• Journal of Wind Energy
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• v.9 no.4
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• pp.57-64
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• 2018

To maximize power generation and reduce the construction cost of a commercial utility-grade wind turbine, the size of the wind turbine should be large. The initial design of the 12 MW University of Ulsan(UOU) Floating Offshore Wind Turbine(FOWT) was carried out based on the 5 MW National Renewable Energy Laboratory(NREL) offshore wind turbine model. The existing 5 MW NREL offshore wind turbines have been expanded to 12 MW UOU FOWT using the geometric law of similarity and then redesigned for each factor. The resonance of the tower is the most important dynamic responses of a wind turbine, and it should be designed by avoiding resonance due to cyclic load during turbine operations. The natural frequency of the tower needs to avoid being within the frequency range corresponding to the rotational speed of the blades, 1P, and the blade passing frequency, 3P. To avoid resonance, vibration can be reduced by modifying the stiffness or mass. The direct expansion of the 5 MW wind turbine support structure caused a resonance problem with the tower of the 12 MW FOWT and the tower length and diameter was adjusted to avoid a match of the first natural frequency and 3P excitation of the tower.

• Wind and Structures
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• v.12 no.5
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• pp.401-412
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• 2009

A method is applied for the estimation of structural damage of tall slender structures using natural frequency and displacements measurements by GPS. The relationship between the variation in the global stiffness matrix (or in the stiffness of each finite element) and the change in the natural frequencies of the structure is given. In engineering practice the number of frequencies which can be derived by GPS measurement of long-period structures will be equal to one, two or three first natural frequencies. This allows us in initial studies to detect damage with frequency changes based on forward methods in which the measured frequencies are compared with the predicted analytical data. This idea, of health monitoring from possible changes to natural frequencies, or from a statement of excessive displacements is applied to the Stuttgart TV Tower.

• International Journal of High-Rise Buildings
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• v.9 no.4
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• pp.343-349
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• 2020

This research aimed to clarify the long-term mechanical performance of a steel truss member strengthened by a carbon fiber-reinforced polymer (CFRP) without protective coating through exposure testing. Strengthening and repair methods using CFRP have been developed in recent years; however, there is a lack of durability research for CFRP-strengthened members, especially mechanical performance investigation according to actual exposure testing. In this study, 10 CFRP-strengthening steel specimens were created in 2015, and elastic bending tests were conducted biannually. Eventually, although resin loss occurred due to environmental effects, the mechanical performance of CFRP-strengthened steel was not degraded, and we propose a calculation method of bending stiffness to evaluate the lower value of stiffness for design.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2013.10a
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• pp.320-324
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• 2013

A crack identification method using an equivalent bending stiffness for cracked beam and committee of neural networks is presented. The equivalent bending stiffness is constructed based on an energy method for a straight thin-walled pipe, which has a through-the-thickness crack, subjected to bending. Several numerical analysis for a steel cantilever pipe using the equivalent bending stiffness are carried out to extract the natural frequencies and mode shapes of the cracked beam. The extracted modal properties are used in constructing a training patterns of a neural network. The input to the neural network consists of the modal properties and the output is composed of the crack location and size. Multiple neural networks are constructed and each individual network is trained independently with different initial synaptic weights. Then, the estimated crack locations and sizes from different neural networks are averaged. Experimental crack detection is carried out for 3 damage cases using the proposed method, and the identified crack locations and sizes agree reasonably well with the exact values.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.3 no.3
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• pp.126-136
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• 1991

This study is concerned with the nonlinear dynamic behaviors of guyed towers for wave loadings. In order to analyze the nonlinear responses of guyed towers efficiently, the main tower is modeled as an equivalent stick, the guyline system is idealized as a spring with nonlinear stiffness in the horizontal direction. and the pile foundation system is represented as a linear spring in the rotational direction. The wave forces on the main tower are evaluated by using Morison's equation. In order to consider adequately the nonlinearities of the guying system and drag forces due to fluid viscosity. the analyses are performed in the time domain. The mode superposition method is adopted for solving the nonlinear equation of motion efficiently. which is based on the Newmark integration scheme. Numerical analyses are carried out to investigate the sensitivity of two major design parameters for guyed towers. i.e., the clump weight conditions and the base renditions of the tower.

• Smart Structures and Systems
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• v.10 no.4_5
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• pp.471-483
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• 2012

In this paper, the structural health monitoring (SHM) benchmark problem of the Canton tower is studied. Based on the field monitoring data from the 20 accelerometers deployed on the tower, some modal frequencies and mode shapes at measured degrees of freedom of the tower are identified. Then, these identified incomplete modal data are used to update the reduced finite element (FE) model of the tower by a novel algorithm. The proposed algorithm avoids the problem of subjective selection of updated parameters and directly updates model stiffness matrix without model reduction or modal expansion approach. Only the eigenvalues and eigenvectors of the normal finite element models corresponding to the measured modes are needed in the computation procedures. The updated model not only possesses the measured modal frequencies and mode shapes but also preserves the modal frequencies and modes shapes in their normal values for the unobserved modes. Updating results including the natural frequencies and mode shapes are compared with the experimental ones to evaluate the proposed algorithm. Also, dynamic responses estimated from the updated FE model using remote senor locations are compared with the measurement ones to validate the convergence of the updated model.

• Journal of the Korean Geotechnical Society
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• v.28 no.2
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• pp.57-70
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• 2012

For soft ground, a pile foundation is typically used as a substructure of transmission tower. However, differential settlement between the foundations can cause structural damage of transmission tower. The connected-pile foundation is a type of group foundation consisting of four foundations connected with beams, and it was suggested in USA and Japan. In this study, a series of 1/8 scale model pile tests were performed to investigate the effect of load direction and stiffness of connecting beam on the responses of connected-pile foundation. As a result, the load capacities of the connected-pile foundation were larger than those of the conventional group pile foundation. For example, under the given test conditions in this paper, the resistibility against differential settlement was improved significantly for connected-pile foundation and its efficiency was maximized when the stiffness of connecting beams is about 25% of the mat foundation.

• Journal of Korean Society of Steel Construction
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• v.23 no.6
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• pp.669-677
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• 2011

The multi-span suspension bridge generally has more than three towers and two main spans. To economically and effectively design a multi-span suspension bridge, the proper stiffness ratio of the center tower to the side tower must be determined. This study was conducted to propose a method of figuring out briefly the structural behavior of the towers in a multi-span suspension bridge. In the equivalent suspension bridge model, the main cable of the multi-span suspension bridge is idealized as an equivalent cable spring, and the external loads of horizontal and vertical forces that were calculated using the tensile forces of the main cable were applied on top of the towers. The equilibrium equations of the equivalent multi-span suspension bridge model were derived and the equations were solved via nonlinear analysis. To verify the proposed method, a sample four-span suspension bridge with a main span length of 3,000 m was analyzed using thefinite element method. The displacements and moment reactions of each tower in the proposed method were compared with the FEM analysis results. Consequently, the results of the analysis of the equivalent suspension bridge model tended to be consistent with the results of the FEM analysis.

• Wind and Structures
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• v.21 no.6
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• pp.657-675
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• 2015

In this study, the feasibility of vibration-based damage detection methods for the wind turbine tower (WTT) structure is evaluated. First, a frequency-based damage detection (FBDD) is outlined. A damage-localization algorithm is visited to locate damage from changes in natural frequencies. Second, a mode-shape-based damage detection (MBDD) method is outlined. A damage index algorithm is utilized to localize damage from estimating changes in modal strain energies. Third, a finite element (FE) model based on a real WTT is established by using commercial software, Midas FEA. Several damage scenarios are numerically simulated in the FE model of the WTT. Finally, both FBDD and MBDD methods are employed to identify the damage scenarios simulated in the WTT. Damage regions are chosen close to the bolt connection of WTT segments; from there, the stiffness of damage elements are reduced.

• Structural Monitoring and Maintenance
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• v.5 no.4
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• pp.507-519
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• 2018

In this paper, damage assessment in wind-turbine towers using vibration-based artificial neural networks (ANNs) is numerically investigated. At first, a vibration-based ANNs algorithm is designed for damage detection in a wind turbine tower. The ANNs architecture consists of an input, an output, and hidden layers. Modal parameters of the wind turbine tower such as mode shapes and frequencies are utilized as the input and the output layer composes of element stiffness indices. Next, the finite element model of a real wind-turbine tower is established as the test structure. The natural frequencies and mode shapes of the test structure are computed under various damage cases of single and multiple damages to generate training patterns. Finally, the ANNs are trained using the generated training patterns and employed to detect damaged elements and severities in the test structure.