Elsa Caetano

Investigation of dynamic cable–deck interaction in a physical model of a cable-stayed bridge. Part I: modal analysis

A description of an experimental investigation involving the study of the dynamic interaction between the cables and the deck/towers system in cable-stayed bridges is presented. The work was carried out on a physical model of a cable-stayed bridge (the Jindo Bridge, in South Korea), whose characteristics of stiffness and mass have been conveniently scaled. Standard modal analysis tests were performed using both an electrodynamic shaker and a shaking table, leading to the creation of a high-quality database, characterizing the dynamic behaviour of the bridge. The study shows the existence of a clear dynamic interaction between the cables and the deck/towers system, associated with the appearance of several groups of mode shapes, at closely spaced frequencies, involving different cable movements, but similar configurations of the girder and towers.

Investigation of dynamic cable-deck interaction in a physical model of a cable-stayed bridge. Part II : seismic response

The present paper describes an investigation of the effect of dynamic cable interaction with the deck and towers in the seismic response of a cable-stayed bridge. This study involved shaking table tests performed on a physical model of Jindo bridge, in order to validate two alternative numerical models, which differ in terms of consideration of coupled cable/deck and towers modes. The response to artificial accelerograms was calculated and correlated with measured data. Additional numerical simulations are presented in order to clarify the role that cables play in the attenuation or amplification of the structural response. It was found that the cable interference with global oscillations may cause a decrease of the bridge response. However, this ‘system damping’ may not develop in the case where a narrow-band excitation is applied, causing large amplitude of vibrations of some cables, with significant non-linearity, and inducing higher-order modes. Copyright

Combined Experimental-Operational Modal Testing of Footbridges

In combined vibration testing, an artificial, measured force is used in operational conditions. This requires the identification of a system model that takes both the measured and the operational excitation into account. Advantages with respect to the classical operational modal analysis approach are the possibility of obtaining mass-normalized mode shapes and the increase of the excitation level and its frequency content. An advantage with respect to the classical experimental modal analysis approach, where the ambient excitation is not modeled, but considered as disturbing noise, is the possibility of using excitation levels that are of the same amplitude, or even smaller, than the ambient excitation levels. In this paper, combined modal testing of footbridges is explored using two case studies: a steel arch footbridge with spans of 75.2 m and 30.3 m and a concrete stress-ribbon footbridge with spans of 30 m and 28 m. The comparison of the modal parameters (eigenfrequencies, damping ratios, mode shapes,...

LabVIEW toolkits for output-only modal identification and long-term dynamic structural monitoring

This paper describes the development of computational tools for modal identification and long term dynamic monitoring in LabVIEW environment. These tools mainly consist of two individual toolkits for structural modal identification (SMI) and continuous monitoring (CSMI), respectively. The SMI toolkit implements the frequency domain Peak-Picking (PP) and Enhanced Frequency Domain Decomposition (EFDD) method, as well as the time domain Stochastic Subspace Identification (SSI) techniques. Based on this toolkit, the user can easily develop the whole process of structural modal identification by simply pushing buttons. The CSMI toolkit was conceived for continuous dynamic monitoring excluding manual interaction. It automatically searches the latest output measurements, detects maximum vibration amplitudes and makes statistical treatment of acceleration time series, generates waterfall plots for depicting the frequency component distribution and identifies modal parameters based on automated EFDD technique. The application of these tools is briefly described based on experimental data collected at Pinhão bridge and Coimbra footbridge.

Evaluation of dynamic effects and fatigue assessment of a metallic railway bridge

In the framework of the national research project ‘Evaluation of the structural integrity of metallic railway bridges’, the Laboratory of Vibrations and Monitoring of the Faculty of Engineering of the University of Porto, Portugal has recently developed a study concerning the assessment of the dynamic behaviour and fatigue condition of a metallic railway bridge on the international ‘Beira Alta’ route in Portugal. This paper describes the numerical simulation of the dynamic behaviour of the bridge under railway traffic, the experimental updating and validation of the finite-element model used, and the estimation of the fatigue residual life using different code specifications and the characteristics of real trains crossing over the bridge.

Vertical Crowd–Structure Interaction Model to Analyze the Change of the Modal Properties of a Footbridge

AbstractIn this paper, a biomechanical crowd–structure interaction model is proposed and further implemented to adequately estimate the energy exchange between pedestrians and a footbridge. The proposed model focuses on the vibrations in the vertical direction, and it allows for taking into account the change of the modal properties of the structure due to the presence of the pedestrians, thus improving the numerical estimation of the response of the structure under pedestrian flows. The model involves two submodels, namely (1) a pedestrian–structure interaction submodel and (2) a crowd submodel. The first submodel follows from a modal projection of a system with two degrees of freedom, which simulates the behavior of each pedestrian, on the vibration modes of the structure. The parameters of this model have been estimated from the accelerations recorded on a real footbridge. For the second submodel, the crowd behavior is simulated via a multiagent method. The performance of the resulting overall model is...

Numerical and experimental studies of the Braga Sports Stadium suspended roof

This paper describes a set of dynamic numerical and experimental studies conducted on the suspended roof of the Braga Municipal Sports Stadium. Modal parameters, calculated from a numerical model of the constructed structure that takes into account the geometrically nonlinear structural behaviour and the progressive application of loads, are compared with the corresponding parameters identified on the basis of an ambient vibration test. Particular attention is given to modal damping identification due to the necessity of analysing the susceptibility of the suspended roof to buffeting effects.

Updating Numerical Models of Masonry Arch Bridges by Operational Modal Analysis

This article aims at presenting and discussing the strategies for updating the finite element numerical modeling of stone masonry arch bridges using operational modal analysis. The study comprehended three bridges: two old ones, the St. Lázaro and the Lagoncinha bridges, and a recently constructed bridge in Vila Fria, Portugal. Updating of the bridge models is performed by comparing the numerical and experimental modal parameters. Three-dimensional detailed numerical models are used to perform modal analysis of the bridges. Experimental modal identification of the bridges is based on the measurement of their acceleration responses during normal operation. The assigned material properties are also based on available results obtained from in situ and laboratory tests and on the results of visual inspection and historical research carried out for both old bridges.

Vandal Loads and Induced Vibrations on a Footbridge

The paper describes a study to characterize intentional human dynamic loads (vandal loads) and their induced effects on a footbridge. A numerical model to characterize the dynamic behavior of the footbridge has been developed, is experimentally validated, and is used to numerically simulate the response induced by groups of pedestrians synchronized at critical bridge frequencies. The vandal load associated with a single pedestrian is characterized and compared with definitions in the literature. The response is then calculated considering the measured load and compared with the measured response to the excitation induced by a single pedestrian or a group with varying dimensions. The definition of a general load model for vandal excitation, including a tentative definition for synchronization is discussed.

Shape Sensing Monitoring System Based on Fiber-Optic Strain Measurements: Laboratory Tests

In the context of the characterization of the shape of flying sails whilst racing, a novel method for real-time form-finding of flexural beams based on strain measurements has been developed and implemented. The very large displacements and deformations occurred on the beam pose specific demands both to the algorithm for shape detection and to the materials and sensors to use in the practical implementation, which are described and addressed in this paper. The developed method is implemented by installation of fiber Bragg grating (FBG) sensors on a beam, which are used to evaluate the curvature at specific cross-sections. The deformation curve is reconstructed by integration of local curvatures and interpolation. The effectiveness of the developed method is verified by tests on a physical model. An image-based monitoring system is further employed to validate the concept and the tests conducted with the FBG sensors. This allows the performance of an additional series of sensitivity tests in order to define the most relevant measurement parameters and characterize the accuracy of the developed method.