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An embedding system of tram and subway rails on polyester segmented supports with continuous concrete foundation.
(Published on July 2006 – JEC Magazine #26)
JASENKO JOSIPOVIC M.MECH.E., & RADOMIR MLADENOVIC M.ARCH.E. ARS CREATIAE LTD. & START 6 CONSULTING
Sipok, a system of tram and subway rails embedded on polyester segmented supports with continuous concrete foundation is applicable for the construction of new or a reconstruction of the existing tramway tracks exposed to extremely high load (over 15 Mt/year). When reconstructing the existing track, the system allows a reduced speed of the tramway traffic. A design of polyester composites and a mechanical design of the Sipok system establish single physical characteristics of the embedding elements material, required for optimal taking over of the wheel’s and rail’s impact energy and its deposit in the embedding elements in the form of energetic capacity (Fig. 1).
Such a designed system has a life span of minimum 20 years for embeddings on straight sections of the track and under the average work load from tramway’s wheel of 70 kN.
By definition, the tramway’s wheel and the rail impact energy is completely taken over by the embedding elements. When incorrectly defined physical characteristic of embedding elements are used, a destruction of elements themselves and transfer of impact energy into the tramway track foundations take place. In the constructions of a tramway rail embeddings as applied so far, there are two basic types of embedding constructions:1. Segmented type a) Sleepers on balance b) Individual embedding on the continuous concrete foundation
2. Continuous type - of a recent date, and occurring in two forms: a) Continuous pouring of elastic polyurethane mass (e.g. SIKA’s Chemie GmbH Icosit ) on continuous concrete foundation b) A continuous prefabricated elastomer mattress on continuous concrete foundation
The basic difference between these two types of embedding construction is the manner of taking over the wheel and the rail impact energy. In segment embeddings, a part of the impact energy is spent on deformation of non-supported rail, while in continuous embeddings all the energy is accumulated in the embedding itself (closed system), which is at the same time the main big deficiency of this type of construction.
The principle deficiency of all segmented embeddings containing in its structure the following sequence: rail - resilient element - metal plate (- base or other resilient element of embedding) is in that the metal plate returns back (reflects) the impact energy into resilient element, instead of directing it further ahead towards the foundations or subsequent embedding elements. Such a double loaded resilient element degrades more rapidly and changes its physical characteristics, which results in negative impact on the behaviour of other elements and the complete system of embedding.
Design of Sipok sytem
In establishing the design claims for the Sipok system a consideration of the existing solutions of embeddings and a lifelong own experience in the field of construction of tramway tracks where taken into account. Deformation share of particular elements in total embedding deformation value directly shows the efficiency of mechanical design and indirectly established what is the amount of wheel’s and rail’s impact energy spent-on/ accumulated-in particular embedding element. Table 1. shows deformation share for the case when the wheel is positioned above of segmented embedding (Fig. 2).
The optimal energetic capacitance is achieved when certain embedding element has its maximal deformation without reaching critical stress. The novelty of Sipok embedding system is in that wheel’s and rail’s impact energy is completely spent on elastic deformation of the Sipok structural elements. This feature is achieved by the demand of mechanical designer to the chemical engineers to develop such materials whose physical properties will satisfy designed mechanical characteristics. A prefabricated polyester plate and also polyester mass poured on site are developed to have necessary physical characteristics and as chemically inert and resistant to outdoor conditions are introduced. By the usage of the numerical tools (such as FEM /FEA simulations, Fig. 3) it can be shown that Sipok system is the only one having deformation share equally distributed between elastomer and polymer embedding members, while by other types of embeddings such difference is much more increased. By its application the occurrence of galvanic currents is disabled, thus disabling galvanic corrosion.
Design tips implied to the elements between rail and concrete foundation: - Higher material density to accomplish “better” energetic capacitance, - Lower modul of elasticity (compression) with higher compression strength, significant toughness; - “Gummifederung” type of elastomer build-in; - “Sequencial” ratio of compression modules (1 elastomer: 5 polymer (: 800 concrete)).
A prefabricated workshoped synthetic slabs (Fig. 3) of several characteristic thickness to fit with uneven spots of the concrete slab are laid above the drilled holes. The composition of the mass to be poured is very similar, as to its formulation, to the components of the workshop prefabricated polyester slab where high reacting polyester type resins of duroplast family whose properties are realized by several fractions of burned quartz sand (natural polymer - SiO2). The chemical design of the polyester composite secure that physical properties of polymer compound, particulary modulus of compression, to be in the range of 100- 150 N/mm² for the average vertical load of 70 kN (vertical load represents real impact between rail and tramway's wheel). The designed density of the prefabricated workshoped polyester slab and poured polyester mass is approx. 2000 kg/m³.
A 6 mm thick styrenne-butadiene tie-plates and 4 mm thick reinforced styrenne-butadiene tie-plates are put on synthetic slabs. The rubber-plate's designed modul of compression (elasticity) is always in the range of 20-40 N/mm² for the average vertical load of 70 kN.
The advantage of Sipok system, referencing to other systems of rails embedding in use today, is in its optimized mechanical design process obtained by very sofisticated chemical solution of polyester composite attributes. The final product is simple for the first installation, has low inital costs and does not need the particular maintenance for a relatively long period of time. Sipok’s designed time of exploatation is 15-30 years for a tram traffic of 15 MGT/year. In practice the Sipok system was applied in Sarajevo in 1991. and has been confirmed by duration until today without any major changes in its structure, in spite of lack a complete maintenance of tram rails and cars.