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The work focuses on the application of modal identification techniques to plan-asymmetric building structures that are subjected to base excitation. The challenging problem related to the identification of high-order modes and, especially, torsional modes is taken into account for different lengths of time and directions of the applied base excitation. The evaluation of the identification accuracy was performed through a simulation approach: analytical models of plan-asymmetric RC frames, which are characterized by different structural eccentricities, were subjected to white noise base excitation. This kind of input is rather idealized for real buildings, which are generally excited by earthquake or traffic loads, but it is commonly adopted in experimental programs related to vibration-based damage detection using scaled or full-size buildings tested on shaking tables. The output-only modal identification was applied using the response time-histories only and the modal parameters, in terms of natural circular frequencies, modal damping ratios and mode shapes, were determined. These identified parameters were then compared with the true values related to the initially assumed analytical models. Firstly, the analyses were performed for one structural configuration, for a fixed direction of the applied base excitation, but considering various durations of such input. The results showed that the accuracy related to mode shapes identification was lower for the first torsional mode with respect to the second longitudinal mode, which is a higher-order mode. In addition, the identified torsional mode shape exhibited major modal complexity than the longitudinal mode, as a result of simulated noise and identification errors. Secondly, the analyses were conducted considering different structural configurations, a fixed duration of the base excitation, but different directions of the applied input. The outcomes showed that the accuracy in mode shape estimations, evaluated by calculating the Modal Assurance Criterion (MAC) between identified and analytical mode shapes, was strongly dependent on the direction of the applied input, especially for high-order modes. It was also observed that these MAC values are directly correlated with the modal participation factors of the structure, evaluated for the different directions of the base excitation. Referring to the torsional mode shapes, the results showed that the number of input directions, for which these modes can be identified, are higher for the structures with major structural eccentricities.

An area of research in civil engineering that has received increasing attention in recent years concerns the application of vibration-based health monitoring and damage detection techniques on structures under ambient vibrations. The dissertation presents research investigations that were carried out in this field, by focusing mainly on building structures and on techniques that belong to a subclass of the modal flexibility (MF)-based methodologies for damage detection. According to these techniques, modal flexibility-based models of structures are estimated from vibration tests, and then, by applying inspection loads to such models, structural deflections are determined and used for detecting damage. Three main problems are addressed in the thesis. 1) In modal testing and identification of real-life structures not all the modes can be usually identified. MF-based deflections are thus estimated using incomplete modal models, leading to truncation effects. To address this problem, approaches are proposed to predict and reduce the truncation effects on MF-based deflections of building structures. 2) In the literature the damage detection methods based on MF-based deflections have been mainly developed for building structures that can be modeled as plane structures. In an attempt to extend these existing methodologies to more complex structures, research investigations were carried out on simple rectangular “box type” 3D building structures characterized by either plan-symmetric or plan-asymmetric configurations. 3) Modal flexibility can be only determined when mass-normalized mode shapes are available. However, such scaled mode shapes can not be directly estimated from output-only data. To address this problem, a MF-based approach for damage detection in building structures that can be applied directly on output-only data with minimal or no a-priori information on the masses is proposed. For all three analyzed problems, numerical simulations and experimental output-only vibration tests conducted on frame building structures were used to demonstrate the effectiveness of the proposed approaches.

ispartof: Forecast Engineering: From Past Design to Future Decision - Graduate Courses for Structural Engineering Application 2015 & 2016 pages:61-83 ispartof: pages:61-83 status: published

The present work aims at performing an output-only modal identification starting from the dynamic responses of a plan-asymmetric RC frame structure that is subjected to a random ground motion along different directions. The main purpose is to evaluate the accuracy of the identified modal model for each direction of the exciting input, which is an unmeasured white noise signal. The Eigensystem Realization Algorithm is the applied identification method and it could be extended to the Operational Modal Analysis by adopting adequate pre-identification procedures. Two of them, which are the Natural Excitation technique and the Random decrement method, are separately applied and compared. All the identification procedures are performed on simulated data and the results show that the identification of the mode shapes of the structure is influenced by the direction of the applied ground motion; however, the main modes of the structure are always identified while some uncertainties occur for the modes at the highest frequencies, especially for the torsional ones. Notwithstanding the classical assumption of the OMA are not completely satisfied by the considered input, the obtained results, compared with the known analytical structural model, demonstrate the effectiveness of the algorithm.

In civil engineering structures it is highly desirable to detect the presence of damage and changes in the global structural behavior at the earliest possible stage, and, among the many existing strategies for vibration-based damage detection, modal flexibility (MF)-based approaches are promising tools. However, in most of the existing studies, the experimental validation of such approaches has been performed on small-scale laboratory structures, where damage has been artificially imposed as stiffness reductions, for example by substituting some structural elements. It is thus important to continue to test the effectiveness of such MF-based approaches on full-scale structures characterized by more realistic damaged conditions. This paper focuses on the methods for output-only damage detection and localization that are based on the estimation of structural deflections from modal flexibility, and the objective of this paper is to test the applicability of such methods for locating damage in a full-scale reinforced concrete (RC) structure that has experienced earthquake-induced damage. The considered structure is a shear wall building that can be modeled as a bending moment-deflecting cantilever structure, and was tested on the large-scale University of California, San Diego—Network for Earthquake Engineering Simulation (UCSD-NEES) shaking table. Two approaches, which are based, respectively, on the estimation of the curvature and the damage-induced rotation from the deflections, have been applied and compared on the data of the considered case study. These approaches have been applied in different scenarios characterized by different data sets and by a different number of degrees-of-freedom measured on the considered structure.

Methods based on the evaluation of structural deflections from experimental modal flexibility matrices are important tools in vibration-based damage detection, especially for building structures. Modal flexibility matrices, however, can not be directly estimated when dealing with output-only vibration data (e.g. in the important case of structures tested under ambient vibrations). One can use an a-priori estimate of the mass matrix of the structure or the added mass method to obtain the modal normalization constants needed to assemble the flexibility matrix, but these operations can be challenging for real-life civil structures. To address this problem, a modal flexibility-based approach for output-only damage detection and localization in building structures that can be applied with minimal or no a-priori information on the structural masses is proposed in this paper. This approach is also able to deal with the general case in which mass modifications, e.g. due to operational variability in structures, are present before and after damage. The approach is based on the estimation (directly from output-only data) of modal flexibility-based deflections of building structures that are proportional with respect to the corresponding true deflections. From analytical investigations it was found that the missing scaling factor between the two deflections can be made theoretically equal to the total mass of the structure, as proposed in the paper. Then, interstory drifts evaluated from the proportional deflections are used for damage detection and localization according to two proposed strategies. The first strategy can be applied with minimal a-priori information on the masses (i.e. using a parameter that quantifies an eventual relative modification of the total mass of the structure before and after damage). The second one is a more advantageous strategy that can be applied from output-only data without any a-priori information on the masses, even in the case in which the masses are varied before and after damage. The effectiveness of the proposed approach was demonstrated using both numerical simulations and experimental vibration tests on frame building structures.

In this paper a modal flexibility-based approach for damage diagnosis is presented and discussed. Modal flexibility matrices of structural systems can be derived from vibration tests and changes in these matrices can be associated to structural damage. One of the main challenges is to apply modal flexibility-based methods on real-life civil structures, to detect damage on structures using ambient vibration data. A recent method has been formulated for damage detection, localization, and quantification of building structures; it is based on the modal flexibility-based deflections of such structures under uniform loads. The method was originally formulated for frame buildings that can be modeled as plane shear-type structures. The objective of the paper is to test this methodology on generic buildings that, in principle, cannot be easily modeled as plane shear-type structures. The method was applied to the ambient vibration data of a steel frame structure that has a monitoring system with acceleration sensors. Various damage configurations were induced to the structure by removing diagonal braces on the external surface of the frame. The results showed that the method is able to identify the stories and the directions of the frame that have been affected by the damage.

It is of interest in the fields of vibration-based structural identification and damage detection to analyze the truncation effects introduced on modal flexibility (MF) based deflections that are estimated using only a subset of structural modes. To address this problem, an approach for truncation error analysis on MF-based deflections of structural systems subjected to a generic load is proposed in this paper. The approach is based on the determination of the relative contribution of each mode to the deflection by means of a proposed load participation factor (LPF). This factor, as derived analytically, depends both on the applied load and on the distribution of the structural masses. The validation of the proposed approach was carried out both on numerical models of shear-type frame buildings and on experimental data of a steel frame structure tested under ambient vibrations (i.e. the benchmark study sponsored by the IASC-ASCE Task Group on SHM). In both cases, results show that the LPF factors can give an a priori indication of the truncation effects expected on the MF-based deflections. The relationship between the proposed approach and the approach based on the mass participation factors, introduced by Zhang and Aktan (1998) for the case of uniform load (UL) deflections, is discussed since the two approaches are equal only if a special load, which is a mass proportional load (MPL), is considered. Thus, the application of this MPL load for mass irregular structures is also investigated. Numerical analyses performed both on a shear-type frame building and on a simply-supported beam, showed that for the great majority of the analyzed configurations, the truncation errors on the MF-based deflections due to the MPL are lower compared to those related to the UL.

The work aims at performing the dynamic modal identification of the long-span laminated timber footbridge built on the Marecchia River near Rimini, Italy. A first sine vibration test has been performed adopting a mechanical shaker just after the footbridge construction in 2000 in order to check the structural behavior that has been assumed in the project. A second test has been replicated in 2005 using the same excitation and almost the same test set-up adopted in the first one. The dynamic modal extraction is performed on the FRFs through both the peak picking method, applied together with the half power bandwidth technique, and the circle-fit method. The orthogonal properties of the identified mode shapes are verified through the Modal Assurance Criterion (autoMAC). Finally, the results obtained for the two tests and through the two techniques are mutually compared. It is worth noting that the FRFs, evaluated for different intensity levels of the exciting force, reveal an inherent non-linear behavior of the footbridge. The analyses also show that the first natural frequencies of the structure are included in the frequency range of the human step and this could lead to unpleasant feelings for the pedestrians.

The present work aims at applying and comparing some methods for vibration-based damage detection of civil structures starting from ambient vibration data. The analyzed procedures are based on a dynamic identification of the modal parameters that is performed in output-only conditions; this is a typical situation that occurs when structural health monitoring strategies are applied to civil structures. The here investigated damage-sensitive features are the modal parameters, the modal flexibility matrix and the damage-induced deflection, due to unitary inspection loads, of the identified structure; possible variations in these parameters can be adopted in order to detect, localize and quantify the damage. A damaged condition is analytically simulated through a stiffness reduction in the elements of a RC shear-type plane frame. The output-only modal identification is performed through the Eigensystem Realization Algorithm, combined with the Random Decrement technique, on the simulated responses due to a white noise ground motion of both the undamaged and the damaged structures. At the end, the effectiveness of the different identified damage features is evaluated and the accuracy related to the identified modifications, due to damage, is determined through a comparison with the initially assumed variations in the structural model.

An output-only vibration test was conducted on small steel frames during the OMA course at the 6th International Operational Modal Analysis Conference (IOMAC 2015). These structures were excited by human-hand-induced vibrations and tested with and without additional masses. This paper presents the data analysis on one of the structures that were tested. The objectives of this analysis are to compare two different OMA techniques (i.e. EFDD and NExT-ERA) and to investigate how the identified modal parameters of the structure are varied in case of added masses. Identified natural frequencies, modal damping ratios and mode shapes are in good agreement between the two selected OMA techniques. The results also show that modal damping ratios increased with additional masses on the structure.