Continuous Welded Rails. Longitudinal Efforts Track–Bridge Interaction

Since the 1960s, with the development of new, high-performance on-site welding techniques and new fasteners guaranteeing effective clamping, the use of welded long rails (LRS) has become widespread, particularly on high-speed lines (LGV).

Long welded rails are made up of a multitude of rails of normal length, conditioned by manufacturing and transport techniques, welded together on site and thus forming a single continuous unit. The distinction between welded long rails and normal bar rails is very clear: the length of normal rails in France does not exceed 36 meters, whereas that of welded long rails can extend over several hundred meters, or even kilometers.

This technique offers a number of advantages, including improved ride quality, less rail wear and, above all, extremely low maintenance costs thanks to the elimination of rail expansion devices, which are among the most expensive. Long welded rails have also made it possible to increase train speeds, which in the early 1960s did not exceed 160 km/h for passenger traffic. Long welded rails also reduce track discontinuities, eliminating wheel impact. This in turn improves train handling and reduces passenger discomfort. Long welded rails also ensure better resistance to vibration and, above all, reduced noise when the train passes overhead.

The stability of such systems, extending over very long lengths, is at the very heart of their study. Resistance to buckling due to thermal expansion and longitudinal forces is the main focus of the study of long welded rails. Uncontrolled lateral deformation could be the cause of catastrophic derailments, such as the Velars-sur-Ouche accident on July 23, 1962. The train was travelling at almost 140 km/h in scorching heat, causing the rails to buckle and derail five cars, one of which ended up crashing fifty meters below, at the bottom of the Combe de Fain. The thermal effects of long welded rails are so severe that they are sometimes referred to as thermal rail buckling. The deformations that occurred during the heatwave of 2003 (36 in all) were so severe that the SNCF reviewed its safety margins and now introduces a "safety margin linked to the 2003 heatwave".

The presence of a structure is certainly not without impact on the behavior of continuous welded rail. As paragraph 6.5.4 of standard EN 1992-2/NA, describing the combined response of the track and structure system to variable actions, clearly states: "When the rails are continuous at discontinuities in the track support (bridge-embankment transition, etc.), the bridge structure and the track jointly resist longitudinal actions due to acceleration and braking. Longitudinal actions are transmitted in part by the rails to the embankment behind the abutment, and in part...