Modal Identification of Damaged Frames

Summary The paper investigates the possibility of identifying localised damages for multi-span and multi-floor linear elastic frames using only natural frequencies measured in the undamaged and damaged configurations. Namely, frames of increasing complexity are studied by exploring one by one their significant substructures (i.e. multi-span beams, floor by floor); the error function is defined and minimised on a database of finite element damaged models that only includes the natural frequencies of the local modes of the substructure, that is, the only modes significantly affected by the localised damage considered here. The performances and limits of the procedure are here discussed by means of numerical simulations on steel frames of increasing complexity; a particular attention is also devoted to the role of noise on the identification procedure. Copyright © 2015 John Wiley & Sons, Ltd

Seismic risk assessment of Trani's Cathedral bell tower in Apulia, Italy

Abstract The present paper deals with the evaluation of the seismic vulnerability of slender historical buildings; these structures, in fact, may manifest a high risk with respect to seismic actions as usually they have been designed to resist to gravitational loads only, and are characterized by a high flexibility. To evaluate this behavior, the bell tower of the Trani's Cathedral is investigated. The tower is 57 m tall and is characterized by an unusual building typology, i.e., the walls are composed of a concrete core coupled with external masonry stones. The dynamic parameters and the mechanical properties of the tower have been evaluated on the basis of an extensive experimental campaign that made use of ambient vibration tests and ground penetrating radar tests. Such data have been utilized to calibrate a numerical model of the examined tower. A linear static analysis, a dynamic analysis and a nonlinear static analysis have been carried out on such model to evaluate the displacement capacity of the tower and the seismic risk assessment in accordance with the Italian guidelines

Non-Destructive Damage Detection and Retrofitting Techniques on a Historical Masonry Tower

The aim of the paper is to detect the damage of the bell tower of the Church “Santa Maria della Natività”, Noci (Bari, Italy) that in 2012 was hit by a lightning and to propose a retrofitting solution. The solution must be designed to improve the constructive regularity, the strength and ductility of the structure, especially on the more resistant structural elements or on the mechanisms of collapse so as to transform them from brittle to ductile ones. The tower is 35 m tall and it is structurally connected to the Church for about one third of its height; the remaining part of the tower is totally disconnected from the structure of the Church. The results of the experimental tests have been analyzed in order to estimate the modal parameters of the tower and to calibrate a 3D finite element model of the tower in order to design ad hoc improvement interventions

The Performance of Empirical Laws for Rebound Hammer Tests on Concrete Structures

The assessment of concrete compressive strength plays a key role in the analysis of the seismic vulnerability of existing buildings. However, the adoption of classical destructive tests is usually limited by their invasiveness, cost and time needed for the execution. Thus, in order to overcome these limits and allow investigations to be extended to a large number of points, the use of the rebound hammer test is investigated here with a detailed analysis of the effects on the accuracy of the strength assessment related to the choice of the conversion model relating rebound index to compressive strength. The analysis has been performed by comparing several empirical laws calibrated with data acquired in an experimental investigation of an existing concrete building. The relationships between the coefficients of the examined conversion models are then established, with the aim of reducing the unknowns in the calibration procedure. Furthermore, the influence of the coefficients of variation of concrete strength and rebound index on the results of the calibration procedure has been analyzed, thereby supporting the assessment of the accuracy of the concrete strength.This research was funded by “Studio del comportamento meccanico e del retrofitting di edifici esistenti: approcci innovativi e Life Cycle Assessment” project Politecnico di Bari FRA 2021

Optimal model through identified frequencies of a masonry building structure with wooden floors

The paper presents the analysis of an important historical building: the Saint James Theater in the city of Corfù (Greece) actually used as the Municipality House. The building, located in the center of the city, is made of carves stones and is characterized by a stocky shape and by the presence of wooden floors. The study deals with the structural identification of such structure through the analysis of its ambient vibrations recorded by means of accelerometers with high accuracy. A full dynamic testing was developed using ambient vibrations to identify the main modal parameters and to make a non-destructive characterization of this building. The results of these dynamic tests are compared with the modal analysis of a complex finite element (FE) simulation of the structure. This analysis may present several problems and uncertainties for this stocky building. Due to the presence of wooden floors, the local modes can be highly excited and, as a consequence, the evaluation of the structural modal parameters presents some difficulties.This work was supported in part by the European Territorial Cooperation Programme “Greece-Italy 2007-2013”, under grants of the project Structural Monitoring of ARTistic and historical BUILding Testimonies (S.M.ART:BUIL.T.)

A preliminary approach of dynamic identification of slender buildings by neuronal networks

The study of the dynamic behavior of slender masonry structures is usually related to the preservation of the historic heritage. This study, for bell towers and industrial masonry chimneys, is particularly relevant in areas with an important seismic hazard. The analysis of the dynamic behavior of masonry structures is particularly complex due to the multiple effects that can affect the variation of its main frequencies along the seasons of the year: temperature and humidity. Moreover, these dynamic properties also vary considerably in structures built in areas where land subsidence due to the variation of the phreatic level along the year is particularly evident: the stiffness of the soil–structure interaction also varies. This paper presents a study to evaluate the possibility of detecting the variation of groundwater level based on the readings obtained using accelerometers in different positions on the structure. To do this a general case study was considered: a 3D numerical model of a bellower. The variation of the phreatic level was evaluated between 0 and −20 m, and 81 cases studies were developed modifying the rigidity of the soil–structure interaction associated to a position of the phreatic level. To simulate the dispositions of accelerometers on a real construction, 16 points of the numerical model were selected along the structure to obtain modal displacements in two orthogonal directions. Through an adjustment by using neural networks, a good correlation has been observed between the predicted position of the water table and acceleration readings obtained from the numerical model. It is possible to conclude that with a discrete register of accelerations on the tower it is possible to predict the water table depth.The authors express deep gratitude to Ministerio de Economia y Competitividad of the Spain׳s Government and the Generalitat Valenciana. This work was financed by them by means of the BIA2012-34316 and ACOMP/2014/289 research projects

Model Updating Based on the Dynamic Identification of a Baroque Bell Tower

The sanctuary of Santa Maria di Loreto (Mola di Bari, Italy) was built in in the sixteen century and it is constituted by a church with a simple hut façade characterized by a fine calcareous rose window and on its left side a high bell tower with heavy bells. In the present study, a non-destructive dynamic identification of the modal characteristics of the structure has been carried out by firstly organising an experimental setup of environmental measurements with several accelerometers placed on different levels of the bell tower. Secondly, the data acquired by the accelerometers have been elaborated by using modern statistic techniques of operational modal analysis (OMA). Finally, these results have been used for calibrating a complete finite element (FE) model of the bell tower structure that may permit to obtain important information about the state of integrity of the medieval structure. This calibrated model was developed to obtain a preliminary evaluation of the seismic vulnerability of the bell tower by performing a nonlinear static analysis

Measuring the modal parameters of a cultural heritage tower by using strong-motion signals

This paper presents the dynamic experimental campaign carried out on a stocky masonry clock tower situated in the Swabian Castle of Trani (Italy). The main objective of this paper is, after estimating the main frequencies and vibration modes of the considered structure, defining the transmission of vibrations along the height of the tower by varying the forced frequency at the base. At this aim, short acceleration records have been acquired simultaneously in 20 points of the tower at different levels, due to a series of sinusoidal forced vibrations applied at the base by using a pneumatic shaker device specifically designed for the tests. The proposed procedure permits to extract for each monitored point the amplitude of the sinusoidal component related to the excitation frequency and the phase shift due to the structure damping. The results of the proposed procedure are compared with the results of a classical operational modal analysis in environmental conditions in order to demonstrate that the short forced tests permit to classify the typology of the structural mode shapes.This work was supported by Structural Monitoring of ARTistic and historical BUILding Testimonies - (S.M.ART. BUIL.T.) Project of the European Territorial Cooperation Programme Greece-Italy 2007-2013 and by PRIN-MIUR 2010 research project entitled “Dynamics, Stability and Control of Flexible Structures”