Nam Hyoung Lim

Stability of continuous welded rail track

Abstract As the use of continuous welded rail (CWR) increases in track structures, derailing disasters associated with track buckling also increase in great numbers due to high compressive thermal stress. A three-dimensional CWR track model is developed in the present study to be used for extensive buckling analysis of CWR tracks subjected to temperature load. The analysis model is encoded into a special purpose program using the finite element method. The CWR track model consists of four elements: a mono-symmetric thin-walled open section beam element with 7 degrees of freedom per node to represent the rail; a solid beam-on-elastic-foundation element having 6 degrees of freedom per node to simulate the tie, including vertical and/or longitudinal ballast resistance; an elastic spring element with two nodes and zero length to stand for pad-fastener system; and spring elements for the lateral or longitudinal ballast resistances. Also, two types of significant nonlinearity are included in the track model: the geometric nonlinearity of the rail element, and the materially nonlinear resistance of the ballast. The validity of the present study is strictly verified through a series of comparative analyses with those by others. The nonlinear analysis results have shown that buckling of the track is a three-dimensional problem, and the 2-D rail–tie model and beam model overestimated the CWR track stability.

Elastic buckling of I-beams under linear moment gradient

Abstract This paper investigates the elastic lateral–torsional buckling of I-beams under linear moment gradient that very precisely incorporates the effects of moment gradient and various end restraints. The elastic critical buckling moments are obtained independently by using: (1) the Bubnov–Galerkin method and (2) the finite element method. The present formula of the moment gradient correction factor cannot satisfactorily predict the buckling capacities of doubly symmetric and monosymmetric I-beams with various end restraints. We propose alternative equations for evaluating the moment gradient correction factor, considering end restraint conditions.

A consideration on intermediate diaphragm spacing in steel box girder bridges with a doubly symmetric section

Abstract Diaphragms in box girder bridges are implemented primarily to prevent premature excessive distortional deformation under torsional loading condition. Distortional warping and transverse bending stresses, which are the major stress components resulting from distortion, should be appropriately limited to a specific level for efficient use of the cross-section by installing adequate intermediate diaphragms. The objectives of the present study are to develop a thin-walled box beam finite element and to propose tentative design charts for adequate spacing of intermediate diaphragms. The developed beam element possesses nine degrees of freedom per node and the validity was intensively verified from a series of comparative studies using a conventional shell element. Also, performed herein are extensive parametric studies for continuous box girder bridges of doubly symmetric steel box section. The design parameters taken into account were the desired ratio of the distortional warping normal stress to the bending normal stress, the number of spans, the span length, the aspect ratio of the box section, and the spacing of the intermediate diaphragms. The results were summarized into tentative design charts indicating efficient spacing of intermediate diaphragms for the various stress ratios.

Experimental Study on the Evaluation of the Human Stability due to Overtopping Waves in the Breakwater

Major loss of life can occur in the breakwater when people are toppled by overtopping waves. Over the last four decades, a number of numerical(analytical) and experimental studies to define the limit criteria of the loss of human stability in flood flows have been conducted. However, the analytical or experimental study to investigate the loss of human stability in overtopping waves has not been performed up to now. In this paper, the experimental investigation of the human stability in overtopping waves is conducted using two-dimensional large scale multipurpose wave flume for wave-current-tide. These new experimental results show that the toppling of human can be expected in lower depth/lower velocity compared to that of ordinary flood flows based on previous experimental study. Also the criteria for the stability of people subject to overtopping waves are proposed.

Reliability Evaluation of the Speed Reduction Scheme in Korean High-speed Train based on the Reliability of the CWR Buckling

In order to prevent the track buckling in the hot summer and ensure the safety of the running train, as a part of the safety control plan for high-speed railway, the train speed restriction according to the rail temperature was introduced in 2004. However, the conceptual and theoretical background of train-speed restriction is unclear. In this paper, the buckling probability evaluation program( ProCWR) developed by this researchers was improved to take maximum and minimum buckling temperature into account for the calculation of allowable temperature. From the theoretical background of the international research and the buckling probability considering domestic railroad conditions, the validity and reliability of Korean high-speed train speed reduction scheme were investigated.

An Analysis of Fall Accidents into the Sea by the Fault Tree Analysis Method

Recently, because the waterfront development of various form have been done in the coastal zone, human`s sphere of activity is approaching gradually to the sea. As a result, the riskiness of the safety accidents is increasing gradually. In addition, the frequency of the abnormal wave such as a heaving wave which is the major cause of the safety accident in the coastal zone is increasing. According to the case analysis of the accident, fall accidents into the sea at the breakwater and the rocks on the seashore is main safety accident. In this paper, in order to provide a basic information about the prevention and the countermeasure for the fall accidents into the sea, the fault tree is developed through the case analysis of the fall accidents. To investigate the frequency of the basic event in the fault tree, the survey against ordinary people and policeman is conducted. In the end, the fault tree analysis(FTA) is carried out to evaluate the risk of the fall accidents into the sea.