Importance of Bending Stiffness of Different Track Forms

Abstract

The most common form of railway track construction is still discrete sleepers on ballast. However, other types of track, sometimes referred to as ballastless, hybrid and/or slab track, may in some circumstances be more cost effective over their life cycle despite their higher initial cost. Current trackbed design methods for different types of superstructure generally specify a trackbed/subgrade support stiffness in terms of an “EV2” value, i.e. the stiffness evaluated from the second load step in a plate bearing test. Specifying the same EV2 value regardless of the track system superstructure creates over-conservative designs that fail to make use of the higher longitudinal bending stiffness of ladder or slab track systems compared with ballasted track. This paper reports the results of computer analyses carried out to understand better the influence of the longitudinal bending stiffness of the track system superstructure on trackbed response. Three-dimensional models of three types of railway track superstructure system (conventional sleepers, ladder and slab track) were set up in finite element simulations. The models were validated against closed-form solutions for a beam on an elastic foundation. Parametric studies were then undertaken to understand how changes in support stiffness influenced peak trackbed deflections and stresses, for the different track superstructures. The results show that, with increasing bending stiffness of the track superstructure, peak deflections are decreased, and the longitudinal extent of the deflection bowl is increased. There are also significant reductions in stresses being transferred into the trackbed. Slab and ladder tracks are also shown to be better at spanning regions of poor/reduced support.