GNGTS 2013 - Atti del 32° Convegno Nazionale

Dynamic control of historical buildings through interferometric radar technique: an useful approach for Structural Health Monitoring on earthquake damaged structures S. V. Calcina, L. Piroddi, G. Ranieri Dpt of Civil Engineering, Environmental Engineering and Architecture (DICAAR), University of Cagliari, Italy Introduction. Structural damage identification is a fundamental element following an earthquake. A correct definition of the damage state of buildings allows us to establish technical procedures and operational standards for safeguarding the structures, aimed at restoring their original conditions. Structural Health Monitoring techniques (SHM) make it possible to deduce the presence of lesions and estimate the severity of the damage to the structures by measuring the mode of vibration of the buildings. This is possible because the dynamic response of structures is strongly influenced by the conservation state of materials and by structural lesions. The scientific literature includes many methods of structural dynamic response evaluation and algorithms for the extraction of main modal parameters (Ivanovic et al. , 2000; Sohn et al. , 2004). Several experimental configurations are classified in function of the number of measured output signals (number of sensors) and of the type of sources used to energize the structures. In this context there are several experimental layouts to modal parameters identification. Specifically, it is possible to differentiate in forced vibration tests (i.e. using mechanical shaker or vibrodines), free vibration tests (i.e. by means of falling objects inside or outside the buildings – impulsive signals) and ambient vibration tests (i.e. using random sources as wind and artificial noise) (Maia et al. , 1997). The last methods are very useful for assessing dynamic behaviour and for a rapid evaluation of the true conditions of existing damaged structures after an earthquake. However, the most common data acquisition techniques use in-contact sensors. Velocimeters or accelerometers arrays are placed inside the building and are used to record time series of ambient vibration. In the last decade, advances in the field of Ground Based Remote Sensing assure remote data acquisition and real-time monitoring of vibrations in critical conditions, as well as dynamic control of severely damaged structures after earthquakes. In this paper the authors propose a procedure for assessing the state of damage to structures in areas affected by earthquakes. This approach is founded on remote monitoring techniques of mechanical vibrations bymeans of interferometric surveys. Thismethod allows us to ensuremaximumsafety conditions during the monitoring of potentially damaged structures. In fact, the assessment of the structural damages during the next phase is extremely delicate due to precarious stability conditions of structures and due to possible further aftershocks during technical inspections by specialists. For this reason a new application of this proposed operational approach will be discussed in the case of the Emilia earthquake (Italy) for the stability control of the bell tower of San Giacomo Roncole using IBIS-S ground-based microwave interferometer. Instrumental features of the IBIS-S technology. Microwave interferometry has recently emerged as a new technology, specifically suitable to remotely measuring the vibration response of structures. Several authors observe that the interferometric technique has proven to be a useful remote sensing tool for vibration measurements of structures, such as architectural heritage structures (Pieraccini et al. , 2005, 2009; Atzeni et al. , 2010), engineering infrastructures (Gentile and Bernardini, 2008 and 2009) and vibrating stay cables (Gentile, 2010 a, b). This radar technology is founded on the combined use of high-resolution waveforms (Wehner, 1995) and interferometric technique (Henderson, 1998). This technique is implemented to compute the displacement of each target through the phase shift Δ of the back-scattered microwave signals collected at different time intervals. The displacement along the radar line of sight d r is computed from the phase-shift with the following simple Eq. (1): 101 GNGTS 2013 S essione 3.2