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Monitoring and Identification of the Seismically Isolated

Seismically Isolated

This paper describes the installation and management of the monitoring system of the “Our Lady of Tears Shrine” in Syracuse, whose dome is an imposing r.c. and prestressed r.c. structure of about 22,000 ton that was seismically isolated by flat sliding devices with hysteretic dampers. The monitoring system, Seismically Isolated representing an upgrading and improvement of an old system never made working, has some innovative features, because it allows to manage with the same dedicated hardware and software both the slow (thermal variations, relative humidity, wind direction and velocity) and the fast acquisitions (dynamic vibrations by wind and earthquake). The monitoring system was inserted among those structures maintained and controlled by the Seismic Observatory of Structures of the National Department of Civil Protection. Some records of low magnitude earthquakes allowed to validate the correct behaviour of the whole structure, as well as to make a dynamic identification of the complex construction and to calibrate a detailed finite element model of the Sanctuary, thus predicting isolators’ behaviour under design earthquake.

In the last decades, a lot of existing or new constructions located all over the world have been object of seismic isolation interventions, aiming to passively control their structural behaviour. The efficacy of such interventions has been tested during recent real ground motions, as well as within laboratory and in-situ testing campaigns. This technology is now recognized as allowing structures to attain higher safety levels with respect to those achieved by conventional structures, even if modern seismic criteria are applied. Its large diffusion allows achieving a sensible reduction of seismic risk and is favoured by reliable codes and optimised computations [1] – [6] .

On the other hand, the new monitoring technologies have now a fundamental role in the structural field and have been progressing by supporting and helping the research through the use of more and more accurate and cheap instrumentations and methodologies: the structural monitoring represents an increasingly accessible reality, so that it can be considered one of the most important mean of decision support [7] [8] .

In its first applications, the need of a structural monitoring derived from the need to determine the safety conditions of historical constructions. At national level, during the last years of the twentieth century, the first case studies were relative to monumental buildings, as a consequence of a rediscovery of the Italian architectural heritage. Nowadays, the application field of the structural monitoring is extended to all the civil engineering constructions, from the transport infrastructures (bridges, dams, galleries, etc.) to the strategical buildings. Such a spread is due to multiple reasons: the technological progress in terms of sensors and data processing; the need to get more information on how a structure behaves, so reducing the uncertainties on its safety; the potentialities in the framework of the emergency management during catastrophic events; the local and/or global real-time control of the structure.

A structural monitoring system is able to acquire (in continuous, if necessary) the values assumed by some significant quantities, to warn appointed technicians on possible anomalies or the overcoming of defined threshold values, and the consequent automatic activation of alarm procedures and/or of suitable emergency measures. These technologies must allow to obtain all those information which points out a damage, an unexpected response or a change in the structural behaviour, and, if necessary, they must provide significant data concerning the environmental input. From the knowledge of these information derives the possibility to actuate eventual maintenance interventions, before the latters become very onerous in economic terms.

The static monitoring is addressed to observe the slowly variable quantities, as those related to thermal variations and meteorological phenomena: in spite of the greater simplicity with respect to the dynamic monitoring, it has huge potentialities, as happens when the data related to wind actions are acquired. The results of the static monitoring allow to the designer to get useful indications for a correct analysis of the structural behaviour.

The most important innovations have been achieved in the framework of the dynamic monitoring, directed to observe rapidly variable physical quantities. Such a kind of system can be designed in different ways, according to the desired objectives: the dynamic characterization of the monitored structure, the assessment of the time varying response of the structure, the transmission of a seismic excitation along a structure, etc.

Some years ago, the Italian National Seismic Service (SSN) started an important project, called Seismic Observatory of Structures (OSS), representing a complex network of structures properly selected on national scale, which include a continuous monitoring system able to provide useful experimental data and to transmit alarms in case of seismic events: bridges, public constructions as hospitals, city halls, churches, schools, isolated structures, etc. [9] . In the specific case of the isolated structures, the monitoring system allows to control the performance of the installed isolators. The OSS network is managed by a central computer, located in the SSN headquarters, where the recorded data flow in real-time: these data are automatically processed in a preliminary phase, and afterwards published on internet. Therefore, the OSS Project is able to provide to the academic and professional community all the data needed to evaluate the real response of a structure in exercise, to verify the performance of a control system installed on the monitored structure, etc. The goal of the OSS Project is that a monitoring system is able to return a picture of the structure, which is updated over time, and allows to determine an estimate of the resources needed to reduce the seismic risk. On the other hand, the OSS network provides the opportunity to compare the real constructions with the ideal project, whenever a check of the existing constructions’ safety is required according to the new codes.