• Wind and Structures
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• v.22 no.1
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• pp.107-131
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• 2016

The need for a more affordable, reliable, clean and secure energy has led to explorations in non-traditional sources, particularly renewable energies. Wind is one of the cleanest energy sources that plays a significant role in augmenting sustainability. Wind turbines, as energy convertors, are usually tall and slender structures, and depending on their location (inland or offshore), they can be subject to high wind and/or strong wave loadings. These loads can cause severe vibrations with detrimental effects on energy production, structural lifecycle and initial cost. A dissipativity analysis study was carried out to know whether wind turbine towers require damping enhancement or rigidity modifications for vibration suppression. The results suggest that wind turbines are lightly damped structures and damping enhancement is a potential solution for vibration lessening. Accordingly, the paper investigates different damping enhancement techniques for vibration mitigation. The efficacy of tuned mass damper (TMD), tuned liquid column damper (TLCD), tuned sloshing damper (TSD), and viscous damper (VD) to reduce vibrations is investigated. A comparison among these devices, in terms of robustness and effectiveness, is conducted. The VD can reduce both displacement and acceleration responses of the tower, better than other types of dampers, for the same control effort, followed by TMD, TSD, and finally TLCD. Nevertheless, the use of VDs raises concerns about where they should be located in the structure, and their application may require additional design considerations.

• Earthquakes and Structures
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• v.10 no.6
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• pp.1451-1465
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• 2016

The effectiveness of base isolation (BI) systems for mitigation of seismic vibration of bridges have been extensively studied in the past. It is well established in those studies that the performance of BI system is largely dependent on the characteristics of isolator yield strength. For optimum design of such systems, normally a standard nonlinear optimization problem is formulated to minimize the maximum response of the structure, referred as Stochastic Structural Optimization (SSO). The SSO of BI system is usually performed with reference to a problem of unconstrained optimization without imposing any restriction on the maximum isolator displacement. In this regard it is important to note that the isolator displacement should not be arbitrarily large to fulfil the serviceability requirements and to avoid the possibility of pounding to the adjacent units. The present study is intended to incorporate the effect of excessive isolator displacement in optimizing BI system to control seismic vibration effect of bridges. In doing so, the necessary stochastic response of the isolated bridge needs to be optimized is obtained in the framework of statistical linearization of the related nonlinear random vibration problem. A simply supported bridge is taken up to elucidate the effect of constraint condition on optimum design and overall performance of the isolated bridge compared to that of obtained by the conventional unconstrained optimization approach.

• Wind and Structures
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• v.23 no.1
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• pp.45-57
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• 2016

Open-cross-section composite beam (OCB) tends to suffer vortex-induced vibration (VIV) due to its bluff aerodynamic shape. A cable-stayed bridge equipped with typical OCB is taken as an example in this paper to conduct sectional model wind tunnel test. Vortex-induced vibration is observed and maximum vibration amplitudes are obtained. CFD approach is employed to calculate the flow field around original cross sections in service stage and construction stage, as well as sections added with three different countermeasures: splitters, slabs and wind fairings. Results show that flow separate on the upstream edge and cause vortex shedding on original section. Splitters can only smooth the flow field on the upper surface, while slabs cannot smooth flow field on the upper or lower surface too much. Thus, splitters or slabs cannot serve as valid aerodynamic means. Wind tunnel test results show that VIV can only be mitigated when wind fairings are mounted, by which the flow field above and below the bridge deck are accelerated simultaneously.

• Earthquakes and Structures
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• v.14 no.6
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• pp.567-576
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• 2018

This manuscript introduces a new damping device which is composed of a water tank and a pendulum. The new damping device can be tuned to multiple frequencies. In addition, it has a higher energy dissipation capacity when compared with the conventional Tuned Liquid Dampers (TLDs). In order to evaluate the efficiency of this new damping device a series of free vibration and forced vibration tests were conducted on a scaled down single-story one-bay steel frame. Two different configurations were studied for the mass of the pendulum that included a completely and a partially submerged mass. It was observed that the completely submerged configuration led to 44% higher damping ratio when compared with the conventional TLD. In addition, the completely submerged configuration reduced the peak displacement response of the structure 1.6 times more than the conventional TLD. The peak acceleration response of the structure equipped with the new damping device was reduced twice more than the conventional TLD. It was also found that, when the excitation frequency is lower than the resonance frequency, the conventional TLD performs better than the partially submerged configuration of the new damping device.

• Smart Structures and Systems
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• v.30 no.1
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• pp.89-106
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• 2022

The negative stiffness of an active or semi-active damper system has been proven to be very effective in reducing dynamic response. Therefore, energy dissipation devices possessing negative stiffness, such as viscous inertial mass dampers (VIMDs), have drawn much attention recently. The control performance of the VIMD for cable vibration mitigation has already been demonstrated by many researchers. In this paper, a new optimal design procedure for VIMD parameters for taut cable vibration control is presented based on the fixed-points method originally developed for tuned mass damper design. A model consisting of a taut cable and a VIMD installed near a cable end is studied. The frequency response function (FRF) of the cable under a sinusoidal load distributed proportionally to the mode shape is derived. Then, the fixed-points method is applied to the FRF curves. The performance of a VIMD with the optimal parameters is subsequently evaluated through simulations. A taut cable model with a tuned VIMD is established for several cases of external excitation. The performance of VIMDs using the proposed optimal parameters is compared with that in the literature. The results show that cable vibration can be significantly reduced using the proposed optimal VIMD with a relatively small amount of damping. Multiple VIMDs are applied effectively to reduce the cable vibration with multi-modal components.

• Structural Engineering and Mechanics
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• v.3 no.6
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• pp.569-581
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• 1995

In this paper, the seismic behavior of a 10-story building equipped with viscoelastic dampers is analyzed. The effects of ambient temperature, the thickness, the total area, and the position of the viscoelastic dampers are studied. Results indicate that the energy-absorbing capacity of viscoelastic damper decreases with increasing the ambient temperature. The thickness and the total area of viscoelastic dampers also affect the seismic mitigation capacity. The thickness cannot be too small, which is not effective in vibration reduction, nor can it be too large, which not only increases the cost but also reduces the seismic resistance. The total area of viscoelastic dampers should be determined properly for optimum damper performance at the most economical design. The mounting position of viscoelastic dampers also influences the structure's seismic performance. Numerical results show that, if properly equipped, the VE dampers can reduce the structural response both floor displacement and story shear force and increase the overall level of damping in structures during earthquakes.

• Geomechanics and Engineering
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• v.20 no.2
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• pp.155-164
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• 2020

In recent years, the stability, safety and comfort of trains has received increased attention. The mechanical characteristics and differential settlement of the foundation are the main problems studied in high-speed railway research. The mechanical characteristics and differential settlement of the foundation are greatly affected by the ground treatment. Additionally, the effects of train load and earthquakes have a great impact. The dynamic action of the train will increase the vibration acceleration of the foundation and increase the cumulative deformation, and the earthquake action will affect the stability of the substructure. Earthquakes have an important practical significance for the dynamic analysis of the railway operation stage; therefore, considering the impact of earthquakes on the railway substructure stability has engineering significance. In this paper, finite element model of the CFG (Cement Fly-ash Gravel) pile + cement-soil compacted pile about composite foundation is established, and manual numerical incentive method is selected as the simulation principle. The mechanical characteristics and differential settlement of CFG pile + cement-soil compacted pile about composite foundation under train load are studied. The results show: under the train load, the neutral point of the side friction about CFG pile is located at nearly 7/8 of the pile length; the vertical dynamic stress-time history curves of the cement-soil compacted pile, CFG pile and soil between piles are all regular serrated shape, the vertical dynamic stress of CFG pile changes greatly, but the vertical dynamic stress of cement-soil compacted pile and soil between piles does not change much; the vertical displacement of CFG pile, cement-soil compacted pile and soil between piles change very little.

• Smart Structures and Systems
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• v.25 no.1
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• pp.65-80
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• 2020

In order to protect a structure over its full life cycle, a novel tuned mass damper (TMD), the so-called semi-active eddy current pendulum tuned mass damper (SAEC-PTMD), which can retune its frequency and damping ratio in real-time, is proposed in this study. The structural instantaneous frequency is identified through a Hilbert-Huang transformation (HHT), and the SAEC-PTMD pendulum is adjusted through an HHT-based control algorithm. The eddy current damping parameters are discussed, and the relationship between effective damping coefficients and air gaps is fitted through a polynomial function. The semi-active eddy current damping can be adjusted in real-time by adjusting the air gap based on the linear-quadratic-Gaussian (LQG)-based control algorithm. To verify the vibration control effect of the SAEC-PTMD, an idealized linear primary structure equipped with an SAEC-PTMD excited by harmonic excitations and near-fault pulse-like earthquake excitations is proposed as one of the two case studies. Under strong earthquakes, structures may go into the nonlinear state, while the Bouc-Wen model has a wild application in simulating the hysteretic characteristic. Therefore, in the other case study, a nonlinear primary structure based on the Bouc-Wen model is proposed. An optimal passive TMD is used for comparison and the detuning effect, which results from the cumulative damage to primary structures, is considered. The maximum and root-mean-square (RMS) values of structural acceleration and displacement time history response, structural acceleration, and displacement response spectra are used as evaluation indices. Power analyses for one earthquake excitation are presented as an example to further study the energy dissipation effect of an SAECPTMD. The results indicate that an SAEC-PTMD performs better than an optimized passive TMD, both before and after damage occurs to the primary structure.

• Geomechanics and Engineering
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• v.29 no.5
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• pp.567-582
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• 2022

In the present study, a series of physical experiments and numerical simulations were conducted to investigate the effects of mode I and mixed-mode I/II cracks on the fracture modes and stability of roadway tunnel models. The experiments and simulations incorporated different inclination angle flaws under both static and dynamic loads. The quasi-static and dynamic testing were conducted by using an electro-hydraulic servo control device and drop weight impact system (DWIS), and the failure process was simulated by using rock failure process analysis (RFPA) and AUTODYN software. The stress intensity factor was also calculated to evaluate the stability of the flawed roadway tunnel models by using ABAQUS software. According to comparisons between the test and numerical results, it is observed that for flawed roadways with a single radical crack and inclination angle of 45°, the static and dynamic stability are the lowest relative to other angles of fractured rock masses. For mixed-mode I/II cracks in flawed roadway tunnel models under dynamic loading, a wing crack is produced and the pre-existing cracks increase the stress concentration factor in the right part of the specimen, but this factor will not be larger than the maximum principal stress region in the roadway tunnel models. Additionally, damage to the sidewalls will be involved in the flawed roadway tunnel models under static loads.

• Wind and Structures
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• v.12 no.4
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• pp.313-332
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• 2009

This paper numerically and experimentally investigates the control performance of the active mass damper (AMD) systems in a 26-story high-rise building in use. This is the first full-scale application of the AMD system for suppressing the wind-induced vibration of a building structure in Korea. In addition, the AMD system was installed on top of the building already in use, which may be the world's first implementation case. In order to simultaneously mitigate the transverse-torsional coupled vibration of the building, two AMD systems were applied. Moreover, the H-infinity control algorithm has been developed to utilize the maximum capacity of the AMD system. From the results of numerical simulation using the wind load obtained from the wind tunnel tests, it was found that the maximum acceleration responses of the building were reduced significantly. Moreover, the control performance of the installed AMD system was examined by carrying out the free and forced vibration tests. The acceleration responses on top of the building in the controlled case measured under strong wind loads were compared with those in the uncontrolled case numerically simulated by using the wind load deduced from the measured data and a structural model of the building. It is demonstrated that the AMD system shows good control performance in reducing the building accelerations.