RAILWAY TRACK-BRIDGE STRUCTURE INTERACTION FOR CONTINUOUS I LONG WELDED RAILS (CWR ! LWR)

Abstract

Most of the railway lines today opt for Continuous Welded Rails (CWR) or Long Welded Rails (LWR) for the benefits they offer in terms of decrease in rail distresses, reduced track maintenance, increased riding comfort and safety. The provision of LWR is imperative for the high-speed railway lines that operate on national networks and intra-city metro networks. The resistance offered by the fastenings and the ballast to the relative sliding of the rails leads to the longitudinal forces in the rails. The free movement of the rails is accommodated by the expansion devices in the breather portion at the ends of LWR. Introduction of a bridge under a LWR track causes it to rest on a surface subject to deformation and movements, hence causing displacement of the track. As both track and bridge are able to move, any force or displacement that acts on one of them will induce forces in the other, thus resulting in track-bridge interaction. It also indirectly results in additional forces at the bearings, which affects the substructure design. The interaction depends on various bridge and track parameters, principal among them being the resistance of the track to longitudinal displacements. Thermal loads, horizontal braking and acceleration forces and vertical traffic loads are the major actions contributing to the interaction effects. The interaction effect is taken as a serviceability limit state with regards to the bridge, and as an ultimate limit state with regards to the rail. Foreign national codes have developed some guidelines and design criteria that involve limitation of the bridge displacements and the additional stresses in the rails. These guidelines are suitable for designs with short or moderate expansion length of the spans and almost uniform stiffness at the supports. For major viaducts and bridges for metro railways, there would be significant variation in support stiffness and a large number of successive decks. It is advisable to go for computer-assisted interaction analysis in such cases. The computer analysis needs to be a multi-step analysis to take into account the elasto-plastic character of the track resistance. An intuitive understanding of the various actions participating in the system, and the interaction behaviour is attempted. The theoretical basis of the track-bridge interaction has been studied along with the other findings as published in the literature. A detailed study of the parameters that affect the interaction is carried out. Codes of other countries and the prevalent practice in India have been reviewed. While well established procedures for the laying and maintenance of LWR / CWR track are in place, the Indian railway standards are silent about the problem of track-bridge interaction. The use of rail-free fastenings on bridges, as recommended by Indian railway guidelines, severely limits the bridge length in higher temperature zones. The study aims at developing a method for simplified computer analysis that can be executed by available commercial software using problem specific parameters. A parametric study is performed using SAP2000 to compute the effects of different actions such as braking loads, temperature variation and vertical loads on support reactions, rail stresses and displacements. The interaction behaviour is evaluated for multi-span bridges. Large support forces and rail stresses are generated for various actions on long span bridges due to the interaction. The findings of the parametric study can serve as guidelines during the conceptual design stage and would aid the detailed analysis of the bridge.