Said to be adaptable to all types of track and terrain, a new sleeper, known as the UniP steel tie, is the result of years of development by Applied Rail Research Technologies of Canada. In addition to structural advantages, other benefits include lower production, installation and maintenance costs and a long service life.
The sleeper is in essence an inverted steel rail. It uses a conventional ‘T’ or ‘I’ shaped steel beam to resist bending without too much reinforcement. The result is a railway sleeper making the best use of the material’s strength and ductility to withstand higher dynamic loads. The current design also does away with a fault observed in earlier versions of fatigue cracks appearing at the rail/sleeper junction. By choosing a t-shaped section for the sleeper ensures an effective transverse load bearing member for the track.
The UniP steel tie design adopts a flat surface of uniform thickness for the flange to ensure improved fatigue resistance over other steel sleepers that have thinner sections in the pressed-up rail seat locations. The flange transfers the rail seat load directly to the ballast and provides a sufficient ballast bearing area to reduce ballast pressure and sleeper. The flange is stiffened with a vertical plate which acts as the web of the sleeper.
The function of the web is to restrict the ability of the sleepers to skew or bunch together by giving longitudinal track stability. There is a bead along the bottom of the web which relieves tensile bending stresses.
A combination of AutoCAD and Solid Works was used for the initial structural design analysis of the sleeper, followed by extensive parametric FEA, using the company’s own field test database for design verification.
The sleeper is manufactured by rolling as a standard wide flange beam H section with two beads, or bulbs, in the centre. The H section beam is cut to the necessary length and then longitudinally sliced between the beads to produce two long T beams each with a bead. These beams are again cut to length and the flange bent down. Adopting this approach allowed optimum use of the thickness of the flange and web to sustain heavy haul loading without sleeper overstressing. It also provided up to 7in. extra clearance in railway tunnels.
Along the edge of the flange on either side of the web, a 2in. strip is bent down 30 degrees to retain the ballast. At either end of each sleeper, baffle plates support the web and stop it buckling. The baffles also offer three or four times more resistance to lateral track buckling to that provided by concrete or wood versions. As shoulder ballast is not needed to give lateral stability, the steel sleeper is said to save up to 700ton of ballast per mile.
Compared to typical wood and concrete sleepers with ballast embedment depths of up to 10in., that of the steel sleeper is less than 1/2in. This increases the effective ballast depth by 9 1/2in. to about 21in. and gives more elasticity and higher energy absorbing ability to the track bed. It also allows a greater spreading of the vertical load through the ballast section to the subgrade.
Superior retention of the rail and maintenance of rail gauge result from the combination of steel sleeper and elastic fastening system. The fasteners, consisting of two-legged hook-in shoulders, double J clips, insulators and canted pads, can be additionally used with concrete or wood sleepers.
Considerably lighter than all other sleepers – weighing 148lb – it is claimed two people can install the sleeper without special equipment. They can be positioned 24in. apart, as compared with the 19-1/2in. needed for wood.
The life cycle cost of the fully recyclable steel sleeper and fasteners is claimed to be approximately $240,000 (US) per mile. For wood sleepers with elastic fastening the figure is given as $330,000 per mile, for concrete $305,000 per mile, and for the inverted trough steel sleeper at $300,000 per mile.
Material from Design Engineering, Canada is acknowledged as the basis of this article.
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