• KSCE Journal of Civil and Environmental Engineering Research
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• v.6 no.2
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• pp.85-91
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• 1986

Fatigue life is an important consideration for the design of offshore structures in deep sea and in hostile environments. In this paper, the effect of the bandwidth of random stresses on the fatigue life estimation of fixed offshore structures is investigated. The dynamic analysis is carried out in the frequency domain by using the equivalent linearization technique. Fatigue damages are calculated by two stress cycle counting methods; i.e., the narrow band method and the wide band method using rainflow counting technique. Example studies are carried out for two different structures. Numerical results indicate that the wide band approach, which is more complex but theoretically more appropriate pridicts smaller values of fatigue damages compared with those by the narrow band approach for all seastate conditions. Such trend becomes more apparent for the cases of severe seastates where the bandwidth of random stresses becomes large.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.19 no.4 s.74
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• pp.375-387
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• 2006

Different from the previous studies which investigated seismic P-$\Delta$ effect in slender columns though comparison of response spectra according to stability coefficients obtained from the analyses based on the assumed moment-curvature relationship, the axial force and P-$\Delta$ effect in RC columns are investigated on the basis of the layered section method which can effectively consider the changes of stiffness and yield strength due to the application of axial force in RC members. Practical ranges of slenderness and stability coefficient are assumed, and sixty sets of horizontal/vertical earthquake inputs are used in the analysis. From the parametric study, it is noted that the maximum deformation of the slender RC column is hardly affected by P-$\Delta$ effect or vortical earthquake but dominantly affected by the applied axial force. Therefore, it can be concluded that no additional consideration for the P-$\Delta$ effect and vortical earthquake is required in the seismic design of a slender RC column if the axial force effect is taken into account in the analysis and design procedures.

• Journal of Korean Society of Steel Construction
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• v.18 no.2
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• pp.173-180
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• 2006

The seismic responses of a building are affected by the base soil conditions. In this study, linear time-history seismic analysis and nonlinear pushover static seismic analysis were performed to estimate the base shear forces of 3-, 5-, and 7-story steel buildings, considering the rigid and soft soil conditions. Foundation soil stiffness, based on the equivalent static stiffness formula, is used for the damper, one of the Link elements in SAP 2000. The base shear forces of the steel buildings, estimated through time-history analysis using the general-purpose structural-analysis program of SAP 2000, were compared with those calculated using the domestic seismic design code, the UBC-97 design response spectrum. and pushover static nonlinear analysis. The steel buildings designed for gravity and wind loads showed elastic responses with a moderate earthquake of 0.11 g, while the elastic soft-soil layer increased the displacement and the base shear force of the buildings due to soil-structure interaction and soil amplification. Therefore, considering the characteristics of the soft-soil layer, it is more reasonable to perform an elastic seismic analysis of a building's structure during weak or moderate earthquakes.

• Journal of the Korea Institute of Military Science and Technology
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• v.26 no.1
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• pp.1-9
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• 2023

The skin structure of sensor pod mounted on the exterior of aircraft can be exposed to the acoustic dynamic load and static load such as aerodynamic pressure and inertial load during flight. Fatigue life prediction of structural model under acoustic load should be performed and also differential stiffness of model modified by static load should be considered. The acoustic noise test spectrum of MIL-STD-810G was applied to the structural model and the stress response power spectral density (PSD) was calculated. The frequency response analysis was performed with or without prestress induced by inplane static load, and the response spectrum was compared. Time series data was generated using the calculated PSD, and the time and frequency domain fatigue life were predicted and compared. The variation of stress response spectrum due to static load and predicted fatigue life according to the different structural model considering mean stress were examined and decreasing fatigue life was observed in the model with prestress of compressive static load.

• Journal of the Earthquake Engineering Society of Korea
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• v.12 no.1
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• pp.79-87
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• 2008

Seismic performance evaluation of structure requires an estimation of the structural performance in terms of displacement demand imposed by earthquakes on the structure. Incremental Dynamic Analysis(IDA) is a analysis method that has recently emerged to estimate structural performance under earthquakes. This method can obtained the entire range of structural performance from the linear elastic stage to yielding and finally collapse by subjecting the structure to increasing levels of ground acceleration. Most structures are expected to deform beyond the limit of linearly elastic behavior when subjected to strong ground motion. The nonlinear response history analysis(NRHA) among various nonlinear analysis methods is the most accurate to compute seismic performance of structures, but it is time-consuming and necessitate more efforts. The nonlinear approximate methods, which is more practical and reliable tools for predicting seismic behavior of structures, are extensively studied. The uncoupled modal response history analysis(UMRHA) is a method which can find the nonlinear reponse of the structures for ESDF from the pushover curve using NRHA or response spectrum. The direct spectrum analysis(DSA) is approximate nonlinear method to evaluate nonlinear response of structures, without iterative computations, given by the structural linear vibration period and yield strength from the pushover analysis. In this study, the practicality and the reliability of seismic performance of approximate nonlinear methods for incremental dynamic analysis of mixed building structures are to be compared.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.8
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• pp.759-769
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• 2013

Electronic equipment has been applied to virtually every area associated with commercial, industrial, and military applications. Specifically, electronics have been incorporated into avionics components installed in aircraft. This equipment is exposed to dynamic loads such as vibration, shock, and acceleration. Especially, avionics components installed in a helicopter are subjected to simultaneous sine and random base excitations. These are denoted as sine on random vibrations according to MIL-STD-810F, Method 514.5. In the past, isolators have been applied to avionics components to reduce vibration and shock. However, an isolator applied to an avionics component installed in a helicopter can amplify the vibration magnitude, and damage the chassis, circuit card assembly, and the isolator itself via resonance at low-frequency sinusoidal vibrations. The objective of this study is to investigate the dynamic characteristics of an avionics component installed in a helicopter and the structural dynamic modification of its tray plate without an isolator using both a finite element analysis and experiments. The structure is optimized by dynamic loads that are selected by comparing the vibration, shock, and acceleration loads using vibration and shock response spectra. A finite element model(FEM) was constructed using a simplified geometry and valid element types that reflect the dynamic characteristics. The FEM was verified by an experimental modal analysis. Design parameters were extracted and selected to modify the structural dynamics using topology optimization, and design of experiments(DOE). A prototype of a modified model was constructed and its feasibility was evaluated using an FEM and a performance test.

• Journal of Aerospace System Engineering
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• v.1 no.2
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• pp.27-36
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• 2007

The Damage Tolerant Design is developed to help alleviate structural failure and cracking problems in aerospace structures. Recently, the Damage Tolerant Design is required and recommended for most of aircraft design. In this paper, the damage tolerant design is applied to tilt rotor UAV. First of all, the fatigue load spectrum for the tilt rotor UAV is developed and fatigue analysis is performed for the flaperon joint which has FCL (fatigue critical location). Tilt rotor UAV has two modes: helicopter mode when UAV is taking off and landing; fixed wing mode when the tilt rotor UAV is cruising. To make fatigue load spectrum, FELIX is used for helicopter mode. TWIST is used for fixed wing mode. Fatigue analysis of flaperon joint is performed using fatigue load spectrum. E-N curve approach is used for picking crack initiation point. The LEFM(Linear Elastic Fracture Method) is considered for analyzing crack growth or propagation. Finally, including the crack initiation and propagation, the fatigue life is evaluated. Therefore the Damage Tolerant Design can be done.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.31 no.2A
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• pp.97-104
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• 2011

This study presents the aerodynamic admittance function of bridge girder under turbulent flow generated from wind velocity spectrum measured at bridge site. Three dimensional buffeting analysis of concrete cable-stayed bridge were performed considering aerodynamic admittance functions obtained from four different methods. It is revealed from the analysis that vertical buffeting responses considering proper aerodynamic admittance functions were just half of that neglecting aerodynamic admittance function. Grid turbulence was found to relatively lower the aerodynamic admittance function at low frequency range, and to underestimate the buffeting wind forces. It is recommended to use the aerodynamic admittance function evaluated from flutter derivatives or measured at active turbulence in order to properly predict the buffeting responses of bridges.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.1 s.173
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• pp.16-28
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• 2000

The steady rotating flows in eccentric annulus has been studied by a numerical method based on the spectral collocation method. The inner cylinder has a constant angular velocity while the outer on e is stationary. Flow between eccentric cylinders is of considerable technical importance as it occurs in journal bearings. In the present work, the governing equations for laminar flow are expressed as Navier-Stokes equations, including the non-linear convection terms. The solutions were utilized i, estimate the effects of the nonlinear terms on the load acting on the rotating cylinder. Based on the half and the full Sommerfeld methods, the load on the rotating cylinder is evaluated with eccentricity, by integrating the pressure and skin friction around the cylinder. The attitude angle and Sommerfeld reciprocal are calculated from the load. Also, the torque on the rotating inner cylinder was calculated. considering the skin friction. The attitude angle and Sommerfeld reciprocal are decreased with eccentricity. Viscous damping coefficient due to the skin friction becomes larger with decreasing the annular space. It is found the non-linear effects of the convection terms on the flow and the load are important. especially on the attitude angle, for relatively wide annular configurations however, the effects on those are minor for very narrow annular ones.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.9 no.4
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• pp.51-61
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• 1989

The classical spectral analysis of random vibration is not applicable to the random vibration of nonlinear structures or the dynamic response of active mechanical systems whose governing equations contain random parametric and inhomogeneous excitations. If the random load is simulated, dynamic responses can be obtained with the application of numerical integration schemes to the governing equations of above problems. Thus, in this paper, efficient and practical methods of simulating nonstationary random seismic ground accelerations are presented by using the fast Fourier transform technique. Typical applications of the simulated ground accelerations are the simulations of the dynamic response of rotor-bearing systems under earthquake excitations. The study of accuracy is presented to determine the applicability and practicality of methods of simulation.