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1 Volpe The National Transportation Systems Center Finite Element Analysis of Wood and Concrete Crossties Subjected to Direct Rail Seat Pressure U.S. Department of Transportation Research and Innovative Technology Administration John A. Volpe National Transportation Systems Center Volpe The National Transportation Systems Center Advancing transportation innovation for the public good Hailing Yu and David Jeong Structures and Dynamics Division

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2 Overview Background Finite element analyses Results Conclusions Future work Acknowledgements

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3 Background Rail seat failure in ties can cause rail rollover derailments Plate cutting in wood ties Rail seat deterioration in concrete ties o Probable cause for two Amtrak derailment accidents in Washington in 2005 and 2006 o Recently observed on the Northeast Corridor Correlation of rail seat failure with rail seat load is needed

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4 Objectives Develop finite element (FE) models for wood and concrete ties in a ballasted track Study failure mechanisms of railroad ties subjected to rail seat loading using the FE models

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5 Current Simplifications Fasteners are not modeled Vertical load is applied as direct rail seat pressure Lateral load is not applied

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6 Directionality in Wood Material L: parallel to fiber T: perpendicular to fiber and tangent to growth rings R: normal to growth rings L R T

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7 Orthotropic Elasticity

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8 Orthotropic Strength Limits SymbolDescription X Lt Tensile strength in the fiber direction L X Lc Compressive strength in the fiber direction L X Rt Tensile strength in the radial direction R X Rc Compressive strength in the radial direction R X Tt Tensile strength in the tangential direction T X Tc Compressive strength in the tangential direction T S LR Shear strength in the L-R plane S LT Shear strength in the L-T plane S RT Shear strength in the R-T plane

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9 Representative Wood Properties E L (psi)E R (psi)E T (psi) 1,958,000319,154140,976 LR LT RT G LR (psi)G LT (psi)G RT (psi) 168,388158,59841,118 X Lt (psi)X Lc (psi)X Rt, X Tt (psi)X Rc, X Tc (psi)S LR, S LT (psi) 15,2007, ,0702,000 Based on properties of the white oak species described in Bergman, R., et al., “Wood handbook - Wood as an engineering material,” General Technical Report FPL-GTR-190, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory: 508 p

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10 Macroscopic Heterogeneity and Material Nonlinearity in Concrete Ties Steel strands/wires Linear elasticity with perfectly plastic yield strength Concrete Linear elasticity followed by damaged plasticity Interfaces Bond-slip depicted in linear elasticity followed by initiation and evolution of damage to bond

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11 Quarter Symmetric FE Models of 8- Strand and 24-Wire Concrete Crossties

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12 Concrete Material Models Concrete damaged plasticity Uniaxial tension: linear elasticity followed by tension stiffening Uniaxial compression: linear elasticity followed first by strain hardening and then by strain softening Multi-axial yield function d t – tensile damage variable d c – compressive damage variable d – stiffness degradation variable (a function of d t and d c )

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13 Cohesive Interface Elements n – normal direction s, t – shear directions Normal traction t n Shear tractions t s, t t Quadratic nominal stress criterion for damage initiation

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14 Support to the Ties Ballast Extended Drucker-Prager model for granular, frictional materials Subgrade Modeled as an elastic half space using infinite elements Transitional layers can be modeled if geometric and material properties are known

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15 Material Parameters All material parameters are obtained from open literature There is insufficient data on the bond-slip properties of steel tendon-concrete interfaces

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16 Analysis Steps Initial condition Steel tendons pretensioned to requirements (concrete tie) First step (static analysis) Pretension released in the tendons (concrete tie) Second step (dynamic analysis) Uniformly distributed pressure loads applied on rail seats (wood and concrete ties)

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17 Deformed Concrete Tie Shape After Pretension Release

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18 Compressive Stress State in Concrete After Pretension Release

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19 Ratio of Pretension Retention

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20 Predicted Failure Mode Under Rail Seat Pressure Wood tie – compressive rail seat failure

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21 Predicted Failure Mode Under Rail Seat Pressure Concrete tie – tensile cracking at tie base

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22 Rail Seat Force vs. Displacement Up To Predicted Failure Absolute rail seat displacement Rail seat displacement relative to tie base

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23 Partition of Tie-Ballast Interface Fifty-one sub-surfaces on lower surface of wood tie Contact force calculated on each sub-surface

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24 Contact Force Distribution on the Lower Surface of Wood Tie

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25 Conclusions FE analyses predict that under a uniform rail seat pressure load, The wood tie fails at the rail seats due to excessive compressive stresses Tensile cracks form at the base of the concrete ties The simplified loading application predicts rail seat failure in the wood tie but not in the concrete ties

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26 Future Work Calibrate bond-slip relations in the steel tendon-concrete interfaces from tensioned or untensioned pullout tests Incorporate fasteners and rails, and apply both vertical and lateral loading

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27 Acknowledgements The Track Research Division in the Office of Research and Development of Federal Railroad Administration sponsored this research. Technical discussions with Mr. Michael Coltman, Dr. Ted Sussmann and Mr. John Choros are gratefully acknowledged.

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