812 research outputs found

    Dynamics of Structures:a workshop on dynamic loads and response of structures and soil dynamics

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    Ambient vibration studies for system identification of tall buildings

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    This is the peer reviewed version of the article, which has been published in final form at DOI 10.1002/eqe.215. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.The performance of a building under wind and seismic loads depends on stiffness and mass distribution, and may be estimated using finite element codes. Experience has, however, shown that such finite element models often fail to predict accurately the fundamental natural frequencies. Usually the frequencies will be underestimated, that is the building will turn out to be stiffer than anticipated, meaning the design would usually be conservative. On the other hand, effects like torsional eccentricity and foundation compliance may not be correctly modelled, which could be less desirable. A full understanding of linear performance under lateral loads can be obtained through experimental evaluation of the vibration modes. Traditionally only a limited range of modal analysis procedures and software has been applied to civil applications and the ‘special case’ where no input forces can be measured has been the usual situation for large civil structures. Recent developments in system identification, which is the set of procedures to build mathematical models of the dynamic structural systems based on measured data, have added significantly to the potential of ambient vibration or ‘output only’ testing. The aim of the research reported here has been to apply and evaluate the procedures on typical buildings. The procedures are briefly explained and two experimental programmes are then described; a long-term tremor monitoring exercise on a 280m office tower and an ambient vibration survey of a smaller office block. The different forms of response data are examined to study the performance of the analysis procedures and expose benefits and limitations in their use. There is a growing interest in output-only modal analysis procedures in civil engineering. The experience reported in this paper has shown that quick and reliable estimation of mode shapes and frequencies can be obtained, even with small amounts of data. Judgement of modal participation and damping ratios requires more detailed study yet the results are at least as convincing as existing and relatively limited frequency domain methods

    THE TECHNIQUE OF DETERMINATION OF STRUCTURAL PARAMETERS FROM FORCED VIBRATION TESTING

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    This thesis details the results of an investigation into a technique for determination of "useful" structural parameters from forced vibration testing. The implementation of this technique to full scale civil engineering structures was achieved by several developments in the experimental and computational fronts: a vibration generator and a computer-aided-testing system for the former and two computational algorithms for the latter. The experimental developments are instrumental to exciting large structures and acquisition of large quantities of useful data in digital format. These data serve as inputs to the computational algorithms whose outputs are structural parameters. These parameters are in either modal or spatial forms which cannot be measured directly but have to be extracted from the raw data. The modal-parameter-extraction method is based on direct Least-Square fitting technique and is simple to implement. The technique can yield good accuracy if the residual effects from out-of-range modes are removed from the raw data before fitting. The spatial-parameter- extraction method distinguishes itself from other conventional methods in the way that the orthogonality property is not explicitly used. This method is applicable to situations where conventional methods are not; i.e. in cases if modal matrices are not square. Some success was achieved in cases in which computer synthesized or good quality laboratory test data were used. Full scale field tests of a tall office block and a slender tower were carried out and their modal models obtained. Attempts to obtain spatial models of these structures were not carried out, however, as this task can be a separate research topic in its own right. Further research in such application is still required

    Prediction of flow induced vibration of a flat plate located after a bluff wall mounted obstacle

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    [EN] Accurate prediction of Flow Induced Vibration phenomena is currently a field of major interest due to the use of lightweight materials in the automotive and aerospace industry. This article studies the turbulent flow around a wall-mounted obstacle, and the induced deformations produced by the pressure fluctuations on a plate located downstream the obstacle. The methodology used is a combination of experimental tests and numerical simulations. On one side, experiments were carried out in a wind tunnel test facility equipped with Particle Image Velocimetry to characterize the fluid velocity field, and laser vibro-meter to measure the vibrations of the plate. On the other side, Fluid-Structure Interaction (FSI-one-way) has been calculated by considering different turbulence modeling approximations (RANS and LES). Finally, numerical results have been analyzed and validated against the experiments in terms of main flow structures and the vibroacoustic response of the plate.This work has been partially supported by Universitat Politecnica de Valencia through the grant Programa de apoyo a la Carrera Academica del Profesorado 2018/03/14 and by the Spanish Ministerio de Economia y Competitividad through Grant No. DPI2015-70464-R. The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center), funded by the Research Foundation Flanders and the Flemish Government department EWI. The Research Fund KU Leuven and the Flanders Innovation and Entrepreneurship Agency, within the SILENCEVENT project, are gratefully acknowledged for their support.Torregrosa, AJ.; Gil, A.; Quintero-Igeño, P.; Ammirati, A.; Denayer, H.; Desmet, W. (2019). Prediction of flow induced vibration of a flat plate located after a bluff wall mounted obstacle. Journal of Wind Engineering and Industrial Aerodynamics. 190:23-39. https://doi.org/10.1016/j.jweia.2019.04.008S233919

    Parameter identification of vibration structures

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    Echinodome response to dynamic loading

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    An Innovative Structural Dynamic Identification Procedure Combining Time Domain OMA Technique and GA

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    In this paper an innovative and simple Operational Modal Analysis (OMA) method for structural dynamic identification is proposed. It combines the recently introduced Time Domain– Analytical Signal Method (TD–ASM) with the Genetic Algorithm (GA). Specifically, TD–ASM is firstly employed to estimate a subspace of candidate modal parameters, and then the GA is used to identify the structural parameters minimizing the fitness value returned by an appropriately introduced objective function. Notably, this method can be used to estimate structural parameters even for high damping ratios, and it also allows one to identify the Power Spectral Density (PSD) of the structural excitation. The reliability of the proposed method is proved through several numerical applications on two different Multi Degree of Freedom (MDoF) systems, also considering comparisons with other OMA methods. The results obtained in terms of modal parameters identification, Frequency Response Functions (FRFs) matrix estimation, and structural response prediction show the reliability of the proposed procedure

    System Identification of Offshore Platforms

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    Dynamic Characteristics and Wind-induced Response of a Tall Building

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    The design of tall buildings requires an accurate understanding of the expected wind loads and the resulting responses. The techniques used to estimate the wind-induced response are subject to uncertainty, which can result in unsatisfactory building performance or an over-designed structure. Altering the structure to rectify unsatisfactory performance can be extremely difficult and prohibitively expensive, while an over-designed structure represents unnecessary cost to the owner. This implies that accurate estimates of wind loads and responses are crucial to tall building design. Two aspects of tall building wind-induced response estimation are investigated: the estimation of natural frequencies and damping ratios; and the understanding of mechanisms causing wind-induced responses. This was primarily conducted via full-scale testing of a tall building. The building used for full-scale measurements is Latitude tower, an office tower located in the Sydney central business district, with a height of 187m above ground and 28m of underground levels. The building has a composite design including a reinforced concrete core, and reinforced concrete floor slabs supported by steel beams spanning between the core and perimeter columns. Outriggers linking the core and perimeter columns, as well as offset outriggers at the facade, are located at mid-height. The full-scale testing was conducted in two parts: vibration testing during construction; and a two year monitoring programme commenced after construction completion. Vibration testing during construction was conducted to determine the natural frequencies and damping ratios as the structure changed. Forced vibration testing and ambient vibration testing techniques were used. The Frequency Domain Decomposition and Stochastic Subspace Identification techniques were used to estimate the natural frequencies and damping ratios from the ambient vibration test outputs. The natural frequencies and damping ratios from the forced and ambient vibration tests differed by less than 5% and 30% respectively. Changes in the fundamental natural frequencies during construction were discussed in conjunction with the structural changes to further the understanding of how changes in the stiffness and mass of a tall building influence the natural frequencies. The measured natural frequencies during the early stages of construction were used to update a finite element model representing the structure at the time of testing. The material properties and floor beams were the primary focus of the model updating. The knowledge gained from partial structure updating was applied to a model of the completed structure, and the natural frequency estimate errors improved from 17% to 7%. The fundamental mode damping ratios measured during construction changed by less than 15% between the first test, conducted when 38% of the tower height was reached, and the final test at construction completion. The wind-induced monitoring programme included the measurement of wind velocities, accelerations, and displacements at the top of the building. The peak events for southerly and westerly wind directions were discussed. It was found that the acceleration response was dominated by the fundamental vibration mode. For southerly winds this corresponded to an along-wind response, but for westerly winds this corresponds to a cross-wind response. The probability distributions of upcrossings for along-wind and cross-wind responses where not significantly different to a Gaussian distribution for both southerly and westerly winds. The slope of the linear least squares fit was greater than two in all cases, which suggested intermittent characteristics were present in the responses. The standard deviation resonant acceleration responses from a high frequency base balance wind tunnel test were within 29% of the measured values

    Dynamic model update program

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    This thesis presents a general method to modify the properties of a finite element model of a structure to better correlate analytical (FE) modal data with experimental modal data. A FORTRAN program to correlate finite element and experimental results of a structure by executing a Cross-Orthogonality check was developed. Sensitivity coefficients of a structure were generated to tune the FE model to match the experimental model. These sensitivity coefficients were generated through sensitivity analysis which illustrates the change in response of a structure for known changes in parameters. This thesis covers the theory, procedure, and application of sensitivity analysis. Sensitivity coefficients were generated through MSC/NASTRAN (SOL 63 and SOL 53) and the coefficients were used to tune a finite element model. A method that employs nonlinear coefficients developed by Wada and Kuo was also used to increase the rate of convergence. The application of Wada and Kuo method was simplified and the calculations needed to obtain nonlinear coefficients were significantly reduced. The rate of convergence of linear coefficient model tuning is compared with the rate of convergence of nonlinear coefficient model tuning
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