Vertical Loads from North American Rolling Stock for Bridge Design and Rating
By
Duane Otter, Ph.D., P.E., and MaryClara Jones
Transportation Technology Center, Inc., Pueblo, Colorado
Abstract
As a part of the Association of American Railroads’ (AAR) bridge research program,
Transportation Technology Center, Inc. (TTCI) quantified the current North American
vertical load environment using wayside weigh-in-motion (WIM) measurements. The
following results were obtained:
•
Freight car truck loads totaling 160,000 pounds or more travel over mainline
routes daily.
•
Three-axle locomotive truck loads totaling 220,000 pounds travel over
mainline routes daily.
•
The heaviest two-axle freight car truck loads are significantly higher than the
heaviest two-axle locomotive truck loads.
The 286,000-pound gross rail load (GRL) car was introduced around 1990, and in
2003 was allowed in unrestricted interchange. Current American Railway Engineering
and Maintenance of Way Association (AREMA) guidelines recommend designing
bridges for a Cooper E-80 design load. This design load was first recommended in the
late 1960s, in conjunction with the introduction of 263,000-pound GRL cars. With the
increase in allowable car load, it is prudent to assess the current vertical load environment
with respect to bridge design loads.
The WIM data used in this study includes traffic from intermodal, mixed freight,
passenger, and various unit trains.
Results from this study are compared to current AREMA Chapter 8 and Chapter
15 guidelines for design and rating of railway bridges.
© AREMA 2009 ®
These loads are also applied to railroad track throughout the North American
network. As such, this information may also be of value to designers of various track
components.
Introduction
TTCI measured actual net truck vertical (NTV) forces on several main lines in North
America using wayside data. The purpose of the measurements was to determine the
vertical forces that are currently being imparted onto railway bridges. The wayside data
provides forces measured at specific track locations for millions of railcar passes.
The 286,000-pound GRL car was introduced around 1990, and in 2003 it was
allowed in unrestricted interchange. But current AREMA guidelines still recommend
designing concrete and steel bridges for a Cooper E-80 design load.1 With the increase in
allowable car load, it is prudent to assess the current vertical load environment with
respect to bridge design loads.
Historical Loadings
In the 1960s, the interchange allowable car load was increased from 220,000 pounds to
263,000 pounds for four-axle freight cars. This was an increase of just under 20 percent
in allowable car load. In conjunction with that load increase, American Railway
Engineering Association (now AREMA) bridge design loads increased from Cooper E-72
to Cooper E-80. This was an increase of about 11 percent in design load.
Presently, most unit coal trains and grain trains have 286,000-pound GRL cars.
Around 1990, the 286,000-pound GRL car was introduced, and in 2003 it was allowed in
unrestricted interchange. This was an increase of almost 9 percent in allowable car load,
© AREMA 2009 ®
compared to the previous interchange allowable car of 263,000 pounds. Compared to the
220,000-pound car, the 286,000-pound car is an increase of 30 percent in allowable GRL.
In addition, the 1995 AAR car specifications for the 286,000-pound car allowed a
minimum car length of less than 42 feet. For the previous 263,000-pound car, the
minimum car length was around 44 feet. This resulted in an increase of about 14 percent
in the car load per foot of bridge length.
Also introduced around the same time was the articulated double-stack car,
consisting of three to six intermodal well platforms, sharing common intermediate trucks.
Most articulated double-stack cars are built with “125-ton” trucks at the articulation
points. The capacity of these trucks is 157,500 pounds. (This equates to 315,000 pounds
on a four-axle car.) While these articulated double-stack cars are not allowed in
unrestricted interchange, they are nonetheless very common on many North American
main lines, moving under contract agreements. There are also a few lines carrying
315,000-pound four-axle cars.
The Cooper E-80 design load recommended for concrete and steel bridges has
vertical forces of 80,000 pounds on the four heaviest axles. The design of shorter steel
spans may be governed by an alternate live load, with vertical forces of 100,000 pounds
on four axles. Introduced in the early 1990s, the AREMA Chapter 15 alternate live load
for design of steel bridges is intended to improve fatigue performance of floor systems
and short spans.
Wayside Measurements of Revenue Service Trains
Wayside measurements are capable of gathering data from a large number of passing
trains including different types of equipment. Wayside detectors are currently in use at
© AREMA 2009 ®
many locations on several railroads throughout North America. This study uses measured
NTV forces from WIM systems on tangent track from both cars and locomotives.
Wayside wheel force data was obtained from 12 different sites, from at least 1 site
on each of the six largest railroads in the USA and Canada. Data was collected over a
12-month period to minimize any seasonal effects. The NTV force was calculated by
taking the sum of the average wheel loads for each wheel of the truck as it passed through
the multiple measurement locations of a WIM site. Average wheel load data was used
from each detector to minimize car dynamics and speed effects. (These detectors are
typically located on smooth tangent track with welded rail to minimize vehicle
dynamics.) Therefore, the data should closely resemble the static vertical load
distribution. The data used in this report excludes effects of wheel defects that result in
high impact forces. (Note: The WIM data was collected from sites commonly known as
wheel impact load detectors (WILD). These detectors now produce WIM and other
outputs in addition to detecting high impact wheels.)
The traffic included in the wayside data consisted of unit, intermodal, mixed
freight, and passenger trains. Locomotives with four axles and locomotives with six
axles were analyzed separately. Over 20-million, two-axle truck passes and over 600,000
locomotive truck passes (from both six-axle and four-axle locomotives) were analyzed.
For reference, the North American railcar fleet numbers approximately 1.5 million. The
locomotive fleet numbers about 25,000. Over the course of one year, many vehicles were
recorded numerous times by the WIM systems included in this study.
© AREMA 2009 ®
Figure 1 shows the truck weight distribution for two-axle freight car trucks from
one wayside site. Traffic at this site includes a considerable number of coal cars, both
21
0
19
5
18
0
16
5
15
0
13
5
12
0
10
5
90
75
60
45
30
15
16%
14%
12%
10%
8%
6%
4%
2%
0%
0
Percent of Occurrence
empty and loaded.
Truck Weight (000 lb)
Gothenburg 2
Figure 1. Truck Weight Distribution for Two-axle Trucks from a Wayside Detector Site with
Primarily Loaded and Empty Coal Trains
Figure 2 shows the truck weight distribution for a site with a considerable amount
of intermodal traffic. Note the more even weight distribution in Figure 2. This is due to
intermodal cars carrying containers or trailers that may reach volume capacity before
weight capacity (i.e., cube out).
© AREMA 2009 ®
16%
Percent of Occurrence
14%
12%
10%
8%
6%
4%
2%
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0
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5
21
0
19
5
18
0
16
5
15
0
13
5
12
0
10
5
90
75
60
45
30
15
0
0%
Truck Weight (000 lb)
Bagdad 1
Figure 2. Truck Weight Distribution for Two-axle Trucks from Wayside Detector Site with
Many Intermodal Trains
Wayside data was sorted by 10-mph increments up to the maximum speed
recorded. The maximum allowable speed was governed by the railroad timetable for
each location.
Figure 3 shows the cumulative distribution plot for NTV forces for four-axle cars
for each speed range for the wayside site with primarily loaded and empty coal trains.
Figure 4 shows the cumulative distribution plot for NTV forces for a wayside detector
with more intermodal cars. Note the data points for train speeds greater than 80 mph,
presumably from Amtrak passenger cars and express cars that traverse this site.
© AREMA 2009 ®
FORCE (THOUSAND POUNDS)
FORCE (THOUSAND POUNDS)
Figure 3. NTV Forces from Two-axle Trucks from a Wayside Detector Site
with Primarily Loaded and Empty Coal Trains
Figure 4. NTV Forces from Two-axle Trucks from a Wayside Detector
with Many Intermodal Cars
© AREMA 2009 ®
The values shown in Table 1 are for two-axle trucks where the truck loads
measured were as high as 193,000 pounds. Note that for many of the wayside detectors,
the NTV forces at the 99.95-percent level are very near or exceed 160,000 pounds, which
is the design level of two Cooper E-80 axles.
Table 1. NTV Forces from Two-Axle Trucks from Wayside Detectors
(Thousand Pounds)
Wayside Sites
BC1
BC2
CA1
CA2
TX
GA
PA
AR
NE1
NE2
NE3
MN
Total
95.0% NTV
146.1
145.7
145.0
136.0
140.9
142.1
141.1
136.9
132.2
149.9
152.3
143.1
99.5% NTV
152.4
151.4
165.2
158.4
151.9
151.2
151.5
149.7
150.2
155.1
156.5
152.2
99.95% NTV
159.7
157.1
188.2
177.6
162.3
160.4
162.6
155.7
157.4
159.7
159.7
156.4
Number of Trucks
1,346,880
1,870,474
1,527,969
1,446,744
830,860
1,294,001
962,709
1,176,512
3,558,536
1,709,420
3,211,479
1,459,446
20,395,030
Figure 5 shows the NTV forces for 12 wayside sites for two-axle trucks,
excluding locomotives. The three frequency levels are as follows:
•
95-percent NTV or 5-percent probability level, approximately 10 occurrences
per train
•
99.5-percent NTV or 0.5-percent probability, approximately one occurrence
per train
•
99.95-percent NTV or 0.05-percent probability, one to 10 occurrences per
day, depending on traffic
Figure 5 shows that each of the 12 wayside sites experiences a NTV force of
nearly 160,000 pounds on a daily basis.
© AREMA 2009 ®
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TRUCK
VERTICAL
LOAD
NetNET
Truck
Vertical
Load
(THOUSAND POUNDS)
(Thousand Pounds)
2 Axle Trucks
200
190
180
170
160
150
140
130
120
110
100
WILD Site
95.0% NTV
99.5% NTV
99.95 % NTV
Figure 5. NTV Forces for Two-Axle Trucks at
12-Mainline Locations for Three Probabilities of Occurrence
The values shown in Table 2 are for four-axle locomotives where the truck loads
measured were as high as 156,000 pounds. Clearly, these forces are less than those from
freight cars.
Table 2. NTV Forces from Four-Axle Locomotives from Wayside Detectors
(Thousand Pounds)
Wayside Sites
BC
BC
CA1
CA2
TX
GA
PA
AR
NE1
NE2
NE3
MN
Total
95.0% NTV
99.5% NTV
99.95% NTV
138.3
141.8
144.7
148.6
144.3
143.3
138.2
148.8
142.1
149.1
149.7
140.7
143.1
144.1
148.2
152.5
147.7
147.2
144.0
150.9
148.9
154.0
155.8
147.5
143.1
144.9
151.2
154.4
151.6
151.7
152.3
154.6
153.2
154.9
155.8
152.8
© AREMA 2009 ®
Number of
Trucks
246
343
5,020
5,348
2,370
2,894
1,580
3,482
800
598
196
7,268
30,145
The values shown in Table 3 are for six-axle locomotives where the NTV forces
measured were as high as 240,000 pounds (for three-axle trucks). Note that the vast
majority of the locomotive passes are from six-axle locomotives. Compared to the freight
car data, there is much less variability in the locomotive NTV forces. This is to be
expected, because their weight remains relatively constant. The primary cause for load
variability on locomotives is the amount of fuel in the tank.
Figure 6 shows the NTV forces for 12 wayside sites for three-axle trucks of sixaxle locomotives.
Table 3. NTV Forces from Six-Axle Locomotives from Wayside Detectors
(Thousand Pounds)
Wayside Sites
95.0% NTV
99.5% NTV
99.95% NTV
Number of Trucks
BC
210.9
215.2
219.2
26,541
BC
215.3
219.3
222.1
47,873
CA1
210.1
215.1
219.9
96,664
CA2
216.1
220.9
227.7
60,268
TX
211.8
216.9
220.5
27,409
GA
213.1
217.9
220.7
44,116
PA
205.6
211.7
217.1
30,422
AR
210.9
216.2
220.8
39,273
NE1
210.3
215.3
220.0
76,374
NE2
212.8
219.0
224.8
46,008
NE3
210.7
215.4
219.2
65,682
MN
218.2
222.3
225.9
30,666
Total
591,296
© AREMA 2009 ®
220
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100
Ar
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NET TRUCK
VERTICAL
LOAD
Net Truck
Vertical
Load (Thousand
(THOUSAND POUNDS)
Pounds)
240
WILD Site
95.0% NTV
99.5% NTV
99.95 % NTV
Figure 6. NTV Forces for Three-Axle Trucks of 6-Axle Locomotives at
12-Mainline Locations for Three Probabilities of Occurrence
Comparison of Axle Spacings
In order to properly compare the WIM data to the design loadings, the axle spacings need
to be considered. Some relevant axle spacings are listed below:
•
Cooper E-Series (80,000-pound axles):
•
AREMA Chapter 15 Alternate Live Load
(100,000-pound axles):
Four axles at 60 – 72 – 60 inches
•
Typical 286,000-pound Freight Car:
Four axles at 70 – 80 – 70 inches
•
Typical 415,000-pound 6-Axle Locomotives:
Six axles at 60 – 60 – 60 inches
- EMD SD70MAC
Six axles at 84 – 80 – 155 – 80 – 84 inches
- GE ES44DC
Six axles at 79 – 79 – 165 – 79 – 79 inches
The axle spacings listed for freight cars and locomotives are for adjacent trucks of
coupled units. These spacings are approximate and will vary with slack conditions and
other tolerances. Because the actual axle spacings for freight cars are somewhat greater
than those for the heaviest four axles of the design loads, the design loads will tend to be
© AREMA 2009 ®
conservative compared to the actual vehicles for the same axle load. For locomotive
loadings, additional axles from the Cooper series will need to be considered (see
AREMA Chapters 8 and 15).1
Comparison of Equivalent Cooper Loads
One way to compare the actual measured loads and axle spacings to design loads is to
compute the equivalent Cooper load effects generated on spans of various lengths. Of
greatest interest for this study are span lengths less than 100 feet, where the effects of
high NTV forces from individual trucks will be most noticeable.
There are arguably many ways to compute the effects of the rare, but heavy rail
car truck loads over bridge spans. Various statistical methods and Monte Carlo
simulations could be performed using the actual distributions. One of the simplest
approaches is to consider a single truck with a 99.95-percent NTV force in a train of
otherwise nominal NTV forces.
Figure 7 compares Cooper load effects for the AREMA Chapter 15 alternate live
load, a string of typical unit train 53-foot cars with 286,000-pound GRL, and a similar
string of unit train cars that includes a single overloaded truck with a NTV force of
160,000 pounds. As expected, the effects of the single 160,000-pound truck are more
pronounced for shorter span lengths. Note that for an 8-foot span, the effect is as high as
the Cooper E-80 loading. In the less likely case of two adjacent trucks at 160,000
pounds, the effects are no different at the shortest span length, and gradually become
higher for longer span lengths. The load effects shown are the maximum of the
equivalent Cooper load effects for shear force, bending moment, and pier reaction.
Because the effects are dominated by the single heaviest truck load, it was deemed
unnecessary to perform any more sophisticated statistical simulations.
© AREMA 2009 ®
AREMA Chapter 15 Alternate Live Load
53-ft cars 286,000 lb with one truck 160,000 lb
53-ft cars 286,000 lb
Figure 7. Comparison of Equivalent Cooper Loads
Figure 8 includes two six-axle locomotives pulling the unit train of 53-foot
286,000-pound cars. The effect of a single freight car truck with NTV of 160,000 pounds
is generally more severe than a single three-axle truck of a six-axle locomotive with NTV
of 220,000 pounds over the range of spans shown. This is not unexpected, because the
locomotive axle load is only 73,300 pounds, as compared to 80,000 pounds for the freight
car.
© AREMA 2009 ®
AREMA Chapter 15 Alternate Live Load
Two Six-Axle Locos 415,000 lb + 53-ft cars 286,000 lb
Two Six-Axle Locos 415,000 lb with one truck 220,000 lb + 53-ft cars 286,000 lb
Two Six-Axle Locos 415,000 lb + 53-ft cars 286,000 lb with one truck 160,000 lb
Figure 8. Comparison of Equivalent Cooper Loads
Implications for Railroad Bridge Design and Rating
It is noted that the AREMA Chapter 15 alternate live load, in conjunction with the
Cooper E-80 loading, provides a relatively uniform margin between the governing design
load and the unit train with the single overloaded truck. This uniform margin is desirable.
For spans designed only to the Cooper E-80 load, the margin decreases as span length
decreases, particularly for spans of 12 feet and under. While the Cooper E-80 design load
is not exceeded, the margin goes to zero for spans of 8 feet and under.
For purposes of bridge rating, it is important to understand the actual loads that
are likely to be crossing bridges. The statistical distributions show the relationship
between nominal loads and actual loads.
© AREMA 2009 ®
Conclusions
This paper presents the current North American railroad vertical load environment as
measured using 12 wayside WIM systems on various main lines. The following force
levels were observed:
•
Freight car truck loads totaling 160,000 pounds travel over mainline routes
daily.
•
Three-axle locomotive truck loads totaling 220,000 pounds travel over
mainline routes daily.
•
The heaviest two-axle freight car truck loads are significantly higher than the
heaviest two-axle locomotive truck loads.
The WIM data used in this study includes traffic from intermodal, mixed freight,
passenger, and various unit trains.
The AREMA Chapter 15 alternate live load, in conjunction with the Cooper E-80
load, provides a relatively uniform margin between effects of design and measured loads
over the range of affected span lengths.
References
1. American Railway Engineering and Maintenance of Way Association. 2008. Manual
for Railway Engineering. Landover, Maryland.
© AREMA 2009 ®
List of Tables
Table 1. NTV Forces from Two-Axle Trucks from Wayside Detectors
Table 2. NTV Forces from Four-Axle Locomotives from Wayside Detectors
Table 3. NTV Forces from Six-Axle Locomotives from Wayside Detectors
List of Figures
Figure 1. Truck Weight Distribution for Two-axle Trucks from a Wayside Detector Site
with Primarily Loaded and Empty Coal Trains
Figure 2. Truck Weight Distribution for Two-axle Trucks from Wayside Detector Site
with Many Intermodal Trains
Figure 3. NTV Forces from Two-axle Trucks from a Wayside Detector Site with
Primarily Loaded and Empty Coal Trains
Figure 4. NTV Forces from Two-axle Trucks from a Wayside Detector with Many
Intermodal Cars
Figure 5. NTV Forces for Two-Axle Trucks at 12-Mainline Locations for Three
Probabilities of Occurrence
Figure 6. NTV Forces for Three-Axle Trucks of Six-Axle Locomotives at 12-Mainline
Locations for Three Probabilities of Occurrence
Figure 7. Comparison of Equivalent Cooper Loads
Figure 8. Comparison of Equivalent Cooper Loads
© AREMA 2009 ®