• Wind and Structures
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• v.22 no.3
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• pp.273-290
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• 2016

The flutter derivatives of bridge decks can be efficiently identified using the experimentally and/or numerically coupled forced vibration method. This paper addresses the issue of inherent requirement for adopting different frequencies of three modes in this method. The aerostatic force components and the inertia of force and moment are mathematically proved to exert no influence on identification results if the signal length (t) is integer (n=1,2,3...) times of the least common multiple (T) of three modal periods. It is one important contribution to flutter derivatives identification theory and engineering practice in this study. Therefore, it is unnecessary to worry about the determination accuracy of aerostatic force and inertia of force and moment. The influences of signal length, amplitude, and frequency ratio on flutter derivative are thoroughly investigated using a bridge example. If the signal length t is too short, the extraction results may be completely wrong, and particular attention should be paid to this issue. The signal length t=nT ($n{\geq}5$) is strongly recommended for improving parameter identification accuracy. The proposed viewpoints and conclusions are of great significance for better understanding the essences of flutter derivative identification through coupled forced vibration method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.10
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• pp.1467-1474
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• 2004

In this study, a new damage identification method for beam-like structures with a fatigue crack is proposed. which does not require comparative measurement on an intact structure but require several measurements at different level of excitation forces on the cracked structure. The idea comes from the fact that dynamic behavior of a structure with a fatigue crack changes with the level of the excitation force. The 2$^{nd}$ spatial derivatives of frequency response functions along the longitudinal direction of a beam are used as the sensitive indicator of crack existence. Then, weighting function is employed in the averaging process in frequency domain to account for the modal participation of the differences between the dynamic behavior of a beam with a fatigue crack at the low excitation and one at the high excitation. Subsequently, a damage index is defined such that the location and level of the crack may be identified. It is shown from the analysis of vibration measurements in this study that comparison of frequency response characteristics of a beam with a single fatigue crack at different level of excitation forces enables an effective detection of the crack.

• Wind and Structures
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• v.12 no.5
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• pp.413-424
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• 2009

A series of wind tunnel free-decay sectional model dynamic tests were conducted to examine the effects of torsional-to-vertical natural frequency ratio of 2DOF bridge dynamic systems on the aerodynamic and dynamic properties of bridge decks. The natural frequency ratios tested were around 2.2:1 and 1.2:1 respectively, with the fundamental vertical natural frequency of the system held constant for all the tests. Three 2.9 m long twin-deck bridge sectional models, with a zero, 16% (intermediate gap) and 35% (large gap) gap-to-width ratio, respectively, were tested to determine whether the effects of frequency ratio are dependent on bridge deck cross-section shapes. The results of wind tunnel tests suggest that for the model with a zero gap-width, a model to approximate a thin flat plate, the flutter derivatives, and consequently the aerodynamic forces, are relatively independent of the torsional-to-vertical frequency ratio for a relatively large range of reduced wind velocities, while for the models with an intermediate gap-width (around 16%) and a large gap-width (around 35%), some of the flutter derivatives, and therefore the aerodynamic forces, are evidently dependent on the frequency ratio for most of the tested reduced velocities. A comparison of the modal damping ratios also suggests that the torsional damping ratio is much more sensitive to the frequency ratio, especially for the two models with nonzero gap (16% and 35% gap-width). The test results clearly show that the effects of the frequency ratio on the flutter derivatives and the aerodynamic forces were dependent on the aerodynamic cross-section shape of the bridge deck.

• Coupled systems mechanics
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• v.1 no.2
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• pp.133-154
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• 2012

In this paper a bridge flutter prediction is performed by using advanced numerical simulation. Two novel approaches were developed simultaneously by utilizing the ANSYS v12.1 commercial software package. The first one is a fluid-structure interaction simulation involving the three-dimensional elastic motion of a bridge deck and the fluid flow around it. The second one is an updated forced oscillation technique based on the dynamic mode shapes of the bridge. An aeroelastic wind tunnel model was constructed in order to validate the numerical results. Good agreement between the numerical results and the measurements proves the applicability of the novel methods in bridge flutter assessment.

• 제어로봇시스템학회:학술대회논문집
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• 1996.10b
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• pp.1087-1090
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• 1996

The objective of this paper is to present a new input torque shaping method for slewing and vibration suppression of flexible structure based on Fourier series expansion. Vibration energy of the structure with shaped control input is investigated with respect to the shaping parameter of the reference torque, maneuver time and the number of trigonometric functions to be included in the series. Analytic expressions of the performance indices and their derivatives are derived in the modal coordinates. Numerical results show the effectiveness of the proposed approach to design the open-loop control law that modifies the shape of input torque for simultaneous slewing and vibration suppression.

• Journal of KSNVE
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• v.4 no.2
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• pp.137-146
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• 1994

An analytical method for linear free vibration of fully liquid-filled circular cylindrical shell with various boundary conditions is developed by the Fourier series expansion based on the Stokes' transformation. A set of modal displacement functions and their derivatives of a circular cylindrical shell is substituted into the Sanders' shell equations in order to explicitily represent the Fourier coefficients as functions of the end point displacements, forces, and moments. For the vibration relevant to the liquid motion, the velocity potential of liquid is assumed as a sum of linear combination of suitable harmonic functions in the axial directions. The unknown parameter of the velocity potential is selected to satisfy the boundary condition along the wetted shell surface. An explicit expression of the natural frequency equation can be obtained for any kind of classical boundary conditions. The natural frequencies of the liquid-filled cylindrical shells with the clamped-free, the clamped-clamped, and the simply supported-simply supported boundary conditions examined in the previous works, are obtained by the analytical method. The results are compared with the previous works, and excellent agreement is found for the natural frequencies of the shells.

• Journal of the Earthquake Engineering Society of Korea
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• v.10 no.6 s.52
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• pp.19-28
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• 2006

Most conventional model updating methods must use mathematical objective function with experimental modal matrices and analytical system matrices or must use information about the gradient or higher derivatives of modal properties with respect to each updating parameter. Therefore, most conventional methods are not appropriate for complex structural system such as bridge structures due to stability problem in inverse analysis with ill-conditions. Sometimes, moreover, the updated model may have no physical meaning. In this paper, a new FE model updating method based on a hybrid optimization technique using genetic algorithm (GA) and Holder-Mead simplex method (NMS) is proposed. The performance of hybrid optimization technique on the nonlinear problem is demonstrated by the Goldstein-Price function with three local minima and one global minimum. The influence of the objective function is evaluated by the case study of a simulated 10-dof spring-mass model. Through simulated case studies, finally, the objective function is proposed to update mass as well as stiffness at the same time. And so, the proposed hybrid optimization technique is proved to be an efficient method for FE model updating.

• Smart Structures and Systems
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• v.14 no.4
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• pp.569-594
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• 2014

Damage detection methods based on modal analysis have been widely studied in recent years. However the calculation of mode shapes in real structures can be time consuming and often requires dedicated software programmes. In the present paper the combined application of proper orthogonal decomposition and gapped smoothing method to structural damage detection is presented. The first is used to calculate the dynamic shapes of a damaged structural element using only the time response of the system while the second is used to derive a reference baseline to which compare the data coming from the damaged structure. Experimental verification is provided for a beam case while numerical analyses are conducted on plates. The introduction of a stiffener on a plate is investigated and a method to distinguish its influence from that of a defect is presented. Results highlight that the derivatives of the proper orthogonal modes are more effective damage indices than the modes themselves and that they can be used in damage detection when only data from the damaged structure are available. Furthermore the stiffened plate case shows how the simple use of the curvature is not sufficient when analysing complex components. The combined application of the two techniques provides a possible improvement in damage detection of typical aeronautical structures.

• Wind and Structures
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• v.10 no.1
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• pp.1-22
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• 2007

The suppression of aerodynamic response of long-span suspension bridges during erection and after completion by using single TMD and multi TMD is presented in this paper. An advanced finite-element-based aerodynamic model that can be used to analyze both flutter instability and buffeting response in the time domain is also proposed. The frequency-dependent flutter derivatives are transferred into a time-dependent rational function, through which the coupling effects of three-dimensional aerodynamic motions under gusty winds can be accurately considered. The modal damping of a structure-TMD system is analyzed by the state-space approach. The numerical examples are performed on the Akashi Kaikyo Bridge with a main span of 1990 m. The bridge is idealized by a three-dimensional finite-element model consisting of 681 nodes. The results show that when the wind velocity is low, about 20 m/s, the multi TMD type 1 (the vertical and horizontal TMD with 1% mass ratio in each direction together with the torsional TMD with ratio of 1% mass moment of inertia) can significantly reduce the buffeting response in vertical, horizontal and torsional directions by 8.6-13%. When the wind velocity increases to 40 m/s, the control efficiency of a multi TMD in reducing the torsional buffeting response increases greatly to 28%. However, its control efficiency in the vertical and horizontal directions reduces. The results also indicate that the critical wind velocity for flutter instability during erection is significantly lower than that of the completed bridge. By pylon-to-midspan configuration, the minimum critical wind velocity of 57.70 m/s occurs at stage of 85% deck completion.

• Wind and Structures
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• v.10 no.5
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• pp.463-479
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• 2007

A series of wind tunnel sectional model dynamic tests of a twin-deck bridge were conducted at the CLP Power Wind/Wave Tunnel Facility (WWTF) of The Hong Kong University of Science and Technology (HKUST) to investigate the effects of gap-width on the self-excited vibrations and the dynamic and aerodynamic characteristics of the bridge. Five 2.9 m long models with different gap-widths were fabricated and suspended in the wind tunnel to simulate a two-degrees-of-freedom (2DOF) bridge dynamic system, free to vibrate in both vertical and torsional directions. The mass, vertical frequency, and the torsional-to-vertical frequency ratio of the 2DOF systems were fixed to emphasize the effects of gap-width. A free-vibration test methodology was employed and the Eigensystem Realization Algorithm (ERA) was utilized to extract the eight flutter derivatives and the modal parameters from the coupled free-decay responses. The results of the zero gap-width configuration were in reasonable agreement with the theoretical values for an ideal thin flat plate in smooth flow and the published results of models with similar cross-sections, thus validating the experimental and analytical techniques utilized in this study. The methodology was further verified by the comparison between the measured and predicted free-decay responses. A comparison of results for different gap-widths revealed that variations of the gap-width mainly affect the torsional damping property, and that the configurations with greater gap-widths show a higher torsional damping ratio and hence stronger aerodynamic stability of the bridge.