Dynamic and vibration monitoring can be effectively implemented as a seismic assessment tool in case of an earthquake or natural disaster, where many buildings in a small area may be affected and damaged. In Italy, through the national network of the Seismic Observatory of Structures (OSS), the Department of Civil Protection (DPC) monitors the oscillations caused by earthquakes in 160 constructions of public property. The paper reports the research activities performed on three monitored structures of the OSS, affected by the Central Italy earthquake swarm of 2016 (max Mw=6,5). The three buildings (i.e. a school in Visso -MC, a former law court in Fabriano -AN and the city hall of Pizzoli -AQ) were constructed between 1920 and 1940 and are made of unreinforced masonry, with a good regularity in plan and elevation. Even though they present similar constructive features, the observed seismic-induced damages were largely different: heavy damages, near collapse, for Visso school; slightly damages for Pizzoli Municipality; and almost no damages for the building in Fabriano. The dynamic behavior of the monitored structures was studied through modal analysis techniques both under ambient vibrations and during the earthquake swarm. Data recorded by the monitoring systems are used to assess modal parameters variation and in particular to study the phenomenon of frequency and damping wandering during strong motion. Some severity indexes and dynamic parameters are calculated as well, including peak accelerations, structural amplifications, interstory drifts and elastic spectra. The results of modal identification will be used to assess the reliability of numerical models and their possible development to improve the predictive capacity in estimating the level of safety of existing structures.

The definition of proper intervention strategies for the selection of optimal (and minimum) interventions on exiting masonry buildings is a quite complex task. The choice has to take into account several aspects such as construction time, cost/benefit analysis, use of the building during the execution of interventions, resilience of the structure in case of exceptional events. Moreover, it is necessary to study the effectiveness of interventions to guarantee adequate safety levels as well as the reliability of numerical tools for structural analysis and design. In this framework the research activities presented here aim at assessing the effectiveness of strengthening interventions executed on an unreinforced masonry building hit by the Central Italy earthquake swarm of 2016 (max Mw=6,5). The structure, constructed in 1940, is equipped with a dynamic monitoring system, managed by the Seismic Observatory of Structures (OSS) of the Italian Department of Civil Protection (DPC), that was able to record the dynamic response during each seismic sequence. Ambient vibrations and strong motion data were used to calibrate and validate numerical models, built with the equivalent frame approach using 3Muri software. Then parametric nonlinear static analyses on the calibrated models were implemented in order to assess the effectiveness of interventions performed on vertical and horizontal structural elements. The incremental application of interventions was also evaluated, and various strengthening technique tested, comparing the response in terms of structural performance and seismic capacity.

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