• Interaction and multiscale mechanics
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• v.5 no.1
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• pp.1-12
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• 2012

Considerable longitudinal rail forces and displacements may develop in continuous welded rail (CWR) track on long-span bridges due to temperature variations. The track stability may be disturbed due to excessive relative displacements between the sleepers and ballast bed and the accompanied reduction in frictional resistance. For high-speed tracks, however, solving these problems by installing rail expansion devices in the track is not an attractive solution as these devices may cause a local disturbance of the vertical track stiffness and track geometry which will require intensive maintenance. With reference to temperature, two actions are considered by the bridge loading standards, the uniform variation in the rail and deck temperature and the temperature gradient in deck. Generally, the effect of temperature gradient has been disregarded in the interaction analysis. This paper mainly deals with the effect of temperature gradient on the track-bridge interaction with respect to the support reaction, rail stresses and stability. The study presented in this paper was not mentioned in the related codes so far.

• Interaction and multiscale mechanics
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• v.1 no.2
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• pp.303-315
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• 2008

In this paper, thermodynamical properties of crystalline silicon under strain are calculated using classical molecular dynamics (MD) simulations based on the Tersoff interatomic potential. The Helmholtz free energy of the silicon crystal under strain is calculated by using the ensemble method developed by Frenkel and Ladd (1984). To account for quantum corrections under strain in the classical MD simulations, we propose an approach where the quantum corrections to the internal energy and the Helmholtz free energy are obtained by using the corresponding energy deviation between the classical and quantum harmonic oscillators. We calculate the variation of thermodynamic properties with temperature and strain and compare them with results obtained by using the quasi-harmonic model in the reciprocal space.

• Journal of Environmental Impact Assessment
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• v.15 no.3
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• pp.193-206
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• 2006

Recently, high ozone episode occurred frequently in Korea. Moreover ozone episode occurred not only in the city but also in background area where local anthropogenic sources are not important. It analyzed frequency exceeding 100ppb ozone at air quality monitoring stations in Seoul and rural area during 1995-2004. This paper reports on the use of the Community Multiscale Air Quality (CMAQ) modelling system to predict hourly ozone levels. Domain resolutions of 30km, 10km, 3.333km (innermost) have been employed for this study. Summer periods in June 2004 have been simulated and the predicted results have been compared to data for metropolitan and rural air quality monitoring stations. The model performance has been evaluated with measured data through a range of statistical measures. Although, the CMAQ model reproduces the ozone temporal spatial trends it was not able to simulate the peak magnitudes consistently.

• Interaction and multiscale mechanics
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• v.1 no.1
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• pp.103-121
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• 2008

This paper presents an optimization-based method for computing a minimal bounding ellipsoid that contains the set of static responses of an uncertain braced frame. Based on a non-stochastic modeling of uncertainty, we assume that the parameters both of brace stiffnesses and external forces are uncertain but bounded. A brace member represents the sum of the stiffness of the actual brace and the contributions of some non-structural elements, and hence we assume that the axial stiffness of each brace is uncertain. By using the $\mathcal{S}$-lemma, we formulate a semidefinite programming (SDP) problem which provides an outer approximation of the minimal bounding ellipsoid. The minimum bounding ellipsoids are computed for a braced frame under several uncertain circumstances.

• Interaction and multiscale mechanics
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• v.3 no.4
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• pp.405-414
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• 2010

The performance of full-scale steel and composite bridge safety barriers under vehicle crash is evaluated by using the nonlinear explicit finite element code LS-DYNA. Two types of vehicles used in this study are passenger car and truck, and the performance criteria considered include structural strength and deformation, occupant protection, and post-crash vehicle behavior. It can be concluded that the composite safety barrier satisfies all performance criteria of vehicle crash. Although the steel safety barrier satisfies the performance criteria of occupant protection and post-crash vehicle behavior, it fails to satisfy the performance criterion of deformation. In all performance evaluations, the composite safety barrier exhibits a superior performance in comparing with the steel safety barrier.

• Interaction and multiscale mechanics
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• v.3 no.4
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• pp.321-331
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• 2010

A new technique is proposed for bridge structural damage detection based on spatial wavelet analysis of the time history obtained from vehicle body moving over the bridge, which is different from traditional detection techniques based on the bridge response. A simply-supported Bernoulli-Euler beam subjected to a moving spring-mass unit is established, with the crack in the beam simulated by modeling the cracked section as a rotational spring connecting two undamaged beam segments, and the equations of motion for the system is derived. By using the transfer matrix method, the natural frequencies and mode shapes of the cracked beam are determined. The responses of the beam and the moving spring-mass unit are obtained by modal decomposition theory. The continuous wavelet transform is calculated on the displacement time histories of the sprung-mass. The case study result shows that the damage location can be accurately determined and the method is effective.

• Interaction and multiscale mechanics
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• v.3 no.3
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• pp.267-276
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• 2010

A nonlocal finite element model is developed for solving elasto-static frictional contact problems of nanostructures and nanoscale devices. A two dimensional Eringen-type nonlocal elasticity model is adopted. The material is characterized by a stress-strain constitutive relation of a convolution integral form whose kernel is capable to take into account both the diffusion process of nonlocal elasticity and the scale ratio effects. The incremental convex programming procedure is exploited as a solver. Two examples of different nature are presented, the first one presents the behavior of a nanoscale contacting system and the second example discusses the nano-indentation problem.

• Proceedings of the KSEEG Conference
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• 2002.04a
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• pp.159-162
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• 2002

We tried to obtain an initial velocity model for prestack depth migration via waveform inversion. For application of any field data we chose a smooth background layered velocity model (v=v0 + k x z) as an initial velocity model. Newton type waveform inversion needs to invert huge Hessian matrix. In order to compute full Hessian matrix arising from full aperture data and full illumination zone, we meet insurmountable difficulties of paying astronomical computing cost. For the layered media, approximate Hessian emerging from single shot aperture data can be used repeatedly for split spread source configuration. In our work of using this Hessian characteristic of layered media we attempted to obtain the approximate velocity model as close as possible to the true velocity model in first iteration.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2007.04a
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• pp.405-410
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• 2007

We present a new global/local analysis with the aid of MLS(Moving Least Square)-based finite elements which can handle an arbitrary number of nodes on every element side. It give a great flexibility in constructing finite element meshes at the specified local regions without remeshing. Compared to other type global/local analysis, it does not require any superimposed mesh or need not solve the equilibrium equation twice as well as shows an excellent accuracy. To demonstrate the performance of proposed scheme, we will show several examples in relation to capturing highly local stress field.

• Interaction and multiscale mechanics
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• v.6 no.2
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• pp.107-125
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• 2013

A strain smoothing meshfree formulation with stabilized conforming nodal integration is presented for modeling the consolidation process in saturated porous media. In the present method, nodal strain smoothing is consistently introduced into the meshfree approximation of strain and pore pressure gradient variables associated with the saturated porous media. Meanwhile, in order to achieve a consistent numerical implementation, a smoothing approximation of the meshfree shape function within a nodal representative domain is also proposed in the stiffness construction. The resulting discrete system of equations is all expressed in smoothed nodal measures that are very efficient for numerical evaluation. Subsequently the space-time fully discrete equations are further established by the generalized trapezoidal rule for time integration. The effectiveness of the proposed meshfree consolidation analysis method is systematically illustrated by several benchmark problems.