Civil Engineering Reference
In-Depth Information
TABLE 5.3
Variable Amplitude Stress Range Cycles per Train
Variable Amplitude
Total Variable Amplitude
Span Length,
Stress Range Cycles
Stress Range
L
(ft)
per Train
Cycles,
N
v
5.3
×
10
6
L >
100
3
10
6
100
>
=
L >
75
6
10.5
×
10
6
75
>
=
L >
50
55
96.3
×
10
6
50
>
=
L
110
192.5
×
Source:
AREMA, 2008, Manual for Railway Engineering, Chapter 15.
With permission.
where
N
v
is the total number of variable amplitude load cycles,
S
E60
is the stress
rangefromCooper'sE60load(characteristicofmodernfreighttrainloads),and
Δ
Δ
S
E80
10
6
trains over the bridge design life) and Equation 5.56 provide the adjusted number of
equivalent constant amplitude stress range cycles over the superstructure design life
as shown in
Table 5.4.
AREMA (2008) recommends reductions in the fatigue design live load as a means
of considering the lower number of cycles on lightly traveled railway lines. On rail-
way lines with less than 5 MGT (million gross tons) per mile per year, AREMA
(2008) recommends a fatigue design load based on Cooper's E40. On railway lines
with 5-15 MGT per mile per year, stress ranges from a Cooper's E65 design load are
recommended. Therefore, for all traffic levels, the number of effective or equivalent
constant amplitude live load stress ranges in Table 5.4 can be used to develop appro-
priate allowable fatigue stress ranges for the design of steel railway superstructure
members and details.
is the stress range from Cooper's E80 design load. Table 5.3 (based on 1.75
×
5.3.2.2.2.2 FatigueStrengthofSteelRailwayBridges
AREMA(2008)recom-
mends a stress-life approach for fatigue strength evaluation of members and details.
This is appropriate for high cycle stress range magnitudes that are generally low
enough to preclude the need of considering yield effect (predominantly elastic strains
TABLE 5.4
Constant Amplitude Stress Range Cycles
Equivalent Constant Amplitude Stress Range
Span Length,
L
(ft)
Cycles Over Member or Detail Life,
N
2.2
×
10
6
L >
100
4.4
×
10
6
100
>
=
L >
75
40.7
×
10
6
75
>
=
L >
50
10
6
50
>
=
L
81.3
×
