Filling the gaps? The problem of integrating the missing elements in historic centres damaged by eartquakes

Some of the earthquakes that blessed Italian territory in the last 40 years (starting from Friuli earthquake in 1976) created strong damages in many small towns and little centers, where the historic network of buildings and roads was often suddenly interrupted or even cancelled. Besides the immediate huge sufferance of many human lives lost, there is also a long-term effect of the seismic destruction: the disappearing of the reference framework of a small community, often accompanied by the displacement of the population from the damaged old center during the emergency phase, followed by the final abandonment of the site and the unavoidable destruction.
In the analysis of seismic damages on historic buildings, many attention was paid in the past to the behavior of the single building, trying to understand the reasons of the peculiar weaknesses that each building exhibited face to seismic actions: this kind of observations give rise to a better knowledge of the seismic behavior of ancient constructions and also to a more appropriate approach to their reinforcement, both after the earthquake (and subsequent damage) and before that a new one can occur.
But only in recent years attention has been paid on buildings that are closely connected together, although often built in different times, and that sometimes strongly rely structurally on the already existing ones, almost for the resistance to the horizontal loads: this is the common way of building in historic centers, where houses are built in rows, or in blocks; and also the rows are connected together, like in Castelvecchio, with some kind of vaults or arches crossing the old paths. Besides the local weakness of the single building, an overall vulnerability of the system of buildings is to be studied, giving rise to the collapse of some parts of the aggregate or concentrating the damage in some of the core resisting elements.
So, despite the difficulty, an interpretation of the response of such systems can be made only taking into account the interaction between the different buildings and their constructive relationships, so that an history of the construction and of the damages occurred and repair provided is to be followed, also before any reasonable structural analysis, based mainly on direct observations of the buildings. Moreover, any proposal of integration of the loosing part of the urban network must take into account the seismic response of the aggregate, and how (or if) the new element is able to interact with the old parts also from the structural point of view; so that the question arise of when is useful (or mandatory) to rebuild collapsed buildings, and when instead the new gaps or the resulting new spaces may offer new possibilities for urban living, and must be regarded as a result of a natural evolution process.