Dynamic resistance and rational design of railway prestressed concrete sleepers
Current design philosophy for railway concrete sleepers is based on the analysis of static and quasi-static stresses resulting from quasi-static wheel loads and essentially the static response of concrete sleepers. In general, cracking can incur when the bottom fibre stress is larger than tensile strength of concrete. Premature cracking of prestressed concrete sleepers has been detected in railway tracks. The major cause of cracking is the infrequent but high-magnitude wheel loads produced by a small percentage of "out-of-round" wheels or railhead surface defects, which are crudely accounted for in any design and test standards (including EN 13230) by a single load factor in design (or k factors for test criteria). These prescribed factors in either design or testing have very little to none of relationship with real behaviours of railway prestressed concrete sleepers, especially considering their entire service life. In fact, its scientific origin is somewhat questionable. Based on the current design methods (either by European EN 13230 or other international standards, e.g. Australia AS 1085.14 or American AREMA Manual Chapter 30), the cracked sleepers must be theoretically replaced by new ones, resulting in a costly maintenance budget each year. In reality, concrete sleepers are embedded in ballast or in mass concrete slabs. Crack detection is neither normally carried out by visual inspection nor any NDT&E approach. This has raised a paradox political game between manufacturers and asset managers about the shared responsibility and risk. Such the critical issue is hidden under other more pressing demands by other highly-publicised rail problems. On this ground, it is important to address such important issues as the realistic spectrum and amplitudes of dynamic forces applied to the railway track, and the rational limit states design concept that is taking care of the realistic loading conditions and the true behaviour and dynamic capacity of the sleepers. This paper presents a rational limit states design concept for prestressed concrete sleepers. The paper highlights the dynamic design guideline and the necessity to shift from static to dynamic consideration for railway concrete sleepers. The numerical investigations and case scenarios have been performed using dynamic analyses of railway tracks calibrated by dynamic testing data of materials and structures. The dynamic design guideline embraces uncertainties on railway tracks in which a simple measurement on railway sleepers might be inadequate to ascertain thorough understanding into their behaviours, and unable to design or manufacture the sleepers safely, efficiently and effectively.