# Dynamic Identification on an Irregular Structure
## 1 Introduction  
The seismicity of the Italian territory and the recently recorded seismic events require the adoption of effective measures capable of mitigating the effects of earthquakes on structures. The high vulnerability of the Italian built heritage is mainly associated with the high variability of the seismic hazard classification that has occurred in recent decades. The consequence of this circumstance is the existence of a great amount of the building stock realized in accordance with the seismic criteria but considering seismic forces lower than those imposed by the new seismic classification, and, in addition, a number of other buildings, the oldest ones, which have been built without any seismic criterion. Thus, in these cases, interventions to adapt the structures to the new seismic hazard are needed. To better design these types of interventions it is advisable to proceed with the structural monitoring before their execution, to obtain useful information on the building under exam. The level of knowledge acquired, in fact, strongly influences the design of structural improvement interventions. Monitoring is one of the most important management tools to guarantee the safety of buildings and to allow an economic and effective planning of maintenance interventions. This is useful in the case of structures that must perform their function for many years and that are utilized for several uses during their service life, different from those initially foreseen. In recent years, the Operational Modal Analysis (OMA) has become one of the most utilized techniques, leading to the identification of the unknown modal parameters of a building from the monitoring output-only analysis [1; 2; 3; 4]. This procedure allows us to accurately evaluate in situ the actual behavior of the structures [5; 6] in standard environmental conditions, without any applied force; for this reason, it is also used for buildings with monumental and historical value [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. In fact, some recent studies [19; 20; 21; 22] demonstrate that the knowledge of the modal parameters of the structure is essential for the evaluation of the seismic vulnerability and the effects of horizontal forces on structural parts and masonry infills during earthquakes. Furthermore, modern structural design techniques require the increasingly important use of mathematical models for the prediction of structural behavior such as the finite element (FE) model. The reliability of the results that can be obtained from these models depends on the underlying assumptions, i.e., the constraint conditions, the mechanical model adopted for the materials, the internal restraints, etc. For this reason, it is important to study data reprocessing techniques and their results that allow for structural identification and related parameters. In the literature, several experimental tests on structures have been studied and carried out to improve the level of knowledge; the final aim is to define the test apparatus capable of validating the FE model and guarantee the acquisition of a high level. However, the application to the analysis of reinforced concrete (r.c.) buildings characterized by irregularities is limited. In these cases, the irregularities influence the dynamic properties that are thus expected to be different from the usual ones and, consequently, an ad hoc definition of the testing set-up is needed. The present paper deals with these issues by analyzing the dynamic behavior of a strategic building, the \"Municipality Headquarter\" of Palagianello (Italy). This building is characterized by an r.c. frame and presents irregularities both in plan and elevation. In detail, this study involved a first phase of monitoring obtained by acquiring data with the installation of accelerometers in ad hoc chosen points of the structure. The second phase was to perform OMA on the acquired data, by means of ARteMIS software [23].