• Earthquakes and Structures
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• 제24권1호
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• pp.21-36
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• 2023

Structural vibrations generated by earthquakes and wind loads can be controlled by varying the structural parameters such as mass, stiffness, damping ratio, and geometry and providing a certain amount of passive or active reaction forces. A Double-Tuned Mass Dampers (DTMDs) system, which is simple and more effective than the conventional single tuned mass damper (TMD) system for vibration mitigation is presented. Two TMDs tuned to the first two natural frequencies were used to control vibrations. Experimental investigations were carried out on a three degrees-of-freedom frame model to investigate the effectiveness of DTMDs systems in controlling displacements, accelerations, and base shear. Numerical models were developed and validated against the experimental results. The validation showed a good match between the experimental and numerical results. The validated model was employed to investigate the behavior of a five degrees-of-freedom shear building structure, wherein mass dampers with different mass ratios were considered. The effectiveness of the DTMDs system was investigated for harmonic, seismic, and white noise base excitations. The proposed system was capable of significantly reducing the story displacements, accelerations, and base shears at the first and second natural frequencies, as compared to conventional single TMD.

• Ocean Systems Engineering
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• 제13권4호
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• pp.401-421
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• 2023

Reducing hawser line tensions and dynamic responses to a certain level is of paramount importance as the hawser lines provide important structural linkage between 2 body TLP-TAD system. The objective of this paper is to demonstrate how MR Damper can be utilized to achieve this. Hydrodynamic coefficients and wave forces for two bodies including second-order effects are obtained by 3D diffraction/radiation panel program by potential theory. Then, multi-hull-riser-mooring-hawser fully-coupled time-domain dynamic simulation program is applied to solve the complex two-body system's dynamics with the Magneto-Rheological (MR) Damper modeled on one end of hawser. Since the damping level of MR Damper can be changed by inputting different electric currents, various simulations are conducted for various electric currents. The results show the reductions in maximum hawser tensions with MR Damper even for passive control cases. The results also show that the hawser tensions and MR Damper strokes are affected not only by input electric currents but also by initial mooring design. Further optimization of hawser design with MR Damper can be done by active MR-Damper control with changing electric currents, which is the subject of the next study.

• Journal of the Earthquake Engineering Society of Korea
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• 제11권3호
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• pp.53-62
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• 2007

This paper presents a new vibration control method for long-span bridges using complex damper system. The new system presents simple mechanical configuration with oil and elasto-plastic dampers which have velocity and displacement dependency in vibration energy absorbing. This system can produce various damping forces according to the applied external forces by the velocity and displacement-dependent characteristics of the dampers. The oil damper dissipates vibration energy for relatively frequent and small amplitude like in the case for small to moderate earthquakes, whereas the elasto-plastic damper system works for rare and large amplitude vibration such as high seismic excitation. Thus, the proposed system exhibits the advantage of low cost with high performance since the roles of the two different dampers are effectively separated. A numerical model is established for the complex damper system, and the response characteristics and effectiveness of the proposed system are presented through numerical simulations. Numerical results show that the proposed complex damper system can significantly improve the seismic performance of long-span bridge structures with much more effective damping mechanism than single conventional passive damper systems.

• Nuclear Engineering and Technology
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• 제17권4호
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• pp.290-301
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• 1985

600MW(e) CANDU Primary Heat Transport System (PHTS) is composed of the two “figure-of-eight” loops and is designed to operate with the 4% Reactor Outlet Header (ROH) quality at its rated power. This existence of the two compressible regions and the positive flow-qualitly-void feedbacks are the sources of the PHTS flow instability. To ensure the PHTS stability, ROH-ROH interconnect pipes are installed as passive systems. This paper describes the investigation of the PHTS flow instability at its design full power condition. Also studied are the interconnect effect and the inherent system damping effect on the system stability. The time domain stability analyses are accessed by using the ATHER/MOD-I code which is the improved version of the KAERI developed ATHER code. Under the most adverse system modelling, the “figure-of-eight” symmetric loop shows divergent flow oscillations. Under with the interconnect, the PHTS stability is remarkably enhanced so that the system becomes stable. However, even under the conservative pressurizer modelling, the PHTS shows the more convergent flow oscillations. With the interconnect and the pressurizer modelling, its stability is highly credited. Conclusively, the inherent system damping by pressurizer itself can credit the PHTS stability without the interconnect.

• Journal of the Korean Society for Precision Engineering
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• 제30권5호
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• pp.559-563
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• 2013

This study aims to quantify the effects of medication (Med) and deep brain stimulation (DBS) on resting rigidity in patients with Parkinson's disease. We tested 10 limbs of five patients under each of four treatment conditions: 1) baseline, 2) DBS, 3) Med, 4) DBS + Med. Rigidity at the wrist joint was assessed using the Unified Parkinson's Disease Rating Scale (UPDRS). The examiner randomly imposed flexion and extension movement on patient's wrist joint. Resistance to passive movement was quantified by viscoelastic properties. Not only rigidity score but also damping constant showed improvements in rigidity by DBS and Med treatments (p<0.05). This indicates that the viscosity can represent the change in rigidity due to DBS as well as Med, which was manifested by UPDRS score.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• 제36권4호
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• pp.346-351
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• 2008

In this paper, the semi-active control of a helicopter landing gear using magneto -rheological(MR) damper is studied. A dynamic model of the MR damper is formulated by incorporating magnetic field-dependent Bingham properties of the MR fluid. The electromagnet of the MR damper is designed and its magnetic field is analyzed using a commercial finite element code. The damping characteristics of MR damper by changing the intensity of the magnetic field are investigated and the dynamic responses of the helicopter landing gear with MR damper are simulated. The semi-active control of the helicopter landing gear is simulated by implementing a sky-kook control algorithm and its performance is evaluated comparing to the passive control.

• Earthquakes and Structures
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• 제9권1호
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• pp.127-143
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• 2015

In conventional seismic design, structures are assumed to be fixed at the base. To reduce the impact of earthquake loading, while at the same time providing an economically feasible structure, minor damage is tolerated in the form of controlled plastic hinging at predefined locations in the structure. Uplift is traditionally not permitted because of concerns that it would lead to collapse. However, observations of damage to structures that have been through major earthquakes reveal that partial and temporary uplift of structures can be beneficial in many cases. Allowing a structure to move as a rigid body is in fact one way to limit activated seismic forces that could lead to severe inelastic deformations. To further reduce the induced seismic energy, slip-friction connectors could be installed to act both as hold-downs resisting overturning and as contributors to structural damping. This paper reviews recent research on the concept, with a focus on timber shear walls. A novel approach used to achieve the desired sliding threshold in the slip-friction connectors is described. The wall uplifts when this threshold is reached, thereby imparting ductility to the structure. To resist base shear an innovative shear key was developed. Recent research confirms that the proposed system of timber wall, shear key, and slip-friction connectors, are feasible as a ductile and low-damage structural solution. Additional numerical studies explore the interaction between vertical load and slip-friction connector strength, and how this influences both the energy dissipation and self-centring capabilities of the rocking structure.

• Journal of Korean Society of Steel Construction
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• 제24권2호
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• pp.149-161
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• 2012

The use of seismic energy-dissipative devices for passive control is increasing exponentially in the recent years for both new and existing buildings. Use of these devices started in and has been somewhat limited to developed countries. One of the current challenges is to promote the use of seismic dampers in earthquake-prone developing countries by lowering the cost of the devices. This paper proposed a new type of seismic damper based on yielding of a cantilever type metallic element for seismic retrofit of existing and new building structures. The hysteretic behavior and energy dissipation capacity of the proposed damper was investigated using component tests under cyclic loads. The experimental results indicated that the damping device had stable restoring force characteristics and a high energy dissipation capacity. Based on these results, a simple hysteretic model for predicting the load-displacement curve of the seismic damper was proposed.

• Structural Engineering and Mechanics
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• 제47권6호
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• pp.881-898
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• 2013

This paper describes analytical investigation into a new dual function system including a couple of shear links which are connected in series using chevron bracing capable to correlate its performance with magnitude of earthquakes. In this proposed system, called Chevron Knee-Vertical Link Beam braced system (CK-VLB), the inherent hysteretic damping of vertical link beam placed above chevron bracing is exclusively utilized to dissipate the energy of moderate earthquakes through web plastic shear distortion while the rest of the structural elements are in elastic range. Under strong earthquakes, plastic deformation of VLB will be halted via restraining it by Stopper Device (SD) and further imposed displacement subsequently causes yielding of the knee elements located at the bottom of chevron bracing to significantly increase the energy dissipation capacity level. In this paper first by studying the knee yielding mode, a suitable shape and angle for diagonal-knee bracing is proposed. Then finite elements models are developed. Monotonic and cyclic analyses have been conducted to compare dissipation capacities on three individual models of passive systems (CK-VLB, knee braced system and SPS system) by General-purpose finite element program ABAQUS in which a bilinear kinematic hardening model is incorporated to trace the material nonlinearity. Also quasi-static cyclic loading based on the guidelines presented in ATC-24 has been imposed to different models of CK-VLB with changing of vertical link beam section in order to find prime effectiveness on structural frames. Results show that CK-VLB system exhibits stable behavior and is capable of dissipating a significant amount of energy in two separate levels of lateral forces due to different probable earthquakes.

• Earthquakes and Structures
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• 제14권3호
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• pp.215-227
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• 2018

Supplemental passive control devices are widely considered as an important tool to mitigate the dynamic response of a building under seismic excitation. Nevertheless, a systematic method for strategically placing dampers in the buildings is not prescribed in building codes and guidelines. Many deterministic and stochastic methods have been proposed by previous researchers to investigate the optimum distribution of the viscous dampers in the steel frames. However, the seismic performances of the retrofitted buildings that are under large earthquake intensity levels or near collapse state have not been evaluated by any seismic research. Recent years, an increasing number of studies utilize genetic algorithms (GA) to explore the complex engineering optimization problems. GA interfaced with nonlinear response history (NRH) analysis is considered as one of the most powerful and popular stochastic methods to deal with the nonlinear optimization problem of damper distribution. In this paper, the effectiveness and the efficiency of GA on optimizing damper distribution are first evaluated by strong ground motions associated with the collapse failure. A practical optimization framework using GA and NRH analysis is proposed for optimizing the distribution of the fluid viscous dampers within the moment resisting frames (MRF) regarding the improvements of large drifts under intensive seismic context. Both a 10-storey and a 20-storey building are involved to explore higher mode effect. A far-fault and a near-fault earthquake environment are also considered for the frames under different seismic intensity levels. To evaluate the improvements obtained from the GA optimization regarding the collapse performance of the buildings, Incremental Dynamic Analysis (IDA) is conducted and comparisons are made between the GA damper distribution and stiffness proportional damping distribution on the collapse probability of the retrofitted frames.