• Journal of the Korean Geotechnical Society
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• v.29 no.3
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• pp.29-40
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• 2013

Seismic response of single degree of freedom system supported by shallow foundation was analyzed by using nonlinear explicit finite difference element code. Numerical analysis results were verified with dynamic centrifuge test results of the same soil profile and structural dimensions with the numerical analysis model at a centrifugal acceleration of 20 g. Differences between the analysis and the test results induced by the boundary conditions of control points can be reduced by adding additional local damping to the natural born cyclic hysteretic damping of the soil strata. The analysis results show good agreement with the test results in terms of both time histories and response spectra. Thus, it can be concluded that the nonlinear explicit finite difference element code will be a useful technique for estimating seismic residual displacement, earthpressure etc. which are difficult to measure during laboratory tests and real earthquake.

• Earthquakes and Structures
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• v.8 no.6
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• pp.1325-1348
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• 2015

Real-time hybrid testing (RTHT) involves virtual splitting of the structure into two parts: physical substructure that contains the key region of interest which is tested in a laboratory and numerical substructure that contains the remaining part of the structure in the form of a numerical model. This paper numerically assesses four step-by-step integration methods (Central difference method (CDM), Operator splitting method (OSM), Rosenbrock based method (RBM) and CR-integration method (CR)) which are widely used in RTHT. The methods have been assessed in terms of stability and accuracy for various realistic damping ratios of the physical substructure. The stability is assessed in terms of the spectral radii of the amplification matrix while the accuracy in terms of numerical damping and period distortion. In order to evaluate the performance of the methods, five carefully chosen examples have been studied - undamped SDOF, damped SDOF, instantaneous softening, instantaneous hardening and hysteretic system. The performance of the methods is measured in terms of a non-dimensional error index for displacement and velocity. Based on the error indices, it is observed that OSM and RBM are robust and performs fairly well in all the cases. CDM performed well for undamped SDOF system. CR method can be used for the system showing softening behaviour. The error indices indicate that accuracy of OSM is more than other method in case of hysteretic system. The accuracy of the results obtained through time integration methods for different damping ratios of the physical substructure is addressed in the present study. In the presence of a number of integration methods, it is preferable to have criteria for the selection of the time integration scheme. As such criteria are not available presently, this paper attempts to fill this gap by numerically assessing the four commonly used step-by-step methods.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.3
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• pp.292-301
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• 2008

A linearized model for hysteretic acoustic nonlinearity of imperfectly joined interface is proposed and analyzed by using Coulomb damping to investigate the characteristics of the reflection and transmission coefficients for harmonic waves at the contact interface. Closed crack is modeled as non welded interface that has nonlinear discontinuity condition in displacement across its boundary. Based on the hysteretic contact stiffness of the contact interface, the reflected and transmitted waves are determined by deriving the tractions on both sides of the interface in terms of the discontinuous displacements across the interface. It is found that the amplitudes of the reflected and transmitted waves are dependent on the frequency and the hysteretic stiffness. As the frequency of the incident wave increases, the higher reflection and lower transmission are obtained. It also shows that the hysteresis of the interface increases the reflection coefficient, but reduces the transmission coefficient. A fatigue crack is also made in aluminum specimen to demonstrate these characteristics of the reflection and transmission of contact interfaces.

• Geomechanics and Engineering
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• v.20 no.4
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• pp.323-331
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• 2020

Pile foundation is widely used for railway bridges in loess throughout northwestern China. Modeling of the loess-pile interaction is an essential part for seismic analysis of bridge with pile foundation at seismically active regions. A quasi-static test is carried out to investigate the hysteretic behaviors of pile foundation in collapsible loess. The failure characteristics of the bridge pile-loess system under the cyclic lateral loading are summarized. From the test results, the energy dissipation, stiffness degradation and ductility of the pile foundation in loess are analyzed. Therefore, a bilinear model with stiffness degradation is recommended for the nonlinearity of the bridge pier-pile-loess system. It can be found that the stiffness of the bridge pier-pile-loess system decreases quickly in the initial stage, and then becomes more slowly with the increase of the displacement ductility. The equivalent viscous damping ratio is defined as the ratio of the dissipated energy in one cycle of hysteresis curves and increases with the lateral displacement.

• International journal of steel structures
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• v.18 no.4
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• pp.1107-1124
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• 2018

The structural behavior of reinforced concrete coupled shear wall structures is greatly influenced by the behavior of their coupling beams. This paper presents a process of the seismic analysis of reinforced concrete coupled shear wall-frame system linked by hysteretic dampers at each floor. The hysteretic dampers are located at the middle portion of the linked beams which most of the inelastic damage would be concentrated. This study concerned particularly with wall-frame structures that do not twist. The proposed method, which is based on the energy equilibrium method, offers an important design method by the result of increasing energy dissipation capacity and reducing damage to the wall's base. The optimum distribution of yield shear force coefficients is to evenly distribute the damage at dampers over the structural height based on the cumulative plastic deformation ratio of the dissipation device. Nonlinear dynamic analysis indicates that, with a proper set of damping parameters, the wall's dynamic responses can be well controlled. Finally, based on the total plastic strain energy and its trend through the height of the buildings, a prediction equation is suggested.

• Journal of the Earthquake Engineering Society of Korea
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• v.12 no.1
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• pp.43-55
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• 2008

The complexity of determining strain associated with shear modulus and damping ratio in torsional tests has been resolved by means of several approaches. Particularly, the modified equivalent radius approach is adequate to when generating the plots of equivalent radius ratio versus strain more effectively over any range of strains in resonant column and torsional shear (RC/TS) tests. The modified equivalent radius approach was applied for hyperbolic, modified hyperbolic, and Ramberg-Osgood models in evaluating damping ratio. Results showed that using a single value of equivalent radius ratio based on conventional equivalent radius approach is not appropriate. A new model was developed to consider the soil damping behavior at small strains as well as hysteretic damping and it was attempted to determine adjustments are required in evaluating strain associated damping when combining the two damping components.

• Earthquakes and Structures
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• v.9 no.1
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• pp.29-48
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• 2015

The main purpose of this work is to compare different criteria for the seismic strengthening of RC framed buildings in order to find the optimal combinations of these retrofitting techniques. To this end, a numerical investigation is carried out with reference to the town hall of Spilinga (Italy), an RC framed structure with an L-shaped plan built at the beginning of the 1960s. Five structures are considered, derived from the first by incorporating: carbon fibre reinforced polymer (FRP)-wrapping of all columns; base-isolation, with high-damping-laminated-rubber bearings (HDLRBs); added damping, with hysteretic damped braces (HYDBs); FRP-wrapping of the first storey columns combined with base-isolation or added damping. A three-dimensional fibre model of the primary and retrofitted structures is considered; bilinear and trilinear laws idealize, respectively, the behaviour of the HYDB, providing that the buckling be prevented, and the FRP-wrapping, without resistance in compression, while the response of the HDLRB is simulated by using a viscoelastic linear model. The effectiveness of the retrofitting solutions is tested with nonlinear dynamic analyses based on biaxial accelerograms, whose response spectra match those in the Italian seismic code.

• Journal of the Earthquake Engineering Society of Korea
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• v.11 no.6
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• pp.1-14
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• 2007

Optimal design method of nonlinear damping system for seismic response control of adjacent structures is studied in this paper. The objective functions of the optimal design are defined by structural response and total amount of the dampers. In order to obtain a solution minimizing two mutually conflicting objective functions simultaneously, multi-objective optimization technique based on genetic algorithm is adopted. In addition, stochastic linearization method is embedded into the multi-objective framework to efficiently estimate the seismic responses of the adjacent structures interconnected by nonlinear hysteretic dampers without performing nonlinear time-history analyses. As a numerical example to demonstrate the effectiveness of the proposed technique, 20-story and 10-story buildings are considered and MR dampers of which hysteretic behaviors vary with the magnitude of the input voltage are considered as nonlinear hysteretic damper interconnecting two adjacent buildings. The proposed approach can provide the optimal number and capacities of the MR dampers, which turned out to be more economical than the uniform distribution system while maintaining similar control performance. The proposed damper system is verified to show more stable performance in terms of the pounding probability between two adjacent buildings. The applicability of the proposed method to the design problem for optimally placing semi-active control system is examined as well.

• 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.

• Tribology and Lubricants
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• v.14 no.4
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• pp.44-50
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• 1998

This paper describes the simulation of the levitation force between permanent magnet and high Tc(critical temperature) superconductor(HTSC). Levitation force is evaluated numerically on the basis of the magnetic vector potential method and the critical state model. The superconductor is approximated to 2-D slab model. By performing computations, the following characteristics have been investigated: the process of the generation of hysteresis, the various hysteretic behaviors. The characteristics of hysteresis are important for the application to magnetic bearing, for the damping and the nonlinear stiffness is related to hysteresis.