Finite Element Analysis of Wood and Concrete Crossties ...

batchquonochontaugUrban and Civil

Nov 29, 2013 (3 years and 6 months ago)

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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

2

Overview


Background


Finite element analyses


Results


Conclusions


Future work


Acknowledgements

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

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

5

Current Simplifications


Fasteners are not modeled


Vertical load is applied as direct rail seat
pressure


Lateral load is not applied

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

7

Orthotropic Elasticity






















































































RT
LT
LR
TT
RR
LL
RT
LT
LR
T
R
RT
L
LT
T
TR
R
L
LR
T
TL
R
RL
L
RT
LT
LR
TT
RR
LL
G
G
G
E
E
E
E
E
E
E
E
E


















1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
0
0
1
0
0
0
1
8

Orthotropic Strength Limits

Symbol

Description

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

9

Representative Wood Properties

E
L

(psi)

E
R

(psi)

E
T

(psi)

1,958,000

319,154

140,976


LR


LT


RT

0.369

0.428

0.618

G
LR

(psi)

G
LT

(psi)

G
RT

(psi)

168,388

158,598

41,118

X
Lt

(psi)

X
Lc

(psi)

X
Rt
,
X
Tt

(psi
)

X
Rc
,
X
Tc

(psi
)

S
LR
,
S
LT

(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.

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

11

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

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
)

13

Cohesive Interface Elements

n


normal direction

s, t


shear directions


Normal traction
t
n

Shear tractions
t
s
,
t
t

bracket
Macaulay

the
is


where
,
1
2
0
t
t
2
0
s
s
2
0
n
n






















t
t
t
t
t
t
Quadratic nominal stress criterion for damage initiation

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

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

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)

17

Deformed
Concrete Tie
Shape After
Pretension Release

18

Compressive Stress State in Concrete
After Pretension Release

19

Ratio of Pretension Retention

0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
8-strand tie
24-wire tie
Average ratio of pretension retention
Relative distance to tie center (1=tie end)
20

Predicted Failure Mode Under Rail
Seat Pressure


Wood tie


compressive rail seat failure

21

Predicted Failure Mode Under Rail
Seat Pressure


Concrete tie


tensile cracking at tie base

22

Rail Seat Force vs. Displacement Up To
Predicted Failure

Absolute rail seat displacement

0
5
10
15
20
25
30
35
40
0
0.05
0.1
0.15
0.2
0.25
0.3
8-strand concrete tie
24-wire concrete tie
Wood tie
Rail seat force (kip)
Rail seat displacement (inch)
(a)
Rail seat displacement relative to
tie base

0
5
10
15
20
25
30
35
40
0
0.005
0.01
0.015
0.02
0.025
0.03
Rail seat force (kip)
Relative rail seat displacement (inch)
(b)
23

Partition of Tie
-
Ballast Interface


Fifty
-
one
sub
-
surfaces on lower surface of
wood tie


Contact
force
calculated on each
sub
-
surface

24

Contact Force Distribution on the
Lower Surface of Wood Tie

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

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

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.