A simplified method for calculating load distribution and rail deflections in track, incorporating the influence of sleeper stiffness

Abstract

When performing railway track stiffness analyses, in current standards sleepers are often regarded as a rigid member. For flexible sleeper materials like timber or polymers, this may lead to underestimating rail deflections up to a factor two and overestimating rail seat loads up to 20%. Calculations incorporating sleeper bending can currently be performed analytically by a two-layer beam-on-elastic-foundation calculation, or by finite element analyses, but a simple approach does not yet exist. This article introduces a simple calculation method to establish rail deflections, track stiffness and rail seat loads, incorporating the effects of both bending and shear stiffness of the sleeper, applicable to new or tamped track. A sleeper flexibility factor fS is introduced as a deformation multiplication factor compared with a rigid sleeper. Validation against current calculation models shows a deformation accuracy within 6% and a load accuracy within 2%. When a track is not maintained, a gap will develop under the sleeper at the rail seat. The size of this gap correlates to the stiffness of the sleeper. Eventually a uniform load distribution can develop under the sleeper. A calculation method is introduced to estimate the gap and the track behaviour at uniform load distribution. When for flexible sleepers the track deflection due to the gap can develop outside of the desired range, timely track maintenance or monitoring is advised.