1,721,024 research outputs found
Improving the speed performance of an Evolutionary Algorithm by a second-order cost function approximation
No full textIn the present paper, dynamic identification problem of a FE structure with unknown parameters is solved by global search method. Response surface methodology is introduced in Differential Evolution algorithm to improve the performance of the algorithm. Differential evolution (DE) is an evolutionary algorithm where N different vectors collecting the parameters of the system are chosen randomly or by adding weighted differences between vectors obtained from two populations. In the modified algorithm, the new parameter vector is defined as the minimum of a second-order polynomial surface, approximating the cost function. Performance in term of speed rate is strongly improved by introducing the second-order approximation; nevertheless, robustness of DE algorithm for global minimum search of cost function is preserved, since multiple search points are used simultaneously. A numerical examples is presented, concerning identification of mechanical parameters of a steel truss girder bridge with unknown values of masses and stiffnesses of bracing and bearing
Parameter Calibration of a Social Force Model for the Crowd-Induced Vibrations of Footbridges
No full textA reliable prediction of the human-induced vibrations of footbridges relies on an accurate representation of the pedestrian excitation for different loading scenario. Particularly, the modeling of crowd-induced dynamic loading is a critical issue for the serviceability assessment of footbridges. At the design stage, the modeling of crowd loading is often derived from single pedestrian models, neglecting the effect of the structural vibrations as well as the interactions among pedestrians. A detailed description of the crowd behavior can be achieved employing a social force model that describes the different influences affecting individual pedestrian motion. These models are widely adopted to describe the crowd behavior especially in the field of evacuation of public buildings, public safety and transport station management while applications in the serviceability assessment of footbridges are less common. To simulate unidirectional pedestrian flows on footbridges, this paper proposes a parameter calibration of the Helbing’s social force model performed adopting the response surface methodology. Parameters of the social force model are calibrated so as to represent the fundamental relation between mean walking speed and density of the pedestrian crowd. The crowd-induced vibrations are then simulated by modeling each pedestrian in the crowd as a vertical load that crosses the footbridge with time varying trajectory and velocity estimated from the calibrated social force model. Finally, results are compared to those obtained from a multiplication factor approach proposed in literature. This considers the crowd as a uniform distribution of pedestrians with constant speed and given synchronization level and the footbridge response is evaluated as the response to a single pedestrian scaled by a proper enhancement factor
Identificazione strutturale di una struttura metallica mediante prove di vibrazione forzata
No full textNella presente nota si riporta un esempio applicativo di una metodologia di identificazione basata su prove di vibrazione forzata. Vengono descritti i risultati di una prova dinamica con vibrodina effetuata su una porzione dell’ampliamento del Politecnico di Torino a struttura metallica: dai dati sperimentali si sono calcolate le funzioni di risposta in frequenza che hanno fornito frequenze proprie e deformate modali. Successivamente si è confrontata la risposta con un modello agli elementi finiti correlando modi propri e frequenze
Vehicle-bridge interactive analyses on the Lagoscuro viaduct
No full textThe FADLESS research project aims at defining innovative technical guidelines for the assessment and control of existing and new bridges with regard to fatigue phenomena induced by vibrations and distortions produced by the passage of trains. For this purpose an improved representation of the vehicle loads is required.
The dynamic interaction between a bridge and the moving vehicles represents a special discipline within the structural dynamics field. In order to simulate the train-bridge interactive dynamics many kinds of two-dimensional and three-dimensional models for train carriages have been presented and adopted, in which the springs and the dashpots are used to describe the interactive effects between wheels and primary suspensions as well as primary and secondary suspensions. The subsystems interact through the contact forces, the forces induced at the contact points between the wheels and the rails surface (of the railway bridge) or the pavement surface (of the highway bridge). Such a problem is nonlinear and time-dependent because of the fact that the contact forces may move from time to time, while their magnitudes do not remain constant, as a result of the relative movement of the two subsystems.
This paper presents the results concerning the dynamic interaction between trains and the Lagoscuro bridge. In the vehicle-bridge interaction (VBI) models [1] the car bodies, the bogies and the wheels are assumed to be rigid and connected by means of springs and dashpots to represent the two suspensions systems.
Dynamic analyses will be conducted using a suitable numerical model of the Lagoscuro railway bridge. First, dynamic analyses of the bridge will be performed using the standard procedure given by the Eurocode 1, where the vehicle-bridge interaction is neglected and only a sequence of moving loads is defined, and the results will be compared with those obtained using the two-dimensional VBI model. Then, vehicle-bridge interactive analyses are performed, considering a three dimensional model for cars and bridge to study also lateral and torsional vibrations of the bridge. Data regarding the suspension systems of the high-speed train ETR500Y given in [2] are used. Finally, parametric investigations have been conducted as regard the travelling speed and the weight of the train
Experimental modal analysis and fatigue assessment on the Lagoscuro viaduct
No full textIn this paper, the preliminary results concerning the fatigue assessment of Lagoscuro railway viaduct is described. First, experimental dynamic tests have been performed in order to identify the modal properties. Then, a FE model is corrected with a model updating procedures, where the uncertain model properties are adjusted in order to have the numerical predictions as close as possible to the measured data. Making use of the FE model, a preliminary fatigue assessment is carried out following the procedure defined in EN1993
A multi-objective optimization approach for FE model updating based on a selection criterion of the preferred Pareto-optimal solution
No full textMulti-objectives optimization problems are often solved constructing the Pareto front and applying a decision-making strategy to select the preferred solution among the Pareto-optimal solutions. With the aim to reduce the computational effort in multi-objective optimization problems, this paper presents a procedure for the direct evaluation of the preferred updated model, without the need to evaluate the whole Pareto front. For this purpose, the objective function to minimize is defined as the distance between a candidate point and the equilibrium point in the objective function space. The choice of the criterion of the minimum distance from the equilibrium point comes from a preliminary study carried out to assess the performances of different selection criteria. The robustness and the efficiency of the proposed procedure are assessed through the comparison with the results obtained from the estimation of the Pareto-optimal solutions and the subsequent selection of the preferred one for two numerical case studies. The proposed procedure is finally applied to the calibration of a complex FE model with respect to experimental modal data. Results show that the proposed procedure is effective and considerably reduces the computational effort. Moreover, the procedure is able to directly estimate the optimal weighting factor that allows to know the relative importance between the selected objectives and can be used to solve the multi-objective optimization with the weighed sum method
Experimental analysis and fatigue assessment on a railway steel viaduct
No full textThe FADLESS research project aims to define innovative technical guidelines for the assessment and control of existing and new bridges with regard to fatigue phenomena induced by vibrations and distortions produced by the passage of trains. The project will employ the most recent experimental and numerical techniques in order to identify the most typical details frequently subjected to high fatigue effects and to draft technical guidelines suitable of controlling such effects during bridge design or assessment.
As part of the research project, this paper describes some results concerning the fatigue assessment of the Lagoscuro railway viaduct. The Lagoscuro viaduct is composed of two parallel steel railway viaducts crossing the Po river. The first viaduct was built in 1948 and is composed of nine single span truss-girder bridges; the upper and the lower chord, diagonals and struts of the main truss girders are composed of four L-shaped steel elements, riveted together by means of plates; stringers and additional elements supporting the railway lines are also riveted. The new Lagoscuro viaduct was recently built in order to improve the railway line; the same geometry has been adopted but, different to the old viaduct, the truss girders are composed of H-shaped elements welded or bolted together in the joints.
First, the results of the experimental dynamic tests [1] are performed in order to identify the modal properties of both viaducts. Then, global finite element models are corrected according to the model updating procedures, where the uncertain model properties are adjusted in order to obtain numerical predictions as close as possible to the measured data, in terms of modal frequencies and mode shapes. An evolutionary algorithm [2] is used. After having identified the members potentially subject to fatigue or local vibrations, a substructure finite element model containing the specific member has been studied. The detailed substructure model has been then excited by prescribing at the boundaries the displacement time histories obtained in the corresponding nodes of the global model. From the stresses obtained in the detailed finite element model, the fatigue assessment is carried out following the procedure defined in EN1993 - Eurocode 3 (part 1-9) and the results are compared to those obtained with the procedure proposed in [3]. In order to obtain the stress ranges on each element, dynamic analyses are performed with traffic mixes defined according to EN 1991 - Eurocode 1. Stress range spectra are then determined and the damage indexes are evaluated according to the Miner role
A simplified method based on improved multiplication factors to assess crowd-induced vertical vibrations of footbridges
Get PDFDue to the high mechanical performance of new structural materials, modern footbridges are usually slender and lightweight structures. It follows that their natural frequencies often fall within the typical range of pedestrian pacing frequencies, feature that renders them rather sensitive to human-induced vibrations. Special care is therefore needed to avoid excessive levels of swaying, both to safeguard pedestrians from being bothered by the sense of discomfort and to prevent the functionality of footbridges to be compromised. However, in contrast with the single person dynamic force, the characterisation of the crowd-induced loading is rather challenging, time-consuming, and thus scarcely compatible with the design stage needs. This paper proposes an analytical model based on the multiplication factor approach, which allows to predict the vertical crowd-induced response from that of a single pedestrian. The work is based on extensive numerical simulations, carried out in due respect of human-human interaction and pedestrian step forcing variability. Besides, the possibility of managing human-structure effects is also foreseen. The method is meant to handle the vibration serviceability check of a wide range of scenarios, including a broad spectrum of footbridge parameters and crowd densities. For validation purposes, model predictions are compared against regulations and experimental results
Bayesian and deterministic surrogate-assisted approaches for model updating of historical masonry towers
No full textEfficient two-step procedure for parameter identification and uncertainty assessment in model updating problems
Get PDFThe model updating procedures employed in vibration-based health monitoring need to be reliable and computationally efficient. The computational time is a fundamental task if the results are used to evaluate, in quasi-real-time, the safe or the unsafe state of strategic and relevant structures. The paper presents an efficient two-step procedure for the identification of the mechanical parameters and for the assessment of the corresponding uncertainty in model updating problems. The first step solves a least squares problem, providing a first estimate of the unknown parameters. The second (iterative) step produces a refinement of the solution. Moreover, by exploiting the error propagation theory, this article presents a direct (non-iterative) procedure to assess the uncertainty affecting the unknown parameters starting from the experimental data covariance matrix. To test the reliability of the procedure as well as to prove its applicability to structural problems, the methodology has been applied to two test-bed case studies. Finally, the procedure has been used for the damage assessment in an existing building. The results provided in this article indicate that the procedure can accurately identify the unknown parameters and properly localize and quantify the damage
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