyses (G1 and G2 according to the Italian code) and (4) full non-linear dynamic analyses with input accelero-gram registered during the 20th May seismic event.

A multidisciplinary insight into the collapse causes of the masonry clock tower and castle in Finale Emilia, Italy

Due to space constraints of the present paper, hereafter only some relevant results coming from pushover and non-linear dynamic analyses are presented. Here it is worth mentioning only that, at present, they represent the most advanced numerical tools that it is possible to apply for the quantitative evaluation of the seismic vulnerability of complex historical struc-tures.

Pushover is conducted by means of Strand7 [6] commercial code, assuming that masonry behaves as an elastic-perfectly plastic material obeying a Mohr-Coulomb failure criterion, whereas non linear dynamic analyses are conducted with the commercial code ABAQUS [7] assuming a smeared crack model with isotropic damage in tension.

In all the analyses carried on, a full three-dimensional (3D) detailed FE model has been developed, starting from the available documentation (photographs and existing drawings as well as direct in-situ surveys), provided by the huge work done by Technical University work team, evaluating the effect induced by different directions for the seismic load and its distribu-tion. Experimental data available for coeval structures in the same zone and code of practice specifics were employed to calibrate elastic and inelastic masonry parameters, to be used within the FE non-linear analyses.

The results confirm an insufficient performance under horizontal actions and extremely high vulnerability of the clock tower, whereas for the castle non-linear dynamic analyses con-firm the collapse of the upper parts.

East-West South- North

Figure 1: Lateral views of the clock tower.

2 MATERIALS ASSUMED IN THE FE MODEL

Isotropic materials are used for masonry, neglecting the typical orthotropy of such materi-als demonstrated many times in the literature [8]-[13], because it has been considered that the texture is quite irregular and constituted by several layers, with a weak interconnection be-tween internal and external layers.


Figure 2: Section, plan view and photo of the castle

-a - c

-b -d

Figure 3: State of damage of the tower (-a & -b) and the castle (-c & -d) after the seismic event.


Such choice allows an easier determination of elastic and inelastic parameters, is made in

agreement with the Italian code and allows the utilization of commercial FEM, where ortho-tropic softening models are not at disposal.

Three different materials are modeled, namely ordinary masonry (external and internal walls and vaults), infill (over vaults), timber beams (belfry sustaining frame in case of the clock tower, floors, rods, etc.). For the castle, the variety of materials and building technolo-gies is larger, because several rehabilitation interventions where executed in the recent (2002) and less recent (1980) past. In particular, some internal vaults were pulled down and substi-tuted with r.c. floors and steel beams. A recent rehabilitation was aimed at increasing the load bearing capacity of some of the external walls (especially one long central wall), by means of standard mortar joints stitching, the introduction of pre-compression carbon fiber elements between external leaves and the introduction of tie rods under the roof, having the desired ef-fect of a more monolithic behavior of the structure. A comparison between the situation be-fore and after the rehabilitation is shown in Figure 4. Such intervention plaid a beneficial role and preserved all those masonry elements that were subjected to the rehabilitation. It was not possible to complete the works on the other parts of the castle, especially on the isolated tower (Mastio) due to economical budget imitations.

Figure 4: comparison between photos kept before and after the strengthening of the central wall.

According to Italian Code NTC [4], Chapter 8, and subsequent Explicative Notes [3], the mechanical properties to assume for the masonry material depend on the so called knowledge level LC, which is related to the so called Confidence Factor Fc. There are three LCs, labeled from 1 to 3, related to the level of knowledge that one has on the mechanical and geometrical properties of the structure. LC3 is the maximum, whereas LC1 is the minimum. For the case at hand, in absence of specific in-situ test results, a LC1 level is assumed. Fc summarizes the level of knowledge regarding the structure and the foundation system, from a geometric and mechanical point of view. It can be determined defining different partial confidence factors FCk (k=1,4), on the base of some numerical coefficients presents in the Italian Line Guides (Table 4.1). Due to the limited level of knowledge achieved in this case, the highest confi-dence factor was used, FC = 1.35.

After visual inspections, values adopted for cohesion and masonry elastic modulus are tak-en in agreement with Table C8A.2.1 Circolare 617 [3], assuming a masonry typology consti-tuted by clay bricks (approximate dimensions 210x52x100 mm3) with very poor mechanical properties of the joint and quite regular courses. Elastic and inelastic material properties are summarized in Table 1.


Table 1: Mechanical properties adopted for masonry and vaults infill

fm N/cm2 tttt0 N/cm2 E MPa G MPa w kN/m3

Masonry 240 ÷

400

6 ÷

9,2

1200 ÷

1800

400 ÷

600 18

Infill 74 3,7 689 296 16 Pushover analyses are conducted with an elastic perfectly plastic material obeying a Mohr-

Coulomb failure criterion. A friction angle equal to 20 degrees is assumed, again in agreement with the Italian code. The utilization of perfectly plastic materials is allowed by the Italian Guidelines when masonry is modeled with 2D and 3D finite elements, i.e. where the drop of the pushover curve for the reduction to a 1DOF system with the area equivalence is hardily reproducible. In this case, the Italian Guidelines recommend carrying on the analyses up to “meaningful displacement”, which reasonably may be associated to an activation of the fail-ure mechanism.

Loads applied when dealing with the pushover analysis are: masonry self-weight, wooden and masonry floors weight (dead and live loads) and seismic action incremented up to failure. The behavior of both structures is investigated on principal directions X and Y.

-a -b Figure 5: Virtual geometrical model of the clock tower (-a) and the castle (-b).

Period T=0.4194 sec Period T=0.4122 sec Period T=0.1680 sec

Figure 6: First six modal deformed shape softhetower

The Italian code prescribes the evaluation of the load carrying capacity by means of two configurations of horizontal forces: the first provides a distribution of forces derived by the assumption of a linear variation of acceleration along the height (Group 1, G1) while for these


conditions assumed a constant acceleration (Group 2, G2). As prescribed by the Instructions for NTC 2008 (Instructions 2009) the first distribution of forces of Group 1 and Group 2 can-beappliedindependentlyoftheparticipatingmassactivatedbythefirstmode. Authors experienced that G2 distributions systematically less safe for the cases analyzed, i.e. provides larger failure horizontal accelerations.

3 BUILDING GEOMETRY AND MODELLING

FE models have been obtained by means of standard 3D automeshing routine after the con-struction of detailed virtual geometrical models, see Figure 5.

Pre-rehabilitation Post-rehabilitation

1st mode, period T=0.398 sec 1st mode, period T=0.394 sec

2nd mode, period T=0.397 sec 2nd mode, period T=0.384 sec

3rd mode, period T=0.383 sec 3rd mode, period T=0.381 sec

Figure 7: First four modal deformed shapes of the castle before and after the rehabilitation intervention


On the FE models so obtained, a preliminary set of modal analyses are performed, with re-sults summarized in Figure 6 and Figure 7 respectively for the tower and the castle. The re-sulting response conventionally represents the behavior of the structure during the earthquake. The eigenvalue analysis is extremely useful in light of the pushover simulations to perform later. Indeed, to reduce the structure to a 1DOF system, the Italian code requires the knowledge of the fundamental mode normalized displacement vector. In particular, in Figure 6, the first three modal deformed shapes of the tower, corresponding to a non-negligible exci-tation of the mass are depicted. Modal deformed shapes are classic for such kind of structure (towers) and may be easily predicted with a cantilever beam approach.

Similarly, in Figure 7 the first three vibration modes of the castle are represented, before and after the rehabilitation of 2002. In this latter case, due to the complexity of the structure, a full 3D FEM approach is needed for a reliable evaluation of the frequencies, even if indica-tions by the Guidelines for the Cultural Heritage [5] on masonry buildings may be useful for rough estimations. As can be seen from deformed shapes of Figure 7, modes are quite global, whilst a certain concentration of deformation may be observed on the towers. Especially the second mode changes considerably in shape assuming improved mechanical properties for the long wall between the two towers of Figure 4.

-a

-b

Control point displacement Figure 8:Pushover curves obtained for distributions G1 and G2. –a: total base shear vs control node horizontal displacement. –b: acceleration ag/g vs control node horizontal displacement.

4 PUSHOVER AND NON-LINEAR DYNAMIC ANALYSES

Control point


Full 3D pushover analyses are conducted on the FEM models of the tower and the castle. Resultant capacity curves of the clock tower are depicted in Figure 8 for both G1 and G2 load distributions, for seismic action along X and Y. Subfigure –a refers to the total base shear vs control node horizontal displacement, whereas in subfigure –b the acceleration ag/g vs control node horizontal displacement is represented. Deformed shapes at peak (or col-lapse) provided by the commercial code are reported in Figure 9. As it is possible to notice, the tower fails for the formation of a flexural-shear hinge near the base of the structure, which appears roughly in agreement to what occurred in reality. The slip surface provided is rather clear and may be isolated manually identifying the elements with maximum plastic dissipation. The difference between behaviors in X and Y direction is obviously very little, considering the symmetry of the structure and the almost square section of the tower.

The safety assessment performed by means of the N2 method for both directions in the less conservative case (i.e. G1, G2 case is also indicated for the sake of completeness) is re-ported in Figure 10, where it can be noticed that the tower exhibits insufficient displacement capacity.

G1 X G1 Y

Figure 9:Clock tower, deformed shapes for G1 distribution.

Figure 10: clock tower, safety assessment by means of the N2 method.


-a -b

Control point displacement Control point displacement Figure 11: castle, capacity curves (-a) along X and Y directions (G1 and G2 distributions) and (-b) along X+ direction (G1) with improved mechanical properties of the central wall in Figure 2 (normalized against total shear at the base with c=0.1MPa)

Similar pushover simulations are performed for the castle (see Figure 11), first assuming

the same mechanical properties for the whole structure and then with improved mechanical properties for the central wall depicted in Figure 4.

A Mohr-Coulomb failure criterion with mechanical parameters consistent with the Italian code and with Table 1 is utilized within a perfect plasticity assumption and associated flow rule.

N2 method provides a positive safety assessment for the castle, Figure 12, in agreement with the global behavior of the structure under the real earthquake sequence occurred. The structure, indeed, did not collapse, albeit presenting considerable damage in the upper part of the towers, near the roof and in the internal arcade, see Figure 3.

acce

lera

tion

[g]

displacement Figure 12: castle, safety assessment by means of the N2 method, G1 distribution X+ direction.

5 NON LINEAR DYNAMIC ANALYSES

In order to capture all possible failure mechanisms, the effect of the earthquake is also considered throughout non linear dynamic analyses, i.e. by applying at the base of the build-ing a real accelerogram. The same 3D models used for pushover analyses in Strand7 [6] are implemented in ABAQUS [7], taking into consideration geometrical (large displacement ef-fects) and material non linearity (elasto-plastic with damaging behavior of the masonry ma-terial), utilizing the available Damage Plasticity Model in ABAQUS [7].

The full dynamic analyses are performed by using a real accelerogram registered not far


from the tower during the first seismic shake (20th May). Here it is worth noting that in the model both the vertical and horizontal accelerograms have been applied for the dynamic analyses. A vertical ground motion, indeed, could be potentially rather important, especially for the tower and generally when the structure is located near the epicentre of the earthquake.

In Figure 13, the tensile damage patch obtained for the clock tower is shown at two time steps (8 seconds, after the accelerations peaks and at 20 seconds, end of simulations) and from different points of view.

From an overall analysis of simulations results, it is possible to notice that the tower ex-hibits significant damage as a consequence of the intensity of the earthquake occurred. Com-paring the crack patterns at the end of the simulations, it appears rather clear that the tower collapses for the formation of a shear-flexural hinge in the lower region, immediately over the ground floor vault, as evidenced by the inclined path of the formed crack.

8 sec 20 sec 20 sec bottow view 20 sec transversal

section Figure 13: Tower. Damage map evolution.

Figure 14: Castle. Damage map after 20 seconds

When dealing with the castle, damage patch at the end of the simulations, see Figure 14,

shows severe damage of the upper part of all the towers and on the arcade, especially at the second floor levels.

In this latter case, results of the dynamic analyses seem in almost perfect agreement with what observed after the seismic event.

6 CONCLUSIONS

In the paper, some of the total amount of numerical results obtained by means of a com-prehensive multi-disciplinar activity on two monuments damaged by the May 2012 Emilia Romagna seismic event have been concisely presented. The two monuments are the clock


tower, completely collapsed, and the castle in Finale Emilia, one of the towns mainly stricken by the earthquake.

In particular pushover and full non linear dynamic analyses were performed with commer-cial codes on detailed truly 3D FE discretizations, obtained after a careful survey of the struc-tures, historical research and CAD modelling. Simulation results showed the insufficient performance of both structures, completely justifying the experienced state of damage. Espe-cially the output provided by non-linear dynamic analyses results particularly satisfactory from a structural point of view, because it was found to perfectly fit what happened in reality.

REFERENCES

[1] M. Acito, M. Bocciarelli, C, Chesi, G. Milani. Collapse of the clock tower in Finale Emilia after the May 2012 Emilia Romagna earthquake sequence: Numerical insight. Engineering Structures, 72, 70-91, 2014.

[2] NTC 2008. Nuove norme tecniche per le costruzioni. Ministero delle Infrastrutture (GU n.29 04/02/2008), Rome, Italy, 14/01/2008.

[3] Circolare 2009.Circolare n. 617 del 2 febbraio 2009 Istruzioni per l’Applicazione Nuo-ve Norme Tecniche Costruzioni di cui al Decreto Ministeriale 14 gennaio 2008.

[4] OPCM 3274 2003. Ordinanza del Presidente del Consiglio dei Ministri n.3274 20/03/2003: Elementi in materia di criteri generali per la classificazione sismica del ter-ritorio nazionale e di normative tecniche per le costruzioni in zona sismica e successivi aggiornamenti.

[5] Linee guida per la valutazione e la riduzione del rischio sismico del patrimonio culturale, Ministero per i beni e le attività culturali MiBAC, Italy 2011.

[6] STRAUS7®, 2004. Theoretical manual-theoretical background to the Strand7 finite element analysis system.

[7] ABAQUS®, 2006. Theory Manual, Version 6.6

[8] G. Milani, P.B. Lourenço, A. Tralli A. Homogenised limit analysis of masonry walls, Part I: failure surfaces. Computers and Structures, 84(3-4), 166-180, 2006.

[9] G. Milani, P.B. Lourenço. A simplified homogenized limit analysis model for randomly assembled blocks out-of-plane loaded. Computers & Structures, 88, 690–717, 2010.

[10] G. Milani, G. Venturini. Automatic fragility curve evaluation of masonry churches ac-counting for partial collapses by means of 3D FE homogenized limit analysis. Comput-ers & Structures, 89, 1628–1648, 2011.

[11] G. Milani. Lesson learned after the Emilia Romagna, Italy, 20-29 May 2012 earth-quakes: a limit analysis insight on three masonry churches. Engineering Failure Analy-sis, 34, 761-778, 2013.

[12] G. Milani, S. Casolo, A. Naliato, A. Tralli, Seismic Assessment of a Medieval Masonry Tower in Northern Italy by Limit, Nonlinear Static, and Full Dynamic Analyses, Inter-national Journal of Architectural Heritage, 6(5), 489-524, 2012.

[13] V. Mallardo, R. Malvezzi, E. Milani, G. Milani. Seismic vulnerability of historical ma-sonry buildings: a case study in Ferrara. Engineering Structures, 30(8), 2223-2241, 2008.

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