Contribution to the knowledge of wide span wooden roofing in Bologna area

This paper deals with the analysis of some examples of large wooden trusses in important churches of Bologna carried out through a survey campaign conducted with a laser scanner. This study helped to develop a wider methodology for instrumental survey, aiming to derive a series of significant information: average deviations of the structures compared to theoretical models, deformations of the elements through time, instability, material state of conservation, structural behaviour both locally and globally. All results highlighted the knowledge that the structural design of these wooden roofing is strictly related to their dimensions and assembly, the understanding of which is functional on any restoration intervention.


INTRODUCTION
The analysis of the wooden roof structures, in particular historic trusses, is featured in scientific literature with several contributions, in the field of both structural engineering and restoration technology.Nevertheless, a few studies have systematically explored the real behaviour of these building systems based on experiences and on-site diagnosis [1].This is due to many factors, including the simple fact that roofing structures are hidden and difficult to access, and not attracting the attention of technicians and researchers except in case of serious deterioration or clear damage with real consequences on people's safety.The roof is a building area easily exposed to material deterioration and fire and it is therefore characterized by strong transformations and large-scale replacements over time.This theme is thus downgraded to a matter of minor importance for restoration, since the subject matter of study is not original anymore or, otherwise, difficult to date.The situation is therefore different whether these structures were exposed to view and decorated or would support valuable ceilings, for example painted surfaces.However, one of the main reasons for this lack of interest, lies in the fact that wooden roofs are typically indeterminate structures, whose safety mainly depends on the quality of the material and on the characteristics of joints and connections.The construction of roofs concerns the art of building, or else, the practice of carpentry and for such reason avoids scientific and analytic interpretations, unless huge approximations are made.The study presented here is based on an accurate survey of the structural components but above all, on the attempt to understand the original construction methods.This research offers a systematic analysis of some significant roofing structures in the metropolitan area of Bologna, based on the wide use of digital survey technologies.The first aim is to give importance to those neglected structural components, identifying new functional types.The second aim is to introduce an effective operational research protocol, with high diagnostic capacity due to the intensive use of new technologies.

STATE OF THE ART
Studies on wide span wooden roofing, in particular timber trusses, are mainly based on important old textbooks [2]; most of the classifications proposed in architectural literature are the result of the technical awareness of the nineteenth-century, that simplified complex and partly structurally indeterminate systems in easier and computable diagrams.In the recent past, the tendency of some historians of architecture and construction, was to identify within those systems a few recurrent structural typologies, highlighting their behaviour that was predicted on their theoretical assumptions [3] [4] [5].
On the other hand, all technical solutions identified for the strengthening or preservation of these structures were based on those simplifying assumptions and preliminary analysis, counting on merely hurried surveys.The logic of intervention pursued the idea to force the behaviour of these structures' to known or easy predictable patterns.
Apart from the current approach for the preservation of the historic roof structures, the assumption of the research is that digital technologies for surveying enable a series of new and original considerations that would be almost impossible following the traditional methods based on direct observations or a simplified architectural survey.While the use of the laser scanner (Fig. 1) in the survey of complex architectural flat surfaces is not new and is widely studied in the field of restoration, rarer, if not absent, is
Our working hypothesis has been to take advantage of the large amount of geometric data, acquired by instruments in form of point clouds, for analysing the trusses in detail and then derive accurate and comparable information on their behaviour.The acquisition of further information allows, in the first place, to trace the logic of the construction and, above all, the assembly of the structures from the very beginning.In fact, the shape of the trusses follows a very precise, and often ignored, construction practice linked to the possibility of raising the elements up to the base of the roof, refining and assembling them through other supporting elements and fixing each piece using metal nails.Nowadays, the system for joining wooden elements is completely changed with the introduction of metal plates and the possibility to check every part of the system numerically; as a matter of fact, retracing the original logic of the junctions is a non-trivial task.Palladian trusses, for example, are divided into classical and composed, but within these categories, there are many variations, which depend on a lot of factors: continuity and assembly of linear elements to form chains and rafters of wider span or larger cross-section, more or less accentuated stiffness of the joints, also linked to the shape of cuts and fasteners, more or less loaded brackets and other details [6] [7] [8].
Extensive and detailed geometric information provide highly interesting data on deformations, rotations and displacements of the structures, to determine messa in sicurezza individuate si sono basate su ipotesi semplificative e su analisi preliminari effettuate limitandosi a rilievi sommari.La logica dell'intervento si è appoggiata all'idea di forzare il comportamento delle strutture a schemi conosciuti o modellabili.A prescindere dall'approccio corrente al problema della conservazione di strutture di copertura storiche, l'assunto della ricerca è che le tecnologie digitali per il rilievo permettano di estrapolare una serie di nuove ed originali considerazioni che sarebbero impossibili, o quasi, seguendo i metodi tradizionali d'indagine fondati sull'osservazione diretta o il rilievo architettonico semplificato.Mentre l'uso del laser scanner (Fig. 1) nel rilievo di superfici architettoniche complesse non è nuovo ed è oggetto di varie ricerche nell'ambito del restauro, più raro, se non assente, è invece lo studio applicato a strutture spaziali nascoste come le coperture lignee; studi in tal senso, almeno in Italia, sembrano non essere presenti.L'ipotesi di lavoro è stata quella di sfruttare la grande quantità di dati geometrici, acquisibili dallo strumento sotto forma di nuvole di punti, per leggere le capriate in modo dettagliato e derivare poi informazioni puntuali e comparate sul loro comportamento.L'acquisizione di maggiori informazioni permette in primo luogo di ripercorrere fin dall'inizio la logica costruttiva e, soprattutto di messa in opera del manufatto.La conformazione delle capriate segue infatti una prassi costruttiva ben precisa e spesso ignorata, legata alla possibilità di innalzare gli elementi fino alla base del tetto, per poi rifinirli, assemblarli attraverso altri elementi di supporto e fissarli tra loro anche con l'impiego di elementi metallici.Oggi il sistema di giunzione degli their real behaviour over time; these variations in shape occur both at local level, i.e. in a single element, and at global level, considering the behaviour of the whole roofing system.Traditionally, in particular in the Italian contexts, roof systems have been studied in a two-dimensional manner, for example, considering the repetition of many identical elements along the aisle of a church, each one characterized by the same behaviour and weakly linked to each other.The global behaviour, provided by movements and reciprocal interferences in the longitudinal direction, has often been ignored, in favour of a greater attention for the understanding of the behaviour of the single truss in its plane.
A further consideration concerns the intervention on this type of structures; a poor understanding of the original construction has led to restoration interventions that brought back the behaviour of trusses to simplified schemes, introducing stiffeners and other superfluous structures that were radically changing the original behaviour of the structure, effectively creating new types of structural systems.Recent major refurbishment interventions on structures that we have investigated, although of decent workmanship, have shown a deep misunderstanding of the behaviour of the structures through time, both locally and globally.Not to mention the adoption of extended replacements that have created problems not only to the roof system, but also to the whole building.

METHODOLOGY
The methodology that is presented here has been developed by successive approximations and corrections after its application in different case studies, in which 3D detection instruments and reverse engineering techniques have been used in field operations.Reverse engineering is the detailed analysis of an object regarding its functioning, its design and its development in order to produce a new object with similar characteristics, perhaps by improving or increasing its efficiency.Starting from a prototype, an object that needs to be rebuilt or specific parts on which to make specific analyses, it is possible to derive reference models in CAD, using the process of reverse engineering.
The model is then easily used to carry out dimensional analyses, modifications of existing parts, and eventually, the development of completely new parts.

Il metodo che si vuole presentare è stato messo a punto per successive approssimazioni e correzioni in seguito all'applicazione dello stesso a diversi casi di studio, in cui sono stati messi in campo strumenti di rilevazione 3D e tecniche di reverse engineering. L'ingegneria inversa consiste nell'analisi dettagliata del funzionamento, progettazione e sviluppo di un oggetto al fine di produrre un nuovo oggetto che abbia un funzionamento analogo, magari migliorando o aumentando l'efficienza dello stesso. Partendo da un prototipo, da un oggetto che si vuole ricostruire o da parti specifiche delle quali si vogliono effettuare particolari analisi,
contexts.The whole path includes a step of acquisition, which is followed by the restitution of the digital acquired data.This last step produces a large set of outputs (photographs, drawings, graphs and models) and contribute to set up operational tools to define and correctly understand the behaviour of the studied building systems and, consequently, suggest consistent design solutions, independently from usual and standardized techniques, if necessary.

Figure 2. Synoptic diagram of the operational protocol
In the acquisition phase the object of interest that is part of a broader topic is identified.In our case, the aim was to analyse the trusses of large span in roofing systems of the seventeenth-century liturgical halls in Bologna.
The use of the same for several case studies allows to compare the results according to a scientific and not to an -empirical approach.Depending on the equipment, a preparatory inspection is essential to plan the phases of survey properly and become familiar with these spaces which, as mentioned before, are often difficult to access.Once the inspection has verified the feasibility of the survey, the scan session is carried out using a laser scanner FARO CAM2 FOCUS 3D.This tool, lightweight and easy to use, allows fast scans with a simple photo tripod.Furthermore, the possibility of using a targetless technique minimizes the preparatory activities inside the spaces to be detected and the dead time between each scan station.The conclusion of the survey is followed by the phase of spatial data consolidation, performed in back office, in which the stations are aligned through a dedicated software to generate è possibile, mediante il processo di reverse engineering, ricavare modelli di riferimento in CAD.Il modello viene poi utilizzato per effettuare agevolmente l'analisi dimensionale, la modifica di parti esistenti, lo sviluppo di nuove parti.I protocolli messi a punto hanno comunque, sin da subito, mostrato tratti definiti e precisi, tanto da poter essere riassunti graficamente in uno schema sinottico che ne rappresenta le sue diverse fasi (Fig. 2).La metodologia usufruisce continuamente di feedback dai casi di studio ancora in corso, mostrando anche buona adattabilità a contesti diversi.Il percorso prevede una fase di acquisizione a cui fa seguito la restituzione digitale dei dati acquisiti; quest'ultima fase produce una serie di output (foto, disegni, grafici e modelli) che vanno a costituire gli strumenti operativi per poter definire ed interpretare correttamente il comportamento dei sistemi costruttivi studiati e proporre conseguentemente la soluzione progettuale più coerente, svincolandosi, ove necessario, dalle tecniche consuete e standardizzate.Nella fase di acquisizione viene individuato l'oggetto di interesse che comunque ricade all'interno di un tema più vasto.Nel caso in esame si è deciso di analizzare le capriate di grande luce in copertura delle aule liturgiche della bologna seicentesca.Utilizzare la stessa metodologia per vari casi di studio consente di rendere confrontabili i risultati ottenuti sulla base di un approccio scientifico e non empirico.In funzione delle strumentazioni utilizzate, è fondamentale un sopralluogo preparatorio per pianificare correttamente le fasi di rilievo e prendere confidenza con questi luoghi che, come detto, spesso sono point clouds, a true metric dimensional information repository of the detected object, to which documentary and photographic information can be associated.This consolidates a real dataset that helps to ensure the traceability of data.
The restitution phase consists of three separate steps, that are chronologically consequential, but may include feedback paths that allow to get back to an earlier phase, if any reconsideration or deeper investigation make it necessary.
Each of the three stages is associated with a specific output that will be useful in the next step of results interpretation.Qualitative analysis involves the careful observation and understanding of acquired data, and shifts to back office all considerations, which, if carried out on site, could be affected by hurry or by the objective inability to control the complexity of the surveyed structures.The digitized spatial data are all completely available and it is not necessary to come back to the survey site to retrieve forgotten information.This analysis allows to identify the specific nature and complexity of the building system under investigation, avoiding what might be called "reductio ad unum", that is the representation of a single truss scheme, whereas there might be slight but significant differences within the same roofing system.Through the direct observation and the extraction of orthophotos from the point cloud an abacus and an overall picture of all the nodes of the different trusses can be drawn up.Some considerations can also The geometric drawing follows logically the first qualitative assessments and gives an effective and very detailed representation of the object of the survey.2D drawings can be traced through operations of "vectorialization" of orthophotos and the amount of spatial data collected is useful to create threedimensional models (Fig. 3).Other than offering the graphic representation of the studied object with far greater speed and precision than in a classical survey, the point cloud gives the possibility, supported by the already performed qualitative analysis, to derive one or more functioning patterns of the construction system.Precise wireframe models can then be provided in order to analyse the behaviour of the structures with finite element calculation software.Reverse engineering software is usually applied to small mechanical parts or design objects to digitize their shape.In order to adapt this software to complex objects such as trusses, it was decided to use the spatial data to create three-dimensional models through a simplified "rectification "of the rods and the identification of an "ideal truss".This truss is the one that best represents the initial, undeformed condition, just as put in operation by builders (Fig 4).
Using the orthophotos, that is the projections in the plane of a single truss obtained from the point cloud, a procedure for a simplified vectorization was set up, in order to help identifying the dimensions of the rods, the barycentric axis and the position of the joints in the trusses.This operation, supported by some simplifying assumptions such as the horizontality between spans, the verticality of the king posts and the absence of rotations in the rods, has enabled the generation of 3D "rectified" models of each truss for several case studies.These simplified models were transported into Geomagic and essere inficiate dalla fretta o dalla oggettiva incapacità di dominare la complessità dell'oggetto rilevato.I dati spaziali digitalizzati sono disponibili nella loro completezza e consentono di non dover ritornare in sito per recuperare informazioni "dimenticate".Questa analisi consente di individuare le specificità e la complessità del sistema costruttivo oggetto di indagine, evitando quella che potremmo definire "reductio ad unum", ovvero la rappresentazione di un solo schema di capriata laddove potrebbero esserci leggere ma significative differenze all'interno di una stessa copertura.Attraverso l'osservazione diretta o l'estrazione di ortofoto dalla nuvola di punti possono essere redatti un abaco e un quadro di sintesi di tutti i nodi che costituiscono le diverse capriate e possono essere fatte delle considerazioni sullo stato di conservazione delle strutture.Il disegno geometrico segue logicamente le prime valutazioni qualitative e consente di dare una rappresentazione efficace, molto dettagliata, dell'oggetto del rilievo.Attraverso operazioni di "lucidatura" delle ortofoto si possono realizzare disegni 2D così come è possibile sfruttare la mole di dati spaziali per realizzare modelli tridimensionali (Fig. 3 compared with their respective portions of the point cloud, representing the real and actual conditions of trusses, and carrying out what was described as "local analysis" (Fig. 5).Through the software, these two three-dimensional objects were aligned by identifying at least three corresponding points on linearly independent planes and by performing a further optimization, in order to ensure the minimum level of deviations and ensure the maximum level of overlapping between the objects.

). Dalla nuvola di punti è possibile ottenere la restituzione grafica dell'oggetto di studio con rapidità e precisione decisamente maggiori rispetto ad un rilievo classico e ricavare uno o più schemi di funzionamento. Possono così essere predisposti precisi modelli geometrici wireframe per analizzare il comportamento delle strutture utilizzando software di calcolo agli elementi finiti. Per poter usare software di reverse engineering, solitamente applicati a piccole parti meccaniche o ad oggetti di design di cui si vuole digitalizzare la forma, ad oggetti complessi quali le capriate, si è pensato di utilizzare i dati spaziali per creare modelli tridimensionali semplificati attraverso una "rettificazione" delle aste e ad una individuazione della "capriata ideale", cioè quella che meglio rappresenta la
Starting from the "rectified truss", the so-called "ideal truss" or "reference truss" is the one that, on the basis of the given assumptions, best represents the original condition at the time of realization.Some simplifications were taken into account also for this operation and some assumptions were made: a common average plane for spans, the perfect verticality of the trusses, the absence of flexural phenomena due to permanent loads and the absence of movements of the king posts out of the plane.The global models of the roofing system, obtained by placing these "ideal trusses" on the original coordinates, were compared with the corresponding portion extracted from the point cloud, always using Geomagic, following similar alignment procedures.Once all the alignment procedures have been completed, the software allows to make a comparative analysis and an evaluation of the differences between the outer faces of the solid geometrical model and the point cloud.The representation shows the size of the displacements with a suitable colour scale.

Partendo dalle "capriate rettificate" è stata poi definita la cosiddetta "capriata ideale" o "capriata di riferimento", ovvero quella che, in base alle ipotesi fatte, meglio rappresenta la condizione originaria all'atto della realizzazione. Anche per questa operazione si sono tenute in conto alcune semplificazioni, ipotizzando un piano medio di appoggio comune, la perfetta verticalità delle capriate stesse, l'assenza di fenomeni flessionali dovuti ai carichi permanenti e l'assenza di sbandamenti nel e fuori dal piano dei monaci. I modelli globali delle
value with a two-dimensional representation.This interpretation provides a significant amount of qualitative and quantitative information and facilitates a precise and widespread understanding of all the movements and deformations that may be encountered in each roof truss.In addition, lowerings and rotations of the king posts can be analysed and highlighted locally as well as torsional phenomena or rotations of the tie rods.Even lowerings of the wall supports and deflections of the struts, affecting the entire gable roof, may be highlighted by this kind of analysis (Fig. 6).
Outputs achieved provide a basis for systematic knowledge that is traceable, objective, comparable and available at any time.This operational method is not linked to the need to classify the complexity of the system within a preestablished standardisation and can take into account the specificities of each construction.Subsequently, it is possible to make targeted suggestions for intervention, not standardized measures but following the understanding of the singularity manifested by each building system.This approach favours the sustainability of the intervention itself in terms of reversibility, compatibility and proper use of current techniques.coperture, ottenuti disponendo queste "capriate ideali" sulle coordinate originarie, sono stati confrontati con la corrispondente porzione estratta dalla nuvola di punti, sempre utilizzando Geomagic, a seguito di procedure di allineamento analoghe alle precedenti.Una volta effettuate le procedure di allineamento, il software utilizzato consente di effettuare un'analisi comparativa e valutare gli scostamenti tra le facce esterne che costituiscono il modello geometrico solido e i punti della nuvola.La rappresentazione attraverso una opportuna scala cromatica rende evidenti le entità degli spostamenti.Inoltre, è possibile selezionare qualunque giacitura ed evidenziare localmente il valore numerico mediante una rappresentazione bidimensionale.Questa lettura fornisce una quantità di informazioni notevole sia qualitativamente che quantitativamente e facilita una comprensione puntuale e diffusa di tutti i movimenti e deformazioni che possono essere riscontrati nell'intera copertura a carico di ciascuna capriata; inoltre possono essere analizzati ed evidenziati localmente abbassamenti e rotazioni dei monaci, fenomeni torsionali o rotazioni delle catene, abbassamenti degli appoggi e flessioni dei puntoni, oltre che fenomeni deformativi a carico dell'intera falda (Fig. 6).Gli output ottenuti costituiscono una base di conoscenza sistematica,

CASE STUDIES
The operating protocol was applied to four specific case studies in the area of Bologna, the Cathedral of St. Peter, the church of St. Salvatore Maggiore, the church of San Giovanni in Monte and the Basilica of San Petronio.The choice depends on the fact that these places of worship were built almost at the same time and all of them are dimensionally significant.The survey phase has already been completed on all four roofings, while the restitution and analysis phases have only been concluded for St Peter and St Salvatore.This has led to get good results and an excellent feedback to continue in the refinement of the method for the case studies that were still in progress.
For St. Peter's Cathedral the survey was carried out by performing, in the roof above the central nave, about 80 scans, with an accuracy of about 7 mm to 10 m away, thus acquiring more than 100 million points.The roof consists of 18 large open-joint trusses that are unique in their kind, as far as it is known, covering a span close to 26 meters.The truss consists of three complementary systems: a bearing trestle, a discharge arc formed by under-rafter and a false counter-rod (interrupted by the central king post) and an external structure, formed by struts, central king post, upper and lower struts and, finally, by the main tie rod in order to close non-deformable triangle.All tension members have almost constant section of about 30 x 30 cm.
In the church of San Salvatore, after climbing on an external staircase, as there is no direct access to the roof, the survey detected 11 trusses following the classical composed Palladian configuration, with a span close to 18 meters.This roofing system is also made of open-joint trusses, with a single rafter interrupted by the lateral king posts, made with members of variable section going from 20 x 20 cm of the upper struts to 30 x 30 cm of the rafters.70 scans were carried out using an accuracy of about 6 mm to 10 m away, acquiring in total more than 130 million points.
The consultation of historical documentation has highlighted that Giovanni Ambrogio Mazenta took part in the projects of the two churches, built in the first half of the seventeenth century, although it is still unclear how much his contribute influenced the two realizations.In any case, both examples highlight that the design solutions adopted are not merely the same.The analysis of data has allowed to define, the functioning diagrams of the two construction systems, which were represented with suitable graphics operations and analysed with simplified calculation programs (Matlab and Sap2000).San Pietro has confirmed a hybrid structural scheme, with a compressed discharging arch and external non-deformable triangle, essentially a rigid statically determined tracciabile, oggettiva, confrontabile e disponibile in qualsiasi momento.Operando in questo modo ci si svincola dalla necessità di dover inquadrare la complessità del sistema all'interno di una tipizzazione precostituita e si possono tenere in conto le specificità di ciascuna costruzione.Conseguentemente è possibile formulare proposte di intervento mirate, non standardizzate, ma frutto della comprensione della singolarità manifestata da ciascun sistema costruttivo favorendo la sostenibilità in termini di reversibilità, compatibilità e attualità dell'intervento stesso.scheme.San Salvatore, instead, showed a "pseudo hypostatic" scheme for asymmetric loads, where the king posts are intentionally allowed to move vertically to follow the movements of the entire building.Regarding the Cathedral of St. Peter, the deformation analysis carried out with the previously described method has brought a bunch of numerical information displaying that the north pitch shows a depression due to flexion of the struts, with a greater intensity in the terminal section, near the supporting wall.This phenomenon is emphasized in correspondence of the last two trusses on Via Indipendenza, implemented in a later stage of construction, during the advancement of the main facade.More precisely, these two trusses show more marked rotations out of the plane compared to those showed by the oldest ones.

Il protocollo operativo è stato applicato a quattro casi di studio specifici nel territorio bolognese, la
Although less marked, all these rotations have the same direction, except for the truss number 8, confirming a displacement of the entire building towards the facade.A uniform trend regarding the lowering of the king posts in the trusses is quite clear, showing maximum values for those on the north side, probably due to heavier accumulations of snow.The profile of the ridgeline is not straight, but highlights a depression exactly where the king posts show a shift downwards in relation to the initial level.This phenomenon is mainly located in correspondence of the facade, where it reaches its maximum value of 20 cm.The intervention hypothesis (fig.7) consists in the construction of some kind of armed beam by providing steel cables spaced apart by new wooden struts located below the main tie rod.In addition, new wooden diagonal struts, discharging on a suitable structure disposed above the main tie rod, are inserted to reduce the span of the side rafters [9].
Regarding San Salvatore, the opportunity to analyse in detail and to compare the extent of the lowering of the supports has provided the fundamental soluzioni progettuali.L'analisi dei dati ha infatti permesso di definire, attraverso opportune operazioni grafiche, gli schemi di funzionamento dei due sistemi costruttivi, che sono stati analizzati con programmi di calcolo semplificati (Matlab e Sap200).
Per San Pietro si è confermato uno schema ibrido, con arco di scarico compresso e triangolo indeformabile esterno, in sostanza uno schema rigido, staticamente determinato.Per San Salvatore, invece, è emerso uno schema "pseudo labile" per carichi asimmetrici, in cui ai monaci è volutamente concesso di muoversi verticalmente per assecondare i movimenti dell'intera fabbrica.Per quanto riguarda la Cattedrale di San Pietro, l'analisi deformativa condotta con la metodologia precedentemente illustrata ha permesso di mettere a sistema una serie di informazioni numeriche che evidenziano come la falda nord mostri un avvallamento dovuto alla flessione dei puntoni, con una maggiore intensità nel tratto terminale in prossimità degli appoggi.Il fenomeno si accentua in corrispondenza delle ultime due capriate su via Indipendenza, realizzate in un secondo momento per consentire l'avanzamento della facciata principale.Proprio queste due capriate mostrano rotazioni più accentuate fuori dal piano rispetto a quelle delle capriate più antiche.Seppur meno accentuate, le rotazioni sono concordi, eccezion fatta per la capriata 8, confermando un cedimento verso la facciata dell'intera fabbrica.È evidente un andamento tendenzialmente uniforme per quanto riguarda l'abbassamento dei monaci all'interno di una singola capriata, con valori massimi per quelli sul lato nord, probabilmente dovuto ai maggiori accumuli di neve.L'andamento della linea di colmo non si presenta rettilineo, ma evidenza abbassamenti proprio laddove i monaci hanno manifestato uno spostamento verso il basso rispetto alla quota iniziale.Tale fenomeno è fortemente localizzato in corrispondenza della facciata, dove si riscontra il valore massimo di 20 cm.L'ipotesi di intervento (fig.7)prefigura la realizzazione di una sorta di trave armata predisponendo cavi in acciaio distanziati da contraffissi al di interpretative basis to understand the peculiarities of its roof.The lowerings occurred significantly more on the south side of this church, adjacent to the ancient monastery, than on the north side, and were generated by the onset of differential foundation settlements, due to known localized effects of subsidence in this area of the city, full of underground canals.The use of a "pseudo hypostatic" system seems thus a conscious choice of the architect, aware of the future evolution of the building.The roof follows the movements of the building and in this perspective should be interpreted the substantial ineffectiveness of the joint stiffening, carried out in the 80s through the arrangement of new metal nailed plates.The truss, designed and built to allow movements and rotations of its tension members, today shows that a large number of those metal plates are almost fully removed and offsite, thus becoming ineffective.The hypothesis of intervention (fig.8) consists of inserting a coaxial sliding sleeve on the side king posts connected with inclined struts to both the counter-rod and the main rafter.This aims to avoid the side king post to slip out of the plane, thus preventing the joints' detachment, while not hindering the movements that the pseudo-hypostatic scheme was originally adopted to avoid.[10].sotto della catena e l'inserimento di saette, che scaricano su un'apposita struttura disposta sopra la catena, a ridurre la luce libera di inflessione dei sottopuntoni laterali [9].Per quanto riguarda San Salvatore, la possibilità di analizzare nel dettaglio e mettere a confronto le entità dei cedimenti degli appoggi ha fornito la base interpretativa fondamentale per comprendere la specificità costruttiva della sua copertura.Sul lato della chiesa, in adiacenza all'antico convento, si sono manifestati abbassamenti decisamente maggiori rispetto al lato nord, anche in ragione dell'insorgere di cedimenti fondali differenziali, dovuti a noti fenomeni di subsidenza localizzata di questa zona della città ricca di canali sotterranei.L'utilizzo di un sistema "pseudo labile" pare quindi una consapevole scelta dell'architetto, cosciente del futuro evolversi della fabbrica.La copertura doveva seguire i movimenti dell'edificio ed è proprio in questa ottica che si può interpretare la sostanziale inefficacia degli interventi di irrigidimento dei nodi, effettuati negli anni '80 mediante la disposizione di nuove piattine chiodate.La capriata, concepita e realizzata per consentire movimenti e rotazioni delle sue aste, oggi mostra un gran numero di questi dispositivi scalzati e fuori sede, risultando, così, inefficaci.In questo caso l'ipotesi di intervento (fig.8), consistente nella realizzazione di un manicotto coassiale scorrevole sui monaci laterali e collegato con saette inclinate sia alla

CONCLUSIONS
It is more and more clear that the analysis of the historical buildings should be undertaken not only by relying on the knowledge contained in established controcatena sia al puntone, mira a limitare il più possibile gli sbandamenti fuori dal piano, impedendo conseguentemente lo sfilamento dei nodi, pur non ostacolando i movimenti per cui lo schema pseudo-labile è stato originariamente adottato [10].

BIBLIOGRAPHIC REFERENCES
textbooks but also by making use of new technologies in a conscious way.Raw data should be interpreted critically but they can make the specificity of each building clear, surpassing the simplification dictated by the use of standard solutions.The proposed operational method is, in this sense, without any doubt effective, because it allows a rational intervention on these structures, counting on the traceability of the surveyed data, and identifying innovative and more sustainable intervention criteria.The operational method itself can also be seen as a way to keep the structure monitored, in anticipation of its conservation through time; the survey, in fact, can be repeated after several years to evaluate the beginning of degenerative processes.The general efficiency of the operative protocol should still be improved.All the different stages shall be provided with clear procedures in order to make them repeatable and comparable.The remaining open case studies are providing, in this sense, very good indications of an ongoing improvement.

Figure 1 .
Figure 1.Image of the roofing of San Salvatore Maggiore extracted from the point cloud (Davide Prati)

Figure 4 .
Figure 4. Identification of the characteristic tension members of a rectified truss in San Salvatore Maggiore (Claudio Demattia)

Figure 8 .
Figure 8. Hypothesis of intervention in the church of San Salvatore Maggiore.(Claudio Demattia)