SEISMIC VULNERABILITY OF MERLONS IN ANCIENT FORTIFIED BUILDINGS

Erica LENTICCHIA1, Eva COÏSSON2, Daniele FERRETTI3

ABSTRACT

The paper investigates the seismic vulnerability of a typical element of the fortified architecture: the merlons. Indeed, the last seismic events in Italy have caused severe damage to a wide range of historical buildings, in some cases even for low values of peak ground acceleration. Among those, the crenellations typical of historical fortified buildings were particularly affected. Based on previous works, in which the seismic damage phenomena on the fortified building typology were collected and catalogued, the merlons proved to be particularly vulnerable elements. For this reason, the present work focuses specifically on the damage mechanisms suffered by these protruding elements, in order to understand their behavior in case of earthquake, to quantify their vulnerability and to provide instruments for their seismic protection. Different case studies referring to merlons, both on crenellated fortified and towers, were illustrated. Examples showed that free-standing merlons are mainly subjected to out-of-plane overturning; however if merlons support a roof, as often happens in which were subjected to a civil reuse in the course of time, also in plane shear failure can occur. Furthermore, some of the case studies analyzed presented seismic retrofitting interventions, whose behaviour under the seismic actions is described and discussed. Case studies showed that the collapse of merlons can occur even with low values of peak ground accelerations (PGA). Though the collapse of merlons is rather common, it received little attention in the literature. The present work focuses specifically on the out-of-plane overturning of merlons. The behavior of the merlons was studied by means of simple mechanical models. In particular, a simple linear elastic model was used to identify the activation of the mechanisms. The subsequent collapse by overturning was studied by means of a non-linear kinematic model. The seismic filtering effect, which is exerted by the supporting or tower, influences the behavior of the merlons. For this reason, proper filtering equations that modify the response spectrum at the ground were chosen. The proposed approach was used for a parametric analysis with merlons and supporting walls. Results permitted to plot curves that relate the slenderness of the merlons to the PGA that leads to their collapse. The proposed work thus defines a simple but reliable procedure than can be adopted by the practitioner for the seismic assessment of merlons and that could be used at a territorial scale in order to identify the most vulnerable cases and optimize the limited economic resources for the prevention of future damage on these elements, which might seem of minor importance but which are particularly meaningful both from the historical and aesthetical point of view.

Keywords: masonry buildings; fortified buildings; seismic vulnerability; merlons

1. INTRODUCTION

The last seismic events in Italy have shown the significant vulnerability of a typical element of the landscape and city centers skylines: the ancient fortified buildings. Fortified architecture is a generic

1Post-doc Research Fellow, Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Torino, Italy, [email protected] 2Associate Professor, Department of Engineering and Architecture, Università degli Studi di Parma, Parma, Italy, [email protected] 3Associate Professor, Department of Engineering and Architecture, Università degli Studi di Parma, Parma, Italy, [email protected]

term, which indicates a very wide variety of buildings, usually characterized by the presence of towers, defensive walls, and other characteristic elements. Frequently, among these features, the presence of a specific type of protruding element particularly characterizes this building typology: the merlons. The merlons are the solid standing part of a or crenellated , typical of medieval architectures or . They are an essential element for the and have been used for centuries, not only for defensive purpose, but also as decorations in order to highlight the social status or the alliance of the castellan. In fact, in more recent configurations, fortified buildings sometimes present false decorative battlements. Based on previous works (Cattari, et al., 2014) (Coïsson, Ferretti, & Lenticchia, 2017), in which the seismic damage phenomena on the fortified building typology were collected and catalogued, the merlons proved to be particularly vulnerable elements. For this reason, the present work focuses specifically on the damage mechanisms suffered by these protruding elements, in order to understand their behavior in case of earthquake, to quantify their vulnerability and to provide indications for interventions aimed at their seismic protection.

1.1 Role of merlons in fortified buildings

The crenellation of a or tower is a defensive technique that was employed since the Ancient Times, but only during the , when the process of castellation spread through the whole Europe, they became a distinctive element of the fortified buildings. In fact, with the spread of the process of castellation through the whole Europe, the crenellation soon became a symbol of , and starting from the Carolingian Empire it was necessary to obtain a license of crenellation in order to build a fortification. A license to crenellate was supposedly a grant that gave permission for a building to be fortified (Davis, 2007). It was employed as a way to control the construction of fortified buildings, and preventing local lords to become too powerful. In medieval England, the license to crenellate was also a tool to demonstrate the lords’ alliance to its monarch (Coulson, 2003).

(a) (b) Figure 1: (a) Merlons and crenels of a battlement (Viollet-le-Duc, 1854 ) (b) Ghibelline crenellation with a wooden panel (Nigra, 1937)

Though the shape and dimensions of the merlons, together with the width of the crenels (the space between two merlons) varied enormously, their purpose was the same: the crenel allowed defenders to shoot arrows and throw down missiles, while the merlons offered shelter from the assailants’ shootings (Figure 1). The geometrical features of merlons depend on the geographical location, the period of construction of the fortification and, most important, by the evolution of the weapons employed. In fact, if at first fortified buildings, and consequently the merlons, were built in wood, during the Middle Ages castles started to be built in stone masonry, and only during the Renaissance in brick masonry. These transformations were a consequence of warfare. During the Roman Empire, the walls of the Empire were mostly represented by its legions (Settia, 2017): were a temporary shelter for the Roman legions, and indeed, in ancient warfare, it was preferred to fight battles in open battlefield than to start 2

a . The siege warfare became a common feature of the Middle Ages, when fortified cities, fortresses and strongholds were necessary to maintain control on a certain area. Crenellations soon became a fundamental defensive element and merlons were designed as thin elements, more tall than wide. Usually, their wideness was sufficient to shelter a couple of men, and they could be provided with loopholes of various dimensions and shapes. This configuration persisted until the introduction of gunpowder artillery. Historians argue about the origins and the initial diffusion of gunpowder. The first appears in the European iconography in an English manuscript of 1326 (Milemete, 1326), represented as an urn-like object. In the fifteenth century, gunpowder artillery came to play a regular, but only occasionally decisive, part in fortress warfare (Duffy, 1979). However, in general the development of and gunpowder made revolutionary changes to siege warfare throughout Europe. If, at first, the artillery was employed in rare occasions, its employment in war became more frequent and, consequently, fortified buildings built in previous periods showed their vulnerability to this new type of action. In fact, when originally the primary strength in fortified wall constructions was due to their height, this then became its weakness since cannonballs could easily damage slender elements. With the introduction of gunpowder artillery, the configuration of defensive walls and merlons changed abruptly: walls became less regular in plan, grew in thickness, and lowered in height; similarly, merlons assumed a stocky shape and their thickness was increased. Merlons have been used until the artillery made them ineffective or even dangerous for defenders. However, they soon started to be employed as a decorative element. Fake merlons and battlements were largely used in the Neo-Gothic Style of the 19th century. Different decorations were used already in the Middle Ages, in order to indicate the alliance to a particular faction. A famous configuration was the one adopted for the Guelf and the Ghibelline factions: the former adopted a normal rectangular shape, while the latter adopted a merlon that ended in the upper part with a swallow-tailed form. In the Middle-East regions, instead, usually merlons were rounded, triangular or shaped with steps tapered at the end (Figure 2).

(a) (b) Figure 2: (a) various forms of merlons (b) the crenellation with rounded merlons of the Bam in Iran.

2. OBSERVED DAMAGE

In order to focus the modeling activities on the most significant aspects of the seismic effects on merlons, a thorough analysis of the damage observed in several Italian earthquakes was carried out. It is not easy to define general rules because each earthquake has its own specific characteristics in terms of frequency content. Furthermore, the fortifications in each area present typical features that depend on: the availability of materials, the soil morphology, the period of castellation, and on the local architectural styles and technical skills (Balestracci, 1989) (Lepage, 2010). Nevertheless, the analysis carried out in (Coisson, Ferretti, & Lenticchia, 2017) on 73 damaged castles pointed out that the observed damage mechanisms of merlons can be summarized in two typologies: the in plane shear failure (mechanism 4a, according to the definition in Coisson et al (2017)) and the out-of-plane overturning (mechanism 4b). These damage mechanisms are reported in Figure 3. The overturning mechanism is more frequent in case of freestanding merlons.

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Many examples can be found for this type of damage: - In the 2016 Central Italy earthquakes: the Castles of Arquata del Tronto, in Figure 3, and Tolentino, both made in brick masonry; - In the 2012 Emilia earthquake: the Castles of Galeazza, Giovannina (San Giovanni in Persiceto) (Figure 3), Pio (Carpi), the fortresses of Finale Emilia and Reggiolo, the Gonzaga Ducal palace in Revere, all in brick masonry; - In the 1980 Irpinia earthquake: the of Monte, in Montella, made in stone masonry; - In 10 out of 14 stone masonry castles hit by the Friuli earthquake and analyzed in (Coisson, Ferretti, & Lenticchia, 2017).

In the castle of Zoppola, made in brick masonry and hit by the Friuli 1976 earthquake, there was overturning of the merlons even if the roof structure rested on them. Unfortunately, the collected information are not sufficiently detailed to explain this behavior. The same behaviour was observed in the fortress of San Felice sul Panaro, hit by the 2012 Emilia earthquake, where some merlons of a walkway over a defensive wall overturned and collapsed although they were covered with a roof (Figure 4). In this occasion, the merlons behaved similarly to the free-standing cases, because of the scant weight of the roof and the inadequate connections between the roof timber beams and the top of the merlons.

When the merlons support a roof, as often happens in castles which were subjected to a civil reuse in the course of time, also in plane shear failure can occur. Examples from the 2012 Emilia earthquake can be found in the Castle of Palata Pepoli (Crevalcore), in the Prigione Tower of Gonzaga, in the fortresses of Finale Emilia, and San Felice sul Panaro (Figure 4b). In the Friuli 1976 earthquake, only the castle of Zoppola (one among the few in brick masonry in this area) showed shear damage. This allows to hypothesize a higher vulnerability to this type of mechanism for the slender brick masonry merlons, although probably some of the numerous cases of overturned merlons previously cited in stone masonry castles could have suffered shear damages before collapsing completely.

Figure 3: Damage mechanisms of merlons: (Mechanism 4a) in plane flexural or shear cracks. (Mechanism 4b) overturning of merlons (Coisson, Ferretti, & Lenticchia, 2017).

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It is also interesting to analyze some of the case studies that presented seismic retrofitting interventions (such as the insertion of rebars, or concrete ring beams on the over standing roofs), because studying their behavior in the occasion of earthquakes allows the comparison of the effectiveness of the different strengthening techniques. In several cases, in facts, concrete ring beams have been introduced in the past on the over-standing roofs, which could be either original or added later on. Despite this type of intervention was aimed at preventing seismic damage, some case studies showed an opposite behavior. The most significant case is the fortress of San Felice sul Panaro: here all the towers that had a concrete ring beam collapsed (Figure 4a), while the only tower that survived, the donjon, had a roof structure entirely in timber. This outcome suggests that the weight that increased the seismic action and the large difference in stiffness between concrete and brick masonry can easily induce stress concentrations on the merlons causing their severe damage and collapse. In other cases, steel rebars were inserted in the merlons in order to improve their connection with the roof and to contrast shear and overturning actions. Once again, the fortress of San Felice sul Panaro shows, in the only survived tower, severe shear damages to the merlons: inside them, through the large cracks, crossed steel bars could then be seen (Figure 5b). In conclusion, the observation of the real damage shows that some interventions, which were intended to limit the seismic damage, actually caused even more damage than the one registered in the unreinforced cases.

The analyzed case studies showed that damage and collapse of merlons could occur even with low values of peak ground accelerations (PGA), starting from about 0.05g, particularly for the overturning mechanisms. Moreover, the overturning collapse is the most dangerous for public safety and has the strongest impact on the conservation of these important cultural assets and on their characteristic skyline. For this reason, the present work focuses specifically on the out-of-plane overturning mechanism of merlons. For this damage mechanism, a simple mechanical model for freestanding merlon is proposed.

Figure 4: (a) The San Felice sul Panaro Fortress after the 2012 earthquake. The only tower survived is the donjon, without concrete ring beam on top (b) Detail of out-of-plane overturning of a merlon

3. MECHANICAL BEHAVIOR OF MERLONS

From the structural point of view, a freestanding merlon behaves like a rocking wall, whose behavior has been deeply studied in the literature; a comprehensive review can be found for instance in Sorrentino et al. (2017) and Lagomarsino (2015). In particular, before cracking the merlon can be modelled as a linear elastic cantilever. After the onset of a crack at the base, it behaves like a rigid block displaying rocking. In the present work, the rocking behavior of merlons was studied by means of the procedure for non- linear kinematic analysis proposed in the Italian Code for Structural Design (2008).

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Figure 5: Shear cracks in the San Felice sul Panaro merlon reveal the crossed steel rebars previously inserted (red arrow).

For the seismic input, it is necessary to consider the filtering effect exerted by the underlying structure (wall or tower) that modifies the response spectrum at ground level. For this purpose, the equations proposed in a draft of the Italian Code Standards were used. Figure 6 shows a sketch of the two response spectra (at the ground and at level z), together with the geometry of the wall and of the merlons being investigated.

Figure 6: Geometry of merlons with underlying defensive wall, and corresponding ground and floor spectra.

3.1 Non-linear kinematic analysis

Merlons have been modelled as rigid blocks rotating with respect to an edge at the base. The behavior of the mechanism is represented by a capacity curve (Figure 7) that relates the seismic acceleration a to the displacement d of a reference point chosen as the center of mass of the merlon. The activation of the mechanism, assumed to be infinitely rigid, occurs for an acceleration t a = m g 0 h m (1) where g is the gravity on Earth and tm and hm are respectively the thickness and the height of the merlon (Figure 6). The acceleration becomes null if the displacement d0=tm/2. 6

In case of free-standing elements like merlons, it is necessary to consider that before the activation of the mechanism, their dynamic response is substantially elastic; therefore it is necessary to introduce a first elastic branch in the capacity curve. This branch links the acceleration a to the displacement d through the period T0: 4 2 π a = 2 d T0 (2) which can be estimated with reference to the solution of a beam with distributed mass: w T0 = κλL Eg (3) where: κ is a coefficient equal to 6.2 for cantilever elements

L is the length of the element (=hm);

λ is the slenderness of the element (=hm/tm , i.e. length to thickness ratio); w’ is the specific weight of the masonry; E is the elastic modulus of masonry. The linear branch defines the capacity curve up to the intersection with the capacity curve of the local mechanism. Therefore, the resulting capacity curve is bilinear, as shown in Figure 7.

The intersection point (ay, dy) corresponds to the limit state of activation of the mechanism. The limit state of collapse LSC is assumed for a displacement dLSC = 0.6 d0 , which seems capable to avoid the dynamic instabilities that occur close to d0.

G d

Figure 7: Overturning mechanism of the merlon and corresponding capacity curve.

3.2 Defining response spectra at different levels of the construction

To perform the seismic assessment of merlons, a correct seismic input must be considered; the motion at the base of merlons is related to the dynamic response of the building and to their position. Usually, merlons are placed at the top of defensive walls or towers. The dynamic behavior of towers and dungeons is similar to the one of belfries (Bassoli et al. 2018), which were studied for example in Curti (2007). Based on the work of Burdisso and Singh (1987a,b), the authors used a “floor response spectrum approach” to define a “floor spectrum” for belfries. Similar expressions have been proposed in a draft of the forthcoming Commentary of the Italian Code Standard, which have been used in the present work.

The adopted simplified analytical formula to assess the acceleration spectrum SeZ(T,ξ) at a given level z is the following:

2 SeZ = ∑SeZ,k (T,ξ, z) (≥ Se (T,ξ) for T > T1) (4) where:

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⎧ −0.5 1.1 ( ) a (z) ⎪ ξk η ξ Z,k T < a Tk ⎪ ⎛ ⎞1.6 ⎡ −0.5 ⎤ T ⎪ 1+⎣1.1 ξk η(ξ) −1⎦⎜1− ⎟ ⎪ ⎝ aTk ⎠ ⎪ −0.5 SeZ,k (T,ξ, z) = ⎨ 1.1 ξk η(ξ) aZ,k (z) a Tk ≤ T ≤ b Tk ⎪ −0.5 (5) ⎪ 1.1 ξk η(ξ) aZ,k (z) T > b T ⎪ 1.2 k −0.5 ⎛ T ⎞ ⎪ ⎡ ⎤ 1+ 1.1ξk η(ξ) −1 ⎜ −1⎟ ⎣ ⎦ bT ⎩⎪ ⎝ k ⎠

2 aZ,k (z) = Se (Tk,ξk ) γ kψk (z) 1+ 0.0004ξk (6) and:

Se(T, ξ) is the elastic response spectrum at the ground level, for the period T and the viscous damping ratio ξ of the mechanism; SeZ,k is the contribution to the floor response spectrum given by the k-th mode of the main structure, with a period Tk and a viscous damping ξk (in percentage); a and b are coefficients defining the range of maximum amplification of the floor spectrum, which have been considered respectively 0.8 and 1.1;

γk is the k-th modal participation factor of the main structure; ψk(z) is the value of the k-th modal shape of the main structure at the level z=hw where the considered local mechanism is placed; η is the damping correction factor that modifies the elastic spectrum for a damping coefficient ξ that is different from 5%; aZ,k is the contribution of the k-th mode to the peak floor acceleration. The adopted formula is based on the dynamic properties of the main structure and on the values of the response spectrum at the base of the building, calculated in correspondence of the significant natural periods of the construction. To compute these natural periods, the defensive walls and the towers were both modelled as linear elastic Euler-Bernoulli cantilevers with distributed mass. The main dynamic properties were computed according to Thompson (1981). In particular, the period Tk is 2π Tk = (7) ωk where

2 EI ωk = βk ρ (8) and 3 I = bwtw /12 is the moment of inertia of the section,

ρ = w twbw ,

βk is a coefficient reported in Table 1.

The k-th participation factor γk and the modal shape ψk(z), normalized with respect to the mass, are c L γ k = k ρ (9) 1 ⎡ ⎤ ⎡ ⎤ ψk (z) = {⎣cosh(βk z) − cos(βk z)⎦− dk ⎣sinh(βk z) −sin(βk z)⎦} ρL (10) where: L=hw and ck, dk are coefficients reported in Table 1.

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Table 1: Coefficients for the dynamic analysis of a cantilever

k β k L ck dk

1 1.87510 0.7830 0.73410

2 4.69409 0.4339 1.01847

3 7.85476 0.2544 0.99922

4 10.99554 0.1818 1.00003

The floor spectra are highly influenced by the degree of non-linearity of the main structure. Indeed, they show a strong amplification in correspondence of the first period of the elastic structure. This amplification reduces significantly when the structure enters the non-linear field. The adopted formulation considers this effect using the equivalent viscous damping ξk and a proper value of the Young modulus E. The spectrum at level z should therefore be calculated with parameters (i.e. Young modulus, equivalent viscous damping) compatible with the damage level displayed by the main structure before the earthquake and modified as a consequence of the action corresponding to the formation of the local mechanism.

3.2 Seismic assessment

For the seismic assessment, it is necessary to distinguish between the activation of the mechanism (LSA) and the collapse (LSC).

For the limit state of activation of the mechanism, the capacity is represented by the acceleration ay in the capacity curve (Figure 7). The demand, in terms of acceleration, is computed considering the filtering effect of the underlying structure (Eq. 4), and the first period T0 of the linear elastic cantilever (Eq. 2). For the collapse, the assessment is performed in terms of displacements, i.e. the displacement demand dE must be compared with the displacement capacity of the mechanism dLSC. Initially, it is necessary to evaluate the characteristic equivalent period of the limit state TLSC, starting from the capacity curve a(d):

dLSC TLSC =1.56π (11) a(dLSC ) Then, for local mechanisms at a certain level z of the building, it is necessary to refer to the 2 2 acceleration spectrum SeZ (TLSC, ξ, z), transformed in displacement spectrum multiplying by T /4π . The displacement demand must be assessed on a displacement spectrum that does not decrease with the period T. Therefore, it would be necessary to compute the maximum displacement attained in the interval [T0, TLSC]. Alternatively, it is possible to refer to the following equation proposed in a draft of the Commentary of the Italian Code Standard: 2 ⎛ 2 2 ⎞ TLSC b T1 dE = SeZ (TLSC,ξ, z) ⎜≥ SeZ (T1,ξ, z) for TLSC > bT1 ⎟ (12) 4π 2 ⎝ 4π 2 ⎠

When the displacement demand for the collapse limit state dLSC is computed, it is important to consider the effect of damping, both on the floor spectrum (non-linearity of the main structure) and on the displacement demand (non-linearity of the local mechanism). Given that no specific evaluations

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have been made in this case, the equivalent viscous damping ξ of the local mechanism has been considered equal to 10% according to (Lagomarsino, 2015), even if more precise evaluations could be adopted (Degli Abbati & Lagomarsino, 2017). Moreover, the damping coefficients ξk should be assessed considering the level of non-linearity reached by the main structure in correspondence of the ag,LSC. Merlons collapse for low values of acceleration, therefore ξk = 5% was chosen. The described procedure for the seismic assessment of merlons is rather tortuous. For this reason, it has been summarized in the algorithm reported in Figure 8.

1. Define the elastic response spectrum at the ground level Se(T, ξ) 2. Define the dynamic behavior of the supporting wall

2.1. Choose a suitable value for damping ξk 2.2. Compute the period Tk , k=1…4 (Eq. 7) 2.3. Compute the participation factor γk , k=1…4 (Eq. 9) 2.4. Compute the modal shape ψk(z), k=1…4 (Eq. 10) 3. Compute the acceleration spectrum SeZ(T,ξ) at the top of the wall (Eqs. 4,5) as a function of its dynamic behavior 4. Define the capacity curve of the merlon in terms of displacement – acceleration d-a (Eqs. 1-3) 4.1. Point of activation of the mechanism (dy, ay) 4.2. Point of Limit State of Collapse (dLSC, aLSC) 5. Perform the seismic assessment 5.1. Evaluate the characteristic equivalent period of the merlon for the limit state TLSC (Eq. 11) 5.2. Compute the displacement demand at the top of the wall dE (Eq. 12) for the period TLSC 5.3. If dLSC > dE merlon is able to withstand the seismic action

Figure 8: Proposed algorithm for the seismic assessment of merlons.

4. PARAMETRIC ANALYSIS

In the present work, merlons on defensive walls were studied, according to the geometry shown in Figure 6. Both the merlon and wall were considered in solid clay brick masonry with lime mortar, with Young modulus E = 2800 MPa and density w = 1800 kg/m3. Two conditions were investigated for the supporting wall: - slender wall: thickness tw = 0.6 m, height hw = 10 m and base bw = 10 m, - squat wall: thickness tw = 2 m, height hw = 6 m and base bw = 10 m. The acceleration spectrum at ground level Se(T,ξ) was defined according to the Italian Code for Structural Design (2008). The spectrum and the corresponding parameters are reported in Figure 9.

3.0

ground 2.5 slender wall 2.0 squat wall (-)

/g 1.5 ag =0.154 g z

S Tcs = 0.27 s 1.0 St = 1 F0 = 2.59 Soil C 0.5

0.0 0 1 2 3 4 5 T (s)

Figure 9. Adopted ground and floor spectra.

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The same figure shows the floor spectra for the two cases of slender and squat walls. The procedure described in the previous section (Fig. 8) was implemented in a Matlab function in order to compute the peak ground acceleration ag corresponding to the limit states of activation and collapse of a merlon of given geometry. Also for the merlons, two limit conditions were considered: - thin merlon: base bm = 1 m and thickness tm = 0.4 m - thick merlon: base bm = 1 m and thickness and tm=1 m. The height hm varied between 0.3 m and 3.0 m. The adopted geometries seem to cover the most recurrent cases. Results of the analysis are reported in Figure 10. The graph represents the peak ground acceleration ag as a function of the height hm for the limit state of collapse (LSC). As could be easily expected, the accelerations decrease as the slenderness of the merlons increase in all cases, but different thresholds can be observed for the different thicknesses considered for the merlons and for the different slenderness of the supporting walls. The worst case occurs for thin merlons on slender walls, when an acceleration as little as 0.05 g causes collapse, in line with the field observations on damaged castles. Furthermore, in case of slender walls (Fig. 10a) the curves present a plateau where results are nearly independent on the height of the merlon. This plateau disappears in case of a squat wall (Fig. 10b) Thick merlons on squat walls are stable and require considerable values of PGA to collapse. The proposed approach thus allows to create graphs for the straightforward assessment of the seismic stability of merlons, given only the geometrical features of the elements. This could be particularly useful for territorial scale analyses, aimed at the identification of the criticalities and at the definition of priorities for the interventions to prevent seismic damage (Coïsson & Ottoni 2012).

1.0 1.0 slender thin (t = 0.4 m) m squat thin (tm = 0.4 m) wall 0.8 0.8 wall thick (tm = 1.0 m) thick (tm = 1.0 m)

0.6 0.6 (-) (-) /g /g g g a 0.4 a 0.4

0.2 0.2

0.0 0.0 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3

hm (m) h (m) (a) (b) m

Figure 10. Parametric analysis for limit state of collapse (LSC) of merlons on a: (a) slender masonry wall; (b) squat masonry wall.

5. CONCLUSIONS

In the present paper, a specific analysis of the overturning damage and collapse mechanisms affecting merlons in the ancient fortified architectures is presented. The parametric analysis takes into consideration the filtering effect produced by walls of different geometries and describes the behavior of the merlons by means of graphs representing the peak ground acceleration leading to the significant limit of collapse of the mechanism, in relation to the slenderness of the merlons. The results show that merlons on a slender wall are particularly vulnerable, regardless of their slenderness. Thin merlons collapse for a pga as low as 0.05 g whereas thick merlons collapse starting from 0.13 g. On the contrary, merlons on squat walls are much more stable: even in case of low thickness, merlons can stand nearly 0.2 g.

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The present paper is limited to the case of merlons placed on defensive walls, but the approach can be easily extended to the case of merlons on towers and it can take into consideration different soil types or different damping coefficients for the supporting structures, depending on the pre-existent level of damage. The proposed work thus permits to define a simple but reliable procedure than can be adopted by the practitioner for the seismic assessment of merlons and that could be used at a territorial scale in order to identify, only based on geometrical data, the most vulnerable cases and optimize the limited economic resources for the prevention of future damage on these elements, which might seem of minor importance but which are particularly meaningful both from the historical and aesthetical point of view.

6. REFERENCES

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