Initial FE Model Research Articles

Condition assessment of a concrete filled steel tube arch bridge using in-situ vibration measurements and an Improved Artificial Fish Swarm Algorithm

This study aims to assess the condition of a Concrete-Filled Steel Tube (CFST) arch bridge using in-situ vibration measurements, Finite Element Model Updating (FEMU) and an Improved Artificial Fish Swarm Algorithm (IAFSA). Dynamic coefficients, derailment coefficients and wheel unloading rates are extracted from the responses of running safety assessment. An updated finite element model using a kriging model and IAFSA is employed to evaluate the strength and rigidity of the structures. The results indicate that the natural frequencies and damping ratios of the CFST arch bridge meet the requirements of design code. The maximum bridge dynamic coefficient is found to be 1.085, indicating that the bridge dynamic performance is good. The wheel unloading rates of vehicle and locomotive exceed the specification limit, indicating that the track condition has to be improved before the bridge is operated. By updating an initial FE model, the errors between the predicted and the measured parameters are reduced to less than 5%.

Assessment of an old reinforced concrete open-spandrel arch railway bridge

This paper presents assessment of a 94 years old reinforced concrete (RC) open-spandrel arch bridge with a total length of 285 m having seven major spans of 35 m, a height of 34 m and a vertical slope of 15 ‰. As the bridge was currently subjected to heavy trains having higher axle loads than it was originally designed, assessment of its actual structural condition was necessary. To determine actual in-situ behavior of the bridge, dynamic tests including a series of acceleration measurements along the bridge spans were performed. Then, 3D finite element model of the bridge was generated by using a structural analysis software based on its original constructional drawings. The initial FE model was updated using Response Surface Method (RSM) technique based on the results obtained from measurement data. The updated FE model was used for structural assessment and evaluation of the bridge under UIC-71 live loading. Rating factor and reliability index for each bridge member based on Load and Resistance Factor Rating (LRFR) approach were obtained. Results of the rating and reliability analysis showed that the most critical member of the bridge was the deck, thus strengthening and retrofitting of these deck members might be needed for live loads greater than those induced by UIC-71 train.

An Improved Method for Dynamic Behaviour Prediction of Carbon Fibre Reinforced Epoxy (CFRE) using Finite Element Model Updating

The dynamic behaviour prediction scheme of carbon fibre composite structures are extremely challenging to be performed due to the geometrical ply-by-ply properties and the ply orientation issues. In this paper, a laminated carbon fibre reinforced epoxy plate with elastic and linear layers was used with aim to investigate the dynamic behaviour of the carbon fibre reinforced epoxy using practical modelling scheme via finite element modelling and updating method. A simplified FE model of the CFRE plate was developed using shell element properties known as PSHELL to represent geometrical laminated properties. Subsequently, the dynamic behaviour of the FE model was calculated using Nastran SOL103. Potential updating parameters of the FE model was analysed and identified using the sensitivity analysis. The model updating method was then used to update the initial FE model of the CFRE plate based on the experimental modal analysis (EMA) result. The comparison of the results were used in verifying the accuracy of the updated FE model of the CFRE plate. The result suggested that the PSHELL properties can be used efficiently to represent the geometrical laminated properties of the CFRE plate without considering the geometrical effect of ply-by-ply properties.

Fragility analysis of a historical reinforced concrete arch railway bridge

In this study, fragility analysis of a reinforced concrete arch railway bridge with a total length of 285 m having seven spans of 35 m, a height of 34 m and 15 ‰ slope were performed. The bridge constructed in 1928 still continues to give service. Because the bridge is located in a seismically active region in the southern part of Turkey and on a road, which is critical and important for national railway transportation, it was aimed to perform a probabilistic seismic assessment of the bridge. For this purpose, firstly, 3D finite-element model of the bridge was generated with the software SAP2000 according to the original constructional drawings. Then, the initial FE model was verified using its natural frequencies and mode shapes obtained from in-situ field acceleration measurements. Nonlinear time-history analyses were performed to obtain the seismic demands for 60 different real earthquake records. Probabilistic seismic demand model (PSDM) was derived to determine relations between engineering demand parameter (EDP) and intensity measure (IM). Lateral displacements of the mid-spans were considered as a damage state for three different service velocities. Finally, fragility curves of the bridge were derived.

Finite element model updating for repeated eigenvalue structures via the reduced-order model using incomplete measured modes

In order to obtain a precise dynamic structural FE model for dynamic analysis, FE model updating is usually used to correct uncertainty parameters for an initial FE model using incomplete measured data. Despite numerous studies concerning FE model updating, the computational cost is still a challenging issue for the repeated eigenvalue structures. Firstly, an improved modal assurance criterion is proposed to evaluate the similarity of mode shapes for the repeated eigenvalue structures in this paper. And then, a novel ROM-based FE model updating framework consisting of an off-line phase and an on-line phase is proposed. In the off-line phase, a reduced-order basis is constructed by extracting primary components of a snapshot matrix using a proper orthogonal decomposition technique. The snapshot matrix represents a collection of static displacement vectors of the FE model under radial nodal loads, which are determined by incomplete measured mode shapes. In the on-line phase, FE model updating is performed via a reduced-order model with much cheaper computational cost. Finally, a numerical example and an experimental example demonstrate the accuracy and efficiency of the proposed framework. The results indicate that the proposed ROM-based FE model updating framework is more efficient and stable than the FOM-based FE model updating framework.

The Prediction of the Dynamic Behaviour of a Structure Using Model Updating and Frequency Based Substructuring

Combining analytical with experimental data to predict the dynamic behaviour of an assembled structure via the frequency based substructuring (FBS) method, is a common practice in the field of structural dynamics. However, the accuracy of the dynamic behaviour predicted from the FBS method relies heavily on the quality of experimental frequency response function (FRF) of the interfaces on which, in practice, it is very difficult to obtain. In addition, the accuracy of the FBS method is highly dependent on experimental rotational degrees of freedom (DOF) which are always found to be very difficult to measure accurately. Therefore, this paper proposes a new frequency response function (FRF) coupling scheme that may uniquely address the difficulties and improve the quality of predicted results of the FBS method. The scheme is formulated based on the finite element method, model updating technique and experimental modal analysis. A simplified finite element model of a physical test substructure is developed to generate rotational FRF data by reconciling the initial FE model using the model updating technique. It was found that the scheme adopted allows generating full translational and rotational degrees of freedom data and leads to a significant improvement in the FRF coupling process between the analytical and experimental model in the FBS method.

Friction coefficient estimation in shaft/bush interference using finite element model updating

Reliable finite element modeling has a great degree of importance for studies related to the mechanical design in industry. Finite element modeling is often associated with some uncertainties. Model updating is a tool which can be used to overcoming these uncertainties. This method uses experimental data of the structural components to modify the initial FE model. A typical use of model updating is for interference joint of a shaft and bush. In the modeling of the interference joint for known materials, an approximate value is assumed for friction coefficient of contact areas. Based on intuition, engineering judgments or previous tests, it is difficult to accurately estimate this coefficient. In this paper, model updating is used to estimate and update the friction coefficient in the contact surface of interference shaft and bush joints. Suitable experiments were conducted and the initial finite element model was modified based on the experimental extraction strength of interference fit joints. Finally, typical values of stress fields were extracted from the new model with an updated value of the friction coefficient.

Numerical simulations of missile impacts on reinforced concrete plates: IRIS-2010/2012 benchmark project

This paper describes the procedures and results of the works done by the Korea Institute of Nuclear Safety (KINS) as a participant of the IRIS-2010 and 2012 benchmark projects, which have been organized by OECD/NEA IAGE working group. Within the scope of the IRIS-2012 project, uniaxial and tri-axial concrete tests were performed and the results were supplied. With these material test data, impact simulations of IRIS-2010 experiments (Punching P1 and Bending B1) were re-performed to improve the accuracy of the simulation results and to reduce the computation time.Numerical impact simulations were performed using an explicit dynamic code, LS-DYNA, and the initial FE model was updated through numerous parametric studies. Considering the symmetry of the structure, an updated model was constructed for ¼ of the structure, and the stiffness and mass of the supporting structure was considered. Loading function with a fictitious loading plate was applied instead of modeling projectiles to reduce the computation time. Three concrete models, Concrete damage rel. 3 (*MAT_072R3), Winfrith concrete (*MAT_084-085), and CSCM concrete (*MAT_159), were used for the impact simulations and the results were compared.

An improved procedure for updating finite element model based on an interactive multiobjective programming

Finite element model updating is an optimization problem to identify and correct uncertain modeling parameters. In conventional model updating, physically incompatible criteria, which designate differences between analytical and experimental results, are combined into a single-objective function using weighting factors. There are no general rules for selecting the weighting factors since they are not directly related to the dynamic behavior of the updated model. Thus, a necessary approach is to solve the time-consuming optimization problem repeatedly by varying the values of weighting factors until a satisfactory solution is obtained. In this work, an interactive multiobjective optimization technique called satisficing trade-off method is introduced to avoid the difficulty. It is relatively easy to state what kind of solutions is satisfactory considering the correlations of the initial FE model with the experimental results, the importance of individual modal properties, and measurement errors, etc. The satisficing trade-off method uses this information directly in the optimization process and finds a Pareto solution which is nearest to the given information. Moreover, as the method provides the tangent hyperplane which approximates the Pareto surface in the neighborhood of the obtained Pareto solution, the desired updated model can be found in a few iterations.

About eigen sensitivity analysis of mechanical structures

Several methods for a calculation of derivatives of eigenvectors with respect to design parameters are described here. These are the finite-difference method, the modal method, a modified modal method, Nelson's method, an improved first-order approximation of eigenvalues and eigenvectors and an iterative method. By combining the other structural reanalysis techniques and one of these sensitivity methods, it is possible to enhance the efficiency and the accuracy of structural optimization techniques for determining the optimum condition of mechanical structure specified by an analyst. The sensitivity approach is based on the prior selection of updating parameters (design variables) in the initial FE model.

Developing a robust SHM method for offshore jacket platform using model updating and fuzzy logic system

Structural monitoring is essential for ensuring the structural safety performance during the service life. The process is of paramount importance in case of the offshore jacket-type platforms due to the underwater structural parts subjected to the marine environmental conditions. This work is an experimental investigation on a laboratory model of a jacket platform with the objective of establishing a baseline finite element (FE) model for long-term structural health monitoring for this type of structures. A robust damage diagnosis system is also developed which is less sensitive to both the measurements and the modeling uncertainties. Experimental vibration tests are conducted on a physical platform model to obtain dynamic characteristics and then, the initial FE-model of the intact structure is developed to determine them numerically. Some differences between numerically and experimentally identified characteristics emerge due to various uncertainties in the FE-model and measured vibration data. To minimize these differences, initial FE-model is updated according to the experimental results. The updated FE-model is employed to predict the changes in the dynamic characteristics under variety of damage scenarios which are imposed by reducing the stiffness at the components of the model. Fuzzy logic system (FLS) and probabilistic analysis is developed for linguistic classification of damage and global damage diagnosis. Incorporation of the FLS fault isolation technique into FE-model updating method are proposed and evaluated for two different FLS methods to develop a vigorous damage diagnosis method. The efficiency of the technique is validated by different damage scenarios foreseen on the physical model. This technique is shown to be effective for diagnosing the presence of degradation and quantify it.

Model Updating for a Welded Structure Made from Thin Steel Sheets

Dynamic behaviour of a welded structure made from thin metal sheets which has a large flat surface and has been assembled together by a number of scattered spot welds is investigated. An impact hammer and roving accelerometers are used in the modal tests to provide data to update the FE model. NASTRAN Solution 103 is used to compute natural frequencies and modes of interest. The large error of the initial FE model is largely reduced mainly by adjusting a crucial parameter of the bending moment of inertia ratio, 12I/T**3.

Model selection in finite element model updating using the Bayesian evidence statistic

This paper considers the problem of finite element model (FEM) updating in the context of model selection. The FEM updating problem arises from the need to update the initial FE model that does not match the measured real system outputs. This inverse system identification-problem is made even more complex by the uncertainties in modeling some of the structural parameters. Such uncertainty often results in a number of competing forms of FE models being proposed which leads to lack of consensus in the field. A model can be formulated in a number of ways; by the number, the location and the form of the updating parameters. We propose the use of a Bayesian evidence statistic to help decide on the best model from any given set of models. This statistic uses the recently developed stochastic nested sampling algorithm whose by-product is the posterior samples of the updated model parameters. Two examples of real structures are each modeled by a number of competing finite element models. The individual model evidences are compared using the Bayes factor, which is the ratio of evidences. Jeffrey's scale is then used to determine the significance of the model differences obtained through the Bayes factor.

Dynamic Parameters Identification and Finite Element Model Updating for Continuous Rigid Frame Bridge

AbstractA three dimensional FE model of a long-span prestressed continuous rigid frame bridge was constructed and the modal analysis was performed by using ANSYS. The real dynamic parameters of this bridge were identified by using the Frequency Domain Decomposition (FDD) method and the Eigensystem Realization Algorithm (ERA) method from the acceleration data recorded by the monitoring system on this bridge. A refined FE model of this bridge was obtained by updating the initial FE model using the identified modal parameters. The research result shows that (1) the modal parameters of this bridge can be identified with good accuracy from the acceleration responses collected by the monitoring system, and the identified modal parameters can be used to update the initial FE model; (2) the updated FE model can accurately reflect the structural dynamic characteristics of the bridge, and the updated physical parameters have good connectivity with the physical condition of the bridge.

A development of data structure and mesh generation algorithm for whole ship analysis modeling system

In the whole ship structure and vibration analysis, the FEA (finite element analysis) model of whole ship structure is required in the early design stage before the 3D CAD model is defined. Because ship structure has a complex curved surface, and many associated structural members, the whole ship analysis modeling job has become a time consuming job. For the effective support of the whole ship analysis modeling, a method to generate the analysis model using initial design information within the ship design process, hull form offset data and compartment data, is developed. To easily handle initial design information and FE model information, a flexible data structure is proposed. An automatic quadrilateral mesh generation algorithm using initial design information to satisfy the constraints imposed by the ship structure is also proposed. With the proposed data structure and mesh generation algorithm, whole ship analysis modeling job for various ship types can be effectively supported and these results are presented.

FE model verification for structural dynamics with vector projection

An FE model for structural dynamic analysis might have as many as three kinds of error—parameter errors, discretisation errors, and configuration errors—that make the dynamic properties of the model different from those of the actual structure. A model updating procedure cannot correct all these errors but the parameter errors of the model. If the effects of the configuration and discretisation errors in an FE model cannot be neglected in a frequency range of interest, the model updating procedure undertaken on the model will result in no improvement for the correlation of the dynamic properties between the model and the actual structure. The model verification procedure, as a part of the overall process of model validation for structural dynamics, verifies the initial FE model in order to establish whether the model can be updated or not. This paper describes a study of the FE model verification procedure. A method for making a configuration check is proposed to address the problems of configuration errors in FE models. The concept of vector projection is used in the proposed model verification method. By projecting reference eigenvectors (usually, experimental eigenvectors) onto a subspace spanned by the analytical eigenvectors from an FE model, and comparing the projections with the reference eigenvectors themselves, it is possible to establish whether or not the FE model can be updated in order to generate the same (or close) eigenvectors as the reference eigenvectors. Two case studies, one based on a simple theoretical model and the other on an industrial structure, show that the proposed method is effective at distinguishing parameter errors in an FE model (i.e. those errors which can be corrected by model updating) from discretisation and configuration errors (those which cannot be corrected by model updating).

Improved 3D coupled calculations of the structural-dynamic behavior of induction furnaces excited by electromagnetic forces using adaptive algorithms

The aim of this work is to make available an automatic procedure for the improved calculation of the structural-dynamic behavior of induction furnaces and handling of meshing information to the mesh generator for the following acoustic analysis. The electromagnetic forces from the electromagnetic FE-model are transferred to the initial mechanical FE-model and a loop of vibration analysis is started, which will be controlled by the maximum error rate and the number of loops. The error criterion is chosen and integrated in the meshing algorithm. The impact of the mesh refinement will be shown by applying the algorithm to a test problem explained in the following. The developed code is created in object oriented language to have the most flexibility for the implementation of the various algorithms.