steel railway bridge fatigue and · 2019. 2. 13. · cooper e80 design loading . 17 cooper e80...
TRANSCRIPT
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Steel Railway Bridge Fatigue and
the Evolution of Railway Car Loadings
The Current State and Future Challenges
of Railway Bridges
Stephen M. Dick, PE, SE, Ph.D.
Hanson Professional Services
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Discussion Items
General bridge features
Railroad car loadings
Steel bridge fatigue
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General Bridge Features
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Common Features
• Simply supported
– “Time is money”
• repairs are easier
• historical precedent
• Standardized designs
– applicable to any type of span
• steel and timber both standardized
• prestressed concrete very popular
– standardization creates economies
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Historical Bridge Types (1890 forward)
• Steel
– rolled multiple beams (up to 30 feet)
– built-up girders (up to 120 – 150 feet)
– trusses (longer than 150 feet)
• pin trusses popular early
• riveted (now bolted) trusses
• welding also used
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Historical Bridge Types (1890 forward)
• Steel – riveted and bolted construction
– rivets common until the 1960’s
– bolting used for repairs on existing bridges
– bolting still used extensively
• truss and girder field erection
• potential fatigue locations
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Historical Bridge Types (1890 forward)
• Steel – welded construction
– became generally used in the late 1950’s
– used in both girders and trusses
– restrictions on details because of fatigue
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Historical Bridge Types (1890 forward)
• Timber
– used extensively for trestle bridges
– span lengths 10 – 15 feet
– used for ballast decks on steel bridges
– timber’s use is declining
• insufficient load capacity for heavier loadings
• supplies of useful timber are declining
• creosote treatment has environmental issues
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Historical Bridge Types (1890 forward)
• Timber
– glulam and hybrid construction is in place for
test bridges.
– results are mixed
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Historical Bridge Types (1890 forward)
• Reinforced Concrete
– used since early in 20th century
– used early for arch and box culverts
– popular in urban areas for grade separations
– substructures
– used for large arches and viaducts in various
locations
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Historical Bridge Types (1890 forward)
• Prestressed Concrete
– used heavily in railroad industry
– replacing timber in trestle construction
– precasting allows faster field construction
– standardized spans up to ~ 50 feet
– Span length limited to ~ 80 – 100 feet
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Historical Bridge Types (1890 forward)
• Foundations
– stone abutments and piers still in use
– older concrete
• many are rubble-filled
• unreinforced concrete
• Gravity design without pile foundations
– newer concrete
• rely on pilings or drilled shafts
• heavy use of steel H-piling
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Future trends
• Not really set
• Refinement of existing technologies
• Modern materials have potential
– durability issues
– cost
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Design parameters
• AREMA Manual
– Timber – Chapter 7
– Concrete – Chapter 8
– Steel – Chapter 15
– Seismic – Chapter 9
– Bridge Maintenance – Chapter 10
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Cooper E80 Design Loading
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Cooper E80 Loading
• Loading still used despite its age (1894)
• Still provides adequate overall moments
for both short and long spans
• Scaleable
• Not entirely satisfactory as a basis for
fatigue checks
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Norm
aliz
ed B
end
ing M
om
ent
8080 1001006060 120120 140140 15015013013011011090907070
1920 Cooper E60
Span Length (ft)
505040403030
2004 Cooper E80
1900 Cooper E40
Design Moments Normalized for Allowable Stresses
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Railway Car Loadings
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Actual Loadings
• Steady increases over time
• Total volume increasing dramatically
• Coal
– 1900 ~ 3,000 pounds per foot
– 2000 ~ 5,400 pounds per foot
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Actual Loadings Important milestone dates
• 1928 - Wood underframes outlawed
• 1941 - Arch bar trucks outlawed
• 1968 - Roller bearings required
Requirements for interchange only
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Actual Loadings Pre-1940
• Car loads from 30 tons (1900) up to 70
tons (1930)
• Majority of car fleet length 40 feet or less
• Very few freight cars exceeding 60 feet
• Passenger cars often the heaviest cars
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Actual Loadings 1940-1960
• Car loads from 70 tons (1940) up to 90
tons (1960)
• Majority of car fleet length 40 feet or less
• Longer car lengths introduced (90 feet)
• Development of railcars for specific
commodities beginning
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Actual Loadings Post-1960
• Car loads from 90 tons (1960) up to 110
tons (1995)
• Wide variety of car lengths available for
commodity specialization
• Long car lengths very common
• Car weights increasing in general because
of specialization of car equipment
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Actual Loadings Total Tonnages
• Tonnage during World War II was highest
for the first half-century
– three to four times the traffic during Great
Depression
• Traffic levels not repeated until 1980’s
• Traffic continuing to increase to historic
levels
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Actual Loadings Unit Train Weights
• 1930’s – 1500 to 3300 plf
• 1950’s – 1200 to 3200 plf
• Empty weight – 1200 plf
– consistent over time
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Actual Loadings Unit Train Weights
2004 data
• Manifest – 1500 to 4600 plf
• Grain – 4700 to 5000 plf
• Coal – 5400 plf
• Automobile – 1500 to 2200 plf
• TOFC – 1200 to 2100 plf
• Double Stack – 1300 to 2600 plf
– empty: assume 900 plf for TOFC/DS/Auto
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Steel Railway Bridge Fatigue
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Steel Railway Bridge Fatigue
• Number of old steel railway bridges is still
very high
• Increased traffic levels on fewer routes are
increasing the number of potential cycles
• Increased axle weights are creating higher
bending moments and cycle potential
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Steel Railway Bridge Fatigue
• Multiple cycles are potentially damaging
from a train
• Need to examine the potential for all
cycles that can occur
• Maximum moment – one overall cycle
• Moment range – one cycle per car
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Moment Trace for a Unit Coal Train on a 50-foot Span.
Time
Ben
din
g M
om
ent Cars Locomotives Midspan
Quarter Point
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Mom
ent
Ran
ge
(k-f
t)
Span Position (x/L)
Moment Range versus Span Position on 50-Foot Span
0
0 0.1 0.2 0.3 0.4 0.5
500
1000
1500
2000
2500
Coal Car
Locomotive
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Moment Trace for a Unit Coal Train on a 100-foot Span.
Time
Ben
din
g M
om
ent Cars Locomotives
Midspan
Quarter Point
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Mom
ent
Ran
ge
(k
-ft)
Span Position (x/L)
Moment Range versus Span Position on for 100-Foot Span
0
0 0.1 0.2 0.3 0.4 0.5
500
1000
1500
2000
2500
Coal Car
Locomotive
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LO
LO - Overall length of railroad car measured over the pulling face of the coupler.
TC - Length between the center pin on the trucks, known as the truck center distance.
SI - Inboard Axle Spacing, the distance between the inside axles of the railroad car.
TC
LO
TC
SO - Outboard Axle Spacing, the distance between the outside axles of the railroad car.
ST - Truck Axle Spacing, the distance between the adjacent axles of a truck.
n - number of axles
P - axle load
ST ST ST ST SO/2 SO/2 SI SO SI
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Mom
ent
Ran
ge
(k
-ft)
Span Position (x/L)
Moment Range versus Span Position on for LS/LO = 1.0
0
0 0.1 0.2 0.3 0.4 0.5
500
1000
1500
2000
2500
Locomotive
Coal Car
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Mom
ent
Ran
ge
(k
-ft)
Span Position (x/L)
Moment Range versus Span Position on for LS/LO = 2.0
0
0 0.1 0.2 0.3 0.4 0.5
500
1000
1500
2000
2500
Locomotive
Coal Car
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Influence Lines For Moment Behavior At Midspan And Quarter Point
Influence Line for LS = 2LO at Midspan
Influence Line for LS = 4LO at Quarter Point
3LO LO
4 3
4 1
X
LO LO
2 1
X 2
1
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Moment Range
General Characteristics
• Cyclical in nature
• Possesses an absolute maximum
• Magnitude can be estimated for integer values
of LS/LO using a similar sine wave approximation
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Moment Range
Absolute Maximum Moment Range
o
ooIAM
L
SSSnPR
44–
4
2
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Equipment Overall LS/LO LS/LO
Type Length 30' span 150' span
SD70 70.00 0.43 2.14
Coal 53.00 0.57 2.83
TOFC 94.00 0.32 1.60
SPDS 71.00 0.42 2.11
APDS - End 65.26 0.46 2.30
APDS - Middle 58.83 0.51 2.55
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Mo
men
t R
ang
e (k
ip-f
t)
0
20
40
60
80
100 M
ax
im
um
M
om
en
t (
k-
ft)
0 0.1 0.2 0.3 0.4 0.5
Span Position (x/L)
100
60
Span Position (x/L)
0.1 0.2 0 0.3 0.4 0.5
20
0
40
80
TOFC Car
Coal Car
Six Axle Locomotive
Maximum Moment vs. Span Position – 80 ft. span
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0
10
20
30
40
50 M
om
en
t R
an
ge
(
k-
ft
)
0 0.1 0.2 0.3 0.4 0.5
Span Position (x/L)
50
30
Mom
ent
Ran
ge
(kip
-ft)
Span Position (x/L)
0.1 0.2
Moment Range vs. Span Position – 80 ft. span
0 0.3 0.4 0.5
10
0
20
40 TOFC Car
Coal Car
Six Axle Locomotive
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Ben
ding
Mom
ent
FIGURE 4. Bending Moment Versus Time For Railcar Loadings Over Long-Span Bridges
Time
Loaded Cars Only
Loaded and Unloaded Cars
Moment Trace for Mixed Empty/Loaded Railcars
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Steel Railway Bridge Fatigue
• Need to “sharpen the pencil” for the
number of cycles to expect from each type
of train
• Need for quicker calculation of moment
range magnitudes other than absolute
maximum
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Additional Research
• Consideration of R ratio in fatigue life
calculations for riveted/bolted members
• Development of very long life fatigue
coefficients for riveted members (over
100,000,000 cycles)
• Retrofit strategies that are economical and
can take full advantage of bridge members
if only details are problematic
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