state of the art

In Europe and especially in Italy, historical town centres are characterized by monuments and minor architecture that are essential parts of the cultural heritage. This heritage is a strategic resource for a sustainable Europe and needs to be preserved [1,2].

Historical centres are mostly made of masonry buildings, often aggregated, which are vulnerable to seismic actions [3], as evidenced by the last strong earthquakes in Italy [4-9]. However, seismic events cause damage to other elements of urban texture (roads, open spaces, utility lines [10-11]). The paradigm shift that sees urban vulnerability as broader than building vulnerability is recent and incorporated into modern theories of seismic risk [12-16]. The Minimum Urban Structure (MUS) [16-17], namely the minimum amount of elements that must remain functional after a seismic event to keep the town open, can allow better management of the emergency and more effective recovery of the urban system.

International institutions promote analyses and planning activities to prevent natural disasters [18] and the concept of resilience as the capacity of a system, community, or society potentially exposed to hazards to adapt to a new scenario [19] can be applied to urban scale [20].

In the last years, research started to investigate seismic vulnerability at the urban scale, by highlighting that management and safeguarding of centres require an interdisciplinary approach [21]. Focusing on structural aspects, several methods have been proposed in the last decades for assessing urban centre building vulnerability [22]. Less accurate, but more practical approaches are empirical/indirect methods. Some of them are based on post-earthquake observations and the definition of Damage Probability Matrices [23]. Other methods adopt preventive simple and quick visual survey of buildings in order to obtain vulnerability index that can be extended to urban scale [24-31]. Simplified vulnerability analyses allow for defining scenarios [32], which are fundamental for risk mitigation strategies and need to be defined at urban scale [11,33], allowing for emergency plans implementation [34,35].

More accurate approaches are analytic ones or direct methods, based on analytical [36-39] or numerical models (equivalent frames [40,41], 2 or 3D finite elements [42,43], limit analysis [44]). These models require a deep knowledge of building geometry, material, and structural characteristics, together with a huge computational effort. For these reasons, they were often applied to specific building typologies [45,46] or small building aggregates [36-44].

Computational multiscale models and methods represent a further accurate approach, applied to plenty of physical and engineering problems [47]. When different spatial and time scales are discretized in a unique framework, rough scales pre-condition fine scales through up/down scaling procedures. Local spectral conditions and energy minimization principles are usually employed to work successfully [48-50]. Very recent contributions provide a multiscale methodology for analyzing masonry buildings; they treat single units or aggregate of few buildings and consider entire panels or macroelements as the finest scale [51]. Computational multiscale approaches working with parallel/distributed computing were already been explored in literature [52], however, they still miss for masonry mechanics, except for some recent contributions in other physical problems [53].

RISK-UE project [54] was probably the first multidisciplinary research dedicated to seismic risk focusing on current, historical, monumental buildings, together with urban system, facilities, lifelines. It focused on several big European cities and tried to involve local authorities. Recently, results of structural vulnerability analyses were extended to the urban scale by considering the Emergency Limit Condition (ELC) [55] as a further performance level requested for buildings [56,57] and focusing on MUS of towns already hit by earthquakes [58]. Multidisciplinary approaches of seismic vulnerability at urban scale have been recently proposed by linking fast but accurate building survey with Geographic Information Systems (GIS), and structural analysis of row buildings [59,60].

An Italian database of historical centres was created in 2012 [61], aiming at monitoring their vulnerability and fostering information sharing between different levels of territorial government. However, subsequent seismic events showed the network ineffectiveness.

National Civil Protection Department [62] established a Seismic Observatory of Structures dedicated to SHM of some public buildings and infrastructures across Italy [63], whereas seismic sensor networks at urban scale have been recently proposed [64,65] to be mainly used as alert tools, without using acquired data for structural modelling and vulnerability evaluation.

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