Seismic response of masonry churches located in the province of Mantua

Italy has been frequently attacked by the earthquake which caused severe damage to the buildings and take away people’s lives during last decades. As one of the richest country in terms of building heritages, the safeguard of historical masonry structures against seismic events has become an urgent problem for Italy. It has been proven that the masonry structures are extremely vulnerable to seismic excitations, not only because they are designed to support mainly gravity loads, but also due to the intrinsic characteristics of this type of buildings. Masonry is a composite system constituted by bricks bonded by mortar. The exact arrangement and size of the bricks and the distribution of the mortar are unknown in most of the cases. Homogenization of the masonry materials according to the experimental data are adopted. The material is characterized by highly non-linear constitutive behavior, which calls for a deeper theoretical study and sometimes its implementation in a computer based code represents a difficulty. For this case study, the behavior of the material is described with the concrete damaged plasticity constitutive model in which takes into account the degradation of the material properties regarding Italian law. Seismic response of the 6 churches and one fortress located in the province of Mantua are studied in this thesis project. Churches are very complicated structure constituted by macro elements, for example façade, bell tower, lateral walls in the nave, triumphal arch, apse and vaults etc., which can be very vulnerable under seismic excitation even with a low magnitude. Nonlinear dynamic and static analyses have been performed to evaluate the seismic vulnerability of the macro elements. The study of buildings starts with the nonlinear dynamic analyses using different accelerograms (accelerogram 1 and accelerogram 2) normalized to a PGA of 0.08g (or 0.1g) and 0.17g (or 0.2g) are consequently performed. In addition, seismic response of structures with different material property is investigated. It is well known that this type of analysis represents the real seismic behavior of the structures in all good manner. The aim of the procedure is to determine the seismic vulnerability of macro elements of the structures, collapse mechanism, if it occurs, and to evaluate the structural performance in terms of displacements, damage, bae shear and energy dissipation under low to high magnitude of seismic excitation considering different mechanical properties. Comparison considers different material properties and different accelerograms of ground motion has been made. The pushover analysis is performed in order to obtain the non-linear base shear-top displacement relationship for critical points on the macro elements respectively, which are different representatives of its local behavior, and to determine the collapse mechanism associated to the most vulnerable macro element. A monotonic loading proportional to the mass and the height was applied. Two types of models, ideal and conservative, are investigated take into to account the level of interlocking between adjacent walls. Former one is assuming ideal interlocking between walls, while it is assumed poor interlocking between walls and the macro element considered as isolated structure in the latter case. Pushover capacity of the macro elements assuming different interlocking level has been compared, and reasonable causes are discussed. Finally, the seismic response of the churches is evaluated with the combination of the dynamic response under different seismic excitation or material and pushover capacity of the macro elements.

Lo studio è effettuato con riferimento a due tipi di strutture murarie storiche -chiese e fortezze, tutte rappresentative della classe di costruzioni in muratura considerata e situate nei pressi di Mantova. In questo progetto di tesi, sei chiese e una fortezza sono state analizzate attraverso l'applicazione dell’analisi dinamica non lineare e dell’analisi statica (pushover). Per semplificare le analisi, le strutture analizzate nella tesi corrente vengono considerate come un unico macro-elemento che può essere: a) chiesa, b) rocca, c) macro-elemento isolato di chiese (facciata, campanile, pareti laterali, arco trionfale e abside). In aggiunta, il materiale è caratterizzato da un comportamento costitutivo fortemente non lineare, che richiede un approfondimento teorico e talvolta la sua implementazione in un codice di calcolo rappresenta una difficoltà. Per questo caso di studio, il comportamento del materiale è descritto con il modello costitutivo di plasticità del calcestruzzo danneggiato. Lo studio degli edifici inizia con le analisi dinamiche non lineari utilizzando diversi accelerogrammi (accelerogramma 1 e accelerogramma 2) normalizzati ad una PGA di 0,08g (o 0,1g) e successivamente a 0.17g (o 0,2 g). Lo scopo del procedimento è quello di determinare la vulnerabilità sismica dei macro-elementi delle strutture, il meccanismo di collasso, se si verifica, e per valutare le prestazioni strutturali in termini di spostamenti, danni, taglio alla base e dissipazione di energia sotto eccitazione sismica di crescente grandezza considerando differenti proprietà meccaniche. L'analisi pushover viene eseguita in modo da ottenere la relazione non lineare “taglio alla base- spostamento in sommità” per diversi punti sulla struttura, rappresentativi del suo comportamento globale, e per determinare il meccanismo di collasso associato al macro-elemento più vulnerabile. Un carico monotono proporzionale alla massa e all'altezza è stato applicato. Due tipi di modelli, ideale e conservativo, sono studiati per tener conto del livello di incastro tra pareti adiacenti.