Report

Finite Element Analysis of Wood and Concrete Crossties Subjected to Direct Rail Seat Pressure Volpe The National Transportation Systems Center Hailing Yu and David Jeong Structures and Dynamics Division Volpe The National Transportation Systems Center Advancing transportation innovation for the public good U.S. Department of Transportation Research and Innovative Technology Administration John A. Volpe National Transportation Systems Center 1 Overview Background Finite element analyses Results Conclusions Future work Acknowledgements 2 Background Rail seat failure in ties can cause rail rollover derailments Plate cutting in wood ties Rail seat deterioration in concrete ties o o Probable cause for two Amtrak derailment accidents in Washington in 2005 and 2006 Recently observed on the Northeast Corridor Correlation of rail seat failure with rail seat load is needed 3 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 4 Current Simplifications Fasteners are not modeled Vertical load is applied as direct rail seat pressure Lateral load is not applied 5 Directionality in Wood Material R L T L: parallel to fiber T: perpendicular to fiber and tangent to growth rings R: normal to growth rings 6 Orthotropic Elasticity 1 E L LR LL EL RR LT TT EL LR 0 LT RT 0 0 RL ER 1 ER RT ER 0 TL ET TR ET 1 ET 0 0 0 0 0 0 0 1 GLR 0 0 0 0 1 GLT 0 0 0 0 0 0 LL RR 0 TT LR 0 LT 0 RT 1 GRT 7 Orthotropic Strength Limits Symbol XLt XLc XRt XRc XTt XTc SLR SLT SRT Description Tensile strength in the fiber direction L Compressive strength in the fiber direction L Tensile strength in the radial direction R Compressive strength in the radial direction R Tensile strength in the tangential direction T Compressive strength in the tangential direction T Shear strength in the L-R plane Shear strength in the L-T plane Shear strength in the R-T plane 8 Representative Wood Properties EL (psi) 1,958,000 ER (psi) 319,154 ET (psi) 140,976 LR LT RT 0.369 GLR (psi) 168,388 0.428 GLT (psi) 158,598 0.618 GRT (psi) 41,118 XLt (psi) XLc (psi) XRt, XTt (psi) XRc, XTc (psi) SLR, SLT (psi) 15,200 7,440 800 1,070 2,000 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. 2010. 9 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 10 Quarter Symmetric FE Models of 8Strand and 24-Wire Concrete Crossties 11 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 dt – tensile damage variable dc – compressive damage variable d – stiffness degradation variable (a function of dt and dc ) 12 Cohesive Interface Elements n – normal direction s, t – shear directions Normal traction tn Shear tractions ts, tt Quadratic nominal stress criterion for damage initiation 2 2 2 t n ts t t 0 0 0 1, where is theMacaulaybracket t n ts t t 13 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 14 Material Parameters All material parameters are obtained from open literature There is insufficient data on the bond-slip properties of steel tendon-concrete interfaces 15 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) 16 Deformed Concrete Tie Shape After Pretension Release 17 Compressive Stress State in Concrete After Pretension Release 18 Average ratio of pretension retention Ratio of Pretension Retention 1 0.8 0.6 0.4 8-strand tie 24-wire tie 0.2 0 0 0.2 0.4 0.6 0.8 Relative distance to tie center (1=tie end) 1 19 Predicted Failure Mode Under Rail Seat Pressure Wood tie – compressive rail seat failure 20 Predicted Failure Mode Under Rail Seat Pressure Concrete tie – tensile cracking at tie base 21 Rail Seat Force vs. Displacement Up To Predicted Failure Absolute rail seat displacement 40 Rail seat displacement relative to tie base 40 (a) 35 30 25 20 15 8-strand concrete tie 24-wire concrete tie Wood tie 10 5 0 0 0.05 0.1 0.15 0.2 0.25 Rail seat displacement (inch) 0.3 Rail seat force (kip) Rail seat force (kip) 35 (b) 30 25 20 15 10 5 0 0 0.005 0.01 0.015 0.02 0.025 Relative rail seat displacement (inch) 0.03 22 Partition of Tie-Ballast Interface Fifty-one sub-surfaces on lower surface of wood tie Contact force calculated on each sub-surface 23 Contact Force Distribution on the Lower Surface of Wood Tie 24 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 25 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 26 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. 27