United States Department of Transportation
i
Finite Element Modeling of Prestressed Concrete Crossties with Ballast and Subgrade Support.
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2011-05-26
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[PDF-877.87 KB]
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Abstract:With the first major installation in North American
railroads during the 1960’s, concrete ties were believed to last
longer than timber ties and have the potential for reduced life
cycle costs. However, their characteristic response to initial
pretension release as well as dynamic track loading is not well
understood. In North America, concrete ties have been found
vulnerable to rail seat deterioration (RSD), but the mechanisms
contributing to RSD failures are not well understood. To
improve such understanding, a comprehensive computational
study of the tie response to dynamic track forces is needed.
This paper presents an initial research effort in this direction
that models concrete crossties as heterogeneous media in threedimensional
finite element analyses, i.e., the prestressing
strands, concrete matrix and the strand-concrete interfaces are
represented explicitly. Damaged plasticity models are
employed for the concrete material, and linear elastic bond-slip
relations, followed by damage initiation and evolution, are
adopted for the strand-concrete interfaces. Further, the ballast
is modeled with an Extended Drucker-Prager plasticity model,
and the subgrade is modeled as an elastic half space. All
material parameters are obtained from the open literature.
Currently the rail fastening systems are not included in
modeling.
Two loading scenarios are simulated: pretension release
and direct rail seat loading. The modeling approach is able to
predict the deformed tie shape, initial interface deterioration,
the compressive stress state in concrete and residual tension in
the strands upon pretension release. The transfer lengths of the
prestressing strands can be readily calculated from the analysis
results. Further predicted are the rail seat force-displacement
characteristics and the potential failure mode of a concrete
crosstie under direct rail seat loading. The responses of two
railroad concrete crossties with 8-strand and 24-wire
reinforcements, respectively, are studied using the presented
modeling framework. The analyses indicate a potential failure
mode of tensile cracking at the tie base below the rail seats.
The results show that the 24-wire tie is better able to retain the
pretension in the reinforcements than the 8-strand tie, resulting
in slightly stronger rail seat force-displacement characteristics
and higher failure load. The effects of the load application
method and the subgrade modeling on the predicted tie
response are further studied.
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