CALIBRATION OF NUMERICAL MODELS TO SUPPORT SHM: THE CONSOLI PALACE OF GUBBIO, ITALY

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Assessment of the rigid behaviour of diaphragms from ambient vibration measurements: application to masonry buildings in pre and postearthquake conditions

The paper presents an inverse procedure, recently proposed by the Authors, aimed at assessing the possible in-plane rigid behaviour of horizontal diaphragms in the global response of existing buildings. The procedure is based on the estimation of floors rigid rotations and angular deformations from ambient vibration data. When the estimation is carried in pre and post-earthquake conditions, it can provide useful information about the loss of rigid behaviour of the diaphragms, possibly linked to a reduction in their stiffness caused by seismic damage. The procedure is tested through numerical simulations assuming different hypotheses on the diaphragms rigidity, with the purpose of simulating increasing levels of damage in the floors. The computational model employed adopts the equivalent-frame approach and is inspired by the geometry and by the structural configuration of the "Pietro Capuzi" school in Visso. The building, continuously monitored by the Italian Seismic Observatory of Structures led by the Department of Civil Protection, has been heavily damaged by the 2016-2017 Central Italy earthquake sequence and was recently demolished. The application to pseudo-experimental response data allowed to identify a variation in the dynamic response of diaphragms due to seismic damage, highlighted by a significant increase in their angular deformations

Testing the dynamic behaviour of floor diaphragms for the seismic assessment of URM buildings

The dynamic behaviour of horizontal diaphragms plays a fundamental role in the seismic response of existing buildings. Indeed, it is well known that the floor diaphragms are responsible for the dynamic coupling and collaboration among the resistant elements, as well as for the redistribution of the seismic action in the nonlinear regime. The fulfilment of these fundamental structural tasks, which depends on the diaphragm in-plane stiffness and on the effectiveness of the boundary connections, can be particularly critical in existing masonry buildings, which are often characterised by timber floors or vaults that tend to exhibit a nonrigid in-plane behaviour. An in-depth knowledge of this subject is mandatory to guide the engineering judgment in the a priori assumptions governing the mechanical modelling, as well as to enable the a posteriori validation of the seismic assessment. To this purpose, the authors have developed a model-driven procedure to investigate the dynamic behaviour of the diaphragms by means of ambient vibration measurements. The procedure, employing at least two biaxial sensors for each floor diaphragm, solves an inverse kinematic problem to discriminate and estimate the diaphragm rigid rotation and macroscopic angular deformation from vibration data. A perturbation approach can be employed to run the procedure also in the typical operational case of minimal sensors availability. The rigid, quasi-rigid or deformable diaphragm behaviour of the observable structural modes is evaluated in the frequency domain, comparing the magnitudes of the power spectral densities associated to the rigid rotation and macroscopic angular deformation. Although the algorithm has already been successfully validated by means of laboratory data and tested through pseudo-experimental simulations against adverse field conditions, this paper investigates its application to full-scale measurements acquired on two existing unreinforced masonry (URM) buildings in Italy: the Pizzoli (AQ) town hall, in Abruzzo region and the Sanremo (IM) town hall, in Liguria region (Northwest Italy). The first application is supported by recordings from a permanent monitoring system installed by the Italian structural seismic monitoring network, whereas the latter by in-situ ambient vibration measurements. Emphasis is given not only to the capability of the procedure to support the calibration of numerical models, but also to its possible repercussions on the seismic assessment, in particular for simplified vibration-based approaches adopting the rigid diaphragm assumption. With this purpose, a more refined equivalent frame model of each structure \u2013 suitably calibrated to match the experimental modal parameters identified from the experimental measurements \u2013 is used to verify how finer rigid-diaphragm discretisations can improve the estimation of the mass participation factors, even for diaphragms commonly assumed as perfectly rigid

Vibration data processing to assess the rigidity of diaphragms in existing buildings

Ambient vibration tests are important tools for the calibration of structural models targeted at damage detection, health monitoring, seismic retrofitting, vibration control. The conceptual process of synthesizing mechanical models unavoidably involves a priori simplifications. Their a posteriori validation is a mandatory step to check the actual reliability of theoretical results. The in-plane rigidity of diaphragms is a common assumption in the seismic assessment of existing buildings, allowing simplified and cost-saving structural analyses. In this regard, the paper proposes a vibration-based procedure to assess the validity of the rigid diaphragm hypothesis. The procedure, requiring at least two bi-axial sensors, exploits a perturbative approach to distinguish and separately estimate the time histories of in-plane rotations and shear deformations of each diaphragm. A frequency domain representation allows the assessment of these variables for the identifiable modes. The effects of measurement noise, errors in sensor position and signal desynchronization are discussed through numerical simulations. The procedure is experimentally validated on a laboratory frame and applied to full-scale vibration measurements of a building. The outcomes highlight how higher modes tend to violate the rigid-body assumption

Structural identification of the dynamic behavior of floor diaphragms in existing buildings

The deformability of floor diaphragms plays a primary role in the structural behavior of existing buildings. Nonetheless, few structural identification procedures are available to investigate this matter from in-situ experimental measurements. Ambient vibration tests can be very useful to the purpose, allowing to assess the importance of the floor deformability in operational modal analyses through model-driven approaches. This information is particularly valuable for unreinforced masonry buildings,often characterized by deformable diaphragms whose effective stiffness is commonly unknown and hard to be evaluated. Based on these motivations, in this paper, a discrete linear model of deformable diaphragm is formulated in a novel fashion. The modal properties governing the free undamped dynamics are analytically determined through a fully general perturbation technique (direct problem). Therefore, a model-based structural identification procedure is proposed to analytically assess the inertial and elastic properties of the deformable diaphragm (inverse problem), assuming the outcomes of experimental modal analyses as known input. Consistently with the perturbation approach, explicit formulas are determined for low-order minimal models and higher-order model updating, accounting for mass and inertial eccentricities. Among the other identifiable mechanical parameters, the focus is put on the first and second-order identification of the in-plane shear stiffness of the diaphragm. The theoretical developments are successfully verified on pseudo-experimental and experimental bases, by applying the identification procedure to (i) the computational model of a prototypical steel frame structure, (ii) the large-scale laboratory model of a two-story composite structure with mass eccentricities, (iii) a permanently monitored masonry building recently struck by the 2016-2017 Central Italy earthquake sequence

Ambient vibration testing of existing buildings aimed to seismic assessment: experiences in Liguria

Ambient vibration tests (AVTs) provide valuable information about the dynamic behavior of existing buildings. Their use is well established in Structural Health Monitoring (SHM) applications to detect modification in structural behaviour after ageing, damage and retrofitting, and surely to calibrate or build numerical models aimed to the seismic assessment. Moreover, vibration-based methodologies assume great relevance within seismic mitigation strategies to prioritize retrofitting actions at urban scale. In this context, the Italian Department of Civil Protection developed the Seismic Model from Ambient Vibrations (SMAV) which relies on modal parameters extracted by AVTs to evaluate the operational level of strategic buildings following a seismic event. Within the application of the previous procedure, this work presents an extensive measurement campaign carried out in Liguria Region in collaboration with Geamb S.r.l in which full-scale AVTs have been performed on seven strategic buildings located in the provinces of Savona and Imperia. The structures under test, four made by reinforced concrete and three by unreinforced masonry (URM), differ greatly for dimensions, constraints and complexity. For each building system identification is carried out using several output-only techniques comparing the results in terms of natural frequencies, damping ratios and mode shapes. Since the measurement chains have been planned according to the assumption of diaphragms in-plane rigidity required by SMAV, the paper investigates how this can affect the accuracy of identified modal parameters and discusses some verification tools

Proposal for an MRPC system with high-precision timing in the LVD structure

The purpose of this paper is to present a project in order to verify -without the need of knowing the distance CERN-Gran Sasso- the discovery made by the OPERA Collaboration concerning the speed of the CERN neutrinos. The project consists of two parts. A simple one and a less simple one. Both have the great advantage of being totally independent of the knowledge of the distance, ≃ 732 km, between the two Labs, CERN and LNGS, where the neutrinos are produced and detected, respectively. The "simple" version of this project is based on the high-energy horizontal cosmic muons, which traverse LVD and OPERA detectors, thus allowing to cross-calibrate the timing systems of both experiments in a way which is totally independent of the TOF measurements of CNGS. This component of the project is being studied in collaboration with the OPERA group, as the time stabilities of both experiments are needed. In fact it is since a long time that the two groups are engaged with this problem. In this paper we will present and discuss the "less simple" part which allows to establish, at the highest possible level of accuracy, if (v > c) effects really exist. © Società Italiana di Fisica/Springer-Verlag 2012