ARCHITETTURE FORTIFICATE NEL MANTOVANO: GLI EFFETTI DEL TERREMOTO DEL 2012

La nota presenta una veduta generale dei danneggiamenti rilevati sul patrimonio delle architetture fortificate delle zone della provincia di Mantova, colpite dalla sequenza sismica della primavera 2012. Si tratta di strutture, in passato molto numerose, ma decimate per le note vicende storiche di progressivo abbandono e successiva demolizione e reimpiego dei materiali in altre opere. Tuttavia permane un patrimonio di edifici, prevalentemente trasformato in torri civiche, di forte impatto simbolico e visivo nei contesti urbani. Pur non riportando i danni evidenziati da altre tipologie costruttive, come ad esempio gli edifici religiosi storici, o i crolli rovinosi registrati a pochi chilometri di distanza in territorio emiliano, gli edifici hanno riportato alcuni significativi danni. Nell’estrema difficoltà del momento e della situazione post-sismica, con priorità di intervento più legate al riparazione dell’edilizia residenziale, produttiva o pubblica, è importante segnalare l’interesse verso tali strutture, spesso ritenute il simbolo della città in ottiche di intervento rispettose della conservazione dell’edificato storico. The research concerns the evaluation of the structural performance of the defence heritage in the Mantua area after the seismic sequence of May 20 and 29, 2012. The earthquake highlighted the high vulnerability of such heritage in the neighbouring Emilia-Romagna region where several brittle collapses of towers and fortification walls occurred, despite the supposed low seismic risk of the area. The interpretation of the surveyed damages in the defensive structure of the southern Mantua’s province showed recursive behaviour. Over the centuries, the area was military strategic due to the complex hydrologic configuration between the river Po, Mincio, Secchia and several artificial canals and ditches. Particularly since Middle Ages, the fixed courses were fortified by an imposing net of defensive structures. Nevertheless, the changes of the political equilibrium and configuration of Italy over time, led to the progressive neglecting of the structures till their almost complete dismantling during the Austrian dominion. Most of the building materials were reused in new hydraulic structures or embankments in order to prevent the frequent floods. The survived structures, mostly donjons, were transformed over time in civic towers or in public buildings, such as town halls or museums. Despite collapses did not occur several damages were documented, and especially cracks located in the upper part, top structures and in the main body of the towers due to in plane and out plane seismic actions. The masonry discontinuities due to the building evolution and the lack of structural connection between the load bearing walls, increased the vulnerability particularly evident in complex structures and palaces such as the Ducal Palace of Revere. According to past experiences and Civil Protection Agency indications, the emergency prompt interventions were aimed at the mitigation of the damages by light structures, mainly ties. At present, progressive interventions are planned according to the preservation of historic architecture; the design has to be based on the deep knowledge of the building, including features, materials properties, structural layout, building technique, damages. In this context, the Municipality of Mantua planned interesting activities of control of the main towers. On the taller tower of the city, the Gabbia Tower, an extensive research program, including dynamic monitoring, was planned and performed to evaluate the structural condition of the building after the earthquake.