Design and Validation of an Accurate Low-Cost Data Acquisition System for Structural Health Monitoring of a Pedestrian Bridge

Structural health monitoring (SHM) is an effective operating technique devoted to enhance the robustness of an infrastructure, and to validate its safety requirements. The aim of SHM is to determine a structure’s reaction when subjected to any type of excitation, by means of identifying modifications in basic vibration measurements and modal parameters such as natural frequencies, damping and mode shapes. Consequently, sensors are mounted on a structure intending to record data on equal time intervals basis prior to, during and after an induced stimulation. Therefore, the necessity to adopt a computer-based data acquisition (DAQ) technique is required in this analytical approach in order to evaluate vibrational signals collected by sensors placed on a structure. In this work an accurate microcontroller-based DAQ system is proposed to monitor a pedestrian bridge located in Athens Greece for the purpose of characterizing the system state and evaluate the modal properties of the investigated structure. Four low-cost yet accurate triaxial accelerometers were systematically placed along the bridge intending to report the system response toward different generated perturbations. The proposed monitoring and computational system was tested in laboratory conditions prior to the bridge assessment. Three triaxial accelerometer were installed on a steel cantilever beam. A comparative analysis between the results of the suggested DAQ system and that of the standard laboratory DAQ system National Instrument DAQ was performed to test the accuracy of the suggested framework

A contribution to the nonlinear stability analysis of multiple parameter systems.

The prebuckling, critical and postbuckling response of five different kinds of models is thoroughly discussed. These models are subjected to simultaneously concentrated loading and temperature variation as well as to other control parameters. More specifically: The first type of model is a rectangular two-bar frame, subjected simultaneously to axial compression and uniform temperature variation along the axes. The prebuckling, critical and post-buckling behaviour under various support conditions is investigated in detail. It is found that the temperature variation does not affect appreciably the critical state of the frame. The second type of model is a simply supported beam-column, made from an open asymmetric angle of thin-walled cross-section under axial thrust. The buckling, critical and postbuckling analysis is performed by using the variational method of Galerkin. The same model is also solved via the Finite Element Method and the results are practically coincident and satisfactory. The advantages and disadvantages of each of these methods are fully discussed. The third type of model is a one-degree-of-freedom (1-DOF) system, which consists of two rigid links of equal lengths pinned to each other. The model, which has an initial imperfection, is supported by a non-linear quadratic spring. The critical and postcritical response have been discussed also in terms of the Catastrophe Theory. The fourth type consists of four models which are one-degree-of-freedom (1-DOF) systems with various control parameters. These models are analyzed using the Catastrophe Theory after being classified into some of the seven elementary types of Catastrophes. Finally, the fifth type of model is a rectangular two-bar frame eccentrically loaded, which is studied by using Catastrophe Theory. The same frame is also analysed making use of the Finite Element Method (FEM), The results obtained from the different methods of analysis are compared and have been proved to be particularly satisfactory

A contribution to the nonlinear stability analysis of multiple parameter systems

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EXPERIMENTAL INVESTIGATIONS ON STEEL ANGLE BEAMS AND COLUMNS STRENGTHENED WITH CFRP PLATES

The results of a testing campaign are presented for rolled steel equal angle sections strengthened with Carbon Fiber Reinforced Polymers (CFRP) plates, targeting the strengthening of existing steel lattice towers. Three-point bending and compression tests have been carried out in order to determine the mechanical properties of members, specifically the bending capacity to principal and geometric axis loading and the buckling strength in the absence or presence of loading eccentricity arising from the connection in one leg. The experimental data include actual material properties, geometrical imperfections, the recorded response to loading in terms of displacements, end-rotations, twist of section, strains in steel or CFRP material, and photos. Also, the experimental set-up, the instrumentation, as well as simple experimental results to indicate the strengthening effect and utility of them to further analyses are provided.Funding was provided by the European Commission through the Research Fund for Coal and Steel (RFCS) project "ANGELHY – Innovative solutions for design and strengthening of telecommunication and transmission lattice towers using large angles from high strength steel and hybrid techniques of angles with FRP strips" (Grant Agreement No. 753993). Provision of the test specimens, consumables and application instructions by ArcelorMittal and Sika France is gratefully acknowledged

Experimental and numerical investigation of the influence of roller bending in rectangular hollow section steel arches

International audienceAn experimental and numerical investigation of the roller-bending influence on the in-plane behavior of rectangular-hollow-section steel arches is presented. Twelve circular roller-bent specimens, grouped in two sets of curvatures, were tested under tensile and compressive loading at the crown. Finite element models were developed to simulate in detail the curving procedure as well as the compression and tension tests and implicit static analyses accounting for geometric and material nonlinearities were carried out. Good quantitative and qualitative agreement was achieved between numerical and experimental results, in terms of load-displacement equilibrium paths, strain-gauge measurements and deformed shapes. The arches under compression exhibited a gradually softening response, attributed to the Bauschinger effect. Locked-in stresses and plastic strains demonstrated a non-symmetrical layout in which significant concentrations were located at the edges of the bottom flange; extended plastification was observed due to the induced shear. The influence of roller-bending on the overall structural behavior was assessed through the comparison of identical roller-bent and stress-free numerical models under various load conditions. Residual stresses were found to have a varying effect up to 10% on the overall response of arches, depending on the axial-bending interaction that takes place in each loading case

Development and Validation of a LabVIEW Automated Software System for Displacement and Dynamic Modal Parameters Analysis Purposes

The structural health monitoring (SHM) technique is a highly competent operative process dedicated to improving the resilience of an infrastructure by evaluating its system state. SHM is performed to identify any modification in the dynamic properties of an infrastructure by evaluating the acceleration, natural frequencies, and damping ratios. Apart from the vibrational measurements, SHM is employed to assess the displacement. Consequently, sensors are mounted on the investigated framework aiming to collect frequent readings at regularly spaced time intervals during and after being induced. In this study, a LabVIEW program was developed for vibrational monitoring and system evaluation. In a case study reported herein, it calculates the natural frequencies as well as the damping and displacement parameters of a cantilever steel beam after being subjected to excitation at its free end. For that purpose, a Bridge Diagnostic Inc. (BDI) accelerometer and a displacement transducer were parallelly mounted on the free end of the beam. The developed program was capable of detecting the eigenfrequencies, the damping properties, and the displacements from the acceleration data. The evaluated parameters were estimated with the ARTeMIS modal analysis software for comparison purposes. The reported response confirmed that the proposed system strongly conducted the desired performance as it successfully identified the system state and modal parameters

Dynamic Response Identification of a Triple-Single Bailey Bridge Based on Vehicle Traffic-Induced Vibration Analysis

Even though prefabricated steel Bailey bridges have been used for more than 80 years, limited studies of their structural features have been conducted, most of which do not consider their response in operational conditions. This study aimed at determining the modal parameters of a 30.48 m length Triple-Single (TS) Bailey bridge based on traffic-induced vibrations and comparing them with numerical results. Low-cost improvised accelerometers recorded and logged the actual response time histories, while a three-dimensional (3D) numerical model was developed to carry out the relevant dynamic analyses. The identification of modal parameters was based on the Operational Modal Analysis (OMA) process and the Frequency Domain Decomposition (FDD) method. Numerical analysis results are in accordance with the operational dynamic response of the Triple -Single Bailey bridge, confirming that the numerical model can effectively be used for extended dynamic analysis. In addition, the analysis of raw time histories through the OMA process indicates that the response is affected by the connections’ condition, in particular, the eventual looseness of bolts and pins. At least five eigenfrequencies were estimated and matched with relevant mode shapes

Dynamic Response Identification of a Triple-Single Bailey Bridge Based on Vehicle Traffic-Induced Vibration Analysis

Even though prefabricated steel Bailey bridges have been used for more than 80 years, limited studies of their structural features have been conducted, most of which do not consider their response in operational conditions. This study aimed at determining the modal parameters of a 30.48 m length Triple-Single (TS) Bailey bridge based on traffic-induced vibrations and comparing them with numerical results. Low-cost improvised accelerometers recorded and logged the actual response time histories, while a three-dimensional (3D) numerical model was developed to carry out the relevant dynamic analyses. The identification of modal parameters was based on the Operational Modal Analysis (OMA) process and the Frequency Domain Decomposition (FDD) method. Numerical analysis results are in accordance with the operational dynamic response of the Triple -Single Bailey bridge, confirming that the numerical model can effectively be used for extended dynamic analysis. In addition, the analysis of raw time histories through the OMA process indicates that the response is affected by the connections’ condition, in particular, the eventual looseness of bolts and pins. At least five eigenfrequencies were estimated and matched with relevant mode shapes