The seismic response of masonry buildings arranged in clusters subjected to seismic strengthening interventions is a research topic still underexplored. This need especially emerged with the drafting of reconstruction plans of the municipalities affected by the earthquakes in L'Aquila (2009) and in Amatrice and Norcia (2016). This paper presents an assessment of the effectiveness of floor retrofit by varying its extent in a masonry urban cluster, using finite element modeling in DIANA FEA. Interventions are applied either to single or groups of SUs, to their full height (unitary interventions), or to individual levels (partial interventions), varying either the number of replaced floors or, when just one of them is replaced, its position. The reinforcement consists in replacing existing floors, modelled as orthotropic shells, with ribbed concrete ones, modelled as equivalent isotropic slabs.  

Results are evaluated in terms of capacity curves from non-linear static analyses, vibration modes of the buildings and tensile strains. 

The case study is a masonry urban cluster located in Castelsantangelo sul Nera (Macerata District), damaged by the 2016 earthquake.  

Both in terms of capacity curves and tensile strains, partial interventions by position show no particular difference from the unreinforced condition, while results worsen by reinforcing the roof only. In particular, numerical tensile strains reflect damage mechanisms observed in post-earthquake scenarios. At the same time, an increase in the number of replaced floors corresponds to almost constant increases in shear capacity.    

Considering unitary interventions with multiple SUs involved and applied to the full cluster or just in a portion of it, the evolution of both local mechanisms and overall displacements of the SUs is strongly affected by the insertion of rigid floors: indeed, these elements introduce a tying effect between piers belonging to different SUs. 

To obtain a refined representation of the structural behaviour, a future development of this work involves the implementation of discontinuities to represent construction phases between SUs. Moreover, the effect of different types of retrofit interventions, i.e. more compatible with the unreinforced structure, can also be explored.