Recovery of stone architectural heritage

9th International Masonry Conference 2014 in Guimarães

Recovery of stone architectural heritage ZERBINATTI, MARCO1; BIANCO, ISABELLA2; FASANA, SARA3; NELVA, RICCARDO4 ABSTRACT: This paper shows some of the main results of the Interreg-Alpstone research project, which focuses on Alpine architecture located between Italy (the Ossola valley and surrounding areas) and Switzerland (the Canton of Ticino). The rich local heritage of stone buildings is often in a state of decline and needs to be renovated. The ongoing study aims to give practical guidelines to professionals and craftsmen in order to give new life to these constructions. This paper describes the methodology of the Interreg-Alpstone research team, some in situ tests and possible sustainable solutions. In particular, it deals with the need of strengthening the original structures (stone walls and roofs, wooden floors) according to current regulations while at the same time preserving their historical and architectonic values. Keywords: Stone buildings, stone masonry, recovery, Ossola valley

NOTATION σv; σ = vertical compressive stress; compressive or tensile stress [Pa] τ = shear stress [Pa] T = horizontal force of traction [N] μ = coefficient of static friction [-] f = ratio T / σv A, f < μ [-] A = area [m2] t = thickness (of the wall) [m] r = radius [m] γ = specific weight (of stone masonry) [N/m3] h = height [m]

1 INTRODUCTION The protection and enhancement of the Alpine vernacular architecture is an issue of shared importance, which involves various players, including local administrations, building owners and conservation experts. Rural architecture is indeed often appreciated for the overall beauty it adds to the surrounding environment. This paper focuses on the Alpine architecture located between Italy (the Ossola valley) and Switzerland (Canton of Ticino). A rich heritage of buildings and infrastructures (e.g. roads, paths, dry walls, bridges) strongly characterises these valleys and reflects the adaptation of past generations to the climatic conditions, the mountain morphology and the available natural resources. There is indeed an intimate relation between the constructions and the surrounding landscape. The area is well known

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Assistant Professor, Politecnico di Torino, Department of Structural, Geotechnical and Building Engineering (DISEG), [email protected] 2) Research Fellowship, Politecnico di Torino, DISEG, [email protected] 3) Post PHD Research Fellowship, Politecnico di Torino, DISEG, [email protected] 4) Full professor, Politecnico di Torino, DISEG, [email protected] th

9 International Masonry Conference, Guimarães 2014

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Zerbinatti Marco, Bianco Isabella, Fasana Sara, Nelva Riccardo

for its quarries of different types of Gneiss (Serizzo and Beola), but less for granite and marble quarrying. These activities represent the main local economic resource. As a consequence, constructions are almost entirely made of ashlars and stone slabs, except for the Walser blockbau buildings, where a large use of wood is also made. The study presented in this paper focuses on stone constructions. These vernacular buildings are characterised by simple geometries (Figure 1a) (almost all have rectangular layouts with some out-bodies), thick stone walls, windows with low height/width ratio and roofs with great values of slope (Figure 1b). They are typically used as stables and barns, often detached from residential buildings/houses. During the decades which followed the Second World War, these mountain areas suffered a continuous depopulation process, which led to an architectural heritage decline and to a widespread loss of the knowledge handed down through the generations. The Interreg-Alpstone international project5 currently studies how to protect these rural buildings and give them a new life. In order to be turned into residences or into tourism accommodation, these buildings have to be adapted to the current needs and regulations, without impairing their architectonic value. In particular, they will have to be modified in order to guarantee a good static and dynamic behaviour. In order to achieve an optimal restoration, a study of the structural element interactions can't be avoided. To this end, the Interreg-Alpstone project will investigate the mechanical properties of their construction materials: the local stone i.e. Gneiss Serizzo and the wood employed as loadbearing components such as floor and roof structures. The research team is currently developing tests and technical solutions for improving the structural behaviour of stone walls (Figure 1c), modillions, roofs and floors. However, other interconnected aspects have been considered for a building's effective recovery, such as hygrothermal behaviour and space management. The section that follow describes the state of the art of the stone building rehabilitation and some solutions developed by the Interreg-Alpstone project. The focus is on technical solutions for a good recovery of Alpine stone buildings. Laboratory and in situ tests are described and considerations are made in regards to the different elements that contribute to the behaviour of buildings. Finally the paper suggests architectural solutions that take into account construction element characteristics and current needs and regulations. Structural and seismic aspects have been investigated by A. Grazzini and E. Quagliarini and early results are also presented in these conference proceedings.

Figure 1. Traditional buildings located in Viganella (a), Bei (municipality of Bognanco) (b) and detail of a stone wall (c) in Veglio (municipality of Montecrestese)

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The “Enhancement of traditional architecture, protection of anthropic environment and of constructions” research project is currently in progress and it is a 2007-2013 interreg cooperation project between Italy and Switzerland. The project (ID 27462783) is headed, for Italy, by the Verbano-Cusio-Ossola Province, and has as partners the C.S.L.- Centro Servizi Lapideo of Verbano-Cusio-Ossola, Gal laghi e Monti, Politecnico di Torino (ISEG Department), Comune di Santa Maria Maggiore, Comune di Beura Cardezza; the Swiss partnership is headed by the Ente Regionale per lo Sviluppo del Locarnese e Vallemaggia, as partners GLAti (Gruppo di Lavoro Artigianato Ticino).

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9th International Masonry Conference,Guimarães 2014

Recovery of stone architectural heritage

2 STONE MASONRY STUDIES AND PRACTICES: THE STATE OF THE ART For centuries stone, dry-ashlar and rubble stone have been the predominant construction materials in Europe and especially in Italy as well as brick. Since the Ist century BC, many authors, wrote on stone masonry, including it in their treatises: starting from Vitruvius, in the De Architectura libri decem, through Villard de Honnecourt, in the 13th century, Alberti in the 15th, Palladio and Scamozzi, in the 16th century, just to mention the best known. Nevertheless, classical treatises generally deal with squared stone masonry. In the 19th century, every building construction manual had specific chapters dealing with squared stone masonry. They were not just descriptive as in the past, but they explained masonry with a scientific approach. Until the beginning of the 20th century, many manuals [1 - 3] systematised the building techniques' knowledge in regards to stone construction, but this was no longer considered as the use of reinforced concrete gradually started to spread. As a consequence, the practical mastery that was handed down through generations by direct experience, quickly disappeared. In recent decades, rural buildings have been universally recognised as a cultural heritage [4, 5]. The public interest in the preservation of these buildings has encouraged the resurgence of the ancient building culture. Therefore, many local and international associations (SPS, Nottingamshire County Council, e.g.) have begun to study specific stone architectures, which are locally widespread. Their goals are mainly the reuse of historical buildings, the rediscovery and innovation of construction techniques and the compliance with current requirements. In the area investigated by the InterregAlpstone project, companies and single experts have studied the local buildings and a large body of specific literature6 has been produced during the recent decades. This mainly consists of historic texts [6, 7], typological anthologies [8, 9] or technical handbooks drawn up for local bureaux [10, 11]. These books generally focus on recognising typical elements of the building and on formal image features, but they rarely give any technical or structural information. At the same time, some authors [12] point out that recent regulations concerning building calculation are generally related to common brick and concrete block building. The term “masonry” covers many kinds of wall, which can differ both in terms of materials (the stones for historic masonries were often quarried locally) and in terms of building techniques (due to stone varieties and historic period of construction). The kind of masonry is also influenced by the geographical position, building's use and the economic environment [13]. As a consequence, recent international standards and guidelines universally acknowledge [14, 15] that structural evaluations regarding the historical masonry technique cannot be drafted ignoring a correct definition of the local masonry and the related “craftsmanship”. This has to be considered especially if seismic rehabilitation of existing buildings has to be carried out. Therefore, specific studies on local stone masonry [16, 15] are needed in order to support the laboratory tests [17] such as the ones required for defining the shear and compressive strengths [12, 18, 19]. In this regard, the latest Italian standard on constructions [20, 21] provides the guidelines for masonry investigation. It lists the various knowledge levels and provides tables with some general mechanical parameters, which have to be used in case of poor knowledge level, implying a limited in situ investigation. The data collection is anyway an important phase because the wall's look may often hide the real interlocking between long stones (diatoni) and/or leaves [16]. Another subject of the present paper is the investigation and classification of traditional construction elements of the Ossola valley architecture (dry stone rubble masonry, but also stone roofs and modillions). This study aims to regain the historical practical knowledge, not yet analysed in theoretical works. Firstly, the main construction elements have been classified according to the materials and building techniques, as described in sections that follow. Secondly, evaluation tests

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The extensive specific bibliography collected by the Interreg research group is composed of more than one hundred titles, generally published in the last two decades. The present paper only reports the more relevant and pertaining, with regard to the current critical approach to architectural heritage recovery and preservation. The whole bibliography will be available at the web page www.alpstone.eu. th

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were carried out in order to assess the masonry's behaviour in relation to the structural improvements suggested by the current guidelines or regulations [15, 22].

3 METHODOLOGICAL APPROACH 3.1. Classification of typical stone masonry The preservation and recovery of this specific rural architecture requires appropriate building techniques. Some examples include: stonework consolidation (mostly dry); fitting in additional openings (to meet regulatory requirements); joint fitting in horizontal structures (in case of missing floors); restoration of parts of masonry which have collapsed or are impaired; routine maintenance. The research team is developing systematic technical solutions and practical guidelines in order to promote these buildings' recovery and to find new uses for them. For this reason, the most common kinds of masonry located in the study area had to be classified. Some classification criteria has been defined according to the stonework's main structural features. Since historical stone masonry consists of two leaves, these features had to be analysed on both sides of the wall. The criteria for masonry classification (related to the different building techniques) are based on the characteristics that follow: 1) Stone-laying surfaces horizontality, that may be strictly, partially or not respected; 2) Staggering of vertical joints; 3) Structural element shape and size. Some examples are: square worked ashlars, generally regular in shape and size; semi-finished ashlars; unrefined stones, freestones, small stone elements used without any additional processing that may change their size or geometry; 4) Transverse interlock with the presence of diatoni (stones of great dimensions that cross the wall’s section entirely or for 2/3 of the thickness). As a consequence, the investigated masonry has been broken down into the categories that follow: A) Masonry with square worked ashlars (Figure 2a). They are characterised by horizontal laying surfaces which are well defined and regular, have staggered vertical joints, staggered ashlars in the corners and the presence of partial diatoni; B) Masonry with semi-finished ashlars from wedged stones (coming from quarries or boulders) with the forms that follow:

Figure 2. Examples of stone masonry in Montecrestese (a, b, c, d respectively) corresponding to types named as A, B.1, B.2 and B.3 in the classification presented

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Figure 3. Examples of stone masonry in Viganella (a), Montecrestese (b) and Calasca-Castiglione (c), corresponding to types named as B.3, C and D in the classification presented B.1 - Masonry marked by the presence of large carefully finished cornerstones and by a stone wall texture between the cornerstones made of smaller split elements. The horizontal laying surfaces are usually well detectable, as a consequence of the building technique (Figure 2b); B.2 - Masonry with well identified laying surfaces but without any cornerstone, made of large and homogeneous blocks (length 0.50 up to 0.60 m, height 0.20 up to 0.30 m, thickness 0.20 up to 0.30 m) (Figure 2c); B.3 - Masonry with split ashlars having an irregular size and shape, with horizontal laying surfaces, which are easily recognisable, although not always very regular (Figure 3a). In these cases, building corners are carried out with transverse overlapping stones and not with cornerstones. Walls are less refined than in case B.1, although they are still built with the technique of simultaneously building both the inner and outer faces (Figure 2d); C) Nineteenth-century masonry, built with semi-finished cornerstone blocks, which are more regular at the corners rather than near the openings. The masonry is quite heterogeneous and it is made with smaller stones compared with the previous categories, even using flakes and materials coming from the processing of larger stones. These walls show horizontal regularisation lines close to the openings (useful for inserting lintels for doors and windows) and floors (useful for making of regular supports for the internal horizontal structures. Figure 3b). D) Masonry made with split blocks or small boulders, having heterogeneous dimensions; stones are not processed, but used as they are extracted. Masonry does not usually have clear horizontal laying planes and larger stones at the corners. It is the less refined masonry (Figure 3c). This classification is strictly connected with the technical solutions under investigation. For this reason, the Interreg-Alpstone team is currently analysing experimental and analytical aspects in order to define stone wall behavioural models. These analysis are in part described in the sections that follow and are in part ongoing (as a consequence, it is not possible to give numerical results at the moment).

3.2. Typical stone roofs and floors of the Ossola valley Traditional roofs located in the Ossola valleys (IT) and in the nearby Ticino (CH) are strictly related to the historical use of local materials. These characterise the look of the different types of roofs, which can be broken down into three categories, corresponding to three main areas, as in the following points: th


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Figure 4. Typical stone roof features: wooden structure in Crevoladossola (a), wind bracing (b), truss top in Crevoladossola (c), pioda tiles on wooden battens in Montecrestese (d) - the area of the beola roofs, made of a stone variety known as schistose serizzo (gneiss), corresponding to the Ossola valley and its surrounding valleys; - the area of the roofs with thin stone slabs or with wood shingles, corresponding to upper areas of the Anzasca, Divedro, Formazza and Strona valleys, generally called the Walser area - the area of roofs with thin stone slabs, brick tiles or, less commonly, thatched roofs, corresponding to the area near the lakes (Maggiore and Orta lakes) This study focuses mainly on the features of the traditional Ossola valleys’ stone roofs, which are made of schistose gneiss slabs, locally known as beolas (lithotype) or piodas (single piece). As they are made of gneiss stone, their mineral composition consists mainly of feldspate and quarz. In this case, it is a fine granitic gneiss (from the Mt. Rosa Plateau), sometimes less commonly amphibolite (Orsalina Moncucco Area), with a mylonitic structure, with very clear planes and fine augen texture (augen-gneiss). The stone slabs obtained by traditional workmanship are about 0.04 up to 0.10 m thick, while there is a certain range of shapes. These roofs have some typical features, as listed below: a. stone slabs with a significant thickness (0.07 up to 0.10 m); b. steep slope (generally corresponding to 88-90%) required by the great overlap of the piodas. These generally have different lengths and they lie on larch battens, on which they are alternatively in contact. The slabs are placed with a low slope because their high thickness and hardness unable the fixing with nails or pegs. For this reason, their shifting out of place has to be prevented by gravity. As a consequence, these roofs are generally very tall and the building's transversal dimension to the ridge line is generally quite limited, i.e. within the 6.00 – 8.00 meters range (Figure 4d); c. very interesting wooden structure, quite unique in the Alpine area, consisting in simple triangular trusses spaced about 1.00 up to 1.20 m apart, laying on sleepers supported by the longitudinal walls. The trusses are often visible even from the outside when the tympanums are open (Figure 1b). Otherwise, the trusses are hidden behind the tympanum, closed with stone masonry (in this case they are independent from each other). Typical trusses are made of only three elements: one horizontal tie beam and two struts, stuck on the tie beam and slanting; their length is equal to 2/3 of the tie beam (from which roofing surface slope is equal to 88%, α = 41° 24'). The top strut joint at the top is a half lap bridle joint, with a laburnum dowel (Figure 4c). The trusses support the battens and lie parallel to the eaves, spaced with a pitch of about 2-3 piodas and fixed with a laburnum dowel (cavich). During the construction phase, some wind

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bracings are arranged between the trusses. The bracing orientation allows them to apply their forces in opposing directions (Figure 4b), and this is also related to the half lap bridle joint direction. The floors are generally made of parallel wooden beams (mainly larch); more rarely the buildings have stone vaults, which were used for the cellars, in the building's lower level. Wooden boards are generally placed on the beams. These can support a flooring made of limestone conglomerate, sand and gravel. Damp walls may have beams supported by stone corbels.

4 RECOVERY PRINCIPLES AND TECHNIQUES FOR HISTORICAL STONE BUILDINGS 4.1. Basic principles for the preservation and buildings reuse The Alpine architecture’s cultural and documentary value requires a careful consideration of the recovery criteria. The importance of these values leads to maintaining the main architectural, typological and distribution aspects. Carrying out limited volumetric enlargements and refurbishment integrations is acceptable when a new use of a building is foreseen. These works have to be consistent with the surrounding constructions and landscape. This can be achieved both by using traditional materials and construction techniques (albeit updated) as well as by employing current materials having an improved performance and a higher sustainability. Traditional techniques will be carried out by skilled workers, with a specific training (at a Building School, for walls, roofs, retaining walls). The materials will be accurately selected among those presently available on the market, e.g. for walls made of stone from quarries or of recycled elements, hydraulic and non-hydraulic lime binders which are consistent with the current regulations (e.g. NHL according to UNI EN 459-1). Timber construction elements will have full and symmetrical cross section in order to encourage the reuse of existing construction elements. Adding tension bars will be carried out according to laying techniques that have been extensively tested during the course of the centuries for the construction of churches, bell towers, high buildings or more recently for their structural rehabilitation. As far as current techniques are concerned, the most common materials are stirrups and metal connectors, mortar and structural adhesives, eco-friendly insulation such as damp proofing and vapour barriers, coarse sand mortars and light weight concrete, synthetic material reinforcement systems. The architectonic distribution can be modified by adding bathrooms and internal vertical connections. The intervention has to be the least impacting as possible: new facade openings have to be limited and plants have to be placed externally to the walls, inside the disused chimney flues or appropriate cavities in the wall. Generally, the former rooms do not have to be subdivided as they are usually small; as a consequence, it is advisable to adopt open space solutions. The most common refurbishment works are the following: - Securing vertical stone structures and strengthening or replacing the timber floors in order to enhance the building’s seismic behaviour; - Adding utility rooms (e.g. bathrooms, kitchens and similar rooms) and possible internal staircases; - Adding or renewing the plumbing and the sanitary fixtures, hot water systems and electrical wiring; - Insulating with new layers placed on the internal walls, basements and roofs; - Reducing the humidity of the retaining walls and of the ground floor by means of an underground ventilated cavity.

4.2. Recovery techniques, tested or under investigation The recovery of Alpine stone buildings requires complying with the current safety requirements and, at the same time, taking into account the preservation criteria described at the point 4.1.

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Figure 5. Sketches of recovery technical solutions for stone buildings As far as seismic response improvement is concerned, some sustainable technical solutions have been tested and used in real cases. These solutions provide a seismic behaviour improvement, but so far there is no experimental demonstration as far as the compliance of the values required by the current national regulations are concerned. Nevertheless, according to the law, the performances that can be achieved with this kind of solutions are acceptable only for historical buildings subjected to cultural heritage protective restrictions. For these buildings, the regulations only require an improvement (and not the full compliance) regards to the reference horizontal accelerations for the area under investigation. The technical solutions that follow provide the safety standards for the stone building elements (walls, floors and roofs). - Addition of metallic tension bars between opposite walls, at floor level. Tension bars can run inside the building and cross so to improve the building’s box action. Tension bars must have an end-plate (preferably circular) in order to provide a better stress distribution on the wall. The wooden beams may not require the use of the tension bars, provided that an end-plate is connected to the beams (Figure 5a and 5b); - Wooden beam stiffening and strengthening by means of: main beams substitution; strengthening of the existent beams; substitution of the plank floor which lies on the beams; stiffening by glueing and/or screwing one or more new plank layouts onto the existing floor. The planks are fixed orthogonally and at 45° in relation to the existing plank floor; connecting the stiffened floors to the perimeter masonry with stirrups which are screwed to the wood and sealed with mortars with a suitable composition (Figure 5c); - Wooden floor stiffening with metal straps. They are arranged both at 45° and as well parallel to the walls and they are fastened to the plank floor; - New NHL (natural hydraulic lime) based concrete laying onto the wooden floor. It is reinforced with corrosion resistant electro-welded mesh and connected to the plank floor and to the beams. The concrete can be made lighter by adding a specific aggregate; - Reinforced wall plates. They lie within the two leaves. In case of roof repairs, the traditional constructive technique is recommended: it consists in adding wind bracings. Additional cross tie-rods at pitch level can be added in order to give greater stiffness to the system. If insulation had to be applied under the roof, the tie-rod may be hidden (Figure 5d) .

4.3. Exploratory evaluation tests on the stone masonry resistance at the tension bar ends The use of tension bars is particularly useful and widespread for improving the static behavior of the existing stone buildings. It is important to know which is the masonry resistance at the tension bar’s end-plates in order to assess the force developed by the bar.

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Experimental exploration tests were carried out on traditional buildings and the results were compared with analytical calculations. This allowed the research team to make a first assessment. A tension bar with a circular end-plate was fixed onto the outer surface of the walls and an horizontal tractive force was applied to it. Mastrodicasa [23] points out that two phenomena can develop: 1) a detachment as a consequence of the friction between the stone ashlars; 2) a detachment due to a loss of the cohesion between mortar joints. The two phenomena may coexist. Nevertheless, the stone walls of the traditional buildings located in the Ossola valley and in the Canton of Ticino were built without any mortar or, alternatively, just with partial layers of mortar (which was only used to close the gaps and to block small stones into place, but it does not surround and bind the largest ashlars). For this reason, this study assumes the friction between the ashlars as the only force contrasting the tension bar’s force. An increasing tensional force T applied to the bar's endplates produces a detachment of part of masonry. In case of almost homogeneous walls, the detached part's shape resembles a truncated cone, having its axis coinciding with the tension bar, its smallest base corresponding to the end-plate on the wall's outer surface and its largest base located on wall's inner surface (Figure 6a). The cone's angle is directly related to the wall's regularity, of the existence of mortar joints and of the stone interlock. A 45° angle from the tension axis can be assumed [23] if homogeneous brick masonry or regular ashlar masonry with good mortar joints, staggered elements and transversely stuck stones is considered. If a vertical tension σv rests on an infinitesimal element of the cone surface of detachment, the corresponding maximum value of T is determined [23] with the integral over the first quadrant (0 ÷ π/2) and throughout the thickness t of the wall (then for symmetry multiplied by 4), which led to the following formula: T = 2 σv μ t (2 r1 + t) where μ is the coefficient of static friction. The same formula can be deduced by taking into account that the force T is proportional to the vertical component (σv · μ) of the tension acting on the truncated cone's lateral surface (partly facing upwards and partly facing downwards). The formula indeed considers the area of the orthogonal projection on the horizontal plane of the truncated conical portion: T = 2 σv μ t [2 r1 + 2 (r1 + t)] 1/2= 2 σv μ t (2 r1 + 2 r1 + 2 t) 1/2 = 2 σv μ t (2 r1 + t) By choosing a cone angle which is different from 45 ° the formula becomes: T = 2 σv μ A, where A is the area of the trapezoid, that is the truncated cone of detachment's projection upon the horizontal plane. Three exploratory tests were carried out on the walls of some buildings located in the hamlet of Veglio –municipality of Montecrestese, in the Ossola valley. A simple devise was built. It consisted of a circular end-plate (Ф = 2 r1 = 0.25 m) connected to a threaded rod, which passed through a hole made in the stone wall. A fixed pulling structure was assembled at the opposite side of the wall. It is made of steel I beams (connected with a plate) with a length that does not interfere with the detachment cone (l = 1.14 m).The pulling force was produced by manually turning, with spanner, a bolt screwed onto the threaded rod. The force was measured with a force washer Ф 31-16 mm (type KMR 200-HBM; measurement accuracy ± 2%). The tests were carried out in 2013, respectively on may, the 24th, on June, the 18th and on October, the 30th, at 11.00 a.m. Weather conditions were good, with no wind or rain; the air temperature and humidity were within the seasonal average. Therefore, stone masonry had no surface moisture. The first test (Figure 6b) was carried out at the first floor of a building built in 1876. The height of the portion of wall above testing point A is hA=5.00 m and the wall's thickness at the same point is 0.55 m. Since the stonework was quite uneven (large and small stones) and without real mortar joints, the angle of the truncated cone from the tension axis was estimated to be almost 36° (semi-opening of cone/wall thickness = 0.72 ratio). The maximum applied force was TA = 36.20 kN without any stone shifting. The test was interrupted for technical reasons. Considering the average compression stress due to the weight of the overlying wall σvA= γ · h = 23.00 kN/m3· 5.00 m = 115.00 kN/m2 = 0.115 MPa,

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Figure 6. Exploratory evaluation tests for the masonry resistance at the tension bar ends and the surface A = 0.55 · (1.04 + 0.25) · 1/2 = 0.3548 m2, the formula can be written as: TA = 36.20 kN = 2 · 115.00 · f· 0.3548 and calculated f = 0.44 < μ7. The second test (Figure 6c) was performed with a slightly improved equipment (with standard steel channel beams and adjustable feet). The wall was 0.62 m thick and the height of the portion of wall above the testing point A was hA = 1.02 m. The maximum applied force was TA = 16.20 kN with the displacement of a stone located at one of the four adjustable feet (B, of dimensions 0.05 x 0.05 m). The portion of wall above the stone's dislocation point B1 was hB = 0.43 m. Considering the average compression stress of the overlaying portion of wall to be σvB= 23.00 · 0.43 = 9.89 kN/m2 = 0.00989 MPa and a surface AB = 0.62 (0.45 + 0.45 + 0.05 + 0.05) 1/2 – (0.27 · 0.37 · 1/2) = 0.2601 m2, the coefficient of friction is deduced from: TB = 16.20/4 = 4.05 kN = 2 · 9.89· μ · 0.2601 and μ = 0.79. In the third experimental test (Figure 6d) the circular end-plate was placed at the same point A (i.e. hA = 1.02 m from the top of the wall), but the equipment was placed horizontally. The height of the portion of wall above B was hB = 0.81 m. The maximum applied force was TA= 32.20 kN. There was no stone displacement (the trial was stopped due to an adjustable foot's deformation). Given σA= 23.00 · 1.02 = 23.46 kN/m2 = 0.02346 MPa and area AA = {0.45 + 0,45 + 0.25 + 0.25] · 1/2} · 0.62 = 0.4340 m2, the friction coefficient is deduced from: TA = 32.20 = 2 · 23.46· f· 0.4340 and f = 1,58 < μ7. The coefficient values obtained in these exploratory tests can be compared with data found in literature and by taking into account the aspects that follow. Mastrodicasa [23, p. 625] had calculated a μ = 0.75 friction coefficient for normal masonry with a 45° angle from tension axis of the detached masonry truncated cone. Borri and al. [24] point out that an interlocking effect arises if not perfectly squared ashlars are used when building the walls; as a consequence the motion requires more energy than the one needed for a simple sliding of flat surfaces in contact. This is due to the need of lifting stones or to the shear force required for breaking the asperities. The value of the fiction coefficient shown is μ = 0.3 ÷ 0.8. Brencich [25, p. 27] suggests friction coefficient values μ between 0.3 and 1.6 according to the masonry laying, with a mean value of μ = 1 and a coefficient of variation between 30% and 50%. The Italian Ministerial Regulation NTC 2008 [21] suggests μ = 0.4 coefficient for brick masonry with mortar joints (sliding surface mortar). Hoek [26] shows a theoretical friction coefficient between "fractured granite rocks", calculated as μ= tg φ where φ = 45° ÷ 50° i.e. μ = 1 ÷ 1.19. As a result of experimental tests at the ISEG Department - Politecnico di Torino (S. Fasana) yielded, for gneiss roofing slabs, values of φ = 30° ÷ 41° i.e. μ = 0.7 ÷ 0.9. The friction coefficient values calculated from the exploratory tests in the Ossola valley are consistent with the values found in literature. The coefficient's value calculated in the third test f = 1.58 is connected with stone interlocking binding feature.

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The applied force was less than the maximum force of static friction and f is less than μ

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Figure 7. Exploratory flexion tests on standard stone samples (a) and modillions (b)

4.4 Stone modillions and slabs for balconies. First static tests. The recovery of Alpine architecture requires some indications so to realise stone modillions able to guarantee the safety over time. There were indeed cases of rupture of new and old modillions. Dimensions of modillions and of balcony slabs were found in historic manuals and these values are about the same found in the investigated vernacular buildings (e.g. modillions with 1.00 m overhang, wedged in the wall for 0.40 up to 0.50 m, 0.30 m high at the joint of the wall, 0.18 m at the base; upper slab 1.10 m width, thickness of 0.08 up to 0.10 m, span till 1.80 m). The research group carried out static calculations of these modillions and it was demonstrated the accordance of the values of flexion and shear with the technical constructive traditions: σ = 2.92 MPa; τ = 0.35 MPa. The values that result from calculations of balcony slabs are lower. The Interreg-Alpstone group tested also the stone material characterisation on standard samples according to many current regulations UNI EN and the flexion on modillions sized of 0.20 x 0.20 x 1.10 m. Standard samples were dried before compression tests; flexion tests were carried out on modillion maintained for one week in the laboratory, under conditions of constant temperature (t = 21C°). The mean values obtained from three samples with horizontal schistosity surfaces were σ = 13.31 MPa; τ = 1.13 MPa. The mean values obtained from four samples with vertical schistosity surfaces were σ = 11.12 MPa; τ = 0.93 MPa. The mean values obtained from four samples with schistosity surfaces orthogonal to the major axis of the modillion were σ = 3.91 MPa; τ = 0.33 MPa. It is clear that the latter position has to be avoided.

5 FINAL CONSIDERATIONS AND OPEN BRANCHES OF RESEARCH The research project is ongoing and this paper presents partial results. Further tests and investigations will be carried out. Some issues that will be developed are mainly: - The realisation of diagnostic systems able to evaluate the entirety of stone construction elements for balconies (modillions, slabs). These systems will be planned for investigations in situ or during the production phase (sonic and ultrasonic tests, penetrating liquids, etc.) - The study and experimentation of reinforcement systems for modillions and slabs. This is in order to improve the safety of placed construction elements and to avoid falls in cases of ruptures. The Interreg-Alpstone project will shortly produce a manual giving a complete view of these recovery issues and providing architectonic guidelines for appropriate conservation of Alpine stone buildings.

ACKNOWLEDGEMENTS The research team thanks the Centro Servizi Lapideo of Crevoladossola, mayors of Santa Maria Maggiore, Montecrestese, Craveggia, the prof. P. Scarzella, the architect A. Scotton, the PhD E. Genna.

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