Carlo Rainieri

Integrated seismic early warning and structural health monitoring of critical civil infrastructures in seismically prone areas

A large part of Europe is exposed to medium/high seismic risk. Throughout the past decades serious structural damage and collapse have occurred in different countries. Examples of structures at risk are existing infrastructure and public buildings. Efficient seismic protection is especially required in these structures. In fact, an earthquake can lead to a high number of injury or death since these structures are often crowded. On the other hand, strategic structures have to be fully operational to manage the aftershock emergencies. This article deals with some results of a research focused on the development of optimized structural health monitoring (SHM) technologies and data processing techniques for critical structures in seismically prone areas. Specific solutions are proposed to take advantage of seismic early warning systems (SEWSs), which are becoming very popular and effective worldwide. The most relevant aspects of seismic early warning (SEW) and SHM systems are herein reviewed and the main issues related to their integration are discussed in order to properly design the final system. Attention is mainly focused on dynamic behavior in operational conditions and on earthquake effects. Hardware and software solutions adopted for the characterization and monitoring of the dynamic response of a sample building are illustrated pointing out the capability of the same architecture to host data and information provided by SEW applications. Finally, datasets in operational conditions are used to evaluate the fundamental modal parameters of the structure by output-only techniques, whose potentialities and limitations in the presence of weakly and heavily nonstationary signals have been also assessed. In particular, they have been applied, respectively, to the records collected during crowded football matches hosted at the stadium located nearby the sample building and during the recent L’Aquila earthquake mainshock.

Some remarks on experimental estimation of damping for seismic design of civil constructions

Experimental estimation of damping is currently not a comprehensively solved problem. Although modal frequencies and mode shapes can be measured confidently and quite easily by means of dynamic tests on civil structures, an accurate identification of damping ratios needs further development. Experimental values can be characterized by large error bounds mainly because damping is strongly influenced by the magnitude of the dynamic response of a structure.

Automated Operational Modal Analysis as Structural Health Monitoring Tool: Theoretical and Applicative Aspects

The aim of structural health monitoring for civil structures is not only detection of sudden or progressive damages but also monitoring their performance under operational conditions or under some particular environmental issues such as earthquakes. Seismic protection of buildings at risk can be reached increasing the knowledge of the structural behavior of existing constructions. This circumstance points out the opportunity of monitoring the performance of civil structures over their operational lives. The present paper deals with automated Structural Health Monitoring (SHM) technologies adopted for the School of Engineering Main Building at the University of Naples “Federico II”. In particular, the attention is focused on the development of an automated procedure based on the Operational Modal Analysis (OMA) that must ensure the continuous monitoring and extraction of the modal parameters of the building. Some numerical examples are then discussed in order to point out effectiveness of the algorithm and relevant issues that need to be improved.

Perspectives of Second-Order Blind Identification for Operational Modal Analysis of Civil Structures

Innovative methods for output-only estimation of the modal properties of civil structures are based on blind source separation techniques. In the present paper attention is focused on the second-order blind identification (SOBI) algorithm and the influence of its analysis parameters on computational time and accuracy of modal parameter estimates. These represent key issues in view of the automation of the algorithm and its integration within vibration-based monitoring systems. The herein reported analyses and results provide useful hints for reduction of computational time and control of accuracy of estimates. The latter topic is of interest in the case of single modal identification tests, too. A criterion for extraction of accurate modal parameter estimates is identified and applied to selected experimental case studies. They are representative of the different levels of complexity that can be encountered during real modal tests. The obtained results point out that SOBI can provide accurate estimates and it can also be automated, confirming that it represents a profitable alternative for output-only modal analysis and vibration-based monitoring of civil structures.

An Integrated Seismic Monitoring System for a Full-Scale Embedded Retaining Wall

Over the last 50 years, data from laboratory tests and post-earthquake reconnaissance have been used to gain knowledge about the dynamic and seismic behavior of geotechnical structures and to improve analysis and design procedures. The scarcity of reconnaissance data has pointed out the need for full-scale and near full-scale tests for research purposes in earthquake engineering. Structural health monitoring (SHM) systems have been applied to different kinds of structures, but although static control of displacements and pressures is quite common, dynamic monitoring is fairly limited in geotechnical engineering. In the present paper, an integrated structural and geotechnical monitoring program aimed at filling this knowledge gap is described with reference to a real flexible retaining wall. The objective of the research is to establish a combination of sensors, numerical analyses, and data processing procedures to turn the monitored retaining wall into a smart geotechnical structure. Attention is herein focused on two aspects related to the implementation of the monitoring system and the dynamic identification of the soil-structure system. Specifically, the paper describes an example of seismic monitoring system for full-scale flexible retaining walls based on sensors embedded in the reinforced concrete piles. Installation details are given together with a description of the monitoring system architecture in its current stage of implementation. Issues and requirements for the measurement chain are discussed, taking into account possible installation drawbacks (i.e., shocks) and the opportunity to monitor the response under operational conditions. Furthermore, a novel and successful application of operational modal analysis (OMA) to such a complex geotechnical system is reported. It allows for the identification of the fundamental modes of the soil-wall system in operation (but before the erection of a building on the excavated side). Implementation of the monitoring system prototype is still in progress, but some interesting results have already been outlined.

Influence of model order and number of block rows on accuracy and precision of modal parameter estimates in stochastic subspace identification

The accurate estimation of the modal properties of civil structures in operational conditions is critical in a number of applications, including structural health monitoring. The available algorithms allow confident estimations of modal frequencies and mode shapes. Damping estimates, instead, are jeopardised by large error bounds, related to the non-linear behaviour of damping as well as to inherent limits of the estimators and eventually poor measurements. In general, the variability of modal parameter estimates is due not only to environmental factors but also to the performance of the estimator. Thus, the accuracy of the estimator has to be optimised to avoid errors in automated modal parameter identification. In this paper, the influence of model order and number of block rows on the accuracy of estimates via stochastic subspace identification (SSI) is investigated. An approach able to provide narrow error bounds and improve the reliability of SSI-based automated modal identification algorithms is discussed.

Integrated non-destructive assessment of relevant structural elements of an Italian heritage site: the Carthusian monastery of Trisulti

The analysis of historical structures in need of preservation and restoration interventions is a very complex task due to the large uncertainties in the characterization of structural properties and detailing in view of the structural response. Moreover, the predictive performance of numerical analyses and simulations depend on the availability of information about the constructional properties of the architectural complex, crack patterns and active degradation phenomena. In particular, local changes in material properties or damage due to past events (such as earthquakes) can affect individual structural elements. They can be hardly detected as a result of the maintenance interventions carried out over the centuries and the possibility to carry out limited or even no destructive investigations due to the historical relevance of the structure. Thus, non-destructive investigations play a fundamental role in the assessment of historical structures minimizing, at the same time, the invasiveness of interventions. The present paper deals with an explanatory case study concerning the structural investigations carried out in view of the seismic assessment of an Italian historical monument, the Carthusian monastery of Trisulti in Collepardo, erected in 1204 under Pope Innocenzo HI. The relevance of the case study is due to the application, in combination, of different NDT methods, such as sonic tests, and active and passive infrared thermography, in order to characterize relevant masonry elements. Moreover, an advanced system for the in-situ nondestructive vibration-based estimation of the tensile loads in ancient tie-rods is described and the main results obtained from its application for the characterization of the tie-rods of the cloister are presented.

Accurate Damping Estimation by Automated OMA Procedures

Systems and techniques for fast damage detection based on vibration analysis are becoming very attractive in different engineering fields. Modal-based damage detection algorithms are well-known techniques for structural health assessment. However, the lack of automated modal identification and tracking procedures has been for long a relevant limit to their extensive use. The development of several automated output-only modal identification procedures in the last few years has led to a renewed interest in modal-based damage detection. However, robustness of automated modal identification algorithms, computational efforts and reliability of modal parameter estimates (in particular, damping) still represent open issues. In this paper, a novel algorithm for automated output-only modal parameter estimation is adopted to obtain reliable and very accurate modal parameter estimates. An extensive validation of the algorithm for continuous monitoring application is carried out based on simulated data. The obtained results point out that the algorithm provides fairly robust, accurate and precise estimates of the modal parameters, including damping ratios. This may potentially lead to a standardized, extensive characterization of modal damping ratios in structures, which is useful to gain knowledge about damping mechanisms in structures and to develop predictive models.

CNT-cement based composites: fabrication, self-sensing properties, and prospective applications to structural health monitoring

Degradation phenomena can affect civil structures over their lifespan. The recent advances in nanotechnology and sensing allow to monitor the behaviour of a structure, assess its performance and identify damage at an early stage. Thus, maintenance actions can be carried out in a timely manner, improving structural reliability and safety. Structural Health Monitoring (SHM) is traditionally performed at a global level, with a limited number of sensors distributed over a relatively large area of a structure. Thus, only major damage conditions are detectable. Dense sensor networks and innovative structural neural systems, reproducing the structure and the function of the human nervous system, may overcome this drawback of current SHM systems. Miniaturization and embedment are key requirements for successful implementation of structural neural systems. Carbon nanotubes (CNTs) can play an attractive role in the development of embedded sensors and smart structural materials, since they can provide to traditional cement based materials both structural capability and measurable response to applied stresses, strains, cracks and other flaws. In this paper investigations about CNT/cement composites and their self-sensing capabilities are summarized and critically revised. The analysis of available experimental results and theoretical developments provides useful design criteria for the fabrication of CNT/cement composites optimized for SHM applications in civil engineering. Specific attention is paid to the opportunities provided by new RF plasma technologies for the functionalization of CNTs in view of sensor development and SHM applications.

Estimating the Elastic Period of Masonry Towers

The fundamental period of vibration plays a primary role for the assessment of the seismic demand on structures. It can be evaluated by numerical analyses, or even according to basic formulations provided by building codes for common structural typologies.